> > On Wed, 25 Jul 2001 Pete Smith <email@example.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
> 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
This effect is even more evident in verticals, I mean in shunt fed
verticals and expecially in those (short) ones loaded with a capacitive
hat and using an omega.
> 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.
As well as for the small loops, this is also valid with all the other
antennas with high Q (small BW) and overall efficiency may get
> Voltage at the open end also increases over the voltage you might
> normally expect.
> 73, Tom W8JI
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