2 mobile interference

[Ett1033]

I have a command car (2000 Expedition) that has 2 mobiles installed. I have 2 1/4 wave antennas on the roof about as far apart as possible. Our main freq is 154.3XX and is PL protected(TK-790). The other radio (CDM 1250) usually is operated on 153.8XX and is not PL protected and won't be. When The main freq is keyed, 80% of the time it causes the other radio to squelch badly. What are some solutions to minimize this interference?

[va3wxm]

How much power is the Kenwood running? It may just be simple overload.

Perhaps a 154 MHz notch filter on the CDM?

[Ett1033]

TK 790 is 110 watt unit, running at 50 watts on that channel

[wa2zdy]

You've two radios, 500 KHz apart with antennas in very close proximity. If the one receiver WERE on PL, you wouldn't have the noise burst, but trust me, I don't see either of those radios hearing while the other is transmitting. Overload is the word for it.

Maybe notch filters would work. Maybe not.

I think you've got a bad situation there. Using PL could eliminate the annoyance of the noise burst, but in the end it won't actually be making the situation much better.

[nmfire10]

In otherwords, the PL would make the problem hidden. You still won't be able to RX on radio 1 while radio 2 is transmitting (and vis-versa).

[jim]

Take the PTT from each radio and run it into the opposite radio's RX AUDIO MUTE.

When one goes into XMT, the other will mute.

[LCFD1]

We are also thinking about installing a second radio, but I know we have not thought of the interference issues yet. We plan on using 1 VHF 45 watt CDM1550 and one 45 watt GM300. Worst case scenerio would be 730 KHz separation, best case 955 KHz. 1/4 wave antenna separation would be 5 feet (on a tahoe) if we are lucky.

What kind of interference would we expect?

I do like Jim's idea of routing the PTT to the Rx audio mute. Some people on our department wont like that though because the purpose of the second radio is to monitor the firefighter down (RIT) channel. You could be transmiting when a mayday occurs.

Thanks
-Sonny

[wa2zdy]

. . .I do like Jim's idea of routing the PTT to the Rx audio mute. Some people on our department wont like that though because the purpose of the second radio is to monitor the firefighter down (RIT) channel. You could be transmiting when a mayday occurs.

Thanks
-Sonny

With that kind of frequency spacing and five feet between the antennas, while you transmit on one radio, you're not going to hear anything on the other radio anyway. So if that's the reason for the second radio, you need to find a different solution. If lives are at stake, this idea could kill someone.

[LCFD1]

Okay,
I definetly understand that a a receiver will desense when a transmitter is working near by. But, would it still be able to pick up a 5 watt HT that is no more than 500 feet away? That might be a dumb question, but I really dont know. When we are operating in that mode, we are only trying to receive signals in the immediate area.

I suppose that we could have a separate channel for the fireground on the main radio that is on the same frequency, but lower power. That way, it will be less likely to desense the other receiver. I would program a button for hi/lo power, but we are using those for other things....

-Sonny

[RRrobby]

LCFD1 worte:

But, would it still be able to pick up a 5 watt HT that is no more than 500 feet away?

Taking into account the inverse distance squared rule of radiation, you are going to get a LOT more signal into the first antenna from a second antenna with 50 watts at 5' than you would get from a 5 watt antenna at 500'. A notch reject filter may work in this situation-RRRobby

[SonicSounds]

Have you thought of fender mounting one of the antennas and leaving one towards the rear of the vehicle on the roof? The vertical seperation might make a big difference.