[Antennas] CushCraft R-7 problems

[JK]

That was some good trouble shooting !


Jay   NE2Q

On Mon, Dec 28, 2015 at 01:24 AM, KD7JYK DM09 wrote:

> : Bad coax or flakey coax connector has been eliminated as 
> possibility??
>
> That reminds me of an issues I had with a 10m rotatable dipole!
>
> It had erratic SWR readings, high, low, variable, different at 
> different times of day, temperature, humidity, and wind.  I usually 
> used it with a tuner from 80m up so didn't know there was a problem 
> until I checked one day.
>
> I swept it with an MFJ-249 and found about a 2.5:1 SWR around 80m, 
> worse elsewhere.  I swept the antenna alone, it was near perfect at 
> 10m.  I thought back to a time I talked with a fellow servicing 
> telephone lines using a time-domain-reflectometer.  He told me he 
> could find a fault within a meter or so based on time and frequency.
>
> I had an idea.  Assuming the coax was bad somewhere, I connected a 50 
> Ohm load at the antenna end.  If the cable was good, I should have a 
> very good reading across the range of the 249.  Still read bad 
> everywhere except slightly better around 80m.  I checked with an Ohm 
> meter, expecting to see the load at the end.  Nothing.
>
> I calculated 1/4 wave at the better frequency and converted that to 
> feet, assuming there was an issue with the co-ax at that point. 
> Fortunately it was about two feet from the antenna.
>
> Upon inspection I found the co-ax was a little swollen there.  A crack 
> formed in the co-ax about half-way around.  This allowed water 
> infiltration. The shielding corroded entirely and the dielectric 
> cracked as well, maybe from fatigue, maybe from ice.  The center 
> conductor was good, but the shield was a bunch of green powder for 
> about three inches.
>
> I removed that section of co-ax, my cable was made of three pieces, 
> replaced it with known, and swept, good cable and got about a decade 
> more out of the system.
>
> I usually monitored with that installation and never noticed a 
> problem. Occasionally I'd tune up on a band for better reception with 
> the less than optimum antenna.  Even with the bad co-ax, things were 
> better when properly "matched".  I could never get into a regional 10m 
> net, but assumed it was due to my location.  Had I not swept the 
> entire system, co-ax and antenna, I never would have known I had an 
> issue.  On the plus side, I do a lot of weak signal work and really 
> optimized my techniques for using a corroded cable!
>
> Kurt
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