Aloha,
Some of you may recall I had raised questions about these
tests right after returning from Dayton, where I heard Steve's
talk. My thoughts were posted in late May, and Steve asked
that we refrain from the particular questions until the report
was published and out. Now seems to be the time, as I,
and many others now have recv'd the report.
I still need some coupling between the tests they ran, which
seem to be measurements of the ground wave energy,
propagating along some 4875 feet of mixed terrain, to
the rcv site. Certainly, as they said, they did not have a
helicopter, so could not really measure the energy up
at the peak of the main lobe. This peak, per the report
figure 2, for an antenna mounted at 50 feet, their chosen
test height, would be up at an angle of 18 degrees per
their model program. Looking at their figure 1B, with the
test antenna mounting at N0AX at an earth terrain elevation
of 200 feet, and the rcv'g site terrain elevation of 125 feet
at W7KT, and both antennas on 50 foot towers, I guess
the elevation of the rcv antenna with respect to the transmitting
antenna must be around only 1 or 2 degrees, surely, no
more than 3 or 4, and it even appears to me that the rcv'g
antenna is LOWER than the transmitting antenna.
It is hard for me to see any antenna pattern energy at such
low radiating angles on their figure 2. Of course, they write
in the report that this was a reasonable test. So, what is
the reasonableness of judging the comparative "goodness"
of these antennas using, what can only be ground wave
energy?
What has that to do with what is going on up at 18 degrees
elevation as seen from the transmitting antenna where
their model says the antenna gains, for 50 foot mounting
height, should be at the manufacturer's design peaks.
That, by the way would be at an elevation of 1584 feet
above the rcv'g antenna site. And, as they write on
page 21, they did not have a helicopter to reach
the main lobe peak, where they write, "the gain
measured is different from that which would be
measured with an elevated probe in the far field
of the main lobe of the antenna." And of course,
their figure 5 illustrates the "smearing" out of
transmitted energy caused by the uneven
terrain. Perhaps that is where the very low
angle energy is from; I can see a bit down
there in the figure, about 5 dBi.
This must be the measured radiated energy, is it
proportional among the various antennas tested?
That is, will the terrain smearing be the same for
all antennas tested?
Just wonder, as I really do not understand how these tests
are valid comparisons, other than for what ever ground
wave energy was intercepted. Is there a clear, accepted
relation between ground wave, (near field ?) energy
patterns and the patterns up at lobe peaks? Or a
clear relationship between the tested antennas and the
TA analysis as illustrated in figure 5, with what appears
to be around 5 dBi of gain along the ground?
Perhaps such range tests of ground wave energy are industry
standard, I am just uninformed in this area. I presume such
range tests are what Cushcraft, Telerex, Hy-gain, Moseley
have all done for years, I guess. Still, what is the relationship,
thanks.
73, Jim, KH7M
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