From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
Mystery Solved
On the theory that you might learn from my mistakes, here's my hint of
the month.
My 80 meter verticals have 1/4-20 shoulder bolts through a steel pipe, to
which are attached all ground radials (placed there with ring-tongue
terminals soldered on, four radials per ring tongue). A shoulder bolt
has a smooth shaft (the shoulder) and ends with about one inch of
threading. Performance of the two element 80 meter vertical phased array
had been declining slowly, and was barely noticeable, until one day last
year a major snowstorm (the famous April Fools day snowstorm of April 1,
1997) broke one of the verticals, and broke the top hat on the other.
Obviously it was time to rebuild and repair.
The verticals were reassembled (one with a new section of tubing, the
other with a repaired top hat) and then it was time to adjust their
lengths so that they would both be resonant at the same 80 meter
frequency. For some reason, one of them wouldn't even come close. It
seemed to resonate at 5.1 MHz.
W1FV discovered the problem by shaking the vertical while applying an MFJ
antenna analyzer. Sometimes the ground system was "there" and sometimes
it wasn't , changing the resonance point dramatically.
Here's the problem: The shoulder bolts had threading starting ever so
slightly BEYOND the steel pipe. When new, the system had points of
contact as the smooth portion of the bolt passed through the pipe at two
points. Now, years later, a bit of rust had broken those points of
contact. The cure was to add three washers to the head side of the bolt,
dragging the thread back toward the pipe. Voila! Solid contact with the
pipe at both ends.
A fairly subtle problem, a dramatic improvement after cure. Lesson
learned: With shoulder bolts (as opposed to full thread bolts), be sure
that you can really tighten down -- not to the end of the thread, but to
put pressure against the surface where you need electrical contact.
Fred K1VR
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