Re: coax question

Michael Daly
 

Hi,

I have copied my buddy Paull Holmes in Farmington about this issue.  He has two concerns.

First..

          My first question is if the cable with problems is genuine Times Microwave LMR-400 cable.  There are a number of companies (mostly Chinese) that are making "Times Microwave" alternatives.  Most of those are OK but do not have the quality control that the Times Microwave company does.  I also worry that some of the Chinese cable will say Times Microwave on it even though it is actually a "knock off". 

 

          If he is sure that the cable he is using is genuine Times Microwave cable then I would suggest that he give them a call and ask them what they believe might be the problem.  The cable should be able to handle a Kilowatt without a problem.  They would likely be interested if their cable is having problems.

 

Then in response to Keith’s point about UF version of LMR cable…

 

            The Times Microwave specs on their Ultra-Flex and the standard LMR-400 is -40 to 185 F degrees (operating).  Hard to imagine that the conditions are worse than that.  I believe that I would send an e-mail to Times Microwave and see what they say about the problem.  There are probably millions of feet of LMR-400 in use and the company has a stellar reputation.  I don’t know what the Time Microwave “knock off” specs are, probably not as good.

 

So more to chew on.  Contacting Times Microwave would seem to be first on the list after confirming it is truly Times Microwave.  And for goodness sake tell us the rest of the story.  I have LMR 400 Ultra Flex on my tower at the top.

 

Mike, n5sj

 

From: nmvhf@groups.io [mailto:nmvhf@groups.io] On Behalf Of Keith Morehouse
Sent: Saturday, August 11, 2018 8:11 PM
To: nmvhf@groups.io
Subject: Re: [nmvhf] coax question

 

Mike, the 'UF' version of all the LMR cable, which is the rubber jacketed, direct bury capable, highly flexible version is notorious for getting soft in direct sun and allowing the inner conductor to push out toward the shield when it is held in a bend or, even worse where it is cable tied and allowed to hang free.  This, of course, causes a increase in VSWR without any other symptoms.

 

I had this problem in EN52 with a 432 MHz LMR600UF feedline twice (it took me two direct failures at the same place to realize what was happening).  As you can imagine, the sun during a EN52 summer may be hot, but I'll bet it's much 'less-hot' then in New Mexico.

 

As a rule, I no longer use LMR 'ultra-flex' in my system unless it is inside.  I'm also VERY careful in planning my rotor loops and flexible RF cable attachments to avoid any locations where there might be a abrupt direction change.

 

-W9RM

 


Keith J Morehouse
Managing Partner
Calmesa Partners G.P.
Olathe, CO

 

On Sat, Aug 11, 2018 at 6:37 PM, WB2FKO <mph@...> wrote:

I'm having an ongoing problem with burning up coax cable feedlines. This has been almost entirely on 6m, where I have been doing the majority of my operating the past few years and primarily during the summer Es season.  I usually run between 500--1000W.  SWR has been very good right up until the moment of failure, which never seems to occur at an opportune time.  Cable does not internally short, so I assume the inner conductor has wandered far enough to send SWR through the roof.  It has been happening every 18--24 months, with the latest cable failure during a meteor scatter run in this weekend's Perseids shower.  I don't trust my cable assembly skills, so have been buying professionally made cable runs from BuryFlex, CableXperts, and recently JEFA Tech, and always a variant of the flexible LMR400.  Stiffer cable would be a substantial pain to install at my station, but I will have to deal with it if this is the solution.  Advice appreciated.  Mike WB2FKO


 


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