Friday, August 14, 2009

YouTube DIY TV antennas

I was fooling around today and ran across a collection of videos on YouTube talking about making homemade "HD" TV antennas. Just about all of them are 4 bay bowtie dipole arrays. Pretty much a Channel Master 4221 without the screen, but usually built by screwing coat-hanger wire to a 2x4.

That'll work ok, but there are a few caveats that I feel duty bound to point out:

1. Measure carefully. All the measurements are fairly critical on an antenna like this. Particularly the length of the elements and the distance between them.

2. The CM4221 has a reflector screen behind it. This gives the antenna F/B ratio that it otherwise wouldn't have. Particularly around here, this is absolutely necessary to reduce multipath. The 4221HD has a modified screen that acts like a VHF-hi dipole. This allows it to receive channels 7-13 in addition to 14-51.

3. Put two of these next to each other (again, measuring the distance carefully) and connect them together and you'll have an 8 bay - the equivalent to the CM 4228, one of the best UHF antennas out there.

4. If you put this antenna outdoors, you're going to either need to build it far more robustly than most of the videos show, or you'll need to replace it every few years as it corrodes.

5. If you put this antenna up indoors, you're fooling yourself. For such an antenna to exhibit its designed gain, it must be mounted several wavelengths away from other objects. Because of the huge amount of multipath involved, indoor reception is a crapshoot. It always has been. It's just that with analog, it was easier to make do with a worse signal, or to turn a bad one into a good one experimentally (what I like to call "Antenna Twister").

6. Some of these video describe this as an "HD" antenna. Even Channel Master is somewhat guilty of that in adding "HD" to the model numbers of some of their antennas. No. Antennas do not care about the nature of the modulation of the signals they receive. Antennas care about frequency. And TV post-transition is using the same frequencies as it was before (fewer of them, actually, since the top 100 MHz of UHF has been reallocated). So a TV antenna is a TV antenna and always has been. And TV antennas have not significantly changed since the TV bands were fixed after World War II.

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