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Antenna Separation for a Repeater



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How far do you want it to work and what power levels are you using ? Ideally you want 20-25' vertical separation for good isolation. The closer they are the less isolation you get. In the end its almost always cheaper to buy a duplexer than run 2 antenna lines and antenna's.

In the LMR world antenna's can be closer but are normally filtered to limit the exposure of each other. On one of our SAR sites we have a DB404 10' below the RX DB408. The receive multi-coupler has not only pass cavities but also notch for the TX channels. The TX combiner has pass cavities for the TX channel only. There is no noticeable decense on the system. But the combiners and multi-coupler cost more than most of you spend on a vehicle.

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Definitely - the CCR's (Inexpensive radios using Direct Conversion single chip design) have sensitive but not selective receivers (ie: Wide open to any and all other nearby transmitters).  Those types of inexpensive radios have receivers that are easily overpowered due to their lack of filtering.  You would be MUCH better off to buy a used LMR radio that has a really good specs for selectivity and sensitivity. Decent used LMR stuff is available at less than $100 per radio - especially when you don't care about the transmit power.

At 20 ft of vertical separation, you would still need some type of a bandpass cavity for the receiver. Get more than 30 ft. and your difference between the transmit and receive coverage begins to get out of hand, not to mention the cost of getting a real sturdy mast that can stay up at over 50 ft. -  just so your lowest antenna  is still 20 ft. off the ground.

There's a chart out there on the internet that's been posted here before, showing the differences in vertical vs. horizontal separation needed to achieve certain levels of isolation. As I remember it, 15 ft. of vertical isolation was equivalent to about 1000 ft. of horizontal.  In other words,  horizontal is not practical at all.


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