The dissipated power in the capacitor depends upon the rf current flowing
through it. You could have a relatively low power application and still
cause significant dielectric heating. This would happen where the impedance
level is low (and therefore the current would be high). So, although Tree's
empirical "touch" test is relevant to his application, it would be incorrect
to generalize from this that ALL uses of 1500W are OK with such capacitors.
Another thing that should be considered is that the breakdown mechanism is
not always attrributable to heating. Dielectric materials can break down by
being over stressed by voltage without significant heating, and oftentimes,
this can be a cumulative effect - i.e. the breakdown doesn't happen
immediately and occurs only after a while. IMHO the best way to be sure that
the coaxial capacitor is being safely used is to determine that the voltage
upon it does not exceed the voltage indicated by the manufacturer's
specification for power and VSWR. If the length is not negligible, then it
will be necessary to examine the capacitor as a transmission line (which it
is).
73,
Riki, K7NJ - 4X4NJ
-----Original Message-----
From: topband-bounces@contesting.com
[mailto:topband-bounces@contesting.com]On Behalf Of Tree
Sent: Thursday, December 16, 2004 9:22 PM
To: topband@contesting.com
Subject: Topband: Coax capacitor losses
Well, I have to admit being concerned about the possible losses in my
coax cable (RG213) capacitor that I am using on my gamma matched tower.
I have perhaps 200 or 250 pf worth of coax coiled up in a loop about
a foot in diameter.
I have never had issues with it melting, so I didn't figure too much
power could be lost - but I was still curious.
So - I just ran a test (and I do mean ran).
I transmitted with full power (1500W) for several minutes on 1805.15 -
key down (yes, that was me at 04:10 UTC). Then I ran out into the antenna
field to measure the temperature of the coax capacitor - using nothing
but my bare hands. It is currently about 40 degrees F outside, and the
capacitor felt pretty cold. I could not detect any warmth anywhere along
the length of the coax - or in the magic device at the end of it that
prevents arcing (see my previous post). After a minute, I could fell
it getting a little bit warmer - but that was from my hand (no - I wasn't
still transmitting).
Therefore, I think it is safe to conclude that the power lost in a
coaxial capacitor due to whatever losses there might be is pretty
minimal and will not have a significant impact on your transmitted
signal strength.
When you figure how cheap one is to make - and how easy it is to make
WX proof - it is still a good bargin. I did use a vacuum variable on
my 80 meter shunt loaded tower, but that was so I could QSY from the
CW to the phone part of the band.
73 Tree N6TR
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