> On Wed, 25 Jul 2001 Pete Smith <n4zr@contesting.com> 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
power.
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
W8JI@contesting.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|