Mobile repeater with out a duplexer

will_545

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I am wanting to put a mobile repeater in a fire truck. I know the easy way is to install a duplexer. But I am wanting to be able to change my transmit channel, so my duplexer would be out of tune and cause problems. What I am trying to figure out is can I separate the antenna far enough they don't cause problems with each other. Also they are both vhf radios.
 

mmckenna

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It can be difficult to get enough separation, but if the truck is big enough, then it's possible. Also keeping power levels low would help quite a bit.
Inband mobile repeaters can be a challenge. You could probably get a wide enough filter to make it possible, but I've never tried personally. Usually out of band is the way to go, but not always possible.
 

prcguy

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If this is for a "vehicle extender" to allow a nearby hand held to talk through a radio back to somewhere else, you can probably make it work with two antennas. This type of setup would have a very low power transmitter on one side. If this is going to be a full blown repeater with power levels over 20 watts then you will probably need a duplexer or band pass filters.
 

mmckenna

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It also depends on the frequencies used.

The mobile repeater channels that often get used are quite a ways separate from the channels commonly used for LMR/Public safety. In the VHF band, 174MHz is often used since that is about as far away as you can get from the 154MHz range.

If you are going to be using multiple channels, you -really- need to have a lot of separation.
 

PACNWDude

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I used to work for a company that provided "emergency communication services" often for hurricanes, forest fires, and other adverse conditions. The poor man's repeater was a kit built with two CDM1250/1550 mobiles and a "controller" box sold by MRE1032 on online auction sites. Each radio had its own antenna, we used 25 watt radios turned down to about 10 watts (have to watch talk-in/talk-out issues). this worked well on large conex box comms trailers, toy hauler trailers, and even on vessels. We kept everything 5 MHz apart and low power as mentioned above. We could even use RG-58 to induce some expected loss, to make it less efficient, still gaining some distance as a repeater for handheld radio users. System worked well, and many videos abound for non duplexer repeater construction.
 

will_545

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I have a motorola rick and cdm. That's another thing that I was thanking was turn the power down. I may try putting one on top of the cab and then put the other antenna in the back under the truck. The truck is about 40 feet. Besides watching swr what should I look for
 
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mmckenna

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Besides watching swr what should I look for
Well, as a417 asked, and I mentioned above, the frequency separation is a huge factor. Trying to do this in-band can be difficult. It's easier and more successful if you have a -lot- of space between the frequencies in use, especially if you want to skip the filtering.
 
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