Tuesday, July 26, 2011

Homebrew TV Antenna

A slight detour from radioland.

Everyone who reads Make Zine or Instructables is handy enough to at least think about making a TV antenna.That's my own home-brew HDTV antenna above. If you look online you'll find several different versions of the same basic antenna descended form the old fashioned "bowtie" model. they're all made with scrap wood, coat hangers, screws, washers and the same $4 radioshack 300Ω-to-75Ω balun. That's standard issue. Mine is not unique. But what's specific is that I actually know why the aerials are a certain length and spacing. Different configurations will all work, but all work differently.

The old UHF and VHF antennas don't work terribly well you may have noticed. After the digital transition channels 53-69 are now gone and in many markets also channels 2-6. So the spacing and frequencies the antennas were designed for has changed. Which is how you get my home-brew model. It's not perfect, but it performs well for what it is. I get 16 channels with that cobbled bit of scrap wood and coat hangers. The place to start is TVFool.com You need to know what stations you can get and where they are. Just throwing up an antenna will get some, but you want to get as much as you can out of all your hard work. I have to re-do mine because I miss-spaced the bays. Oops.

So here are the rules:

1. Size matters
2. Elevation matters.
3. More bays = More better
4. Reflection is directional
  • 1. Size: The length of the aerials in each bay is specific to the frequencies you're seeking to tune in. In a 4 or 8 bay antenna the aerials or whiskers could be 9.5 "inches with 9" bay spacing. That's tuned for the middle of the VHF band. In a 4 or 8 bay set up, a 10" whisker should be spaced 9.5 inches for lower VHF and channels.  In a 4 or 8 bay antenna the  9" whisker bowtie with 8 1/2" bay spacing works best in the the upper VHF band. This website has many of these configurations drawn out. This is recommended if you want to avoid math. Some designs will have evenly spaced bays, some have them unevenly spaced, but none of them are staggered. See this one here.
  •  2. Elevation: The second floor is better than the first floor, the attic is better than the second floor. Outdoors is better than indoors. Every barrier, every wall and every shingle is an obstruction to your TV signal reception. This is elementary.
  • 3. Bays: These are are the "Vs" made from bent coat hangers you see in my picture above. There are designs with 10, 16, or even 32 bays. These get very large and are effectively all outdoor antennas. It's called ganging. When two identical antennas (bays) are mounted together, wire together and pointed the same way, in the same direction you can get up to a 3 dB improvement in signal strength. as I covered previously dB are logarithmic so that's twice the signal strength. In reality in combining systems there is always some loss in the wiring and connectors.More bays is better, but you get less of an increase in strength each time. I don't see any significant reason to try much more than 4 - 8 bays.
  • 4. Reflectors: Radio waves reflect off of a large conducting plane like light off of a mirror. It's not focused liek a parabolic reflector, but it still improved gain. Mine does not have a reflector. That's a coincidence of placement. All the stations I am trying to receive are in the same direction, and all of them from behind the TV. This means I can use the rear of the TV as a reflector. You may or may not pull off this kluge solution. TV signals are directional, and if you use a reflector you are only improving signal strength in one direction. In short it has pros and cons.