antenna spacing

[jim]

I'm installing a 45 watt transmitter at my UHF site.
The UHF repeater is a Mastr II 100 watt (452.xxx/457.xxx) with a single antenna and duplexer. The VHF is going to be a tone controlled remote unit with a 45w SM50 operating on 151.xxx. What is the minimum that should be between the antennae if they are side-by-side? Would it be better to use vertical seperation? I can sacrifice VHF height on the pole. I'd like to see the two systems play well together.

[kf4sqb]

Definately better to use vertical separation. The main thing that would concern me would be the possibility of the UHF system's transmitter de-sensing the receiver of the VHF. The UHF system, being a repeater, shouldn't have this problem because of the cavities, mainly if the RX side is a band-pass type. The VHF, unless you went to the extra expense, wouldn't have this luxury.

[RFdude]

Hard to tell you "by the numbers" what you should do without specifics. If you are in the middle of nowhere and there is no one to which you can cause interference despite yourself, then some basic separation would be OK. But in a dense urban environment, note that your setup is ripe for IMD. 3rd harmonic of 151 is 453 MHz, mixed with your 452 MHz TX and away you go polluting the airwaves. A duplexer is not a narrow Bandpass device and might not offer much attenuation to the mix.

I investigated a situation like this many years ago identifying a couple VHF and UHF paging transmitters (one BP can only) horizontally spaced on a roof top. Actual IMD emissions were -55 dBc. FCC investigated, saw the IMD, and walked away since they were down sufficiently. However, another VHF repeater high on a hill ~60 miles away was receiving the IMD, albeit just above the receiver threshold. The path was over a lake, so free space loss only.

Good design practices please gentlemen!