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[TowerTalk] coax as capacitor

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Subject: [TowerTalk] coax as capacitor
From: (Tom Rauch)
Date: Thu, 26 Jul 2001 03:22:32 -0400
> On Wed, 25 Jul 2001  Pete Smith <> writes:
  Is it feasible to use coaxial cable as the series capacitor in a 
> shunt feed
> system, simply pruning the length of cable as appropriate?  Or is 
> there
> some reason why not?
> 73, Pete N4ZR

Assuming a cable no more than 1/4 wl long:

The cable always looks like more capacitance than it measures at 
dc Pete, because any *series* inductance decreases capacitive 
reactance (same as increasing capacitance).

This effect is minimum with electrically short cables and peaks at 
1/4 wl where reactance is zero (effective capacitance becomes 
infinite), so a cable always looks more capacitive than the dc 
calculation. Fortunately the error is in a direction that let's you trim.

Voltage is always HIGHER at the open end than at the connection 
end, again that amount varies with length and peaks at 1/4 wl. This 
effect can be dramatic as you approach 1/4 wl. That's why gamma 
capacitors on yagi's seem to arc and fail at lower than expected 

Q decreases and loss increases as you approach 1/4 wl, and Q 
can be much lower than conventional well-designed lumped 
components produce. That is because the line operates with a near-
infinite SWR.  This is why you can't use a coaxial capacitor in a 
high-efficiency small loop antenna, and why a high-Q capacitor has 
to be "boxy shaped" with multiple plates. (Multilayer ceramic chips 
have Q's in the ten's of thousands while single layer ceramics have 
Q's in the hundreds.) Coaxial stubs can have Q's in the 20-50 
range when used as capacitors.

These effects may or may not be important depending on the 
application, but they are very very real.

Voltage at the open end also increases over the voltage you might 
normally expect. 
73, Tom W8JI 

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