Two repeaters at a site
Would it work to protect each repeater from the other (against intermodulation & desense) by placing an additional notch reject duplexer (terminated at antenna port and using TX&RX ports to create a two-port device) or just a double-can filter tuned purely to reject the other TX frequency between each antenna and its duplexer?
A. A two repeater RF site is sometimes a difficult thing. On one side you have proper design and on the other side you have cost and practicality.
You did not say whether the two repeaters will be part of a trunked system (i.e. Capacity Plus or Capacity Max). If they were, I would say that you need to be mindful of the fact that having two separate antennas will mean that the repeaters at that site will have slightly different coverage. A radio might be within range of Repeater 1 but may have a weaker or no signal if required to move to Repeater 2.
If the two repeaters are not part of the same trunked system, you can ignore the above statement.
Someone who works for one of the manufacturers can correct me on this point but in my assessment, a duplexer is not really the best piece of hardware for providing additional filtering. In my experience, a cavilty filter would provide much better results and, when multiple cavities are used, multiple nulls and/or peaks could be achieved.
More importantly, what is needed in my opinion, is to carry out a site survey which includes an intermodulation calculation. This calculation must take into account all existing equipment on site, as well as the two additional frequency pairs.
Without more information, my initial thought would be to simply use two antennas and two duplexers. Since this is UHF, the required vertical seperation will not be too large and so the difference in coverage between channel 1 and 2 will not be that much.
The think with RF filtering is that there is usually more than one way of doing things. If anyone reading this has a better idea, I'd be keen to hear about it.
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