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The evils??? of LMR cable

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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desperado
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The evils??? of LMR cable

Postby desperado » Sun Feb 05, 2012 7:23 pm

OK, I hear so many things about how this stuff is evil and show be avoided in certain situations. What is the real skinny on this stuff?
I have heard it generates noise due to dis-similar metal corrosion. I have heard that it generates noise on RX in a full duplex (repeater) environment.
I have heard that it's only bad if you are in a large co-located environment with many repeaters. I hear it can and can't be used for duplexer to radio jumpers, duplexer to main antenna jumpers. So say never use it at all. So what's the real deal, based on real experience and not just what others have said.

I have run the stuff numerous places, in any number of uses including all the above mentioned. I have only had one issue with any system I have installed it on and I am not sure that it's the real issue with it. (5 site simulcast system, as jumpers between the duplexer and the polyphaser to the main antenna. Issue is the narrow band radio has noise on the audio at times and seems to signal fade.)

I would love to hear what others have experienced.
Keith
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Josh
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Re: The evils??? of LMR cable

Postby Josh » Sun Feb 05, 2012 7:40 pm

I've used LMR-400 cable for about 10 years. I have a run that's that old in service right now with zero problems or issues with it whatsoever. The cable works and acts (still) as if it were new, all the things I could receive with it before I still can.

But, I believe I had a second run of LMR400 cable, but then again it may have been simply RG-8 that I didn't seal the connector on the antenna end properly, probably because it was supposed to be a temporary installation. After a year or so, I noticed that the signal was pretty poor when it rained. I disconnected the cable from the radio end to test for a short, but found moisture in the connector- the whole cable was water damaged.

Interesting, but my first run of LMR is still great, even though I've been slowly trying to get things upgraded to 1/2" hard line.

desperado
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Re: The evils??? of LMR cable

Postby desperado » Thu Feb 23, 2012 5:36 pm

Well, this seems to be an answer without being an answer.
I am guessing that either no one has actually had issues, or they weren't significant enough to post here.
Keith
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Bill_G
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Re: The evils??? of LMR cable

Postby Bill_G » Thu Feb 23, 2012 5:40 pm

I've had problems with the connectors, but not the cable itself.

desperado
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Re: The evils??? of LMR cable

Postby desperado » Thu Feb 23, 2012 6:33 pm

The Times captive pin connectors or something else?
Keith
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Bill_G
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Re: The evils??? of LMR cable

Postby Bill_G » Thu Feb 23, 2012 8:06 pm

The RF Industries crimp on NM center pin taper is too steep, and does not fully mate in some NF.

Jim202
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Re: The evils??? of LMR cable

Postby Jim202 » Fri Feb 24, 2012 3:37 am

desperado wrote:Well, this seems to be an answer without being an answer.
I am guessing that either no one has actually had issues, or they weren't significant enough to post here.



This subject has numerous threads on many of the different chat groups. Those of us in the service field shy away from using this type of cable in repeater operation. It just cost too much to have to bring back in a tower crew to replace a cable that will cause noise and desense problems in repeater operation. If it is a well document issue, why would you even want to consider it.

Do some homework before going too far into your project. Don't let the cost of the LMR cable sway your decision.

Jim

desperado
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Re: The evils??? of LMR cable

Postby desperado » Fri Feb 24, 2012 1:01 pm

Jim,
I wouldn't run the stuff up a tower for anyone. If it's on the tower, it's going to be heliax, period.
I have used it between transmitters and combiners, control stations, and between duplexers and surge suppressors in the vault.
Not sure that I should have, but that was the push from the company, and I was low man and had no say.

I was really looking for the specific stories on what people had experienced so I would know what to look for based on those observations.
Keith
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Re: The evils??? of LMR cable

Postby kcbooboo » Fri Feb 24, 2012 1:41 pm

The most common problem is that when LMR is used outdoors, the heating and cooling cause the cable to expand and contract. This causes the braid shield to rub on the foil shield. Eventually this scraping causes noise in the receiver which is most noticeable in full-duplex (repeater) applications. Flexing of the cable by the wind, by not being securely tied to the support, will cause the same problem. Sharp bending may also crack the foil shield, thus reducing its effectiveness.

Indoors or in temperature-controlled environments, this usually is not a problem.

Bob M.

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Re: The evils??? of LMR cable

Postby khw » Sat Feb 25, 2012 2:14 pm

I believe a lot of the comments come from a paper written by Motorola regarding their recommendations about cable and connectors. That paper related mostly to iDEN and what Motorola did to resolve IM issues at sites. It does have some interesting reading and seems to based more on real world experiences. A link is available from the Repeater-Builder website:

http://www.repeater-builder.com/antenna ... r-iden.pdf

Kevin
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desperado
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Re: The evils??? of LMR cable

Postby desperado » Sat Feb 25, 2012 4:27 pm

Well I can see that on a cellular network where a cable may be being fed by a TX combiner with a number of 800Mhz Digitally modulated frequencies.
Heck, anyone that ever owned one of those consumer grade phones knows the havoc it can create when it's transmitting the low power it creates. Run that power up to a level of a cell site transmitter and the IM properties would be crazy.

Again the right cable for the application, and LMR isn't it for iDEN systems.

I have heard so many stories, and how bad the stuff is and was concerned with the work I had done with it, thinking I shouldn't have ever agreed to use the stuff at all. The n the stories about the connectors and how horrible they were. And I know that some crimp connectors are horrible. The ones I have always used were the Times EZ connectors that were crimp on captive pin design. I have never personally seen a problem that could be traced back to a jumper that was a root cause of a problem. RAD Communications even ships microwave jumpers that are LMR 400/600 with these ends. But then again, I have a pretty low opinion of RAD Communications, or at least their support. But I have found that is mostly due to working for a premier Motorola partner and being able to call with ANY Motorola product issue and getting immediate assistance with the issue at hand with no questions asked. RAD's service isn't anywhere close to what Motorola's is.

All that being said, I figured that I should at least ask about what others had seen so I had some real world opinion and not a stories parroted about what someone else had heard from someone else.

There is alot of knowledge on here. I figured I would tap into it and get it from the horses mouth as opposed to the "I knew this guy that experienced...." sort of thing.
Keith
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