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

> -----Original Message-----
> From: nec-list-bounces+p.s.excell=bradford.ac.uk_at_robomod.net
> [mailto:nec-list-bounces+p.s.excell=bradford.ac.uk_at_robomod.net
> ] On Behalf Of Jim Lux
> Sent: 03 January 2005 16:17
> To: nec-list_at_robomod.net
> Subject: 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

IMO this is only like an electric-field version of a traditional
transformer: that is an EM-coupled device which has -ve impedance (i.e.
a power source) at one port. I would expect such a situation to occur
routinely in most arrays, unless very widely spaced.

Fritz Landstorfer (Stuttgart Univ.) did some plots of power flow around
array elements some years ago that might give the insight you're
seeking: they were mostly in the Archiv fuer Electronik und
Uebertragungstechnik.

-- 
Peter Excell, University of Bradford, UK
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