Here's the situation. I'm going to be installing a couple of VHF repeaters at a site and am now choosing antenna placement on the tower. My understanding is that to determine the separation of TX/RX antenna, there should be enough isolation/attenuation so that the TX signal is less than Receive Sensitivity plus the Adjacent Channel Rejection at the RX input. So referring to the Quantar specs, (here), where:
Receive Sensitivity = .25uV ~ -119dBm
Adjacent Channel Rejection = -80dB
Preselector/Receiver Bandwidth = 4MHz
Requirement for TX to RX isolation = -119dBm + 80dB = -39dBm
So referring to the antenna separation curves, (here), the best I can achieve is 50dB of isolation with a vertical separation of 45 feet. So, (assuming no cable loss) if the repeater is transmitting at 50dBm (100 Watts), with the 50dB antenna separation/isolation the TX signal at the receiver is 0dBm, so I'd need to add another > 40dB attenuation with an external pre-select filter.
So is my understanding of how to deal with this situation correct? How does the 4 MHz pre-selct filter factor in to this? Do I only have to protect against signals that fall within the 4 MHz window, including other transmitters? Does the situation change if the TX frequency on the repeater pair is outside of the 4MHz window of what the RX frequency is? It of course makes sense to me that it would, but the specs don't give a figure for what the roll-off of the Rf front end is.