Steve Reinhardt wrote:
. . .
Cool. The neat part about the football field is that the nearest
reflection is well over 1.4 times the distance between source and
measurement antennae. It's flat with no RF hard surfaces around the
perimeter. That's not to say there are no other sources of measurement
error, just that I think their contribution will be small.
. . .

I'm not at all an expert on antenna measurement. I know just enough to
realize that it's extremely difficult to do with even moderate accuracy,
and that some professionals with the very best equipment tend to trust
modeling more than measurement. Besides problems with reflections, you
also have the problem of assuring a constant real power into antennas of
different impedances, feedline radiation, and a host of other
confounding factors.

That being said, I'm sure you'll learn a lot in the process, and you
might be able to get useful results in spite of the difficulties.

I was really saddened to see, some years ago, a published group of
measurements like you're proposing, on a bunch of different antennas.
The results, while quite believable, showed some pattern skewing and
other artifacts which almost certainly weren't really due to the
antennas themselves. What saddened me was that the people running this
detailed, meticulous, and time consuming project hadn't thought to
include a measurement of any antenna with a well known pattern and gain
such as a dipole. Please include some reference antennas which are well
known and/or easy to model! Otherwise, the accuracy of all the results
is purely guesswork. A dipole might not be the best reference because
its broad pattern is more subject to reflections than a Yagi. But a good
reference Yagi (such as one of the NBS standard Yagis) or two could be
constructed and included.

You'll get strong reflections from the ground between the antennas.
It'll be easy to calculate the elevation angle of the signal from the
transmitting antenna which will arrive at the receiving antenna after
reflecting from the ground. If your antennas are far apart, this angle
will be more nearly horizontal, where antenna gain is higher, than if
they're close. So there may will be a stronger reflection if the
antennas are farther rather than closer (although most moderately sized
horizontal Yagis have a broad elevation pattern, not very different from
a dipole). This ground reflection could have a profound effect on
measured gain, and could cause some large differences with only small
differences in, say, antenna height. If the antennas are close and the
angle steep, then the pattern at the elevation angle being reflected
might be quite different from the horizontal antenna pattern. This will
result in a distortion of the measured pattern relative to the free
space pattern. This is something which will show up more clearly in a
plot of a reference Yagi with gain closer to the actual antenna under
test than something like a dipole with a simpler pattern.

Have fun! I'll be eager to see how well your reference antenna
measurements come out.

Roy Lewallen, W7EL