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

I was looking for a more qualitative explanation or tutorial that I could
point to. Perhaps in some antenna textbook or lecture notes?

As you say, one can fairly easily set up a system in almost any modeling
tool and show that negative powers occur. Grant Bingeman has an example on
his web site with 2 "ground plane" 1/4 verticals. You can do the same with
various and sundry analytical models of mutual Z between resonant dipoles,
also. However, such exercises don't give one much insight into what's going
on, and why.

For instance, in the superdirective array (with passive parasitic elements),
there's a lot more stored energy in the reactive near field than in the
non-superdirective array.

I was really hoping for a more generic statement along the lines of:

negative powers occur when the array is .....

"David W. Young" <dwyoung2_at_mindspring.com> writes:
> 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
> >
> > --
> > The NEC-List mailing list
> > NEC-List_at_robomod.net
> > http://www.robomod.net/mailman/listinfo/nec-list
>

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