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I've read that one should be careful when mounting two (unidirectional) antennas on the same mast because their electrical fields may interfere with each other. Further advice stated there should be a vertical distance of at least 2-4 feet between them.

Question: if both antennas aren't amplified, will electrical interference still be an issue?

orlando21
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  • They will always interfere, the question is will it matter? How different are the frequency ranges of the two antennas? What are they being used for? Transmit and/or receive? – kponz Dec 22 '18 at 14:16
  • It's more of a theoretical question, but I imagined two identical yagi-type outdoor TV antennas set up to (only) receive from transmission towers that are too far apart (i.e., greater than 20-40 degrees separation) to get with just one antenna. – orlando21 Dec 22 '18 at 15:42
  • Many old antennas "old school" had both uhf and vhf on the same array if on the same mast pointing in different directions I have not seen interference problems, the height of the pole depends on what is in the area other structures or hills in the way. My last home was on a hill I mounted antennas on the side of the chimney pointing at the transmitters and received all the local broadcast channels. – Ed Beal Dec 22 '18 at 16:53

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For the wavelengths you're trying to receive, keep everything else more than 1 wavelength away. So for VHF FM radio keep other antennas or metal that would act like an antenna 10 feet away if you're trying to pick up far away stations. Same with UHF TV, 2-3 feet away is good for a far away station.

In reality though, most signals/stations are so powerful that a little signal attenuation won't make a difference. With digital TV there is error correction as well, so you either get it or you don't unlike analog TV where you deal with all kinds of noise.

If you need to share a mast, then just do it. Ideally you'd have a 300 foot tower too, but most signals are so strong you don't have to. Worst case just get a higher dBi one to offset any losses.

Dotes
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  • Thanks for answering. I didn't realize VHF made such a difference. If one antenna was UHF and the other VHF, would it still be feasible (in most cases) to mount them on a reasonable-length pole (say 10 feet high)? You mentioned getting a slightly higher gain antenna to offset losses, could we install a good preamp instead (on one of the antennas)? – orlando21 Dec 22 '18 at 15:51
  • Sorry for so many questions, but what kind of interference between the two antennas are we getting; is it multipath interference? – orlando21 Dec 22 '18 at 15:55
  • For a horizontally polarized signal, the other antenna on a mast would act as a ground plane and would both absorb and reflect some of the signal. No it wouldn't be multipath interference because a signal reflection from that close isn't enough distance to have any real measurable delay since RF travels at the speed of light. Are both of these antennas pointing in the same direction? They make combo VHF/UHF units. – Dotes Dec 22 '18 at 21:05
  • A preamp is almost always a good idea, especially if you're talking about outdoor/mast antennas and/or many feet between your antenna and your receiver. A preamp on one or both will not affect anything in regards to interference, and will help reduce signal loss from the RF cable. A preamp will not make the antenna more sensitive or a better antenna. – Dotes Dec 22 '18 at 21:09