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.

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

I used 300 ohm twin lead wire (2 equal lengths) direct to 300 ohm input/ 75 ohm output preamp to eliminate balun/combiner loss.

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.

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.

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.

Parts were lying around and if it didn't work, no loss other than time. It was fun.


Actually, the crappy one I built works better than the one I spent more time on.

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.

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!

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?

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!

I have't seen a commerical pre-amp with only a 300 Ohm input.

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.

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

Some of the Channel Master Spartan series do.

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.

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.

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

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.

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.

Anyone interested in a DIY antenna should take a look at:

http://www.lumenlab.com/forums/index.php?showtopic=9613

More than 45 pages of information covering more than a dozen antenna designs for UHF/VHF.

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

 

Ants_4221 comparison.zip 3.126953125k . file

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