[PVRCNC] W4WS feedline line check ups

[Jim K4QPL]

The instruction manual for the popular MFJ "box" gives clear instructions on
measuring distance to a fault, even for non-techs who think impedance is
being rude to the teacher.

Jim
----- Original Message ----- 
From: "alsopb" <alsopb at gloryroad.net>
To: "PVRCNC" <pvrcnc at mailman.qth.net>
Sent: Wednesday, September 08, 2004 3:00 PM
Subject: Re: [PVRCNC] W4WS feedline line check ups


> Henry,
>
> Looks like some things got ugly there.
>
> Jim Lawson (W2PV) kept TDR records of his multi-array, stacked array
> antenna farm in Schenectady, NY.  His antenna farm at that time was
> really pushing the envelope outwards.  So did his MM operations.  He
> was able to troubleshoot the system with a TDR trace comparison and
> pinpoint the fault within a foot or so.  It saved them lots of time.
> Lots of his work was done by crane.  Knowing where to look helped.
>
> Another useful tool for this kind of work is an antenna analyzer-- 
> especially if one can get at both ends of the coax.
>
> 1) Cable loss can be readily computed by shorting one end, varying the
> frequency and finding the minimum Z exhibited closest to the freq of
> interest.  The Z will range from thousands of ohms to some minimum
> value.  The loss in db can be directly computed given the min value
> and compared to spec's.   The free transmission line program TLW
> allows one to plug in the coax type, length and frequency.  It spits
> out the spec loss.  It also can compute the added loss due to a load
> mismatch.  It is interesting to mismatch the load to produce a 3:1 SWR
> and note how little additional loss one incurs at HF with typical coax.
>
> 2) Another scenerio is:  You've just made up a new length of coax.  It
> measures shorted.  Which connector do you cut off and replace?  This
> is simple to determine with the antenna analyzer.  Put the antenna
> analyzer at one end and vary the frequency.  If the impedance rises
> and falls as one sweeps over a 5 to 10 MHZ spread, the short is at the
> other end.  If the impedance is constant and low value, it's at the
> analyzer end.  I believe one can determine the position of a short in
> a length of cable by noting the impedance vs freq behavior.
>
> Interesting stuff.
>
> 73 de Brian/K3KO
>
> Henry Heidtmann wrote:
> > Thought some of you might be interested in this- we've been repairing
> > damage from this winter over at W4WS. In the meantime, Don WS4NC got a
> > good deal on a TDR- what an amazing tool for finding problems-
> > Don put together a nice report of all our bad news- its posted on the
> > W4NC website- here is the link-
> > Note his technical discussion of the fact that no one brought beer to
> > the test range, and his description of the "landscape equipment damage"-
> > better known as a lawn mower.
> > Have fun, stay dry-
> > Henry, W2DZO
> >
> > http://www.w4nc.com/images/TDRW4WS.pdf
> >
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> >
>
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