eskipper411 said:
JHBrandt said:
You may be OK. You probably have an antenna with a high F/B ratio, which should attenuate the weaker signal from behind more than 15 dB below the stronger one. But 'burbs SE of Dallas (e.g., Ferris) may see the interference you're talking about, since there'd only be about 60 degrees separation between the signals.
Silly me. That assumed you'd rather watch KNAV than KAUZ. Even though KNAV isn't on the air yet, that seems unlikely. If you're more interested in KAUZ during tropo episodes, interference from KNAV is a bigger problem: it'd probably be the stronger signal, which means you'd need a F/B ratio well above 15 dB. You may need to gang together two antennas with a 1/4 wavelength (just under 6 inches at RF 22) N/S separation and a 1/4 wavelength delay line to null out KNAV.
What is an F/B Ratio? And I do have 2 antennas. One at 40 feet and one at 20 feet. Actually surprised that the 70 mph winds we had on the 20th didn't tear it down.
Glad to hear your antennas are undamaged. I'm guessing the 40-footer is pointed at Wichita Falls and the 20-footer at Cedar Hill, but feel free to correct me if I'm wrong.
F/B (front/back) ratio is simply the ratio of an antenna's gain toward the front to its gain toward the back. It's not always a useful spec, but in the KNAV vs. KAUZ case (where you're trying to tune in a signal in front of the antenna while rejecting another signal coming from behind) it gives you an idea of how strong the interfering signal can be before you have trouble. Antennas usually have negative gain toward the back; if so, the F/B ratio would be greater than the gain itself.
With digital signals, we need a minimum 15 dB signal/noise ratio to get a lock, and an interfering signal will look like noise. So if we have a reasonably strong signal in front and the antenna has a 15 dB F/B ratio, the signal from the back can be almost as strong as the signal in front before it prevents us from getting a lock.
OTOH, if the signal in front is marginal, then it'll take less signal from the back before there's too much noise to get a lock. That's why I was expecting that, in your case, KAUZ wouldn't interfere too badly with KNAV (assuming KNAV actually broadcasts something worth watching someday), but KNAV would probably interfere with KAUZ.
Finally, let me clarify about using two antennas to "null out" an interfering signal. This is easiest if the two antennas are identical and at the same height, so they both receive equally strong signals. The idea would be to point both antennas toward the signal you want (let's say KAUZ) but arrange them so the signal reaches one antenna a little bit sooner than it reaches the other. Ideally, you want the signals 90 degrees out of phase. For KAUZ, one antenna (I'll call it "antenna A") will be 90 degrees ahead of the other ("antenna B"), but for KNAV, antenna A will be 90 degrees
behind antenna B.
Then, you make the coax from antenna A about 4 inches longer than the coax from antenna B, then combine the signals with a common signal splitter hooked up in reverse. The extra coax delays the signal from antenna A by about 90 more degrees, so the signals from KAUZ arrive at the splitter in phase - but the signals from KNAV arrive 180 degrees out of phase, and cancel out!
To do this, you need to move one of the antennas a few inches north or south until you find the "sweet spot" that makes the cancellation work. It's a lot of effort, but if you really want to watch KAUZ, and you can't once KNAV lights up, it might be worth the trouble.