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Duplexer Jumpers


Jake313
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Many Radio Tech's will tell you that the mini UHF SMA connectors are a disaster waiting to happen.  The compact thread design is prone to stripping out when connectors are being tightened and may even snap off laterally under certain circumstances. Motorola started using these SMA connectors for proprietary reasons so other brands of equipment could not directly be used to replace Motorola equipment without additional labor costs.

 

You may wish to look at these adaptors for SMA to N applications:

 

http://www.amazon.com/Male-Plug-Female-Jack-Adapter/dp/B00D6HG8B4

 

However, when ANY SMA connector strips out it's threads, (and many will), a quick fix is to cut down some RG8U copper braid about 1/2 inch long and remove a few strands. Use the strands across the stripped SMA threads while you are reattaching the connector so the connector has something to grab on to. Use caution to not allow strands to lap over the end of the connector and get anywhere near the center conductor.  This may possibly salvage a much bigger job and provide a positive connection... until the next time that the connection has to be removed... and then  you start all over again.  Thank you, Motorola.... :rolleyes:

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Thanks PG. Yeah Motorola was trying to be proprietary but just made things more complicated :). Too bad you can't just swap out the mini connector on the back of the radio and put an N connector on.

They designed the area surrounding the SMA radio mounted connector so there isn't any room for anything larger.  I know some techs who actually hard wire a Kenwood output cable and connector to Motorola radios for the sake of reliability.

 

 Good luck on your project.

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     One reason I can think of is that LMR400 is a solid core coax cable which does not flex well, and at a 1 foot length, you have to be careful to keep the looping to a wide relaxed bend.  When it is flexed it likes to return to straight so the loop will cause a permanent stress on the connectors, so check it from time to time. 

 

     Also, if I remember correctly, do not cut any jumpers at 3 foot.  I think with the 400 MHz area the harmonic at that length can cause the jumper to go hot and radiate RF.  I can't remember exactly so as a rule I just never cut jumpers to 3 feet.  Please someone chime in if I am wrong.

 

Jerry

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twice I have encountered what we call a resonant cable.power/SWR look good at the bottom(radio) and no power at the top. 

first time I called a buddy of mine, he asked what freq  told him 460 he said cut 6" off the bottom and it'll be fine.

basically the cable becomes it own own matched load, cut 1/4 wavelength off and it goes open.

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I'm just going to throw this in there…

 

Maxtrac's use Mini UHF, not SMA. Nothing wrong with the Mini UHF as long as you keep it below 120W and below 1 GHz. 

 

The GR series repeaters initially used RG58. Any RG8 or RG58 is a very poor cable in a duplex environment.

 

Best option would be RG142 or RG400 (for the RG58 size). Cut your line to odd quarter wave lengths (roughly) of the tuned frequency (the lines need to be within 1/4" or less of one another).

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