[Antennas] CushCraft R-7 problems

Mon Dec 28 01:24:14 EST 2015

: 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 