[BC] efficiency vs height

[tosenkowsky@prodigy.net]

Two things to bear in mind when discussing tower height. First, you must
account for an approximate 8 to 10 percent increase in height due to
velocity of propagation. In other words, a 90 degree physically tall tower
behaves as a 98 degree tower. Proof? Theoretically a 90 degree infinitely
thin radiator over a perfectly conducting ground plane has an impedance of
36+j21 ohms. Evere measure this in practice? The capacitance to ground,
base insulator, height of base above ground and the steel/face diameter all
play into the equation. A physical height of 225 degrees is actually TOO
TALL and the groundwave will be diminished. Bob Jones wrote an article
years ago titled "Tall Towers. Too Much of a Good Thing" addressing this
very subject. As I recall, they removed a top section of a new tall tower
and the signal increase was very noticeable.

Second, short towers have low base resistances. Any ground loss at low R
becomes significant, hence a lower efficiency in practical terms.

Tom Osenkowsky, CPBE

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