There were some good responses to this question, but I'd like to
add one more thing. This also applies to ANY antenna.
If the feedline is more than a few degrees electrical length between
the antenna and the measurement device, the reactance zero crossing
point might not be the frequency of "antenna resonance".
The conditions of true element resonance (ignoring instrument
tolerances) using a 50 ohm bridge are:
SWR 1:1, Impedance 50 ohms, X=0 with random length 50 ohm lines
-or-
X=0 (any value of R or SWR ) with any impedance line that is an
exact multiple of 1/4 wl long (including zero length).
It is important the line be an exact multiple of 1/4 wl at the
frequency of measurement, not a few percent F lower or higher or even
"pretty close to a multiple of 1/4 wl" !
If a "tuned" or "resonant" feedlines (in terms of electrical 1/4
waves) become more frequency selective when made longer. Small
length errors caused by a change in frequency accumulate in each 1/4
wl section.
(when feeding 50 ohm antennas with very long low loss 75 ohm CATV
cable, the long mismatched feedline will narrow VSWR bandwidth even
if carefully cut to a multiple of 1/2 wl. The same effect occurs when
measuring impedances, a long low loss line adds selectivity to the
system and increases errors if frequency is not exact)
Keep this in mind when using odd impedance lines or measuring things
some distance from the feedpoint!!
73, Tom W8JI
w8ji.tom@MCIONE.com
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