Topband: 200pF HV-capacitor for shunt feeding

[R. Kline]

I have used teflon coax that is approximately the same dimensions as RG213
(sorry forgot the number) for resonating an inverted L on 160M.  It worked
well for a while, and then it broke down at the open end during a contest!
This in spite of the fact that I tried to minimize the voltage gradient by
peeling the shield back about 1.5 inches from the open end and flaring it
outward.  Also, I weatherproofed the open end.  What happened was that there
was an arc-over between the center conductor and the shield that burnt
through the teflon (not at the open end). The teflon dielectric had
obviously been subjected to excessive stress.  The power level at the
antenna was about 1500 watts - the legal maximum in 4X.  After that, I
replaced the coax with a vacuum variable capacitor that worked for many
years with no problems. I believe that polyethylene (as used in RG213) is a
less robust material than teflon, so in a similar application, I wouldn't
expect it to last too long. This technique may work in a bind, but if you
use it - at least have a back up piece of coax ready (if you need the
antenna for a contest).  Perhaps you could consider two coaxial capacitors
in series.  This would be accomplished by shorting the shields together at
the ends, and connecting to the inner conductors.  This should halve the
voltage gradient on the dielectric material allowing the combination to
withstand about 4 times the power.  The disadvantage, of course, is that the
overall length would be double.  Another thing to keep in mind is that you
need to keep the electrical length short for this to work as a lumped
capacitor.  If it is too long, you will not be able to use the simple lumped
capacitance model.  If I remember correctly, a general rule of thumb is that
the lines need to be less than about 1/20 of a wavelength in order to behave
as simple capacitances. Assuming that my memory is not deceiving me (hi),
this of course would be about 8 meters or about 25 feet maximum for 160M.
If the coax has about  28 pF per foot (memory again), then you should  be
able to get up to a bit over 600 pF with a single coax.  If you use the two
paralleled coaxes, then this will still allow you to get up to about 300 pF,
so again it should be satisfactory.
73,
Riki, K7NJ - 4X4NJ

-----Original Message-----
From: topband-bounces at contesting.com
[mailto:topband-bounces at contesting.com]On Behalf Of Tree
Sent: Tuesday, December 14, 2004 6:21 PM
To: Johannes Wilke
Cc: topband
Subject: Re: Topband: 200pF HV-capacitor for shunt feeding


On Tue, Dec 14, 2004 at 08:51:32AM +0100, Johannes Wilke wrote:

> Does anybody know where to get a 200pF HV-capacitor for shunt feeding a
> tower on 160m? Is it possible to use RG213-cable as capacitor?

Yes - I do this on my shunt feed tower.  It works very well.  However, if
you are running high power, you will need to take care with the end of
the capacitor as it can arc very easily.

Please see this posting for more info:

http://lists.contesting.com/archives//html/Topband/1997-12/msg00110.html

73 Tree N6TR
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