En Yuri Blanarovich va escriure en 10 Mar 2004 14:47:54 GMT:

The
apparently easier and safest way to go is not switching at the
very edge but at some 70% of the element.


Why do you want use pneumatic switches to disconnect part of the antenna?
Why not use the LC traps? They provide "automatic" switching.

Because, appart from being dificult to build and tune (to me)
they restrict a lot the useful bandwidth. I have not found a
trapped design that didn't present a very narrow SWR curve.

With a switched diople for 10/15/17/40 I should be able to
operate full-band moderatelly well on all of them (specially, all
of 28-30 MHz), even if the antenna detunes a bit with aging.

As for pneumatic operation it is for simplicity. I would have
used relays but then you have to start thinking on RF chokes and
decoupling both the feeding and the control line. Compressed air
tubes are non-conductive = no-problem. Again, I like the
simplicity of it. Price is not very diferent.

Toni - EA3FYA