[Antennaware] Testing coax w/MFJ 259

[Guy Olinger K2AV]

Water wicked into the shield weave present in coax by being broadly lossy,
which LOWERS swr rather than elevating it.  It's a fairly weird problem if
it only shows on one band. Water logged coax CAN change swr under power, but
since its a resistance and dieletric issue, should affect all bands.

Analyzers don't run enough output to change line characteristics by heating,
and so will only be able to see more loss than there ought to be.

An analyzer usually has a procedure in its manual for measuring coax loss,
normally a measurement done with the far end shorted or open. The loss
measurement is best measured at whatever frequency(s) has the highest SWR
looking into the open coax.  These frequencies depend on the length and
velocity factor of the coax and may not fall into ham bands or customary
operating frequencies.

Computational analyzers like the AIM 4170, which have a program running in a
PC which analyzes frequency-scanned data fed from the analyzer over a serial
port, will provide the loss figure directly.

Personally, I think a core with a physical defect could cause saturation
problems, as would the accidental use of core material not intended by the
manufacturer. Ferrite looks like ferrite, looks like ferrite, and having a
mistake in a box of cores or a mistaken box of cores on the manufacturing
floor is plausible.

I have personally smoked and shattered ferrites not suitable to the use  :>)

73, Guy

On Wed, Sep 23, 2009 at 1:35 PM, Arthur Trampler <atrampler at att.net> wrote:

>
> Hey Gang,
>
> Some of you have followed my tale regarding the Hy-Gain AV640.  Hy-Gain
> suggested that rather than saturation, it would be moisture in the coax or a
> bad solder joint on the 40 meter coil.  I was not able to force an SWR climb
> on 20 meters after 10 minutes of nearly constant "CQ" at 1.5KW out (dead
> band).  Again, this is fed with about 125' of Davis RF LMR400, so at 14mhz I
> still have a good deal of RF at the antenna.
>
> I lowered it, resoldered those connections but rushed it...thunderstorms
> coming in...and will re-do it.  However, Steve Davis was good enough to
> respond to a question I sent him and agrees that moisture seems unlikely if
> it only occurs on 40 meters.  He suggested testing the coax with something
> like an MFJ259 or similar.
>
> My question: what would I be looking for to indicate a problem with the
> coax?  Would it be a change in the velocity factor?
>
> I'm sure I can borrow one locally, but unsure what tests I ought to run to
> check the coax and if any will reveal the likelihood of moisture in it.
>
> My last step would be to see if I can borrow 100' of RG8, 213,
> 9913/whatever to run from my surge arrester/ground to the antenna in place
> of what I have.  I guess I am trying to give Hy-Gain the benefit of the
> doubt that the problem is not with its matching unit.
>
> So the question: What tests are recommended to test coax integrity and
> specifically the likelihood of moisture in it?
>
> Thanks,
> Art, KØRO
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