I believe that all three people (Richard, Roy, and Fred) who have
commented on this topic have misread the figure in Kraus. The gain at
one-half wave spacing from the reflector is *zero*, not zero dB
(relative to a dipole), nor zero dBi.

As expected from the simple mirror image model there is complete far
field cancellation in the phi = 0, theta = 0 direction. That is the only
direction referenced in the figure.

73,
Gene
W4SZ



Roy Lewallen wrote:
The only way to get 0 dBi maximum gain from a lossless antenna is to
have a perfectly circular pattern in all azimuths and elevations. It's
not possible to have a maximum gain (that is, gain in the best
direction) less than 0 dBi unless loss is present. This is something
everyone with a basic understanding of antennas should know.

So it seemed to me very unlikely that a dipole spaced a half wavelength
from a reflector would have a perfectly circular pattern and, therefore,
it must have gain greater than 0 dBi in some direction. I don't have
Kraus' third edition (yet), but there's a diagram on p. 546 of the
second edition which I suspect is the same as the one Richard is
referring to. The caption under the graph clearly says that the gain at
0.5 wavelength is 0 dB *relative to a half wave dipole in free space*,
or about 2.15 dBi, not 0 dBi.

If the third edition really says that the gain of a half wave antenna
spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and
should be brought to the editor's attention so it can be corrected.

I also believe that while you might draw some possible parallels, you
can't directly apply the characteristics of an antenna in proximity to
an infinite reflecting plane (as in Kraus) to those of an antenna in
proximity to a tower.

Roy Lewallen, W7EL

Richard Harrison wrote:

. . .
The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in
field intensity versus spacing from a flat reflector. At 0.5 wavelength
the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone
which has about 2.14 dB gain over an isotropic.

The graph shows a gain of about 2.14 dBi with a spacing of only 0.1
wavelength spacing. So, anything greater than 0.1 wavelength from the
tower should be fine. That`s 20 cm in the 2-meter band, or about 8
inches. More distance means less coupling and should be better.

Best regards, Richard Harrison, KB5WZI