I'm working through the design issues that would be involved in converting
an existing linear-loaded 2-element 40m beam (18-foot boom, 38-foot
elements) to coil loading at the same place where the existing loading
lines attach. The electrical design seems to be coming fine, but I'm a
little puzzled about the loading coils. Analysis that Steve, VE6WZ has
done seem to point to the need for air-wound coils, because low Q kills the
performance of the antenna. In his work, these have been left open, even
though that means some change in performance with weather.
I'm wondering if it would be feasible to enclose such coils in a plastic
enclosure. The material would have to be reasonably UV-resistant, strong,
somewhat flexible, and fairly easy to fabricate. The insulated gap in the
elements would be fiberglass rod, and the element would be supported just
outside of the loading coils by a Phillystran truss, but experience with
this antenna in a linear-loaded form suggests that a good deal of element
movement and flexing is inevitable.
I'm aware, of course, that the wind area of the coil covers will introduce
some additional load on the element. Have I missed other mechanical
issues? Any suggestions for a good coil cover material?
73, Pete N4ZR
The World HF Contest Station Database was updated 2 April 03.
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