Topband: Shunt fed verticals

[k6se@juno.com]

Rudy, N6LF wrote:

"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."
==========
The Morrison and Smith paper describes feeding a grounded tower with a
slanting shunt wire rather than the usual method used today with the
shunt wire parallel with the tower.  The slant feed is just as effective
but, as the paper describes, the pattern is skewed a bit in the direction
of the sloping feed wire.

This pattern skew also exists, but to a much lesser degree, with the
shunt wire parallel to the tower.  The amount of skew increases as the
spacing between the shunt wire and the tower increases.

The correct height that the shunt wire taps to the tower to "see" 50 ohms
resistive at the desired frequency varies depending on the electrical
height of the tower, the sze (diameter) of the shunt wire, and the
spacing of the shunt wire from the tower.

Although the structure does not need to be resonant to work well with a
shunt feed system, greatest SWR bandwidth, lowest tap height, and lowest
RF voltage across the "gamma" capacitor all result when the structure is
electrically 1/4-wavelength.

73, de Earl, K6SE