[TowerTalk] Flat top vs Inv Vee

[T A RUSSELL]

The  definitive paper on dipole efficiency over  imperfect earth
appeared in the November 1972 issue of the IEEE
Transactions on Antennas and Propagation, p. 766
with important corrections on p. 755 of the  September 1973 
issue.    It is entitled "Radiation Efficiency of a Dipole Antenna
above an Imperfectly Conducting Ground,"  by Peder M. Hansen.

Maximum efficiency occurs at heights well above one wavelength.
Examples:

Conductivity	Peak Efficiency

Salt Water	85%
Good Earth	75%
Poor Earth	65%

The most significant information for our discussion is the effects
of imperfect earth for antennas below 0.3  wavelength ( WL).  

Dipole  EFFICIENCY  below 0.3 WL appears to decrease almost
linearly between (average) height above ground from 0.3 WL down
to just above the ground.

Therefore, an inverted vee whose average (or effective) height 
is lower than a flat dipole at the same height as the apex of the
inverted vee, will have proportionally LESS  EFFICIENCY than
the flat dipole.   On 80M,  this applies to antennas below 84 ft.
which covers most city lot installations.

These effects are NOT included in most antenna modeling programs
(except NEC2 based programs).   Earlier programs use PERFECT
EARTH models to calculate antenna currents, even when imperfect
earth is specified for the far field reflections.   That is why the
simpler
modeling programs will tell you that  only 2 elevated radials below
an elevated GP antenna are as efficient as 120 buried radials.
With PERFECT earth (and perhaps saltwater), this is true.  Over
REAL earth, the losses go up by 3 to 6 dB as  W8JI  has told us
over and over based on his Field Strength  MEASUREMENTS.

Thanks to my mentor,  N4NO,   Jerry Cross, PHD in Communications
and Electromagnetic Field Theory and experienced RADAR specialist
for this reference and explanation.

de  Tom  N4KG


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