Hello Skip,
Couldn't help but notice your message. I have used capacitors made from
coaxial cable in several of my antenna designs and haven't had any problems.
My current 160-20 meter, multi-band vertical uses such capacitors to tune
the antenna on 3 of the bands. I use fixed mica capacitors on the others.
An observation that I have made with top-loaded antennas is that the
physical length of the antenna may result in an electrical length different
than expected. This is due to the ground characteristics and dielectric
constant of the earth/air medium in close proximity to the antenna. I
describe this in an article "A Close Look at the Flattop Vertical Antenna,"
in the ARRL Antenna Compendium VI. It may be that an electrical
3/8-wavelength antenna will require a different physical length at your
particular location. My "guess" is that the antenna is electrically too long
and the series inductive reactance may be large. I am surprised that you
weren't able to tune the reactance out using the variable capacitor.
Does your MFJ259 allow you measure both the resistive (R) and reactance (jX)
of your antenna? I have an MFJ259B and it does. If so, replace the coaxial
capacitor with a short and measure the antenna impedance. The resistive (R)
component of the impedance shouldn't be too far off of the value given in
the article (55 Ohms) if the antenna is electrically close to
3/8-wavelength. However, if you find the reactance is much different than
the +j590 Ohms, than that is a clue. Providing the electrical length is
established near 3/8-wavelength the coaxial capacitor should work in your
application.
The MFJ259B allows for a maximum impedance of 650 Ohms, so that might be a
limitation. Also, my MFJ259B does, at times, overload from BC station
energy. When that happens the impedance and SWR measurements are not
accurate. Sometimes I wait until sundown to make measurements on 160 - after
the BC stations lower their power, and change their transmission patterns.
Please note that I do not have a tower, and I have only used masts with my
antennas. There may be a coupling issue between the vertical antenna wire
and the tower that is affecting the electrical length. Someone else may be
able to comment on their experiences.
Best Regards, Thomas - AC7A (Tucson)
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