On Mon, 17 Nov 2008 17:51:07 -0500, Gary Smith wrote:
>Problem is the capacitor. I have several Vacuum variables
Don't assume that you MUST use a specialty capacitor. I'm tuning
my top-loaded vertical with a parallel combo of some fixed caps
that I bought at a surplus house in Silicon Valley for under a
buck each. Works fine. I calculated what I needed using NEC, then
played with the combinations while watching SWR until I got a nice
match. Handles my Titan amp just fine, even when high duty cycle
contesting.
The surplus house I visit is HSC, also known as Halted. They have
an internet presence. The caps I'm using are HV discs. They've got
a fairly good selection. Not complete value range by any stretch,
but it's easy to put together just about any reasonable tuning
value you might need. You just have to be sure that the caps you
use can handle the current. Not all the HV caps they sell can, but
at $.50 each you can buy a bunch, run power to them, and use the
ones that don't heat up.
BTW -- my Tee vertical is tuned long so that it looks like 50 + j
xxx without the cap, and the series cap tunes out the L and yields
50 ohms. As I recall, I've got about 250-300 pF in it.
BTW -- it's also entirely practical to get values like this from a
moderate length of coax, and I've done that too. You just need to
figure out a way to rig it. I've used RG8 like this at a high
voltage feedpoint and it held the voltage at legal power. This is
a low voltage feedpoint.
73,
Jim K9YC
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