Topband: Handheld Impedance Analyzer
shristov at ptt.rs
Mon Mar 28 20:23:43 EDT 2016
"Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
> I don't believe the above assertion.
> Can anyone give a specific example of an antenna having
> an SWR at the resonant frequency, (where X=0), that is higher than
> the SWR at frequencies slightly above or below the resonant frequency?
> I submit that a local minimum in the SWR will always coincide
> with X=0. This is easily proved on a Smith chart, where X=0
> corresponds to the X-axis. Adding reactance or susceptance moves
> the impedance farther from the origin in all cases, meaning SWR goes up.
It would be exactly so if the real part of the antenna
input impedance did not change with frequency.
The real part of dipole impedance rises quite a bit with frequency, so the resonant
frequency as defined by X=0 will be different from the minimum SWR frequency.
With multi-element beams, R may change with frequency in more complex ways.
But for a given coax impedance, what should be aimed for is the lowest
maximum SWR in the working frequency range (subject to preferences).
The resonant frequency itself, defined as X=0 or otherwise, is irrelevant.
Sinisa YT1NT, VE3EA
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