Look at the initial wave propagating out the transmission line to the
antenna. Along the transmission line it radiates little because of the
canceling field from the adjacent wire. Along the antenna it sees more
resistance from radiation. At the end of the antenna it sees an open
circuit, e.g. high impedance. If your feed line plus half the antenna is
a quarter wavelength together you have a low impedance reflected to the
tuner. And nearly resistive. Easy to tune. If your feed line plus half
the antenna is a half wave you have a high impedance at the tuner, easy
to tune with some tuners, again nearly resistive. This was a fundamental
recommendation back in the 1930 when antenna tuners were invented to be
separate from the transmitter. Before that, they tapped the antenna feed
onto the PA tank (sometimes with a series capacitor to keep DC off the
antenna) and didn't worry about harmonics or RF in the shack, and so
long as the PA tuning would dip, it was considered good.
A feed line an even multiple of a quarter wave, that's a half wave is
not a length to be avoided, its handy that it repeats the antenna feed Z
at the tuner. Its a pain when the antenna Z is high while the feed line
Z is low (like coax without balun) and the feed line is an odd multiple
of a quarter wave long because then the tuner sees a very low Z, on the
order of a few ohms if the antenna is a full wave long and the 50 ohm
feedline is a quarter wave long (in the coax, not in air) which is often
outside the range of a tuner. It takes a large tuning capacitor to match
5 ohms on a low band.
73, Jerry, K0CQ
--
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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