How do I determine the necessary isolation and antenna spacing between TX and RX antenna for repeater

Question

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.

Answer 1

I don't know the exact answer to this question and I'm far from being an expert on the matter, but here's a couple naive ideas that may be helpful:

First, consider the fact that the noise floor at the site is likely quite a bit higher than the rx sensitivity of your hardware, and there's usually no benefit to working from the cited figure if it's going to be below the noise anyway. In short, the "practical sensitivity" number to start from is (off the top of my head) maybe 6-10 dB below the noise, which is likely higher than -119 dBm. That being said, if you it that way, you might want to add an input attenuator to the RX feedline in order to bring the noise-governed figure down to the receiver's sensitivity.

Second, remember that while you can always add a carefully tuned high-order filter, either an LPF or a notch, to further reduce the particular frequency (your repeater's TX freq) that you are worried about, this filter will likely attenuate the input frequency a bit as well. But, as mentioned above, that is unlikely to significantly deafen the repeater overall when the noise floor at the site is taken into consideration.