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Note, with clearer screen captures:

50-ohm cal: S11 reads 304.88 ohms real as a folded dipole shoud (on the
right of the chart right where it sould be for 300-
ohms real)

300-ohm cal: S11 reads 53.5 ohms (in the center of the chart since you
have reset the cal position with a 300 ohm cal)
Also note that 53.5 ohms X 6 = 321-ohms, very close
to what you measured with the 50-ohm cal.

This is how a normalized chart works when calibrated to something other
than 50-ohms. You could just as well have taken the chart center as 1, or
unity. With a 50-ohm cal, the center becomes 50-ohms. With a 300-ohm cal,
the center becomes 300-ohms. The values in a normalized chart must be
multiplied by the system impedance of the cal. This is a good method to
make more meaningful measurements when dealing with extremely low and
extremely high impedances. However, the ultimate limitation will always be
the bridge or the directional coupler within the measurement instrument.

The HP 8753 uses a very carefully designed and meticulously machined
directional coupler to accomplish its published specifications (the HP
85047 S-Parameter Test Set mated with the 8753). I am continuously amazed
at how well the little NANOVNA (and all its variants - minus some clones)
compare to this very expensive piece of HP test gear! Credit to the
engineers who made it happen!!!!!!

As far as the additional dips in SWR, the antenna must be in free space to
show only resonances (the horizontal axis on the Smith Chart). Resonance
is defined as +jX = -jX, leaving nothing but real resistance which, for an
antenna, is the sum of ohmic losses in the conductors which make up the
antenna and the radiation resistance which is a 'magic' result of all
things electromagnetic ?? (you don't really want to know- *lots* of
calculus). The higher in frequency you go, the smaller geometries become,
and the easier it is to test in something resembling 'free space'. At FM
broadcast frequencies, its rather difficult without a semi-anechoic chamber
in which to test (read, lots of $$$$$$).

Dave - W?LEV

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*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*