No disagreement ... *BUT* make sure the capacitor is rated to handle
at least 10A of RF CURRENT (or more with a short, heavily loaded)
vertical. The feedpoint of a vertical is a high current, low voltage
point - the shorter the vertical, the lower the radiation resistance
and the higher the required current. The voltage handling is only
important in the ability to withstand static/lightning.
73,
... Joe, W4TV
On 2025-03-17 4:12 PM, Richard (Rick) Karlquist wrote:
It is very poor practice to make a capacitor out of a long piece of tiny
coax. It will be very lossy (conductor loss, not the teflon
dielectric). It is easy to simulate the bad news with Simsmith, etc.
It is also easy to buy a 1206 NPO (Q>1000) 1000pF 1000V 1% capacitor
from Digikey, etc. for less than $1.
73
Rick N6RK
On 3/17/2025 12:42 PM, Joe Subich, W4TV wrote:
On 2025-03-17 2:01 PM, ARTHUR BERNSTEIN via TowerTalk wrote:
For a capacitor I used a long length of Teflon, thin coax @ 39 pf/ft
> (?)Actually coiled it up on a 1” form.
Do you mean 29 pf per foot? That's typical for the small diameter
50 Ohm coax - RG58, RG303, RG316, RG393, RG400, etc.
> Total capacitance was about 1000pf +/-.
With capacitance that high (reactance that low), one would be better
served to calculate the length of cable as an open stub (even if it
is rolled up) rather than simply as capacitance per unit length.
> It might have been RG-302 or similar 50 ohm? Very thin, 1/8”?
RG303 is 0.17", RG316 is .098", RG400 is 0.195" and RG393 is 0.39"
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