After the digital television transition several years ago, we bought an omnidirectional antenna from RadioShack and I mounted it on the rear eave of the house. It’s plastic and is saucer shaped. As the snow collected on it this winter we started losing the signal, so last weekend I tried to clear the snow off it, as I did last year, by gently shaking the mast until the snow slid off. This time, as the snow slid toward the edge, it put too much pressure on the bottom of the disc, where it’s screwed onto an L-shaped bracket that holds it to the mast. The plastic broke, it’s now tilted about 45 degrees, and probably won’t survive another winter. And, it’s still having trouble picking up even our strongest station.
A recent post on Lifehacker about a powerful interior antenna (the Mohu Leaf) resulted in an Internet investigation of antennas and whether something like the Leaf would work for us. I didn’t think it would for two reasons: our house has foil-backed insulation and reflective glass windows, and half our stations are in the high-VHF range (7–13) which isn’t what the Leaf is designed to pick up.
Here are the details of our television stations in Fairbanks:
|Station (network)||Channel||Frequency (MHz)||Direction|
KUAC is only three miles away, and the rest of the stations are about five miles from the house. Two stations are high-VHF and the other two are UHF.
My plan was to build either a DB-4 or M-4 style UHF antenna (lots of plans are available on the Internet if you search for those two terms) and then add a VHF dipole to improve FOX and PBS.
Before I went to the trouble of building something complex like the DB-4 / M-4, I thought I’d start with a simple folded half-loop dipole designed for the range in between channels 7 and 9. If that got me KFXF and KUAC, I’d know what I needed to add to the UHF antenna to pick them up. Magically, the dipole was all I needed.
You can read about the theory and calculations on the Internet, but the length of the dipole (in inches) can be found with the following equation: 468 / MHz * 12". For the middle of channels 7 and 9, this works out to a dipole length of 30 11/16". This could be built in a variety of ways, but I chose a folded half-loop configuration, which means a single piece of wire, bent to form a loop stretched such that it’s 30 and 11/16th inches from tip to tip. The two ends of the wire are connected to the two leads of a 300 to 75 Ohm balun transformer (RadioShack part 15-1230), which typically has a coaxial connection on the other end.
Here’s a schematic:
I made mine from the bare wire inside a 72" section of 12-2 Romex, bending the ends around a screwdriver, which resulted in a separation of about 3/4" between the top and bottom of the loop. Each end was twisted around a brass screw, and tightened with the transformer tabs between two brass washers. The center of the dipole was screwed between two pieces of wood for support, and screwed to the roof of the arctic entryway. The photo shows the antenna and support. The main roof of the house is in the upper left corner.
I tested it in two orientations, one where each dipole was pointed at the stations, and one where it was perpendicular to the stations. Based on the signal strength numbers, I got slightly better results when it was close to perpendicular.
Surprisingly, this single loop of copper wire is all that we needed to pick up all the stations in our area. I got a signal strength of 100 for KUAC and KTVF, 84 for KFXF and 78 for KATN. Not bad for a $6 transformer and six feet of surplus wire!