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I need to mount a VHF receive-only omni (166Mhz to be specific) and a UHF receive-only omni (434Mhz) on a pole and plan to use a T-bracket at the top to space the two antennas apart. How much distance do they need from one-another in order to avoid significant interference?

I read a lot of recommendations but they all pertain to TX/RX interference or to diversity antennas, and both of these configurations have different issues vs. two unrelated RX-only antennas.

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2 Answers 2

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If antennas are end to end, they can be very close, nearly touching without any problem.

If the antennas are side by side, they will not interfere with each other for receiving, but one antenna may shadow the other slightly in some directions, making them directional. The distance between the antennas will control how directional and in what direction.

In general, the minimum distance to completely eliminate this effect would be on the order of 2-10 wavelengths. However, there will be shorter specific distances that may enhance directionality in line with the antennas and other distances that will minimize that effect.

Note also that if these are mounted to a metal tower or pole, the distance from that will also cause this effect, with the longer metallic object generally acting as a reflector, and that may have more effect than any interaction between the two receive antennas.

Note that except in very special cases, the interaction between two receive antennas will be very weak, rarely completely eliminating reception in any direction, but it may strengthen or weaken it somewhat in some directions. It may actually be difficult to measure the difference.

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  • $\begingroup$ if they are close side by side, of course they interact and "interfere"; one acts as reflector / director to the other! That's why they become directive, not "shadowing" (which doesn't exist at these sizes) $\endgroup$ Commented Apr 2 at 20:37
  • $\begingroup$ It depends on the distance too. At some distances, the reflector doesn't reflect. $\endgroup$
    – user10489
    Commented Apr 2 at 22:45
  • $\begingroup$ sure, at half wavelengths; but since the two antennas are operating at different wavelengths, you can't make the distance half of both wavelengths. $\endgroup$ Commented Apr 2 at 22:48
  • $\begingroup$ True. It's not a perfect world. $\endgroup$
    – user10489
    Commented Apr 2 at 22:51
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For receive only, no real problem. If you had transmitters, you could not separate them far enough horizontally. You would blow out the receiver's front end in the adjacent antenna without some serious filtering to isolate the two. Vertically, if you have at least 1/2 wavelength separation, you are in the null that creates -40 Db of isolation.

For receive, it is no problem having them side by side. The effect is so minimal that you can ignore it.

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  • $\begingroup$ Until you accidentally (or on purpose) make a yagi out of them. But, as I said in my answer, this is very special and easy to avoid. $\endgroup$
    – user10489
    Commented Mar 29 at 5:07

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