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GMRS Repeater antenna



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May depend upon repeater output power and antenna gain.

Better might be to mount them in a column, one above the other. Presuming they have a moderate horizontal beam, it would mean the receive antenna is in the null of the transmit antenna.

{pardon nasty ASCII art. upper is your proposal, lower is my idea}

<<|>>                                                  >>|<<
transmit                                               receive

<<|>>      transmit

>>|<<      receive


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50' of vertical separation is the standard here in Illinois using UHF and 2M VHF.  Some taller towers (figure 500' tall) have 100' of vertical separation for even better RX/TX isolation with RX ALWAYS being on the top.  Many simply invert antennas and that isn't enough in my testing.  It helps to have a folded dipole antenna, NOT a fiberglass stick as they radiate power up into space and down into the earth VS folded dipoles radiate 995% of the power along the horizon and work far better than fiberglass sticks as such and work even better when isolated as mentioned below:


For the commercial towers that my systems are on for the one I use 2 separate antennas with no duplexer.  The sites master RX antennas are at the VERY TOP - 300' and extend upwards to 310 - 320' up depending on the length of them.  All transmit antennas have 50' of vertical separation between the bottom of the RX antenna and the top of the TX antenna.  Breakdown below:


300 - 320' = RX

DB420 TX is 20' long so it would be mounted at 230' extending up to 250' = 50' between the top of it to the bottom of RX

DB408 TX antenna is 10' long so it is mounted at 240' extending up to 250' = 50' between the top of it to the bottom of RX

Hope this helps!

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Here is some more information I found in my Excel sheet database that I think will help show how much better vertical separation is VS horizontal separation:

In short you achieve the same level of separation at 50' vertically VS basically 1,000' horizontally.  Most 4 cavity Band Bass / Band Reject duplexers offer about 80 - 85 db of isolation VS 6 cavity BP/BR duplexers are about 100 db of isolation.  Note that the chart below is for dipole antennas, not fiberglass sticks which are far worse with radiation going up into space and down into the earth.


450MHz Channel - Vertical Separation - Dipole Antennas 450MHz Channel - Horizontal Separation - Dipole Antennas
2.5' = 25db 10' = 32db
3' = 28db 20' = 38db
4' = 33db 30' = 42db
5' = 38db 40' = 44db
10' = 48db 80' = 50db
20' = 60db 200' = 58db
30' = 65db 300' = 61db
50' = 70db 500' = 66db
  800' = 70db
  1000' = 71db
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