Re: NEC-LIST: Negative driving point impedances in phased arrays

Jim,

My experience is with long end fire arrays which are easily simulated
with NEC-4, GUI GENEC. The simple fact is that while the intent is to
excite all the elements, some of the elements do not receive power from
the intended driving sources but generate power instead [back toward the
source]. To stop this you have to adjust the impedance of the elements,
particularly the elements near the ends: Forward and AFT. You need a
full wave simulator for this.

Locate, say a long end fire array of monopoles on an infinite GP and
play with the lengths of the monopoles and spacing in the end fire
direction. Start with the lengths electrically short and then increase
the lengths. That should expose the nature of the beast.

Regards,

David Young
dwyoung_at_ieee.org

Jim Lux wrote:
>
> 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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