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John S · September 25th 13, 11:42 AM posted to rec.radio.amateur.antenna

On 9/25/2013 4:27 AM, John S wrote:
On 9/24/2013 9:04 PM, David Ryeburn wrote:
In article ,
(David Platt) wrote:

When you lengthened the hairpin, you added inductance... probably too
much, so you've not only cancelled out the capacitive reactance from
the DE, but have left some excess inductance shunted across the DE.

Backwards. Too much inductance to resonate with the effective (parallel)
capacitance would have resonated with a somewhat smaller capacitance
than you actually have. (The product of inductance and capacitance has
to be the same, for a given resonant frequency.) So you can think of the
actual capacitance present as consisting of however much would be needed
to resonate with the (too large) inductance, in parallel with more
capacitance which does NOT get cancelled out by the inductance. Result:
the actual hairpin, plus the effective (parallel) capacitance the too
short driven element presents, is capacitive, not inductive (in parallel
with the desired 50 ohms).

This is just the opposite from a series resonant circuit where too much
inductance gives an overall inductive result.

Otherwise, I agree with everything David Platt wrote.

David, VE7EZM and AF7BZ

No, David Platt is correct. You are assuming the effective capacitance
is in parallel. The antenna equivalent circuit is a *series* RC. Put in
too much *shunt* inductance and the combination looks inductive at the
feed point.

John KD5YI

I'm wrong. Please disregard.

Sorry.

John

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