On 10/3/2012 7:20 AM, Tom W8JI wrote:
I would agree with your idea about multiple points making it
meaningful if you also moved the probes deep below ground, to a depth
where the field diminished to unimportant values. I think this is a
case of not being able to do things right, so we dismiss what probably
is a problem.
I am in complete agreement with Tom on the importance of measuring to
skin depth at the frequency of interest, and for mere mortals, measuring
radials from a nearby broadcast station is the only way I can think of
to do that. While the transmission line method is fine, and the DG8SAQ
VNA is a winner (I have one), it is still limited to the depth of the
length of its probes.
But it is also important that we remember that there are at least three
primary effects of ground. They are 1) loss in the near field; 2)
reflections in the far field; 3) ground wave propagation. We can
minimize #1 by heroic effects with our ground system, and this shows up
in the impedance measurements of our antenna system. The only thing we
can do about #2 and #3 is to move.
73, Jim K9YC
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