Any ham should be able to remember these factors below, that affect
antennas, and determine the cause of high SWR on the 20 meter section.
Likely 20m uses all of the conductor of the antenna, plus any loading
inductors. What is the total length of antenna conductor close to, as
installed?
If the inductors are close to metal ducts, that could be the cause.
Close meaning paralleling ducts, or electrical wiring Romex runs.
Anything 16 feet or less away would be "close".
Don't overlook a chimney that has a tin flu insert, since that is a
conductor vertically close to being a quarter wave or half wave at 20m.
Is there an attic ridge pole vent? Those may be of tin or flashing
aluminum and would interact with a 20m antenna.
Is there is metallic paint coating or insulation inside the roof that is
foil backed, that could interact with the 20 meter section?
Vent pipes of metal might be an issue if the vent is tin and near the
antenna. (Many vent pipes today are metal only atop the roof, and
plastic drain pipe below).
Wiring that might go up to peak of roof for outside lighting, or for
vent fans, could be interacting.
Indoor TV antennas in attic could be a problem to the 20 m match.
Likely you could just get up on a step ladder and poke your upper body
into the attic scuttle hole and survey visually, any possible issues
with your 20 meter antenna conductor.
If someone from the maintenance dept. installed the antenna for you;
they may not have appreciated the need to keep from touching other
wiring, running close to metal, etc.
With only 3 plus to one measured SWR, you can buy an inexpensive L-
network tuner from MFJ to tune out the remaining SWR, if the internal
tuner of the rig does not take care of it. Or you could add coax to the
transmission line, and see which direction that takes the SWR.
(I am making the assumption you are not running more than 100 watts.)
Of course, many higher powered tuners are on the market or in the ham
literature. There may still be some kits available. Any ham swap meet
will have a variety of tuners for sale that would remedy the 20m match.
Stuart Rohre
K5KVH
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