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Duplexer Tuning / Distance Without Duplexer



I have a GM300 setup, but now I need a duplexer.  Is it possible to get one tuned for RX "467.xxx" and TX "462.xxx" and not the /exact/ frequency? I would like to hop between frequencies as I travel without having to pull out a duplexer for each pair.


Also, if I want to scrap the duplexer idea altogether and just do two antennas, any thoughts on distance between the two (at 10watts)?





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you can "getaway" w/about 3/4 Mc but there will be degradation on both sides as you go out from center.

theoretically w/a good duplexer that is properly tuned to 462/467.6375 you "could" span all the channel from 550 to 725.

I have seen this done once in the past and I serviced it for a few yrs but it was 2 transmitters and 2 receivers set up as a quasi combiner w/duplexer to operate off one antenna.

now to the other option

2 antennas can be done "if" you have enough isolation. horizontally at 10W you would need about 30' or so w/unity antennas. vertically you could do about 10' w/lower ant inverted.

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Hi, Robert and welcome to MyGMRS Forum.


Based on my limited repeater experience, the duplexer has to be 'super tuned' in order to work as designed with the rest of the system. The duplexer's tollerances are such that it won't respond well to something that it isn't tuned precisely for.


I believe that one of the members mentioned about 5 months ago that 30 feet separation for a non duplexer repeater might be acceptable - but I'm not certain as to what ERP was that was being discussed at that time.


Others may have info about a multi frequency, user selectable duplexer that I am not aware of...

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I'll ditto on the tuning. I have a set of older Motorola style cans, 2 RX, 1 TX that I had tuned up on the main freq I use by my buddy when he still worked at M a few years ago. I use them in my MTR2000 machine. 


I have a small cellwave that was tuned for .700 at one time, and I have tweaked on it a time or two. I can "tune" it the hard way if I need to...bird power meter, and a TX I can reduce to almost no power to check the desense. Works good enough. I have it running 10w on .650 right now.


You can do split antennae, I used to have a setup with the Rx separated 30 feet vertical and about 75 horizontal from the Tx. (Rx on a house, Tx on a barn, wireline between them).



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I have two duplexers coming in from China that are supposedly designed for the GM300.  Each tuned to unique frequencies.  I will test both deviation from those, and separately, using two different antenna's.  I will report back my findings here.


Thank you everyone for the swift input!


Here is the rig when everything gets here (waiting for ant's and dups):

1x GR300

2x GM300

1x ID-O-Matic II 

2x DPD GMRS Tuned 4dbi Ant

1x 50Watt UHF 6 Cavity Duplexer RX 467.725 TX 462.725

1x 20W UHF 6 Cavity Mini Duplexer RX 467.650 TX 462.650

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Personally, I wouldn't replace the RICK with the ID-O-Matic as the RICK has remote knockdown capabilities. Is just feed the ID-O-Matic into the RICK's accessory and use it just for ID. Since you have one, you might download the service manual if you don't already have a copy.




Repeater builder also has some info about split antennas versus duplexes and how far apart they should be.

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