[Antennas] Expanding foam and antenna tunning

[Demetre Koumanakos]

Hi Harvey,

Thanks for the reply and the information, also thanks to all who 
replied my post offering help.
As everyone has concluded it will not be a good idea to fill the 
fiberglass fishing rod with expanding foam, therefore I'll look for 
other means of re-enforcing the base of the rod where it is supported 
on the U bolts. PCV tubing probably.

73
Demetre - SV1ENS


> Let me relate to you a case from my actual experience:
> I worked for the US Army Strategic Communications Command in the
> 50's and was tasked to come up with an antenna for 108 MHz at 50
> kW that would put all the energy in a pattern straight up in the
> air. After I designed the antenna an had the prints drawn up it
> was sent to the US Navy Yard, Washington, DC, to be built. Because
> the matching section where the antenna (a dipole made of 3� inch
> copper tubing) connected to the 3� inch Spyroflex coax was all
> designed for air dielectric and was all open to the air, I
> specified that the antenna should be covered with a "radome" as
> used on many radars of that day. After the Navy Yard had the specs
> and drawings for a few weeks I got a call from the man in charge
> of constructing the antenna stating that instead of a radome
> covering the entire antenna, he was going to use a polyfoam to
> seal up the part that I had designed for air dielectric. I told
> him I didn't think that was a viable alternative, because of the
> difference in dielectric constant between all-air and the foam
> stuff, and I wrote him a memo for record so stating. The Navy Yard
> went ahead and used the foam filling against my opinion. And the
> antenna was sent to NASA, the intended user. I heard later,
> through the government "grape vine" when the antenna was fired up
> it lasted about 3 minutes and burst into flames!! Conclusion:
> don't use the foam filler without redesigning the antenna to take
> into consideration the difference in dielectric constant between
> air and the foam and considering the loss that may be introduced
> in the form of heat in the foam dielectric. Harvey/W4TG
> 
> - - -
> 
> Your moderator for this list is:
> Larry Wilson KE1HZ antennas-owner@mailman.qth.net
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