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[TowerTalk] Radials, resonance and the MFJ259

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Subject: [TowerTalk] Radials, resonance and the MFJ259
From: (Leeson)
Date: Thu, 18 Dec 1997 11:25:51 -0800
A last comment...

Pat, AA6EG, raises a question about SWR bridges and transmission lines.

If an SWR bridge had perfect directivity and was insensitive to the
actual impedance level (ideal, but not realized in practice), then at a
given frequency the SWR would read the same no matter what the line
length to the load.  But most practical SWR bridges have some
sensitivity to the actual impedance level, hence the location on the
transmission line.  This is no big deal, especially since it goes away
at 1:1 SWR.

Because both the antenna impedance and the effective line length change
with frequency, it's quite possible to get one measurement of SWR and
frequency of the minimum at the antenna and quite another at the end of
a transmission line.  This can drive you nuts, and the advantage of the
MFJ259 and the like is that you can actually get the bridge to the
antenna feed point.

Why all this fuss about SWR anyway?  Well, the main reason for me is
that if the antenna is matched over the frequency range I'm using it, I
can tell immediately if something isn't right and I can switch among
antennas and modes without the amp going bonkers.  This is especially
important for amps and rigs that have SWR cutouts.  Plus, the Zen of
antennas drives many of us toward some form of perfection, and the dot
in the center of the Smith chart is a unique impedance point, the only
one that isn't sensitive to line length.

Dave, W6NL

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