At 11:32 AM 4/7/2003 -0700, Jim Lux wrote:
>>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?
>The wind loading might be less with a suitable cover over the coil than
>just the coil hanging in the wind.
One could wrap the coil in something that has good RF and UV
properties. Some sort of UV inhibited Polyethylene might be good. Saran
Wrap (Poly Vinylidene Chloride - PVDC) has nice mechanical and RF
properties (particularly in very thin layers), but doesn't have very good
UV properties.
Kapton (polyimide) has good properties, but seems to have some sort of
problem with a combination of UV, moisture, and oxygen.. after 4000 hours
of Florida aging, the elongation strength is almost gone. It's also a bit
expensive.
PET (Polyethylene Terephthalate).. which is what the soda bottles are made
from, is available in a UV stabilized form (not the soda bottles... they
get brittle in sunlight), but has a high dissipation factor (0.01 at 1kHz!!)
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