Sunday, December 7, 2008

Building a VHF TV dipole

After 2/17 of next year, there will be only two broadcasters below channel 14 here in the San Francisco market: KGO on channel 7 and KNTV on channel 11.

You could make do with a UHF corner Yagi... but what about those two channels?

Well, the ChannelMaster CM2016 is a good solution. It's a traditional UHF corner Yagi antenna combined with a VHF-hi dipole. But if you already have a UHF corner Yagi, there's no need to replace it. You can simply add your own VHF-hi dipole. Even better, you could make a relatively high quality folded dipole from copper plumbing pipe.

After February, the two channels that matter will be 7 and 12. 7 has a bottom of 174 MHz, 12's top is at 210 MHz. Halfway between those two is 192 MHz. A half wavelength at 192 MHz is 2.43 feet, or 29 1/4 inches, but we're going to put some fittings on each end, so subtract two inches, for 27 1/4". You'll need two pieces of 1/2" copper pipe that long, 2 90 degree 1/2" "street" elbows and two 90 degree 1/2" normal elbows. "Street" elbows are designed so that one side is the same diameter as the pipe itself, so that it can mate with one of the ordinary elbows to make a 180 degree turn.

Cut a half inch off one of the long pieces of copper and then cut that piece in half. Assemble the two ends out of one of each type of elbow. Put one of the combo elbows on each end of the long piece, and then stick a short piece in each remaining open elbow connection. The two short pieces should have a half inch gap between them. Once it is all dry-fit properly, use a plumber's torch and sweat the whole thing together. Use sheet metal screws to attach a standard TV balun on each side of the gap.

The only trick remaining is to mount the thing. You want the dipole to be at a right angles to the direction from your location to Mt. San Bruno and Sutro Tower. You need to use a piece of wood or plastic or something insulating to attach it to your mast. You also need to keep it at least a couple feet away from any other antenna on your mast, if you can.


netarc said...

Thanks for these - I'm in SF as well, and currently using only a DB2 antenna (UHF14-69), so I guess I'm going to have to get a new VHF antenna or follow your mod if I want to keep getting ABC and KNTV.

Do you have any pix of the finished set up, I read the instructions, but am having a hard time visualizing the thing.

Nick said...

One other note... From what I can see in the FCC database, it's probable KTVJ-LD will be operating on channel 4 (centered at 69 MHz) post-transition. It might be worth attempting to pick them up here in the South Bay. A folded dipole for channel 4 can be built in exactly the same way, but the length will be 6 feet, 9.3 inches.