[TowerTalk] 1/4 Wave Coax

[David & Barbara Leeson]

In his posting Re: 20 Phased, Frank Donovan mentions the unique feature of
a quarter wave (or odd quarter wave multiple) line that the output current
is determined only by the input voltage and the characteristic impedance.
It is not affected by the load impedance.  This allows making a feed 
manifold for multiple element antennas that controls the currents in each
element, typically to make them equal.

However, if the coax is not connected directly to the (split) dipole or
monopole element, but is connected through a matching network such as a
tee-match or beta match, all bets are off because it is the total current
from the coax, and not that portion of the current in the radiating
element, that is determined.  And even if the stacked beams are identical,
the difference in height above ground and the mutual coupling changes the
otherwise symmetrical situation.  Fortunately, it doesn't make a lot of
difference in the resulting pattern and performance, and at least a slight
resistive mismatch won't be transformed to some oddball reactance if the
cable lengths are odd multiples of quarter wavelength.

To teach this in my Stanford classes, I use the ABCD matrix for the 1/4
wave line to show the conversion from input voltage to output current.  I
believe Fred Collins, W1FC, first brought this concept to ham radio antennas,
but it was no doubt familiar to the giants of the 1930's, and has been
featured in the writings of W7EL and Chrisman (can't recall the W8 call).

73 de Dave, W6NL (ex-QHS)

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