Re: coax question

Keith Morehouse
 

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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