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Old 04-22-2013, 2:42 PM
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Default Two Mobiles Same Vehicle

I am looking for some guidance on installing two VHF CM300's in the same vehicle an ambulance to be exact. I don't have the budget right now for a dual head setup due to it being a last minute purchase of the ambulance to be used as backup ambulance. I do have two CM300's I am looking to install, one in the front and one in the back. Is there some way I can eliminate them from feedbacking when transmitting since they will be on the same frequency? I read somewhere online about linking them together via the External PTT pin and the RX Audio pin?

Thanks!
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Old 04-22-2013, 3:50 PM
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Why would you need redundancy for the same freq? I'm not sure I understand.

If they were off freq from each other they'd probably desensitize each other during transmit if they have their own antennas.
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Old 04-22-2013, 4:22 PM
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When I was a volunteer EMT, we had one radio in the truck, with two speakers and microphones (one in the front and one in the back). The only downside was the driver or passenger up front had to change the channel.

Just my two cents...

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Old 04-22-2013, 4:25 PM
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KC8esl Ambulance's use two radios and some time's there are on the same frequency. The reason for this is, the driver can respone to dispatch's and make contact with the medical locations about ETA's while the person in back can talk to the doctors/nurses while in route.

ajmanz there is no way to cross connect the two radios to prevent feed back. the only way to do that is two turn down the volume on the radio that is not currently being used.

As long as the volume on radios are at a resonable level, normally there is no feed back.

All the medic units and ambulances in my area run radios this way and have no problem.
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Old 04-22-2013, 4:31 PM
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@KC8ESL,

In many areas it is common for EMS Unit dispatch and area hospitals they serve to be on the same frequency for their day to day operations.

The driver needs to talk to dispatch and the medic needs to talk to the hospital. A long mic cable that can be reached by the medic is not a safe option. The only safe options are either a dual-head (not cheap) radio or two separate radios.

@ajmanz,
Good luck in finding a useable solution for your agency.
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Old 04-22-2013, 4:32 PM
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Ah yes the whammie effect. I don't think there is a way you can stop it without turning the volume down. I think just maybe if you get a noise canceling microphone then it may prevent it. It also depends on the repeater delay. I know some departments have gotten use to it and just down care that they sound like they're in a tunnel. Also maybe depending on the placement you may not get any feedback.
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Old 04-22-2013, 4:45 PM
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@KB5ILY,

We did things differently. The person in the back was the only person talking to the hospital and the same nurse/doctor who gave you the orders was the same person you gave your ETA to. Once you told dispatch that you were en route to the hospital, they didn't expect to hear from you again until you checked arriving at the hospital.

Nowd, a checking en route, on scene, etc is done with a single push of a button on an MDT.

There really is no clear cut answer for everyone, things are done differently everywhere.

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Old 04-22-2013, 5:35 PM
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I've heard of private ambulance operators in my area using a GM300 mobile mounted in the front of the vehicle with a second mic and speaker connected via the rear accessory connector and long wires ran to the rear area. Yes, you have to change channels from the front, but I don't see how this would be a major pain in the rear. Of course, I'm sure they have updated the radio model since I last heard, and you may be able to do this with your CM300.
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Old 04-22-2013, 6:06 PM
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We have 3 separate CDM1550's in our ambulance; one in the cab, one in the passenger's side rear of the module, and the third in the driver's side front of the module. As long as no one cranks up the volume too much on any of them, there's no feedback problem at all.

And yes, the CM200's, CM300's, and PM400's can all be set up with a remote mic and speaker quite easily. Remote channel select (up to 4 channels plus revert options) is also possible through the accessory connector. With just a bit of tinkering and a DTMF mic, you could easily have channel change available to both the driver and the medic in an ambulance.
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Old 04-22-2013, 8:58 PM
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If you do end up using 2 separate radios, make sure you get as much separation between the two antennas as possible.

Around here this isn't an issue as dispatching is done on the Fire VHF channel, and ambulance to hospital is done on the UHF MED channels. Some had dual head UHF radios with a head in the front and one in the back, but only one RF deck.

Maybe rather than buying two CDM1550's, you could look at doing a dual head set up with some Kenwood radios. Those will be a bit less expensive than Motorola, and would be easier, one power feed, one antenna, etc.
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Old 04-22-2013, 11:01 PM
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Quote:
Originally Posted by ajmanz View Post
Is there some way I can eliminate them from feedbacking when transmitting since they will be on the same frequency? I read somewhere online about linking them together via the External PTT pin and the RX Audio pin?

Thanks!
The CM300 has a programmable input for receive audio mute and you'd have to cross wire that to a transmit indication (PTT) of the opposite radio. You can't use external PTT as this is not an output. You'd have to go into the radio and hunt for the proper logic.

As you can see, there are many ways to set up an ambulance. If what you have now is two CM300s I'd slap them in and not worry about trying to cross-mute. Proper placement in both the cab and the module for the operators will keep volumes at reasonable levels and feedback should not be a problem. The rigs I know that run dual radios almost never have them on the same channel at the same time. The only problem is desense, so the operators need to pay attention to what the other guy is doing and be courteous.
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Old 04-23-2013, 6:57 AM
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Thanks for all the input. I think I will put them in and see how it goes. Next year I will budget for a PM1500 dual head setup which is what I have in my other two rigs.

AJ
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Old 04-23-2013, 7:41 AM
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Smile 2 radios in a squad

is dispatch and the hospital on the same channel?
I use to do installs at a Motorola shop and had no trouble installing 2 radios 1-for driver 1-in the rear box.
I'm a volunteer and i drive a squad our main radio is UHF and the channels for the Hospitals are VHF so no feed back.Yes make sure the antennas are far apart maybe if possible mount the cab\driver radio antenna on the roof of the cab if possible and the other on the roof of the box.your also going to need a thick mount coax for the box they make a 3\8 inch hole mount for the box mount.
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