[TenTec] "A low vswr will kill you ! "

Tue, 23 Jan 2001 22:30:39 -0600

I guess we think about resonance in the antenna being better for
radiation, but there have been "magic" easily tuned lengths (and antenna
and feed) in antenna articles and handbooks ever since the use of tuned
feeders to arbitrary center fed antennas were created in the 1920s.
Sometimes those lengths are proposed as the only ones that work which is
wrong. Just those lengths are more convenient to tune but a truly
versatile tuner (which leaves out everything from MFJ and EFJ) will tune
any length of antenna with any length of feeder. I prefer the more
general rule on selection of antenna and feed line being: Antenna length
to fit BETWEEN the supports and feed line length to reach from the
middle of that antenna and the radio. The older tuners (as chronicled by
Cebik's web page) will tune the general rule antenna most anywhere from
antenna length less than 1/4 wave end to end up through VHF (with
suitable tuners for each VHF band). Sometimes the tuner settings will be
extreme, but the antenna and tuner can be tuned.

Magic lengths for convenient tuning are nice, but not when adherence to
them would keep the amateur off the air because the magic lengths don't
fit his particular premises.

I've worked 160 through 10 with 80 feet in the flat top, and 160 through
2 with 326 feet in the flat top and on MARS frequencies too.

I think the fundamental measure for the magic lengths is that the length
of feed line plus one half the antenna is not far from an odd multiple
of a quarter wave at each frequency of interest. That way the feed
impedance is more towards low than high and not too reactive for
convenient tuning.

73, Jerry, K0CQ

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