[TenTec] Re. [Ten Tec] --- and balanced fed verticals

Stuart Rohre 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 
parallel line.

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.

Stuart Rohre

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