radio overloaded due to antennas proximity

[ZL2CZE]

Hello there!

We have two or more VHF mobile radios set up for events in one building, with antenna separation of maybe 8 meters / 25 ft.
They often both listen on a VHF repeater.
Typically when one transmits, the other is deaf.
There's not much room on the roof. Having them both omni keeps the setup practical for multiple repeaters.

What is the most effective way of mitigating this without drastically decreasing the RF output on said radios?
I'm thinking of pass filters but not too experienced with that kind of stuff.

 

[kayn1n32008]

Your options are very limited.

Reducing output power to minimum needed would be the easiest.
Stacking them vertically would be the most effective.

 

[popnokick]

It's called "receiver desensitization" and yes.... it gets directly into the receiver's front end components. And if the signal is strong enough from the nearby transmitter, it won't even matter if you disconnect the antenna from the receiver; it will still be de-sensed. So filters of nearly any type aren't going to help. However, some radios are less affected by receiver de-sense than others. What are the VHF mobile radios that are in use in close proximity to each other? Are they all the same make / model? Any chance of doing a lot of radio swapping / shopping to see if you can find a combo that is less affected by desensitization?

 

[cmdrwill]

Yes depends on how good the receiver is in your radio.

 

[Firebuff880]

If it were my system I would be looking at a Base Station Combiner from DB Spectra or RFI Wireless..

An yes you will have loss on the TX side.

 

[prcguy]

What are all the frequencies involved? Are the radios always set for a specific repeater freq and never change? If so and if the frequencies pan out you might be able to use a notch cavity on each mobile radio. You would tune the cavity on one radio to notch the transmit frequency of the offending radio and do this for each radio.

If this is for 2m amateur with 600KHz offset it may not work out due to the notch width, but for commercial freqs with wider TX/RX spacing its easy to do.

Hello there!

We have two or more VHF mobile radios set up for events in one building, with antenna separation of maybe 8 meters / 25 ft.
They often both listen on a VHF repeater.
Typically when one transmits, the other is deaf.
There's not much room on the roof. Having them both omni keeps the setup practical for multiple repeaters.

What is the most effective way of mitigating this without drastically decreasing the RF output on said radios?
I'm thinking of pass filters but not too experienced with that kind of stuff.

 

[R8000]

Less power or more separation. That's how RF/Physics works. There's no other way to sugar coat it.

 

[majoco]

You would tune the cavity on one radio to notch the transmit frequency of the offending radio and do this for each radio.

What PRC says +1. Common practice for co-sited VHF stations - although it's more common to have the transmitters on one site and the receivers some distance away, say 2 miles or so. Often the receivers are all located on a 'quiet' RF site - the audio is fed down telephone lines. The transmitter antennas are all grouped quite close the transmitters themselves to reduce power losses.