Problem with two radios

Alittle background. I attempt to operate two radios, both Kenwood d700's from a motorhome with two vertical attennas that are mounted on the roof, that operate on repeaters 147.270 and 147.300 without desensing each radio when the other radio transmits. ? Other repeaters are available but out of the geographic area. So far this has been a fruitless excercise. I could come up with 100 feet of coax to separate the two antennas somewhat but this has not given good results. Better but no gold rings. The radios are used in a race event where contact must be maintained sometimes on separate frequencies. If you are familiar with a Performance Rally, cars and volunteers are on different stages and different frequencies are used because of terrain and the distances .

Looking for ideas on how to operate in this situation.

Being that close physically and on the same band, desense is inevitable without expensive notch filters.

Your best option is one on 2m and one on 440. We've ran into this at disaster drills, and other events where FD/PD high power VHF rigs blank out our 2m rigs at Incident command. We moved to UHF and the problem, logically, went away.

I see no practical solution within the motorhome without spending several thousand $$ on custom BP-BR filters. Each filter would still have to pass the repeater input frequency and output.

You might consider a cross band repeating radio some distance from your motorhome with one side on the 2m repeater you need and the other side on a simplex UHF freq. You could then access that with a UHF radio in the motorhome which will not desense the 2m radio.

Physical separation is your friend here, but you will need more distance than the motor home allows.

Others have made good suggestions. I will add using some type of IP based remote setup, so you can place the radios further apart.

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You would need to have better isolation between your transmitters and receivers. Your radios can't break down into separate transmit and receive ports, so using helical resonators or a duplexer of some configuration won't work for you. You can't hang a helical resonator or pass-reject cavity (or series of cavities) on a single-port radio that's not operating simplex or a very narrow split within the response of the filters. It's not that 147.27 and 147.30 are too close together, your transmissions from 147.87 and 147.90 (I'm guessing it's a + 600 split) are getting into the receivers. Those radios are designed for wideband receive, unlike the old days of radio where Motracs and Micors had preselectors that only let in the frequency you needed. Ditching the Kenwoods and going with one of these heavy old monsters crystalled up on the two repeaters might also solve your problem.

If I were in your situation, I'd go with krokus' plan and move the mobiles as far apart as possible. If you use IP connectivity, they can pretty much be anywhere you have access. Check out BridgeCom Systems - The MV Series

FOUR ANSWERS THAT CONFIRM my findings, but some possibilities that I have not thought up. While I have cross banded in the motorhome, mainly for a demo, I had not thought of taking a another vehicle, driving it across the lot, and runing cross band there to acheive separation. This might work. Thanks.,

teufler said:

FOUR ANSWERS THAT CONFIRM my findings, but some possibilities that I have not thought up. While I have cross banded in the motorhome, mainly for a demo, I had not thought of taking a another vehicle, driving it across the lot, and runing cross band there to acheive separation. This might work. Thanks.,

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Another vehicle is an option, but you could use a pole with an antenna on it, with the radio sitting in small enclosure. If there is no power run to that site, a solar array and a battery could be your power source.

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A recent discussion on cross band, with some options and considerations:

http://forums.radioreference.com/showthread.php?t=309738

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Two ideas come to mind. If you are stationary, spate the two antennas vertically-that is put the antennas in the nulls straight up and down from each other (and as far apart as feasible). Also as you are using repeaters, remember to use the minimum transmit power you can to 'hit' the repeater(s). Backing down the transmitters can go a long way to dealing with desense.

73

I think bring the power down and moving one radio away from the other radio.. In station setup, I must try to get the antennas also separated, about 100 ft one way and 100 ft the other way. When operating out in the wild so to speak, sometimes this can not be acheived due to trees and terrain, but separation than what we had this year will be am improvement.

Best so far

wyShack said:

Two ideas come to mind. If you are stationary, spate the two antennas vertically-that is put the antennas in the nulls straight up and down from each other (and as far apart as feasible). Also as you are using repeaters, remember to use the minimum transmit power you can to 'hit' the repeater(s). Backing down the transmitters can go a long way to dealing with desense.

73

Click to expand...

Best suggestion so far.
This is what I was thinking of suggesting as I read thru the thread.
Separating two antennas vertically is going to do more for you than separating them horizontally.