NEC-LIST: Negative driving point impedances in phased arrays

I'm looking for good conceptual explanations (potentially backed up by
models) for when negative driving point impedances (or negative power)
occurs in a phased array.

There are lots of generalized statements that this occurs (and, in fact,
some fairly unexceptional appearing systems wind up with a "negative
element"), but not a whole lot of simple qualitative explanations of when it
occurs.

One can look at a two element system (say a couple of dipoles less than 0.25
wavelength apart) and come up with some ratios of feed currents that result
in power fed from one element to another. This implies that you've got some
reactive power circulating in the system (since it's obviously not
radiated...).

Is it related to superdirectivity, i.e. you've scanned into the invisible
region? Off hand, I'm not sure, because you can have superdirective arrays
(that is, the phase advance is more than the spacing of the elements) that
don't have negative power on some elements.

Jim Lux

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