Hello,
There is a classic paper on shunt fed verticals which is very
informative:
Morrison and Smith, The Shunt-Excited Antenna, IRE proceedings, June 1937,
pp. 673-696
They and other references point out a small distortion in the radiation
pattern but it is not serious for ham purposes.
Some time back I ran a series of experiments and modeling of shunt fed
verticals. I found that I could attach the shunt wire at a wide range of
points along the tower and by adjusting the wire length and the value of the
series capacitor, obtain a good match. Nothing critical at all. I did find
that the match bandwidth was wider when the length from the tower base to
the feed-point was roughly the same as the height of the attachment point on
the tower. Tall-narrow (like a gamma match!) and low-narrow triangles gave
narrower match bandwidths. At one point while fiddling around, I was able
to create a classic double-hump SWR curve that covered much of the 160 m
band. Basically I was tuning the matching network to a slightly different
frequency than the vertical.
A bit of this information is in the Spring 1997 Communications Quarterly,
page 37-38.
73, Rudy N6LF
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