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Question How to build a UHF antenna... ( AVS Forum HDTV Technical )
Updated: 2008-05-23 09:32:18 (751)
How to build a UHF antenna...

Found this site while researching antennas. Worked great for me. Check it out if you need a cheap UHF antenna.

http://uhfhdtvantenna.blogspot.com

Answers: How to build a UHF antenna... ( AVS Forum HDTV Technical )
How to build a UHF antenna...

Sadly the reflector is steel, so the CG is a bit far back. I get better signal leaning back, so I can't complain. I'll keep a look out for thicker (hopefully straight!) wire; it goes in the right direction and is probably required to survive outdoor use. Probalby no point going outdoor without a rotor ... and if I go for that, a real CM-4221 is probably warranted.

fbov

How to build a UHF antenna...

I like the erzatz-4221. If it's all-aluminum, it's probably a lot lighter than the real 4221, and may be just as sturdy. The 4221 is constructed mostly of galvanized steel.

The real 4221 does use significantly thicker wire, though -- looks like 10 gauge or even 9 gauge. Thicker elements should provide better bandwidth, theoretically anyway.

nybbler

How to build a UHF antenna...

I'm glad you guys posted; it brought this thread to the top.

It seems I'm not crazy in making a 4-bay last weekend. I already posted some stuff on performance;
http://www.avsforum.com/avs-vb/showt...3#post13270043
the attachments are in the following post. Here's a bit on construction.

I cloned a CM-4221.
http://www.hdtvprimer.com/ANTENNAS/cm4221.html

I literally took dimensions from a picture and scaled to overall dimensions. When in doubt, I went big, to try and move the peak gain to lower channels. The screen's made from 24" wide 1x1" cage wire cut to 39". Wiskers and contacts are 18 gauge hanging-ceiling wire. Mounting blocks are delrin-like plastic, machined to wrap the pole and threaded for wisker mounting. Pole is 5/8" aluminum with cut-down and bent angle brackets for reflector mounting. Block spacing is 9" and wisker length is 8".

I made a little bending jig for the wiskers; you want them parallel under the screw so you can trap the signal feed. High molecular weight plastics are easy to machine and will take threads but deform under pressure; I used a 1/2x1/2" backer bar from a guillotine paper cutter.

I tried to minimize metal edges that might radiate, so there are no washers, and all the contacts are terminated tightly. I used rivets for the bar mount but found I needed threaded fasteners to hold the elements. Reflector mounts are behind the bar, but insulated from the reflector so there's no connection to the mounting bar which is, inturn, insulated form the elements.

The Channel Master signal feed layout is critical in my opinion, as it has two features.
1) The twist at top and bottom makes this a parallel/serial combination of 4 elements which has the same impedence of a single element, in this case a 300 ohm dipole. Add a 4:1 balun and a short cable run and I should have good signal transfer.
2) Remaining as perfectly vertical as possible minimizes these conductors' interference with the active elements. Maintaining consistent separation, especially in the twist at the ends, maintains impedence match between the elements, so the impedence match works. I only wish my mounting blocks could have held the leads higher off the pole, so there was more room for the twist.

Unlike some other projects here, this was not done on the cheap; I spent $25 at my local Lowe's. That's not exactly expensive, but it is close to the price of the real thing for an antenna that's not near as rugged and may not work as well. Thankfully I mount in an attic, so robustness isn't an issue, and performance is great so far. More importantly, as others have noted, I had fun.

And for $10, I can make a second one. That's starting to look like a CM-4228 ...

Have fun,
Frank

fbov

How to build a UHF antenna...

I did the same thing last year...
Built a 4bay with existing materials around the house. Used home electrical wiring, chicken wire and a 2X4. It worked great as compared to my existing 1980's vintage VHF/UHF antenna on top of my house. Then I splurged on a $32 Winegard 4bay and that even worked better. I bolted it below my existing antenna and got about 30% stronger signals as compared to the homebuilt.

If you want to save some money and want a fun afternoon project, the homebuilt is a good option.

One improvement I would recommend on the "http://uhfhdtvantenna.blogspot.com" post is that the copper wire that connects each bay should be bare copper. With insulated wire you lose a very small amount of signal. You also don't want the wires to touch, so just bend it up and bridge over it.

armand1

How to build a UHF antenna...

I have built a homemade DB-2 and it even picks up Digital VHF-hi ch 8 and 11. I have it indoor in 2nd floor closet pointing due east and I get DT-8 @ 25kw at 43 miles away northeast and DT-11 @ 15kw at 60 miles away northwest. I run though the Channel Master preamp 7777 in combine VHF/UHF setting.

-Willie

WillieAntenna

How to build a UHF antenna...

Quote:
Originally Posted by NightHawk
I have't seen a commerical pre-amp with only a 300 Ohm input.
Some of the Channel Master Spartan series do.

nybbler

How to build a UHF antenna...

I don't have the rf technical background like many of you, so please forgive my newbiness. I'm going to give this antenna a try, but I've got a few questions.

Some of you speak of "bays". 4-bay, 8-bay, etc. Is the antenna on the uhfhdtvantenna.blogspot.com site a 1-bay?

Getting onto the roof of my house is a scary proposition, so I like the idea of putting it in the attic. I live almost 30 miles from the transmitting tower. The signal has to go through a few hills and trees to reach me. Assuming I'm not going to gain much height by placing the antenna outside, does the signal lose much by having to pass through the wall of my house to get into the attic?

The uhfhdtvantenna.blogspot.com antenna uses tin foil. I see others using chicken wire and other light fencing. I've got all of them lying around. Is one material better than the others?

At the point where the coax cable must connect to your antenna, there is a coax-to-dual-wires gizmo. What is this called? (It's the only thing I don't have, and I'll have to pick one up at Radio Shack.)

The cable going from the antenna to my tv will be approximately 70' long. Is there some rule of thumb about when I'll need an amplifier?

Thanks for any help.

enchant

How to build a UHF antenna...

I use one of these to watch Huntsville AL HDTV (about 47 miles away). I have it in a window facing the towers, and I run the signal through a 24dB VHF/UHF amplifier to boost it up a bit.

My signal checker app (DVIco) reports 23-27 dB for every station except one. It wouldn't last a week outdoors, but it's just right sitting in my window.

A picture is in order (thanks to ImageShack for Free Image Hosting).

=====

I would love to find a plan for a good single frequency VHF antenna. I have found lots of ham radio plans, but when I resize them for the MHz I need, they end up being a bit large.

Kafei

How to build a UHF antenna...

Quote:
Originally Posted by lemmalone
I assumed that MClever has only a 300ohm input on his amp, and thus saved unnecessary conversions from 300 to 75 to 300 to 75 again
I have't seen a commerical pre-amp with only a 300 Ohm input.

NightHawk

How to build a UHF antenna...

Quote:
Originally Posted by Whidbey
I wondered the same thing but I think the only way to get an answer is to do it myself. Gotta admit if I had known about how I could have built my own 4228, I may have given it a shot before buying. Oh well, the 4228 looks better on my roof than 2 x 4's and chicken wire.

James
I built a 4228 and have purchased a new 4228. The factory one still works better than your home made one. Also the factory one will stand up to severe weather conditions. The home made one works great in the house for a set top antenna.Generally I don't have to turn it for the other stations, it receives then even from the back. I built one with solid copper wire and to my surprise the one I built with coathangers worked better. Don't waste your time building a Yagi antenna!! I built one , but it does not work at all! I have a 91XG factory antenna and it works better the the 4228, but a rotor is a must!

dxernut

How to build a UHF antenna...

Quote:
Originally Posted by NightHawk
You're not saving anything there. By using the 300 Ohm input your simply going through an internal balun anyway. The pre-amp itself will be unbalanced and the balanced line must be converted somewhere.
I assumed that MClever has only a 300ohm input on his amp, and thus saved unnecessary conversions from 300 to 75 to 300 to 75 again, but using two twinleads to connect to the amp rather than two baluns. I have the CM Spartan 3, which has a 300ohm input and a 75ohm output, I believe, so that is how I would combine two antennas, and it is one way I would join two halves of an 8bay. Does that make sense?

lemmalone

How to build a UHF antenna...

Quote:
Originally Posted by MClever
I used 300 ohm twin lead wire (2 equal lengths) direct to 300 ohm input/ 75 ohm output preamp to eliminate balun/combiner loss.
You're not saving anything there. By using the 300 Ohm input your simply going through an internal balun anyway. The pre-amp itself will be unbalanced and the balanced line must be converted somewhere.

NightHawk

How to build a UHF antenna...

Yeah, it's funny...I'm pretty close (within 30 miles) of most of the transmitters, but my little RCA antenna just wasn't cutting it. The antenna I built fit perfectly in my garage attic, and luckily, all of the transmitters are within 15 degrees of each other from here, so I rarely have to tweak the rotator. My 3-year old son and I built this antenna from uhfhdtvantenna.blogspot.com in less than an hour and I'm MORE than happy with it!! Since he watches PBS Kids here in St. Louis (and we don't get to watch hardly anything else) it's perfect...and FREE!

seismic744

How to build a UHF antenna...

Quote:
Originally Posted by MClever
For me the DIY 8-bay would not pull in the long distance stations as the 4-bay, so I've stopped messing with it. It could be the multipath or probably an error in assembly on my part.
I also found that my 8bay did no better, and maybe worse, than a 4bay. Kind of a puzzle to me. On the weatherproofing subject, I think you can make it about as weatherproof as you want. One tip would be to have some kind of sturdy positioned horizontally a few inches above the highest element of the antenna for birds to perch on. I have noticed that alot of old commercial-grade antennas have broken elements, and I've seen alot of newer antennas with birds sitting on the elements.

lemmalone

How to build a UHF antenna...

Quote:
Originally Posted by nybbler
Right, they aren't weatherproof at all. The other thing is that the CM4221 is pretty cheap so you're just not saving all that much if you don't have all the parts lying around. It's fun to play with.
Parts were lying around and if it didn't work, no loss other than time. It was fun.

Quote:
Originally Posted by nybbler
I decided to see just how crappy an antenna would work .
Actually, the crappy one I built works better than the one I spent more time on.

MClever

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
This was for a 4-bay, right? With an 8-bay, I suppose I could do this, but I would still have the radiating feeder lines to deal with. Running 300 ohm twin lead to each side would not seem to be practical?
On the 4-bay, I have a balun at the feed point to a CM7777.

On the 8-bay, I have a piece of 300 ohm twinlead from each 4-bay section connecting at a central point on back of the antenna (which actually is my 300 ohm preamp input). I tried it w/2 baluns to a combiner to the CM7777, but it didn't perform as well as the twinlead setup.

For me the DIY 8-bay would not pull in the long distance stations as the 4-bay, so I've stopped messing with it. It could be the multipath or probably an error in assembly on my part.

MClever

How to build a UHF antenna...

Quote:
Originally Posted by MClever
I used 300 ohm twin lead wire (2 equal lengths) direct to 300 ohm input/ 75 ohm output preamp to eliminate balun/combiner loss.
This was for a 4-bay, right? With an 8-bay, I suppose I could do this, but I would still have the radiating feeder lines to deal with. Running 300 ohm twin lead to each side would not seem to be practical?

Falcon_77

How to build a UHF antenna...

Right, they aren't weatherproof at all. The other thing is that the CM4221 is pretty cheap so you're just not saving all that much if you don't have all the parts lying around. It's fun to play with, though. I decided to see just how crappy an antenna would work so I made four elements out of aluminum foil and cardboard, stapled them to a shoebox, ran aluminum foil between them, and taped a balun in the middle of the foil "phasing line". Worked quite well, getting in UHF stations from Allentown (about 30 miles away over hilly terrain) from my second story, at least as long as the balun stayed attached.

nybbler

How to build a UHF antenna...

Quote:
Originally Posted by seismic744
I just built the one from uhfhdtvantenna.blogspot.com, mounted it in my attic with a Magnavox rotator, and it works great! I went from an average of 50% signal strength to consistent 80 and 90% ranges.
These DIY antennas work better than what most people will give credit. I do believe their application is only for indoor use and fail to see how they will hold up to the weather/wind even if 1"x2" screen is used.

I had my CM4221-copy professionally tested (my neighbor is a retired brodcast engineer that now does OTA installs and has a patent-pending antenna design of his own). I'm getting similiar db readings (hand held meter) as a commercial-built CM4221 (4-bay). He was shocked to see this and was amazed I was locking digital stations up to 75 miles away out of my attic (using a balun w/CM7777 preamp).

He had some thoughts on possibly improving this DIY design for fringe areas:
1. get rid of the foil reflector. Run either no reflector or install 1x2 screen.
2. Use copper wire for the connecting wire and > < 's
3. Use 300 ohm twin lead to 300 ohm input preamp to eliminate balun loss before preamp.

I'm on the edge of reception, so I need to be outdoors where the reception db measurements were favorable. This DIY project will not be installed outside. But I can see it working well for people that are closer to the towers than I.

MClever

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
How does your 8-bay build compare with a CM4228? I've been eying the feeder lines on mine between the 2 sides which I've heard causing return loss/radiation problems. As your build appears to have solved that problem, I am curious as to the result.
I used 300 ohm twin lead wire (2 equal lengths) direct to 300 ohm input/ 75 ohm output preamp to eliminate balun/combiner loss.

MClever

How to build a UHF antenna...

I just built the one from uhfhdtvantenna.blogspot.com, mounted it in my attic with a Magnavox rotator, and it works great! I went from an average of 50% signal strength to consistent 80 and 90% ranges.

seismic744

How to build a UHF antenna...

Quote:
Originally Posted by Rick0725
mine works better.

going to find better screen and clean it up alittle.
How does your 8-bay build compare with a CM4228? I've been eying the feeder lines on mine between the 2 sides which I've heard causing return loss/radiation problems. As your build appears to have solved that problem, I am curious as to the result.

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
How does your 8-bay build compare with a CM4228? I've been eying the feeder lines on mine between the 2 sides which I've heard causing return loss/radiation problems. As your build appears to have solved that problem, I am curious as to the result.
I wondered the same thing but I think the only way to get an answer is to do it myself. Gotta admit if I had known about how I could have built my own 4228, I may have given it a shot before buying. Oh well, the 4228 looks better on my roof than 2 x 4's and chicken wire.

James

Whidbey

How to build a UHF antenna...

I didn't buy anything. I built one of these myself in like 1/2 hour and it works perfectly.

bp238

How to build a UHF antenna...

Thank you....

bp238

How to build a UHF antenna...

bull...dont buy it.

especially with the The Mitzu CRA-5000 VHF/UHF antenna scam in one of the pictures.

my picture came from another friggin blog.

Rick0725

How to build a UHF antenna...

mine works better.

going to find better screen and clean it up alittle.

Rick0725

How to build a UHF antenna...

Mine is working perfectly...don't think your's or any other can be better than that. Your's looks a bit bulky and awkward.

bp238

How to build a UHF antenna...

Yeah, but Bob just crosses his, so it can't be a strong driver, more a best practice. I'm a copycat; I cloned the CM-4221, partly because their design details made sense. I find a certain joy in trying to make things that match the requirements of the physical world.
----
with apologies to cpcat ...

fbov

How to build a UHF antenna...

Quote:
Originally Posted by fbov
Regardless, see if my element base geometry improves your feedline capture issues. If it does, add upper and lower elements at the same element spacing you've already established, with a twist to phase match the other element on that feedline. This series-parallel arrangement maintains the dipole's native 300 ohm impedence as well as signal phase. It just strikes me as one of those designs-that-work!
So, when I re-build the feed lines for the top and bottom bays, I should keep the lines parallel and only have a sudden twist at the end, right? Previously, I had them crossing over in the middle (between the bays).

Falcon_77

How to build a UHF antenna...

Looks like I should have stopped at Lowe's instead of Home Depot ... 12 ga. is closer to the commercial gauge than the 18 ga. I used, and you got it straight!

Optimum antenna size depends on the channels you can receive. 10" should have peak gain around Ch 38, and would be great if all stations are Ch 50 and lower. I saw a drop in signal strength for my UHF 58 (my weakest at -77.5dBm) using a 10" design to mid-70's compared with mid-90's with the 7" spacing. YMMV, and we know Bob's does!

For my "experimental" set-up (read nice looking) I run the balun straight out the back. The feedlines are on cross-pieces mounted on the spine, so I just put a nut on the back side and attach there. I see no reason this approach won't work.

There's also a method behind CM's feedline routing. As I recall, optimum propagation requires a specific spacing between parallel conductors that's a function of feedline diameter. That's why the insulator in twin-lead hold a constant spacing. This is clearly a second order effect, given the results of those who ignore it, but one CM just as clearly built into the CM4221/4228. CM's approach also gives you equal length feedlines, another plus.

It may be time to stop at Lowe's ...

Frank

PS Mike, good catch on the balun; a practical reminder to check for continuity in coax, baluns, splitter/combiners, etc.

fbov

How to build a UHF antenna...

Quote:
Originally Posted by johnied
May I ask what material you are using for your bowtie elements.
It is aluminum suspended ceiling wire, 12 gauge. I found them in Lowe's (thanks to a tip from Frank). They are sold in bundles of 50 6' straight pieces and have been easy to work with. They were about $8 for the bundle. This is cheaper per foot than the 100' coil that I have yet to use (which was about $7).

The following is the part number:

181510 6' USG Hanging Wire 12GAU

Falcon_77

How to build a UHF antenna...

Well, I spent a lot of time rebuilding the feedlines tonight only to find out that the balun is faulty. I discovered that signal strengths fluctuated significantly depending the angle that I held the cable, which caused me to find out that the leads into the balun were loose. Replacing the balun helped significantly, but I had already dismantled the other two bays by then.

However, not all is lost as I've discovered that mounting the feedline to the back may actually be viable (see pic). After I make sure the 2-bay is ok again, I will re-build the other two bays... but not tonight!

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Well, I spent a lot of time rebuilding the feedlines tonight only to find out that the balun is faulty. I discovered that signal strengths fluctuated significantly depending the angle that I held the cable, which caused me to find out that the leads into the balun were loose. Replacing the balun helped significantly, but I had already dismantled the other two bays by then.

However, not all is lost as I've discovered that mounting the feedline to the back may actually be viable (see pic). After I make sure the 2-bay is ok again, I will re-build the other two bays... but not tonight!
May I ask what material you are using for your bowtie elements.
These look like they might be something i want to give a try.
Thanks .


John.

johnied

How to build a UHF antenna...

Bob, thanks for the tips.

I'm using 10" elements and 10" spacing on my UHF bow-tie build, to optimize reception for "band IV" as it's rarely called in the US.

I will try to re-work the feedlines and see what happens. Right now they are perhaps too close or bending one instead of the other causes the lengths to be different. Regardless of the problem, it seems clear that these bays are out of phase at the moment.

Falcon_77

How to build a UHF antenna...

Well, my attempt to upgrade the 2-bay to a 4-bay didn't go so well. Attached are pictures of my build, but I think the problem lies in the feed-lines. They don't touch at the cross-over but perhaps they are too close. Also, I have separate pieces for the top and the bottom and not one continuous line.

The results are about 50% worse than the 2-bay build, so I guess it's back to the drawing board.

I think a Hoverman is easier to build than a 4-bay and I'm having very good results with that (picture also attached).

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Well, my attempt to upgrade the 2-bay to a 4-bay didn't go so well. Attached are pictures of my build, but I think the problem lies in the feed-lines. They don't touch at the cross-over but perhaps they are too close. Also, I have separate pieces for the top and the bottom and not one continuous line.

The results are about 50% worse than the 2-bay build, so I guess it's back to the drawing board.

I think a Hoverman is easier to build than a 4-bay and I'm having very good results with that (picture also attached).
Mike: Thanks for the pictures. On the 4 bays that I experimented with, I bent the wires up and over, and also laid them nearly flat, with tape on the crossovers. I found the latter to work better.
On your 4 bay, it looks like you went with 8" whiskers & 8" spacings. This design was my poorest effort, even though Frank's tests showed that it should work good on all but the highest RF #s.
Before you tear down your good looking 4 bay, try routing the connecting wires like I did in post #94. I leave the spacing far apart, then bend sharply towards the crossover.
Keep us posted on the Hoverman. It looks like fun.

spokybob

How to build a UHF antenna...

AJ: How clever. Thanks for the great pics.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by Beerstalker
Would I be better off ditching the 300-75 ohm converter and soldering one of the adapters like the Youtube antenna uses?

How about attaching multiple bowtie antennas together kind of like the Youtube antenna? Or should I just leave well enough alone?
partment complex.
I trimmed off the wires from a transformer like this and soldered them where the twin-lead went before (see my post on page #4). I found that it improved reception when compared side-by-side with an unmodified bowtie connected to one similar to your first picture. My guess is by reducing the amount of twin-lead it reduces the amount of signal picked up by the wire after the antenna and thus reduces multipath.

Since they are so cheap, it's worth trying.

cpldc

How to build a UHF antenna...

Quote:
Originally Posted by Beerstalker
I found that the bowtie antenna that I used as a kid to get Fox before we got to cable actually worked great for me with it sitting inside on top of my entertainment center.

I have a 4 bowtie antenna in the attic but found that I get much better reception with a simple $5 Radio Shack bowtie placed in a window. I have topper windows in my 10-foot ceiling living room so the antenna is about 8 feet above ground level. I use the same type 300-to-75 matching transformer as shown in your first picture, connected to RG-6 run behind the topper curtains and down the side of the lower windows behind the blinds. I get all the local OTA channels (one of which is VHF), ranging from 14 miles to 37 miles distance, 302 to 4 degrees direction (antenna facing midway between), -45.0 to -81.0 dB. Under certain weather conditions I have seen a few (very few) dropouts on the -81 dB station, but never on any of the others.

I am amazed by that little antenna's performance. A couple of pictures are attached.

A J

A J

How to build a UHF antenna...

Quote:
Originally Posted by Beerstalker
How about attaching multiple bowtie antennas together kind of like the Youtube antenna? Or should I just leave well enough alone?
If you are getting what you want now, you can probably leave it alone. Looking at the stations on your list, they are staying on UHF. However, if there are any VHF stations you will want, you may need to add a VHF antenna, but that would be a topic for another thread.

Falcon_77

How to build a UHF antenna...

I made a conventional 2 bay to test against the 2 bay that was a 4 bay with the center 2 removed.
I have been using this 2 bay for 2 days with a few dropouts on the 2 weakest channels. The newer 2 bay looks a little more directional because 12.1 is off to the side. I think this 2 bay design may have some potential in urban area, setting on top of the convertor box.

May 7_9 test
Date 7-May 7-May 7-May 9-may
Time 9:00 PM 9 PM 9 PM 9 PM
Rec RcaCECB RcaCECB RcaCECB RcaCECB
Loc bedroom bedroom bedroom bedroom
antenna Cm4220 4 bay old 2 bay new2bay
pointed 70 deg 45 deg 45 deg 45 deg
amp no no no no

58 4.1 47 42 32 37
56 6.1 76 50 44 52
38 8.1 67 56 44 55
45 12.1 50 40 32 24
49 18.1 62 55 37 49
23 24.2 35 26 18 18
Old 2 bay

newer 2 bay

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by spokybob
This is ugly, but this is a test board. I have always used washers under the screw heads. I found a few of these fender washers. Mechanically they work well. I'm not sure if they affect the signal.
Thanks. The washers I have are probably just too small. Some nylon ones might be a good idea as well. I can't think that having the feedlines getting buried into the wood is a good thing.

Falcon_77

How to build a UHF antenna...

You guys seem to really know about this antenna stuff so I wanted to ask for some of your input. I tested a bunch of different antennas last night with my new DirecTV AM21. The two that worked the best were the RCA ANT806, and the Phillips MANT940, if I had them outside on my balcony. They did not work well indoors. I realize they are not the best antenna's but they were the ones I could buy locally.

However I also decided to give the old school a try as well, and I found that the bowtie antenna that I used as a kid to get Fox before we got to cable actually worked great for me with it sitting inside on top of my entertainment center. It didn't get quite as good of a signal as the RCA or Phillips but it was only 5-10% lower, and by using it I didn't have to worry about drilling another hole through the exterior wall of my apartment to run a cable in. Right now I am getting the following channels using it (I have also included signal strength)

6 (45-55%) out of the Quad Cities
8 (70-75%) out of the Quad Cities
18 (60-65%) out of the Quad Cities
19 (95-100%) Peoria Local
25 (100%) Peoria Local
31 (85-95%) Peoria Local
43 (85-90%) Bloomington Local
47 (95-100%) Peoria Local
59 (55-65%) Peoria Local

This is using this bowtie antenna and this converter to hook to a 300-75Ohm converter, to a male to male RF connector to a 12' Monster RF cable (I know Monster is a ripoff but this is an old cable I had laying around).



Would I be better off ditching the 300-75 ohm converter and soldering one of the adapters like the Youtube antenna uses?

How about attaching multiple bowtie antennas together kind of like the Youtube antenna? Or should I just leave well enough alone?

If you want to look up my antenna web stuff my address is
7150 N Terra Vista Dr
Peoria, IL 61614

I'm on the top floor of a 3 level apartment complex.

Thanks guys, I'm looking forward to your input.

Beerstalker

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Attached is my attempt to convert my 1-bay into a 2-bay. I had problems getting the feed lines to stay in place. They kept sliding out when I would tighten the screws. I ended up placing them flat against the wood, but that caused them to sink it when I tightened them. I don't imagine that is the best thing for reception. Are there any tricks to keep the feed lines in place?
This is ugly, but this is a test board. I have always used washers under the screw heads. I found a few of these fender washers. Mechanically they work well. I'm not sure if they affect the signal.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Attached is my attempt to convert my 1-bay into a 2-bay. I had problems getting the feed lines to stay in place. They kept sliding out when I would tighten the screws. I ended up placing them flat against the wood, but that caused them to sink it when I tightened them. I don't imagine that is the best thing for reception. Are there any tricks to keep the feed lines in place?
I would try some flat washers. That should keep them from embedding into the wood. You can try nylon if metal is a concern.

wblynch

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Attached is my attempt to convert my 1-bay into a 2-bay. I had problems getting the feed lines to stay in place. They kept sliding out when I would tighten the screws. I ended up placing them flat against the wood, but that caused them to sink it when I tightened them. I don't imagine that is the best thing for reception. Are there any tricks to keep the feed lines in place?

With this 2-bay, I recorded strong signals for some channels, but the SNR didn't improve at all and the higher channels (mostly the 50's) saw a drop vs. the 1-bay design...

Perhaps there is a mis-match. I can complete a 4-bay design if I can get the top and bottom feedlines to work somehow. What is the reason that they shouldn't be straight all the way up and down? The top and bottom need a twist for impedance matching reasons?
As long as the elements have good electrical contact to the feedlines, sinking into the wood won't hurt; the screw already does that far better. I made my elements a little differently, attached, so the feedline doesn't actually go under the screw head, but lots of the element does. This element base geometry was obvious to me, but no one else seems to do it.

You've ganged them together in the normal manner for 2-bays. The twist in the 4-bay is to phase-match the bowtie elements on each leg of the feedline. Your elements have independent feed lines, so you just have to make sure the feedlines are the same length (terminals in the middle) to phase match.

As to response, impedence won't be 300 ohms, and will likely vary with frequency. You're using oversize elements as I recall, so upper channels may suffer more, and you're comparison is time-shifted - you used the same parts - so perhaps there were signal field changes?

Regardless, see if my element base geometry improves your feedline capture issues. If it does, add upper and lower elements at the same element spacing you've already established, with a twist to phase match the other element on that feedline. This series-parallel arrangement maintains the dipole's native 300 ohm impedence as well as signal phase. It just strikes me as one of those designs-that-work!

Frank
----
johnied, I think you're right, too! 1/2 wave due to the twist and 1/2 wave due to the element spacing at the design frequency.

fbov

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Attached is my attempt to convert my 1-bay into a 2-bay. I had problems getting the feed lines to stay in place. They kept sliding out when I would tighten the screws. I ended up placing them flat against the wood, but that caused them to sink it when I tightened them. I don't imagine that is the best thing for reception. Are there any tricks to keep the feed lines in place?

With this 2-bay, I recorded strong signals for some channels, but the SNR didn't improve at all and the higher channels (mostly the 50's) saw a drop vs. the 1-bay design...

Perhaps there is a mis-match. I can complete a 4-bay design if I can get the top and bottom feedlines to work somehow. What is the reason that they shouldn't be straight all the way up and down? The top and bottom need a twist for impedance matching reasons?

Sort of, but not for imepdance but for "phasing" so all antennas(which each bowtie is really)
"in phase" Imagine the Current and Voltages peaking at different points on
each bowtie or.antenna without phasing . I believe the phasing puts a half wavlenghth at opposite polarity(the twist) to the wave. ,making the wave identical on all the bowties.. I think im right on this :P..

johnied

How to build a UHF antenna...

Attached is my attempt to convert my 1-bay into a 2-bay. I had problems getting the feed lines to stay in place. They kept sliding out when I would tighten the screws. I ended up placing them flat against the wood, but that caused them to sink it when I tightened them. I don't imagine that is the best thing for reception. Are there any tricks to keep the feed lines in place?

With this 2-bay, I recorded strong signals for some channels, but the SNR didn't improve at all and the higher channels (mostly the 50's) saw a drop vs. the 1-bay design...

Perhaps there is a mis-match. I can complete a 4-bay design if I can get the top and bottom feedlines to work somehow. What is the reason that they shouldn't be straight all the way up and down? The top and bottom need a twist for impedance matching reasons?

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by spokybob
...
I don't understand theory, that's why I like all your comments. It helps me learn about this strange world of antennas.
Bob 61231
Bob,
Consider that the IRRL will not accept antenna advertising based on empirical data, only NEC modeling. Why? Because you can arrange test conditions to prove almost anything, and can never (especially in a consumer location) control all the variables.

You, on the other hand, are living in an empirical world; whatever works is best, regardless what theory says. That's what makes this so difficult, and so interesting!

Frank

fbov

How to build a UHF antenna...

Yes. On this particular session, I was able to rotate for best reception, without moving the antenna to various locations. 24.1 is usually the weakest, but I have to be check to see if 4.1 is still OK.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by spokybob
Yes. On this particular session, I was able to rotate for best reception, without moving the antenna to various locations. 24.1 is usually the weakest, but I have to be check to see if 4.1 is still OK.
All of the signal meters I have used, 100 would be perfect and 0 would be no signal. Actually, my 622 box with a 4221 on the roof, does have 100, with 62 the drop out point. My RCA box has never exceeded 76, and the dropout is around 16 or so.
In all my previous tests, I have mounted the Youtube on a wall,either facing east/west or north/south because the elderly folks I am helping either want to hide the antenna, or do not have the room for a floor mount so it must be hung on the wall. I have done 4 installations so far with good results with my 4 bay. There has been 2 days with strong winds that have caused pixelation, oddly enough our 6.1 was the worst one of those days.
Now I am beginning to understand that db & measured signal strength are not related as least as far as measured numbers. I don't understand theory, that's why I like all your comments. It helps me learn about this strange world of antennas.
Bob 61231

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by spokybob
I did my testing tonight in the bedroom with my RCA CECB and the CM4220, 4 bay and a modified 2 bay. All channels lock in.
I noticed your chart showed that the cm4220 was the strongest (if higher numbers mean better)

I also noticed that the cm4220 was pointed in a different direction than the other 2.

Were the directions that you had listed where you got the best signal?

mclapp

How to build a UHF antenna...

Bob,
With the single assumption that your 4220 has it's reflector, your results line up perfectly: 4220 > 4-bay > 2-bay, as one might expect. Add a reflector to the 4-bay and I expect it'll beat out the 4220, but you clearly don't need it!

And Falcon_77's right; no correlation between signal readings and dB's when using consumer products. Test equipment and professional products are another story, and another zero on the price!

Falcon_77,
Positional differences are far more likely in our diffracted signal fields than in the LOS fields Bob is working in. He may not see a positional effect at all!

Thanks for clarifying your data; it really helps to know if better is up or down.

Frank

fbov

How to build a UHF antenna...

Quote:
Originally Posted by spokybob
I did my testing tonight in the bedroom with my RCA CECB and the CM4220, 4 bay and a modified 2 bay. All channels lock in.
As for these test results, I don't know how to assign dB differences to the different signal strengths.

On my Sony, which has an AGC figure, strong signals are between 39 and 44, but weaker signals will go from 50 to 100 with relatively little change. Using stock attenuators can shed a little light on the difference, but exact measurements will remain in the realm of much more expensive instruments.

Falcon_77

How to build a UHF antenna...

I did my testing tonight in the bedroom with my RCA CECB and the CM4220, 4 bay and a modified 2 bay. All channels lock in.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by spokybob
Now the 6+db is something I can not seem to understand.
I just meant that it's quite possible to find spots that are weaker or stronger by a factor of 4. Actually, a factor of 10 difference is probably easily achieved as well, especially in an attic. One spot could be 10dB weaker than one a few feet away, etc.

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Ken H
Baluns have been evaluated and discussed here, and there are performances differences between them. In general, cheaper ones don't work as well.
That makes sense Ken. Although I paid $5 for the first one at Radio Shack, I wonder if it is a cheaper one. On my next YouTube build, I will test that theory against my Ebay baluns.
Frank: Thanks for the FAQ post. I did a CECB/Youtube install this morning at ZIP 61260. As an afterthought, I tried her rabbit ears with the CECB. It would only pickup 38,49,56 with the ears carefully positioned next to the glass slider.The Youtube is now hidden behind the couch and gets 38,49,56,58,23,45,& 25.

spokybob

How to build a UHF antenna...

That channel reception pattern is interesting, especially in light of TVFool giving worst-case predictions, what's called P(90,90). (90% probablity with 90% confidence, iirc)

P(90,90) is something like 10 to 12 dB lower than P(50,50), so the actual signal received with rabbit ears (38,49 and 56 have signals >-80.7 dBm) was more like -70 dB, right at TVFool's indoor limit. Add an antenna with roughly 10 dB gain and you're getting stations will into the -90dBm range, although I'll bet that KWKB-25 and WQPT-23 are the first to fade when conditions worsen. It'll be interesting to see what happens.

Frank

fbov

How to build a UHF antenna...

Well, update for all you folks. I got wire from a buddy I visited on Friday last week (he was running his Tesla Coil - neat stuff!). 14 gauge household electrical wire. I measured it and bent it for use as a Single Gray-Hoverman, as I didn't want to cut anything. Finally bought a Phillips balun from K-Mart today, for 3.99. Took my wires, a wooden yardstick and screwed them down to it. I only put screws in the center point where they were needed for the balun, just to test it out. I check the TV using my DTX-9900, and WOW.

All of my channels are coming in with a minimum of 45, maximum signal level is 83!

The new G-H antenna is also about 3 feet lower than the rabbit ears sat, as it has no lead cable. For being a quick-and-dirty build, incomplete, and not mounted where I want it, I'm thoroughly impressed!

Now, to work on getting more channels......

aethyrmaster

How to build a UHF antenna...

Quote:
Originally Posted by fbov
That channel reception pattern is interesting, especially in light of TVFool giving worst-case predictions, what's called P(90,90). (90% probablity with 90% confidence, iirc)

P(90,90) is something like 10 to 12 dB lower than P(50,50), so the actual signal received with rabbit ears (38,49 and 56 have signals >-80.7 dBm) was more like -70 dB, right at TVFool's indoor limit. Add an antenna with roughly 10 dB gain and you're getting stations will into the -90dBm range, although I'll bet that KWKB-25 and WQPT-23 are the first to fade when conditions worsen. It'll be interesting to see what happens.

Frank
I again thank you for sharing your knowledge with us. Yesterday we had strong winds and she said 24.1 and 4.1 were pixelating. (She picks up new words very easily.) LOL. Amazing that 12.1 off to the side still was OK. Her antenna is on the south wall of the house, resting on the floor.
I have reverted back to the original Youtube design with 7" whiskers and 5 3/4 spacing. I conducted a coathanger drive and used these also. At the crossover point, I used black tape & did not raise one wire up.
Testing on my garage CECB and using the DISH 622, the numbers are up all across the band. On the next windy day I will take my latest example down to 61260 to see if this is better than hers. The Insignia does not have numbers on the meter, only bad/good.
Here is Vanna & Youtube on my HDTV. They both look good.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by fbov
PS I should mention that a channel-frequency table will tell you the frequencies of the channels you want. Those numbers, in megahertz, divided into 300 (speed of light in m/MHz) yield the wavelength in meters. As an example, 600 MHz, Ch 35, would have a 0.5m wavelength, so 1/2 wave is 9.86", but ch 35 a bit low to be practical.
Thanks for the info ... very interesting and educational (for me anyway). I have a question regarding how to calculate the peak frequency. Running the numbers thru the formula, the 1/2 wave for channel 66 (the peak for a CM4221) is 7.5", not the 8" one would expect. 8" would equate to channel 58. Or am I looking at this the wrong way? I have a CM4228 and I noticed that the whiskers are spread approx. 5-1/2". And if I measure horizontally from the point of the whisker to the outer edge, the horizontal measurement is 7.5".

So my question, does it make sense that the peak corresponds to the horizontal measurement of the whisker out from the bend point?

tblue01

How to build a UHF antenna...

Quote:
Originally Posted by fbov
That channel reception pattern is interesting, especially in light of TVFool giving worst-case predictions, what's called P(90,90). (90% probablity with 90% confidence, iirc)

P(90,90) is something like 10 to 12 dB lower than P(50,50), so the actual signal received with rabbit ears (38,49 and 56 have signals >-80.7 dBm) was more like -70 dB, right at TVFool's indoor limit. Add an antenna with roughly 10 dB gain and you're getting stations will into the -90dBm range, although I'll bet that KWKB-25 and WQPT-23 are the first to fade when conditions worsen. It'll be interesting to see what happens.

Frank
Here is the F(xx,yy) comparison chart andy.s.lee prepared for an idealized smooth earth:
http://www.avsforum.com/avs-vb/attac...0&d=1195561571
http://www.avsforum.com/avs-vb/showt...&#post12266426

He didn't do F(90,90), but it's probably more than 10-12 dB lower than F(50,50).
The difference is also very dependent on range (and topography...which he did not include).

BTW: TVFool uses F(99,99) for Rx(dBm)....which is perhaps 20-30 dB lower than F(50,50):
http://www.avsforum.com/avs-vb/showt...&#post10661475
http://www.avsforum.com/avs-vb/showt...&#post10661305

There is the also an issue re how much Land Use Clutter Loss needs to be added to L-R results.
[TvFool also does not include foliage loss, building/attic/indoor loss, antenna gain, degradation
due to VSWR mismatch, cable/splitter loss and any improvement due to using Preamp....

holl_ands

How to build a UHF antenna...

Quote:
Originally Posted by tblue01
Thanks for the info ... very interesting and educational (for me anyway). I have a question regarding how to calculate the peak frequency. Running the numbers thru the formula, the 1/2 wave for channel 66 (the peak for a CM4221) is 7.5", not the 8" one would expect. 8" would equate to channel 58. Or am I looking at this the wrong way? I have a CM4228 and I noticed that the whiskers are spread approx. 5-1/2". And if I measure horizontally from the point of the whisker to the outer edge, the horizontal measurement is 7.5".

So my question, does it make sense that the peak corresponds to the horizontal measurement of the whisker out from the bend point?
There are a bunch of things that go into the calculation as all the wires (elements) interact with each other. Diameter of the element,spacing,ect..

The spacing of the elements to each other (bay to bay) and reflector spacing along with element length all factor in. Changing any one of those factors will change the frequency responce.

mclapp

How to build a UHF antenna...

When doing an NEC modeling exercise, the size and location of every bit of metal has to be included. However, once that's done, there is a much simpler option that retains a high degree of accuracy over a limited range - scaling.

In my case, I started with a CM4221, which when properly measured has certain relationships between physical size and frequency response. Peak gain is at a frequency whose wavelength equals the width of the bowtie when wiskers are spread. This is an observation only, but it fits with how EM radiation interacts with matter.

If one changes all dimensions when making a copy, you get a scale model of the original, and in antennas, that means you can scale the frequency axis. If it's twice the size, just divide each frequency in the gain table by 2 to get the new gain curve. (But 2x is a BIG change! Scaling may not apply this far out.)

Conversely, if you know an antenna's peak gain frequency and you want to move it, your scale factor becomes the frequency ratio. In this case, a CM-4221 peak gain is Ch 66, 785 MHz, and you want to move it to Ch 51, 695 MHz. The ratio is 785/695=1.129x, which is real close to 1 1/8 (1.125).

The key is that ALL DIMENSIONS have to change by this amount. Some things, like wire diameter, have smaller effects, but basic building blocks like element size and location have to all change by the same factor to maintain any degree of predictability.

Frank

fbov

How to build a UHF antenna...

Quote:
Originally Posted by fbov
When doing an NEC modeling exercise, the size and location of every bit of metal has to be included. However, once that's done, there is a much simpler option that retains a high degree of accuracy over a limited range - scaling.

In my case, I started with a CM4221, which when properly measured has certain relationships between physical size and frequency response. Peak gain is at a frequency whose wavelength equals the width of the bowtie when wiskers are spread. This is an observation only, but it fits with how EM radiation interacts with matter.

If one changes all dimensions when making a copy, you get a scale model of the original, and in antennas, that means you can scale the frequency axis. If it's twice the size, just divide each frequency in the gain table by 2 to get the new gain curve. (But 2x is a BIG change! Scaling may not apply this far out.)

Conversely, if you know an antenna's peak gain frequency and you want to move it, your scale factor becomes the frequency ratio. In this case, a CM-4221 peak gain is Ch 66, 785 MHz, and you want to move it to Ch 51, 695 MHz. The ratio is 785/695=1.129x, which is real close to 1 1/8 (1.125).

The key is that ALL DIMENSIONS have to change by this amount. Some things, like wire diameter, have smaller effects, but basic building blocks like element size and location have to all change by the same factor to maintain any degree of predictability.

Frank
I scaled a four element quad from ARRL Antenna Handbook specd for 145 MHZ
I made a Channel 46 scaled antenna from its dimensions.. It worked pretty darned good... I agree Scaling is a nice concept and Does work and ALL dimensions need to be scaled wire diameter needs to be scaled but it isnt as critical. ....................

Johnie

johnied

How to build a UHF antenna...

Attached is a picture of a 1-bay UHF bow-tie that I constructed. I made the elements 10" long to optimize UHF reception under channel 35 and to attempt to improve upper VHF reception on a UHF antenna. My initial tests are very positive. Has anyone else tried 10" element lengths on a bow-tie?

A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.
When I dis assembled my Youtube with 8" whiskers, I too was surprised when I could still receive my 3 strongest stations with only 2 bays remaining. RF 56,49,38
Bob 61231

spokybob

How to build a UHF antenna...

Boy, I can see another set of experiments in minimalist antennae - how low can you go? The large size of these UHF-low centered bowties will compensate to some extent for their reduced number.

Bob, did you remove the end elements, or the bottom half? The former looks like the way commercial 2-bays are ganged.

I need a free night without t-storms to do some playing ...

fbov

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.
I have a similar result here. A 4221 performed worse than a 4220 (both non-DIY), which in turn performed worse than a Radio Shack 15-234 bowtie. I may have a very unusual situation, but this is what works.

The only DIY part of the setup made a great difference: I removed the twin-lead wires from the 15-234, chopped an inch or so off of a RS 15-1140 transformer and soldered the resulting leads directly to the bowtie ends where the twin-lead connected before. This greatly improved reception on digital channels. I verified this by comparing to an unmodified bowtie I later bought.

It might be worth trying if you have strong signals before attempting to build one of these or buy the Channel Master antennas. You can use a pencil and pencil holder to keep it upright. My setup consists of a whip antenna with a magnetic base (came with a USB HD tuner) plugged into the VHF input of a CM7777, and the modified bowtie clipped onto the whip and plugged into the UHF input. With the preamp I can feed both of my TV's with a splitter and two long-ish runs, 25ft. and 50ft.

This is my most effective setup yet--gets all DC stations (~6 mi. away) and thanks to a strong reflection somewhere gets almost all Baltimore stations from ~30 mi. away. I don't have any digitals on VHF yet but will next year, but I think given the analog picture I get from the whip antenna I should be OK when that happens. This is indoors on a windowsill, much smaller than the CM4220 it replaced.

cpldc

How to build a UHF antenna...

Quote:
Originally Posted by fbov

Bob, did you remove the end elements, or the bottom half? The former looks like the way commercial 2-bays are ganged.
Frank. I wish I would have documented with pictures and numbers.
I photshopped this just to show you what it looked like.

spokybob

How to build a UHF antenna...

cpldc,
Welcome to the world of multipath! If I understand you correctly, your antenna mod eliminated the twin-lead and let you run shielded coax to the tuners. In a strong signal field, that's a big change because twin-lead makes a better antenna than coax.

On the antenna forum, one poster found that his reception was virtually the same when he disconnected his attic-mounted antenna and used only the twin-lead downlead as an antenna. Reception improved markedly when he replaced the twin-lead with coax.
http://www.avsforum.com/avs-vb/showt...8#post13779298

This leads to a bunch more questions, most out of curiosity, but the bottom line is you have something that works for you, and that's what matters!

Frank

fbov

How to build a UHF antenna...

Nice photoshop job, Bob, but you made it a 1-bay! Can I assume you meant to leave a second set of wiskers at the other end, with feedlines unchanged? Separated by ~1.5 waves, these should act like separate antennas. Any chance you tried just one (or was the picture accurate)?

Dont' feel bad about the lack of pictures; I took none when I did my 10"-to-7" series, even though that's the antenna that's in the attic these days.

Frank

fbov

How to build a UHF antenna...

Quote:
Originally Posted by fbov
cpldc,
Welcome to the world of multipath! If I understand you correctly, your antenna mod eliminated the twin-lead and let you run shielded coax to the tuners. In a strong signal field, that's a big change because twin-lead makes a better antenna than coax.
Yep, that's exactly it. Since I can get the DC stations with a paperclip (literally) it only made sense that the cable was bringing in more signal and not in the constructive manner.

Quote:
This leads to a bunch more questions, most out of curiosity, but the bottom line is you have something that works for you, and that's what matters!
Frank
If you wish, ask ahead--I'm pretty much becoming obsessed with OTA thanks to DTV so I don't mind helping others' habits as well I enjoy getting the channels more than actually watching them.

cpldc

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Attached is a picture of a 1-bay UHF bow-tie that I constructed. I made the elements 10" long to optimize UHF reception under channel 35 and to attempt to improve upper VHF reception on a UHF antenna. My initial tests are very positive. Has anyone else tried 10" element lengths on a bow-tie?

A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.
Falcon,

What all channels can you pick up with the single bowtie? How much difference from the 4228?

Thanks, Bill

wblynch

How to build a UHF antenna...

Frank. The pic is accurate.I was so surprised at what was happening. While watching the strength meter, I removed the 2 middle bays, then the bottom one, leaving the long connectors as shown.
The three channels I received are at 70 degrees, 21 miles. The modified youtube was facing north, while hanging flat against a cabinet.
Oh yeah! Despite the file name, it is a one bay.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by wblynch
What all channels can you pick up with the single bowtie? How much difference from the 4228?
Attached are my test results. I can add some pictures later or send you some, if they are of interest.

The 4228 tests were from a few days earlier, but I did not notice any tropo events on any of these tests.

I'm thinking it is time to get that Leader signal meter or something like that. These tests are on my Sony TV, which show SNR and AGC. The downstairs results for the 1-bay are w/o a pre-amp. In the attic, I substituted it for the 4228 into the 7777.

Channel 42 had a problem when the 1-bay was up against the window. Moving it a bit solved the problem as I discovered after the test.

The 1st 1-bay column are the upper VHF 30" bow-tie results.

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Attached are my test results. I can add some pictures later or send you some, if they are of interest.
Cool check list. You're really getting into this hobby! Thanks.

wblynch

How to build a UHF antenna...

I need to do more tests with the 1-bay. It's not doing nearly as well as the office where the 2-bay 4220 performed very well (for UHF). I have not tried the 4228 at the office, but then, getting it out of the attic is probably not going to happen.

The 1-bay was only about 2' from the 4228 in the attic, but I mounted the 4220 in its place to see how it does in the exact spot. Location makes such a difference... and maybe the 4228 isn't in the best spot?

Falcon_77

How to build a UHF antenna...

Falcon_77,
There are two numbers recorded out of the three reported on my Sony (I have an 34XRB970). Is it a signal range, or are you recording AGC or S/N as well?
Frank

fbov

How to build a UHF antenna...

[quote=spokybob;13807532]Frank. The pic is accurate. ...
[quote]

I think we're having calibration issues. Antenna changes are 2x factors in an arena where 10x factors are common. A 4-bay should have 6dB more gain than a 1-bay, but signal strength varies by 50+dB from one station to another, local terrain is a 20-30dB factor, and there's another 20-30 dB in reception probabilities between 50% and 99%.

6dB is 4x
20 dB is 100x
50 dB is 100,000x

Simply put, antennas may be under our control, but they aren't the big drivers of reception that one might expect when all factors are considered.

Frank

fbov

How to build a UHF antenna...

[quote=fbov;13816174][quote=spokybob;13807532]Frank. The pic is accurate. ...
Quote:

A 4-bay should have 6dB more gain than a 1-bay

Frank
Let me think about this statement. Looking at my TVFOOL. Forget RF41. It is not on the air.
RF38= -47
RF56= -48
RF49= -49
RF23= -59
RF58= -60
RF45= -67
I receive all these stations reliably with the 4 bay. Then with the 1 bay I receive only the strongest three. Are my results consistent with your statement?
If that is the case, I am going to be able build a smaller antenna for those folks that live closer to the towers.
I am very interested in Mike's test

spokybob

How to build a UHF antenna...

Bob,
Go up a bit to post #96 where holl_ands nicely summarizes what TVFool is predicting - worst case signal levels that might be 20-30dB weaker than typical, but with no accommodation for a bunch of very real signal attenuators.

That was my point; there are a lot of factors involved that swamp antenna gain differences. You may get three stations with the 1-bay because you're sitting at the threshold today, but will get all six of them tomorrow or next week. In your case, I'd err to the 4-bay side unless you were very close (or the VHF bowtie work pans out).

Frank

fbov

How to build a UHF antenna...

Thought I would chime in... stacked phased antennas have a narrower beamwidth imagine the flashlight beam being narrower in height and width... thats where the gain comes from... in physics there is no free lunch :P

johnied

How to build a UHF antenna...

Quote:
Originally Posted by fbov
Bob,
Go up a bit to post #96 where holl_ands nicely summarizes what TVFool is predicting - worst case signal levels that might be 20-30dB weaker than typical, but with no accommodation for a bunch of very real signal attenuators.

That was my point; there are a lot of factors involved that swamp antenna gain differences. You may get three stations with the 1-bay because you're sitting at the threshold today, but will get all six of them tomorrow or next week. In your case, I'd err to the 4-bay side unless you were very close (or the VHF bowtie work pans out).

Frank
I just read holl ands report again & my skull is just as thick as it was before. Back to my previous post, if -50 is the threshold for a one bay, (per my results) would I be fairly safe in building it if someone's TV Fool showed everything better than -45? I know. I know. The bigger the better.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by fbov
Falcon_77,
There are two numbers recorded out of the three reported on my Sony (I have an 34XRB970). Is it a signal range, or are you recording AGC or S/N as well?
Frank
I'm recording SNR/AGC on the spreadsheet. I don't have a 3rd number to record, but am considering a more informative testing device.

As for signal strengths, at my house I have no problem receiving those down into the -90's, regardless of what I'm using.

Adding the 30dB attenuator tends to split the strong stations from the weak a bit more, but KTLA has to be about 40dB stronger than KCOP, which would indicate that my actual signal strengths at home are probably between -65dBm and -105dBm and not -88 to -102 as the TV Fool plot shows. Attic reception probably shouldn't be so easy at these signal levels, but not having any trees in the line of sight for half a mile probably helps, even though I'm at 2 edge diffraction.

Moving an antenna around can easily make a difference of 6+dB, which makes it clear that location is more important than antenna size, especially indoors.

Falcon_77

How to build a UHF antenna...

Quote:
Originally Posted by Falcon_77
Moving an antenna around can easily make a difference of 6+dB, which makes it clear that location is more important than antenna size, especially indoors.
Thanks Mike, I understand finding the sweet spot. Now the 6+db is something I can not seem to understand. I am running tests tonight. My highest reading on my CECB in the bedroom with the CM4220 happens to be 76 on the meter. Placing the youtube in the same spot shows that channel at 50. How many db would that be? The channel I speak of is RF56.
My dropout number on this box is about 16.

spokybob

How to build a UHF antenna...

Quote:
Originally Posted by fbov
... The screen's made from 24" wide 1x1" cage wire cut to 39".
I had thought my 4221 clone was a little wide ... I misread the dimensions, took the 39" width of the 4228 as the height and scaled everything from there. What should be a 20x36" reflector is 24x39". My entire antenna is 1/9th oversize - element spacing, wisker length, everything - plus I made all errors to the large side, to reduce peak gain back closer to the 14-52 UHF channel range.

If I take a 4221 gain vs. channel plot, scale it down in gain and frequency 10%, and recalculate channels, it now peaks at channel 50 and is ~1 dB higher across the band, even with a 10% (0.5 dB) reduction.

My question: post transition, is anyone going to re-optimize their antenna designs for the reduced frequency range? I fyou look at any of the gain-vs-channel curves, nearly all peak in the 60's, channel wise, where there's no longer any TV to receive.

Just a thought ...
Frank

fbov

How to build a UHF antenna...

Quote:
Originally Posted by enchant
I don't have the rf technical background like many of you, so please forgive my newbiness. I'm going to give this antenna a try, but I've got a few questions.

Some of you speak of "bays". 4-bay, 8-bay, etc. Is the antenna on the uhfhdtvantenna.blogspot.com site a 1-bay?

Getting onto the roof of my house is a scary proposition, so I like the idea of putting it in the attic. I live almost 30 miles from the transmitting tower. The signal has to go through a few hills and trees to reach me. Assuming I'm not going to gain much height by placing the antenna outside, does the signal lose much by having to pass through the wall of my house to get into the attic?

The uhfhdtvantenna.blogspot.com antenna uses tin foil. I see others using chicken wire and other light fencing. I've got all of them lying around. Is one material better than the others?

At the point where the coax cable must connect to your antenna, there is a coax-to-dual-wires gizmo. What is this called? (It's the only thing I don't have, and I'll have to pick one up at Radio Shack.)

The cable going from the antenna to my tv will be approximately 70' long. Is there some rule of thumb about when I'll need an amplifier?

Thanks for any help.
You brought this thread up to the top; you deserve a response, albeit from someone of limited RF background as well.

Bays
A "bay" is a single dipole antenna, the bow-tie like thing that clips to rabbit ears. Some have straight elements, others a pair of V-shaped elements, the principle of all dipoles is the same. What you need to know is that they are "300 ohm" elements and that the width of the dipole determines the frequency it receives best.

The antenna at UHFHDTVAntenna is a 4-bay: 4 pairs of dipole wiskers connected in a special "series/parallel" arrangement that gives you a 300 ohm antenna. 2-bays and 8-bays don't retain a 300 ohm impedance, but the 2-bay is smaller and the 8-bay has higher gain. But all require that the elements of each bay be in the same plane, so all signals add constructively.

Attics
Yes, the signal is greatly degraded by mounting under the roof compared with on top. Here's some data:
http://www.avsforum.com/avs-vb/showt...32#post5410432
By way of calibration, each 3dB is a factor of 2 (-6 dB = 4x less). The chart at the bottom shows the CM-4221 antenna had a -20 db to -30 dB loss that's
- 1/2^7 to 1/2^10, or
- 1/128 to 1/1024 as much signal, or
- 99.22% to 99.91% signal loss in his attic.

That said, mine's in the attic and it's fine for my weakest station, a -77.8 dBm aerial signal strength. You need to find out how much signal strength you have. That's at:
www.tvfool.com
This will show you all the stations you might possibly receive, along with signal strength, distance and direction to each. They're sorted by signal at the antenna height you specify, best reception first.

Reflectors
The size of the holes in the reflector should be small compared with the radiation it should reflect, and it must be metal. The shortest wavelength is Channel 83, 887 MHz, a 338mm wavelength, so holes smaller than 34mm look solid to the wavefront. I used 1x1" cage wire, others have used solid (Al foil) or chicken wire; Channel Master uses 1x2".

The more important requirement is that it be flat like the bow tie arrays, or possibly bent to a slight curve like the actual 4221.

Balun
That gizmo where the antenna connects to the feed line is a matching transformer called a "balun" for "balanced/unbalanced" line interface. It connects the "balanced" 300 ohm antenna with the "unbalanced" 75 ohm coaxial cable. Without it, most of the signal stays in the antenna. Get a good one; Radio shack is a crap shoot, or make one.

Line Loss
See:
http://www.hdtvprimer.com/ANTENNAS/basics.html
about half way down, right before Baluns ... a mast amp will more than accommodate 70 ft. of coax.

In the situation you describe, I'd proceed in steps
- buy or make an antenna and try it in the attic
(This is where I declared victory and stopped.)
If you don't like what you see ...
- add a mast amplifier to reduce noise, primarily
If you don't like what you see ...
- mount it on the roof.

But first, see what your TVFool profile looks like (post it here, if you'd like) so we have some idea what you need.

Have fun,
Frank

fbov

How to build a UHF antenna...

Frank,

Thanks for all the good information!

Since my first post, I made the antenna as per the directions on the blogspot page. My first test was with the antenna right next to the tv, connected with a 6' cable. I was pretty impressed with the results. Then I put it up in the attic and ran a 100' coax from it to the tv. The results were far less satisfactory. So now it's back sitting next to the tv.

Once we get a little deeper into spring, I'm going to take a shot at making a weatherproof antenna using some extra backyard fence (1" x 2") that I've got.

enchant

How to build a UHF antenna...

I wonder if it's the attic or the cable ... an easy test before the cable's run.

You lose signal either due to cable losses or attic losses. 100 ft. of cable should cost you 5-6 dB; next step is a mast amp. Why not test an amp in the attic now and possibly avoid the roof?

Get a good amp, too. Many people recommend Channel Master and Wineguard, but avoid Radio Shack

Pictures of projects are welcomed, too!

fbov

How to build a UHF antenna...

We have UHF channels all the way up to Ch 69 so the full bandwidth is required. YMMV

donnyjaguar

How to build a UHF antenna...

Only until next Feb. ...

fbov

How to build a UHF antenna...

Not for Canada.

milehighmike

How to build a UHF antenna...

The "700 MHz" band channels are being repurposed to new services, such as mobile TV,
including enhanced feeds targeted to handheld TVs in sports statiums--and at home.
Also transferring 1080p movies to laptops/PCs, traffic updates and other
wireless data distribution services that may become "must-haves".

You may want to maintain upper channel coverage post-Feb2009.

Qualcomm's MediaFlo service on CH55 is well on it's way to coast-to-coast service.

holl_ands

How to build a UHF antenna...

Gentlemen,
I can see a variety of reasons for antenna makers to keep their products the same while claiming they're "optimized for HD/DTV." Canadian reception and speculative future services are not among them.

Nor do these two have anything to do with antenna requirements for DTV reception. This thread is about making antennas; if the market won't serve us, there are a few of us who will serve ourselves.

So, instead of telling us why we need something we can't use, how about helping us meet our needs? That would serve all concerned by allowing the builder to aim for the spectrum of interest to them, be it post-transition domestic DTV or broadcast on-demand video.

fbov

How to build a UHF antenna...

Posted by fbov:
Quote:
Gentlemen,
I can see a variety of reasons for antenna makers to keep their products the same while claiming they're "optimized for HD/DTV." Canadian reception and speculative future services are not among them.

Nor do these two have anything to do with antenna requirements for DTV reception. This thread is about making antennas; if the market won't serve us, there are a few of us who will serve ourselves.

So, instead of telling us why we need something we can't use, how about helping us meet our needs? That would serve all concerned by allowing the builder to aim for the spectrum of interest to them, be it post-transition domestic DTV or broadcast on-demand video.
I'm not sure what point you are trying to make. Is it that UHF TV antennas that are currently made to receive channels 14-69 won't be good enough once DTV vacates channels 52-69? Don't tell the folks where I grew up in Buffalo, NY that they shouldn't have an antenna to receive Canadian stations on channels above 51. And I suppose that at some time in the future when one is interested in receiving the MediaFlo service on channel 55 that one will be expected to purchase another antenna, since they only have an antenna that will tune domestic frequencies up to channel 51?

From your post at the top of this page, you apparently believe antenna manufacturers should consider redesigning their antennas to "peak" at a channel below 51 to increase gain somewhat less than what one loses by insertion of a splitter. Probably not worth the effort.

There are issues of substance and issues that are trivial. If you want to make an antenna limited to certain frequencies, which you claim is the subject of this thread, make it as you like. But someday you may be making it again to receive the 700 MHz band.

IMO, Canadian reception and speculative future services are reasons for antenna manufacturers to keep their products the same, not redesign them for "domestic DTV", as you put it. New and improved is not always what it's hyped up to be.

And I have built my own yagi antenna which I have mounted on a 35 foot mast. And I don't care that it probably receives channels above 51, it works great for me.

milehighmike

How to build a UHF antenna...

Quote:
Originally Posted by milehighmike
... it works great for me.
Mike,
That's my point; what works may be different for each of us ...

I'm working on a little analysis, and if it works out, I'll post it here. I've used the actual CM-4221 GIF file used for the HDTV Primer model to grab dimensions, and the correct basis dimensionts gives an 8" bay spacing and wisker length. The wisker spread results in a dipole that's 1/2 wave at the model's peak gain. Seems like a good fit.

Since I've already got a 9" spacing with 8" wisker lengths, I'm going to try the experiment - 7" through 10" bay spacings and wisker lengths. In effect, 87.5%, 100%, 112.5% and 125% of the commercial design. Assuming no gain differences, the curves should look like the attached.

I added in the local stations and TVFool signal projections that I'll use for testing. 58 is my hardest, and should be bad only with the 125%. 16 and 28 should show a drop at whatever the TV's tuner threshold might be. It'll be interesting to see if these predictions pan out.

As to what works, that depends on your needs;
- the 125% gives low channels 2.5 to 5dB more gain in my projections, at the cost of any gain above ch 52. It gets ch 14 well, but not 64.
- somewhere between 112.5% and 125% is an optimum with maximum gain for US post-transition channels
- For my part, I'm going to lengthen my wiskers so I'm a full 112.5%, 9" bay spacing, which should give me 12 dBi gain out to ch 63 ... and down to ch 20.

It's a matter of understanding the tradeoffs. I'd like to talk about it; anyone else?

Frank

fbov

How to build a UHF antenna...

Today I made and measured a series of 4-bay antennas at a range of sizes based on the CM 4221. The commercial design is an 8" element length and bay spacing. I set up a stand where I could vary the element spacing and length, and tested 7", 8", 9" and 10" antennas. This corresponds to peak gain channels of 88, 66, 51, and 40, respectively.

I tested on my local stations:
Station Channel Chnl Rx
21.1 / 16 -70
31.1 / 28 -73.3
8.1 / 45 -72.6
10.1 / 58 -77.5
13.1 / 59 -73.5

The results are about what you'd expect.
- the lowest frequency, ch 16, is also the strongest signal; I had to remove the reflector from the 7" to get any fall-off, despite a large predicted change in gain. You can't beat a good aerial signal!
- channel 28 is noisy, but the trend is to less signal with the shorter antennas
- channel 45 shows almost no change
- the weakest station is ch 58, and its signal gets stronger as the antenna gets smaller, as expected.
- its neighbor, ch 59, shows a similar trend, but with an opposite reaction when the reflector was removed.

I will also note that results with my "good" antenna are consistently better than what's shown here, save for a couple points on 58 and 59 against the 7" antenna. It was an accident, but one I'm happy I made.

Bottom line, you can move the peak gain of an antenna to one end of the UHF band, but only at the expense of gain at the other end. That's no surprise. What's interesting is the ability to tune this type of antenna by a simple scaling of element spacing and length.

HAve fun,
Frank

fbov

How to build a UHF antenna...

FYI: Dipol made a 4-Bay where they extended the length of the two TOP whiskers,
which would help the response at lower freqs (see special version 3 on pg1):
http://www.dipolnet.com/kat01.pdf
Note that they also added some (too small?) DIRECTORS in front of bow-ties...
Gain charts are for various 4-Bays with DIRECTORS, but not for extended whisker version.

Dipol's webpage home:
http://www.dipolnet.com/
BTW: World-wide TV frequencies are more or less same, but channel #'s vary country-to-country.

This opens up a series of questions as to which whiskers are best to extend.
What about extending all FOUR whiskers on the top bow-tie?
What about extending all FOUR whiskers on both top and bottom bow-ties, but not middle two?
What about extending only one set of whiskers in EACH bow-tie?
Exercise for the student using NEC simulation.....

A set of metal back-to-back "sockets" could assist in temporarily adding whiskers.

holl_ands

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