Hi Roy,

A couple of comments on your excellent input into this discussion.

Roy Lewallen wrote:

I found another source (Ramo et al) which directly gives the ratio of
power in the load of a matched receiving antenna to the power applied to
a transmitting antenna, in terms of the effective apertures of the
antennas. This doesn't require the intermediate step of calculating
field strength. The equation is:

Wr/Wt = (Aer * Aet) / (lambda^2 * r^2)

whe
Wr, Wt are received and transmitted power respectively
Aer, Aet are the receiving and transmitting antenna effective apertures
lambda = wavelength
r = distance between the antennas


Kraus "Antennas" also describes this equation. He refers to it as the
"Friis transmission formula" on pages 48 and 49 of the second edition.



One final note, regarding the NEC applied plane wave. My earlier
statement that the resulting field is twice the plane wave source
magnitude when a ground plane is present is true only when the plane
wave is applied over perfect ground at exactly grazing incidence (zenith
angle = 90 deg.). If applied from other angles the resulting field
strength will be different. If you apply a vertically polarized wave
over a ground plane, I believe the resulting field strength will look
like the pattern from a vertical radiator over a perfect ground plane --
strongest when applied at the horizon, decreasing when applied at higher
angles, and dropping to zero if applied from directly overhead. I
haven't confirmed this, but believe it's necessary in order to get a
receiving pattern that's the same as the transmitting pattern. So use it
with caution when a ground plane is present, and don't casually make
assumptions about the resulting field.


I believe a better way to describe this situation is that the plane wave
field strength does not go to zero, but rather the effective aperture of
the antenna goes to zero as the plane wave is applied from overhead.
This does not change your conclusion with respect to antenna patterns.

The oblique-incidence plane wave equations are slightly messy, but they
are well described in treatments of waveguides.

73,
Gene
W4SZ