Building an HDTV Antenna
From Fozzee.net
Contents |
Overview
My friend Jason came across this video on Youtube, describing how to build yourself an HD antenna. I'd bought an HD Tuner for his PC a while back, and was ready to venture onto the airwaves.
As noted on the HDTV page, I've got an external HD tuner, and had been using just a 6' piece of coax as an antenna. That setup netted me about 4 channels at "fair" signal strength. It should be noted that, with Digital, signal strength and quality are not directly related to image quality.
Anyhow, Jason, by his own admission, has limited skill in making things, and an even more limited number of tools. So he convinced me to help him with building the antenna. I agreed, as long as he'd help me build one, and take some pictures.
I'd like to start by noting that the idea and design for this antenna is entirely based on the one in the video above. My big beef is that, for someone like me who builds from the pictures, the video was a bit lacking. Jason's antenna did deviate in a few places (mine a bit more, as a proof of concept) but again, I did not come up with this idea on my own. Heck, I didn't even verify his element sizing calculations. Without further ado, queue the photos.
Materials
The elements themselves are built from coat hangers. It takes the following pieces of coat hanger:
- 8 14" segments (you seem to get 2 of these from a standard hanger) The lengths of these are, theoretically, critical. Like I said, I didn't check the math to know for sure.
- 2 ~20" segments (these arc a bit, and depending on how you bend them, their length varies.. Start long and trim)
Additionally, you need a board to fasten the elements to. The original video says a 1"x4" that is 2.5-3 feet long, but I think that's a bit too long. The maximum distance between the end elements is about 18 inches, so /technically/ a 20 inch board should work fine. The point (imho) of the extra length is to make the board stand up nice. It really all depends on how you plan to mount it.
You'll also need a way to fasten the elements to the board. The video suggests screws and washers, which worked fine, but I used a pneumatic stapler on mine, and that definitely has some different advantages. Mainly, it's faster to assemble. The disadvantage is that it's a PITA to disassemble or tweak. Also, make sure your staples wont penetrate the board :)
Finally, you'll need a way to connect this mess of steel to your tuner. This particular antenna is designed at 300-ohm, (Theoretically could be adapted for gamma match... I'm not really sure) I opted to use a common 300 to 75-ohm balun, which connects the two sets of elements to a single, standard F connector.
Step 1 - Cutting
This step is pretty straight forward. Cut the coat hangers as described. On the 14" segments, you'll want to file, sand, grind, etc the paint off the coat hanger for half an inch or so on either side of the center point.
Step 2 - Bending
Next, you'll need to bend each of the 14" elements into a V shape, such that the ends are "about 3 inches apart". Again, not sure how critical that 3" is, but it seems about right to make them mount nicely, given the stock spacing. The video shows using pliers to bend them, but I did it barehanded, using the shaft of a screwdriver in a vice as a mandrel; This resulted in about a 3/32" bend radius. If you have a protractor, this works out to be 25 degrees.
Step 3 - Fastening
Now this thing starts to look like an antenna. You'll fasten the wire V's to the board so that they hang off opposite sides, 4 on a side. The separation between the vertexes of the Vs isn't all that important, but I found that about 1.5 - 2" seems about right. The Vs will be in 4 rows (two to a row), separated by 5.75". The video suggests fastening them to the board using screws and washers. Don't tighten the screws all the way, as you still need to add the connecting rods.
The connecting rods are the two remaining, longer pieces of hanger. They connect the Vs into two circuits, each consisting of the end two on one side, and the middle two on the other (Right-Left-Left-Right and Left-Right-Right-Left respectively). Again, you'll need to remove the paint where the connecting rods meet with the Vs, to insure signal transfer.
It should be noted that the connecting rods cross in two places; Depending on the state of the paint on your hangers, you may or may not need to additionally insulate those connections, such that the two antenna circuits are electrically isolated from each other.
Step 4 - Cabling
Finally, you'll need a way to hook this mess up to your tuner's 75-ohm input. Remove the paint from the middle of the two connecting rods, and (using screws or solder, as you prefer) attach one leg of the 300-ohm side of the balun to each rod. I'm not certain, but I believe that selecting the "very" center of each rod is important. You can wing it, measure with a ruler or (most anally) use an ohm meter to determine electrical center.
Once that's done, you're all set. Check your V measurements again (3") and flatten all the Vs so they're entirely on one plane.
Enjoy the Digital Revolution.
