Hi, all:
After reading some of the comments on shunt-feeding a tower on 160m, I
thought I'd put my 2 cents worth in.
Bob Kile's (KG7D) comments were quite interesting, but I felt that his
mention about 1/4-wave resonance might lead some of you to believe that
1/4-wave resonance (at 1.8 mHz) of the structure being shunt-fed is a
necessity, while it is definitely not.
Bob merely shoots for a structure which is 1/4-wave resonant at 1.0 mHz,
or about 0.45 wavelengths at 1.8 mHz. NEC shows that Bob's antenna has
about 1.2 dB gain over a 1.8 mHz 1/4-wave vertical, but perhaps more
significant is that this gain is at an elevation angle of about 18
degrees compared to 23 degrees for the .25-wave vertical.
As a matter of interest, I calculated gains and radiation angles for
various verticals with the following results (referenced to the 1/4-wave
vertical & all measured over the same ground):
Height (wavelength) Gain Elev Angle of Max Radiation
---------------------------- -------
-----------------------------------------
.125 -0.29 25
.25 0.00 23
.45 1.18 18
.50 1.61 16
.625 2.92 12
.65 2.70 13
Note that max gain comes from the classic 5/8-wave vertical. (Don't we
all wish we could put one of those up on 160?!) Also of interest is that
the 1/8-wave vertical is not too much worse than the 1/4-wave vertical.
Getting back to shunt-feeding -- Even a short tower resonant (using Bob's
method of measurement) at 3.6 mHz can be successfully matched to 50-ohms
by the gamma match method. Even shorter structures can be matched using
the omega match method.
What I'm trying to emphasize is that almost any vertical structure can be
made to efficiently radiate a signal on top band. That it be resonant is
not a necessity, but, as a general rule -- the bigger, the better (top
loading!).
When shunt-feeding, important guidelines which should be followed are:
1) Route beam feedlines and rotor cables all the way down to the ground
(preferably inside the tower) before they leave the vicinity of the
tower.
2) Insulate guy wires from the tower and break them up into short
electrical lengths with insulators (or use Phillystran).
3) Install a good ground radial system (even for that 5/8-wave vertical)!
I consider this to be as important as the radiator itself.
My two top-loaded (with beams) towers measure close to 1/4-wave resonance
at 1.82 mHz. Maybe that accounts for why the signal from Bob's .45-wave
structure is bigger than my signal, hi!
73 es DX, de Earl, K6SE
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