[TenTec] Re. [Ten Tec] --- and balanced fed verticals
rohre at arlut.utexas.edu
Thu Jan 26 12:36:51 PST 2012
I agree with you, but the original post defined a possibly balanced or
near balanced vertical system, a quarter wave antenna with at least two
radials. (This was the classic RCA design by Brown et al, when the
ground plane antenna as it came to be called was invented.) (Later the
marketing department wanted and added more radials
"to look more symmetrical".)
If placed well above earth it may approach what you and I consider a
balanced antenna, (like a dipole at optimum height and installation).
Very close to certain types of earth, you naturally have parallel line
and low radials coupling from the RF reflection off earth, and balance
may not be achieved.
Yes, the main advantage of feeding with parallel line would be lower
losses in the line, IF you can match the likely low impedance of the
quarter wave vertical with whatever is attached at the source end of the
However we can gain an idea of how well this works, as L. B. Cebik W4RNL
(sk) examined the extreme case of use of parallel line and balance: The
end fed Zepp. There, one side of the parallel line dead ends at an
antenna insulator, while the other side feeds the high impedance at the
end of the Zepp. His modeling found that there was only 10 per cent
imbalance in line currents (presumably measured close to the rig), even
with one side of the line open circuit at the antenna.
Practical access issues with open wire lines have kept many hams for
considering parallel line feed when the vertical is mounted on the
ground, or not elevated out of reach. If one were to attempt such a
feed, they would be well advised to twist the parallel line and keep it
in tension to have equal coupling effects to the earth underneath its
path. And to use insulated wires as line conductors. In old ham days,
parallel line feed, often home made, was done with T poles supporting
the run of feed line to an antenna, much like outdoor clothes drying
lines were formerly strung.
In more recent times, some users of parallel lines I know have used PVC
pipe to insulate and keep separated their line feed, One user did this
to a rotary log periodic. To by pass the rotor area, the center
conductors of two pieces of coax were used in a balanced line manner to
pass the signal to the antenna feed point while keeping the coaxes
equi-spaced even with antenna rotation.
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