Just ran some quick NEC4 models. 4 10 meter tall verticals at
(10,0),(0,10),(-10,0),(0,-10) meters (x,y coordinates). A 2 meter tall probe at
(0,0) excited with 1 volt. Just looking at the current at the base of the
verticals (shorted to ground).
Excited it at 7.1 MHz
Current was 6.58E-5 Amps at -137.1 degrees, in all elements. (well, not the
probe. it was about 1.2 mA at 90.5 degrees.. basically it looks like driving a
capacitor)
Now, I put in a big tower (a wire 0.15 m radius) at (20,20), that was 30 meters
tall (trying to make it roughly resonant, eh?) not connected to anything else.
The new currents and phases were
5.96E-5, -135.3 degrees (for the two elements closest to the new tower)
6.70E-5, -132.9 degrees (for the two elements farthest from the new tower)
This is a change of about -.87dB or +.15dB, and a change of about 2-5 degrees.
What does this mean? I think if you have a VNA, you can probably just measure
the effect of the big tower 20-30 meters away (i.e. about 3/4 wavelength away)
with a simple nonresonant probe.
Whether a 1dB change in power or a 5 degree change in phase in this test setup
is important kind of depends on what your phasing strategy is. I didn't
calculate mutual Z or anything else. To a first order, I would expect the
change in the signal received from a notional far field source to be
comparable. 1dB amplitude error is pretty big if you're expecting deep nulls,
but it's not going to move the main lobe very much.
If I get ambitious, I'll actually tune the elements and put loads on them, to
simulate what you'd actually see on a VNA.
jim, W6RMK
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