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Interaction between multiple FM receiving yagi antennas.

B

Brian Bowers

I need to monitor FM stations in two local markets, with each market being in the opposite direction of the other. I'm planning on installing 2 yagis on one building mounted 8 foot (basic Radio Shack TV antenna) pole. Each yagi will be pointed in the opposite direction. I'll install a simple coax antenna switch to select between the antennas, to feed an FM tuner. My question is, is it critical to leave space between these antennas, so that there isn't any negative reaction between the two, to get the best reception from each market? Keep in mind, I'm trying to get as much height out of each antenna, so if I can leave only inches between the 2 antennas (or one mounted directly under the other one), that would be my preferable choice. I've often wondered this, as I've been in this situation before. No need to discuss notch filters, or any other bandpass filtering, as I'm already familiar with what I need to do there.
 
I assume you're mounting these antenna horizontally. I order to reduce the amount of interaction between the antennas to almost nil, you would need to give them around 10 feet of separation. Sounds like that isn't an option, so the other option would be to mount them directly behind each other back to back. No doubt it will reduce the gain and obviously front to back ratio for both antennas, but it doesn't sound like a critical application anyway.
 
Yes mounting horizontally. OK, stacking one right on top of the other will not work. Thanks. Guess I'll end up giving each one a dedicated mount. I actually think that that is what I've done in the past too.
 
Yagi's pick up a lot of signal from the rear. It is not as much as from the front, but still significant. If the two stations are more or less 180 degrees from each other, you may find that one antenna actually picks up both stations reasonably well. You might try just one antenna pointed at the weakest signal and see what happens. The worst that can happen is you need to buy a second antenna and mast.

If you want antennas that reject signals from the rear, then look into log-periodic antennas. As others have said, separate mounts is probably going to work best if you need two antennas and don't have 10 feet or so of separation on the mast.
 
>>>you may find that one antenna actually picks up both stations reasonably well<<<

Hmmm, OK, before going through the work of installing a second antenna and mast, I will investigate that first. I embarrassingly admit, while being a radio broadcast engineer for over 20 years now, I have never utilized or researched FM Log-Periodic antennas (for receiving) until you just suggested them as an option. Too bad, as I have had the budget for some of them in the past, which would have been a great time to experiment, but not right now.
 
This is a good link. The FM-6 has a substantial back lobe, but perhaps not enough. Perhaps if the stronger signals are off the back. It's something like 16 dB down from the main lobe on average. In the old days, TV antenna manufacturers would make antennas for TV like you want for FM for certain areas, called "area specials". Would a simple folded dipole work, or do you need a lot of gain? Are the stations in the opposite directions on cochannels or first adjacents which are problematic? A rotator is not an option, I assume?

http://ham-radio.com/k6sti/fm6.htm
 
Wow, lots of useful information. Coincidentally, about a week ago, long before your reply, I ordered two Antennacraft FM-6's for my application. They haven't arrived yet. I'll have to see how much time I have to tweak, as I am pretty busy these days, but the modifications do look very inviting. Thanks!
 
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