Re: NEC-LIST: Which excitation mode should I use ?

At 5:40 AM -0500 3/10/07, Phil Ede wrote:
>Modelling cluster of two or three quarter wave monopoles using an EX 0
>command with NEC2 gave an obviously incorrect radiation pattern, so I
>inserted an NT command, which I think converts EX 0 to a current feed.
>This seemed to produce the expected radiation pattern.
>
>I would be pleased if anyone could explain why I have to do this, and
>provide some criteria to decide which form of excitation I should use, when
>using NEC4..

I'm not qualified to answer your "why" question, but I know a way of
getting as many current sources as you want.

First, here is how to get any number, N, of current sources having
the same current magnitude and phase: Create very short (say, 0.001
wl) wire segment somewhere out of the way, and put a voltage source
in it. Connect one end of each of N transmission lines to this
segment. Each transmission line must have the same characteristic
impedance (the value doesn't matter here), and an electrical length
of one-quarter wavelength, i.e., 90 degrees of phase. Connect the
opposite end of each transmission line to a segment of your array
antenna where you want a current source.

Second, here is how to get any number, N, of current sources having
arbitrary current magnitudes, and equal phases: Do as above, but
make the characteristic impedances of the lines inversely
proportional to the current magnitudes that you want.

Third, here is how to change the phase of any one or more of the
current sources by 180 degrees, in other words, how to reverse the
polarity: Do as above, but increase the electrical length of the
respective transmission line(s) by one-half wavelength. WARNING: You
can not obtain any arbitrary phase shift by this method. The only
phase shift that you can get is 180 degrees, except in very special
circumstances.

Fourth, here is how to get N current sources of arbitrary magnitude
and phase: Create N voltage sources of arbitrary magnitude and
phase, and connect each one to a segment where you want a current
source, by means of a one-quarter-wavelength transmission line. Make
all transmission-line characteristic impedances equal.

-Chuck W1HIS

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