[PVRCNC] W4WS feedline line check ups

[alsopb]

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
> 
> _______________________________________________
> PVRCNC mailing list
> PVRCNC at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/pvrcnc
> 
>