Re: NEC-LIST: Negative resistance

In a message dated 98-03-06 00:07:22 EST, John Yorko wrote:

> I am doing a NEC2 simulation feeding 46 identical 2.8 m long dipoles
> (28 segments each) at equal heights (8 feet) above real ground with 46
> separate sources (equal voltage). They are spread out but fairly
> close together (no closer than 4 feet). Some of the sources show a
> negative resistance portion of impedance. I assume this is for the
> reason stated in the above FAQ answer. But is this really "correct"?
> The simulation was done at 30MHz, although I have seen this happen at
> other frequencies. In real life each of these antennas has its own
> matching network and is fed by its own 50W transmitter. With that
> kind of variation in feed point impedance, is there a possibility that
> the matching networks will not work as they do with a single isolated
> antenna? Can I expect anything unusual in my measurements?
>
> Thanks,
>
> John Yorko
>
John:

It seems to me that that is exactly what may happen. My quick take is
that assuming the excitation of each transmitter is coherently fed
from a single frequency source, although possibly with varying amounts
of phase shift, the feed point impedance can indeed take on "strange"
values and, in general, will be different than it would be for an
isolated antenna. This is not an artifact of the NEC program, but an
actual effect, well understood by AM broadcast antenna designers,
among others.

Regards,

Jacob Schanker, P.E.
Received on Mon Mar 09 1998 - 12:43:48 EST