transmitting feedback

[haxsaw]

I have searched on this but not able to find a good solutions, here's what's happening, I have a 800MHz first responder radio, bottom radio, its a XTL2500 and my 2 meter radio, top radio, its a CDM1250, and the 800MHz radio has a remote speaker mounted on top of the CDM1250, I am getting feedback in the remote speaker when I transmit on the 2 Meter radio, I have not had the opportunity to transmit on the 800MHz radio to see if that causes any feedback in the 2 meter radio.

does anyone else have a setup like this and how do I prevent the feedback?

 

[tweiss3]

What feedline are you using and how is it terminated. I know in my office, my computer speakers don't like 2m transmission near them at all from an HT, but the 2m rig connected to 400MAX cable doesn't bleed RF and mess with the computer speakers only 24" away.

My guess is there is some shielding gone somewhere that 2m RF is leaking and going straight into the amplifier (inside the 800MHz radio) and causing the feedback. If you flip the radios, does it do the same thing?

 

[haxsaw]

What feedline are you using and how is it terminated. I know in my office, my computer speakers don't like 2m transmission near them at all from an HT, but the 2m rig connected to 400MAX cable doesn't bleed RF and mess with the computer speakers only 24" away.

My guess is there is some shielding gone somewhere that 2m RF is leaking and going straight into the amplifier (inside the 800MHz radio) and causing the feedback. If you flip the radios, does it do the same thing?

I'm using the standard rg58 coax that came with the antenna, the antenna is mounted on a hood bracket on the driver side of my truck.. I have not tried flipping the radios, the XTL2500 has a different mounting bracket and would make it harder to flip them, i may need to simply relocate the remote speaker.

 

[prcguy]

When you say feedback, is it a squeal in the speaker like you would get in a PA system when the mic is too close and you can hear your voice? Or is it just some noise in the speaker when you transmit? Is it an amplified speaker or passive? Does it happen on multiple 2m frequencies across the band or just one? Can you change power levels on the radio and affect the feedback?

Your FM radios should not have any detectable voice in the speaker from RF getting into speaker leads, etc, and getting rectified into audio, that usually only happens in AM mode. I would rule out coax leakage because thats going to be in the 40+dB range per cable and the coupling between your 2m and 800 antenna are going to be much less, like less than 20dB if on the same vehicle.

Its possible to have a 2m harmonic land in the 800 band and the radio picks it up like any other signal on channel, but that should only be a problem on one 2m frequency. Otherwise you need to find the source of the problem before you can fix it and you need to run some tests to determine the cause.

The easiest would be changing to very low power on the 2m radio to see if the feedback is affected. Can you temporarily move the 2m radio away from the 800 radio and all its cabling for a test? Do you have another 2m radio of similar power you can use outside the car but tap into the antenna cable for the 2m radio causing the problem? Do you have a mag mount antenna you can stick on the car and move around away from the 800 antenna? Things like this will start ruling out potential problems and hopefully discover the cause so you can snuff it out.

 

[haxsaw]

When you say feedback, is it a squeal in the speaker like you would get in a PA system when the mic is too close and you can hear your voice? Or is it just some noise in the speaker when you transmit? Is it an amplified speaker or passive? Does it happen on multiple 2m frequencies across the band or just one? Can you change power levels on the radio and affect the feedback?

Your FM radios should not have any detectable voice in the speaker from RF getting into speaker leads, etc, and getting rectified into audio, that usually only happens in AM mode. I would rule out coax leakage because thats going to be in the 40+dB range per cable and the coupling between your 2m and 800 antenna are going to be much less, like less than 20dB if on the same vehicle.

Its possible to have a 2m harmonic land in the 800 band and the radio picks it up like any other signal on channel, but that should only be a problem on one 2m frequency. Otherwise you need to find the source of the problem before you can fix it and you need to run some tests to determine the cause.

The easiest would be changing to very low power on the 2m radio to see if the feedback is affected. Can you temporarily move the 2m radio away from the 800 radio and all its cabling for a test? Do you have another 2m radio of similar power you can use outside the car but tap into the antenna cable for the 2m radio causing the problem? Do you have a mag mount antenna you can stick on the car and move around away from the 800 antenna? Things like this will start ruling out potential problems and hopefully discover the cause so you can snuff it out.

its is a noise, almost sounds like a guitar amp hum before you plug the guitar in, the noise does change if I lower the power but does not go away, it is on all frequencies but worse on some than others. if that makes since. it does not affect the transmitting of the radio at all..

I moved the speaker temporarily and it got much better but did not go way, I may try a ferrite choke on the speaker wire to see if that eliminates it.

 

[prcguy]

Is it an amplified speaker that's connected to power?

its is a noise, almost sounds like a guitar amp hum before you plug the guitar in, the noise does change if I lower the power but does not go away, it is on all frequencies but worse on some than others. if that makes since. it does not affect the transmitting of the radio at all..

I moved the speaker temporarily and it got much better but did not go way, I may try a ferrite choke on the speaker wire to see if that eliminates it.

 

[haxsaw]

Is it an amplified speaker that's connected to power?

no, it is not an amplified speaker, the XTL2500 does not have a built in speaker and requires an external one.

 

[haxsaw]

no, it is not an amplified speaker, the XTL2500 does not have a built in speaker and requires an external one.

the noise has stopped since I rerouted the speaker wire, I'm still going to use a ferrite on the speaker sire to make sure it does not come back.

 

[prcguy]

Interesting. For VHF suppression a 43 mix snap on ferrite bead with two turns through it or a Laird 28 mix is good. You need at least two turns around the core otherwise it takes about three beads in series to be effective. Place then right at the radio.

the noise has stopped since I rerouted the speaker wire, I'm still going to use a ferrite on the speaker sire to make sure it does not come back.

 

[haxsaw]

Interesting. For VHF suppression a 43 mix snap on ferrite bead with two turns through it or a Laird 28 mix is good. You need at least two turns around the core otherwise it takes about three beads in series to be effective. Place then right at the radio.

so the noise was back today, strange, i think the ferrite will take care of it,

 

[cmdrwill]

the antenna is mounted on a hood bracket on the driver side of my truck..

There IS a possible problem. Not enough groundplane and or bad VSWR mismatch.

 

[ff026]

RF from the antenna mounted on the hood bracket getting into the external 800 speaker.