Coax for repeater

[stmills]

I have a UHF GMRS repeater that I was given with an antenna and feed line. Now that I have the system running I want to replace the antenna and feed line with new line -I suspect the current coax is 20+ years. The coax run is 20' and I am currently putting out 10Watts of power. What I am trying to figure out is with a fairly short coax run- how extreme do I need to get with the type of coax - I want a good cable but don't need overkill.

 

[n5ims]

Get some LDF4-50A. With your length you can get end-or-reel or short runs at good prices (perhaps even free). Avoid LMR-400 and its varients (will cause intermod on duplex operations like repeaters).

 

[Project25_MASTR]

Some quarter inch heliax would probably be appropriate as well for such a short run.


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[johndjmix]

I would run hardline, especially for a short run. Not too expensive for so short.

That said.....I ran 40' of LMR-400 up to our repeater, which is putting out 50 watts. Its crystal clear. Others have said not to do it and I know its not what would be ideal, but since I had already purchased it i went with it. My frequency's are about 7mhz apart (VHF) so possibly that's why im not having issues.

Hit the repeater from 45 miles out (on a high elevation mountain) crystal clear. On flat ground its not totally clear but hit acceptable from 25 miles away.



--John

 

[kayn1n32008]

You want to avoid LMR-400, will it work? Sure but you are inviting issues because the braid and foil are not the same material. With movement in the coax you can generate noise in duplex operation. With as short a run as you have 1/2" hard line will be a much better choice, should be cheap for as short a length you need.

 

[n8zcc]

Here is a good read discussing the dissimilar metals in a feed line.

"I repeat - any cable that has dissimilar metals pressed together, even inside a jacket, will sooner or later create wideband noise (sometimes called duplex grunge) when hit with RF power. Even something as simple as a 1-foot-long jumper between the feedline and the antenna at the top of the tower can cause major desense problems (and for a long time the major antenna manufacturers were shipping 9913 jumpers with their antennas). 9913 is usable in an indoor simplex environment, but you will find that 9913 or any dissimilar metals cable, especially LMR-(any 3-digit or 4-digit number), is a disaster just waiting to happen on a duplex system."

Antenna Systems Information - Coax and Connectors

 

[prcguy]

When building repeater systems I even avoid the use of nickel plated connectors anywhere in the RF path due to possible low level IMD generation, which can eat away at your receive sensitivity. When you have a lot of time and $$ invested in a repeater system and want the very best performance you don't skimp on anything, you don't use LMR coax or nickel plated connectors anywhere.

You also don't use mobile duplexers anywhere near other repeaters, you always use an isolator on the transmitter and you use the biggest, baddest a$$ antenna you can find. Don't want to spend the time and money on using the right components? Then expect problems and low performance.
prcguy

Here is a good read discussing the dissimilar metals in a feed line.

"I repeat - any cable that has dissimilar metals pressed together, even inside a jacket, will sooner or later create wideband noise (sometimes called duplex grunge) when hit with RF power. Even something as simple as a 1-foot-long jumper between the feedline and the antenna at the top of the tower can cause major desense problems (and for a long time the major antenna manufacturers were shipping 9913 jumpers with their antennas). 9913 is usable in an indoor simplex environment, but you will find that 9913 or any dissimilar metals cable, especially LMR-(any 3-digit or 4-digit number), is a disaster just waiting to happen on a duplex system."

Antenna Systems Information - Coax and Connectors

 

[johndjmix]

Once again, not to say hardline isnt a better choise. But this weeked talked to the guy who setup and runs the amateur 443.6500 repeater over at Universal Studios Orlando. Hes running LMR-400 also. No issues and its been up quite a while. Key it up if your down there and have a HAM license, ive been on there all week. Crystal Clear.

Again...not saying anyone is wrong, but simply saying there are quite a few repeaters out there running 400....

--John