On Sat, 1 Nov 97 09:55:30 -0500 John Kaufmann <kaufmann@ll.mit.edu>
writes:
>At 11:09 PM 1/31/97 -0500, Larry Higgins wrote:
>>I tried coax stubs several years ago. RG8 worked fine on the
>exciter.
>>When I fired up the amp there was a blinding flash at the base of the
>>tower. The RG8 had metamorphosed into charcoal. The coax was foam
>>dielectric; never thought of using teflon coax; that sounds like a
>great
>>idea, and it's proven.
For a capacitor, a solid dieletric is often the best because of voltage
breakdown. One point to remember when constructing a coax based capacitor
is to always trim the braid back about an inch; you want an open air path
between inner and outer conductor that is equal to at least 2X the
maximum voltage rating of the cable. Then tape it all up and seal from
moisture.
>Another viewpoint: I used to use coax stubs as capacitors but stopped
>doing so after I made some measurements which showed significant
>resistive loss (several ohms) in long stubs using RG 213. I never
>fried any lines but the idea of tolerating any loss in a vertical
>antenna
>system did not appeal to me. The teflon dielectric lines or hardline
>would probably be better although I haven't measured any. Since the
>loss tends to decrease with shorter lengths, using coax for short
>stubs
>is probably OK, however.
>
>73, John W1FV
Good point John, but remember that often capacitor and stub applications
have different requirements.
A stub in a multi-op contest station for instance wants to have the
lowest C per foot in order to give the sharpest null. In your case you
were possibly in a Catch-22 and trying to balance things between the
capacitor requirements but much longer than a conventional coax
capacitor.
In many stations 3/4" foam hardline is used....even on receive. A small
RG-58 cable is a poor choice for this application. At this QTH, I use
CATV RG-11 for the stubs (remember, the actual cable impedence is not
important) and actually have to add short lengths when going from CW to
SSB contests on some bands. At 1500W I have never had a voltage
breakdown problem in the 10+ years the stubs have been in use.
At HF there is virtually no loss advantage in going from solid poly to a
Teflon dielectric in similar sized cables. Voltage breakdown increases
with the Teflon but the loss benefits are at UHF and above.
Teflon RG-213 size "Plenum Cable" would make excellent shunt fed
capacitors, again remembering to provide that long air path between
conductors.
Also short lengths of 7/8" Andrew or similar hardline are often available
for the asking from 2 Way radio shops.
Ceramic RF capacitors appear to have a poor reputation according to
comments. At my prior QTH I arced a 4500V variable and replaced it with
a 200pf 9KV one. It did not have enough C and I used a 200pf 15KV 857
series doorknob in parallel. Never a trace of drift but with such a high
voltage my current was probably a bit lower. A math friend computed the
RF voltage to be 7200V peak in my particular configuration !!
73 Carl KM1H
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