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[antennaware] Q: Extended Double Zepp with tuner and open wir e feeders

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Subject: [antennaware] Q: Extended Double Zepp with tuner and open wir e feeders
From: (L. B. Cebik)
Date: Tue, 3 Nov 1998 17:50:45 -0500 (EST)

An antenna is resonant when the source/feed impedance is purely resistive,
regardless of how high or low that impedance is.  Hence, a dipole that is
resonant on 40 meters with a low impedance will also be resonant on about
20 meters, but with the high impedance of a 1 wl center-fed wire--and that
resonant point will be very sharp, compared to the broad resonance of the
40 meter test.

On the other hand, a system using any kind of feed line may show a low
impedance at the shack entry point--even with no reactance--as a result of
the transformation of the impedance along the transmission line.  This may
permit a transfer of power to the system and antenna--even with the tuner
out of the line.  However, this is not antenna resonance.  The 80 meter
dipole is still a 1/4 wl center-fed wire with a very high impedance at its
feed point.  However, since the 60' of transmission line you used is about
1/4 wl long, the very high impedance was transformed to a low impedance.
Had you used 1/2 electrical wl of line, you would have re-encountered the
high impedance of the antenna, since impedances repeat themselves
(slightly altered due to line losses) every 1/2 wl along the line.

The effectiveness of the antenna will be still a function of its type,
length, and height, regardless of the ease or difficulty of effecting a
match at the ATU.  Of course, if the impedance values at the ATU are
outside its range of tuning, then the game is over, since even with a good
antenna, one cannot get much power into the system.  But that can be
corrected by modifying the tuner or modifying the line length until the
impedance can be matched.

Low (but not too low) impedance values at the tuner terminals are a
convenience for matching, but do not themselves say anything about the
performance that can be anticipated from an antenna.  If your tuner will
effect a match, then your lines are tuned in the only sense of the term
that counts.  But that does not say how the antenna will perform.  Look to
the antenna itself for that.



L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
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