NEC-LIST: Which excitation should I use ?

You're trying to get the fields from the elements to be in some
magnitude and phase ratio. The fields from the elements are proportional
to the currents in the elements, not the voltages at the feedpoints. So
you need to drive the elements with currents which are in the correct ratio.

The EX cards create voltage sources, so by using them you drive the
elements with voltages which are in a particular ratio. Due to mutual
coupling, the feedpoint impedances are different, so the element
currents aren't in the same ratio as the element voltages.

If you have a copy of the _ARRL Antenna Book_, this is explained in
detail in Chapter 8. Another good reference is Johnson's _Antenna
Engineering Handbook_ or its predecessor edited by Jasik.

A secondary effect not often mentioned is that the current distribution
on the elements is also impacted by current mutual coupling, so the
total element fields aren't necessarily proportional to the feedpoint
currents. The deviation is small for thin radiators of a quarter
wavelength or shorter height, but can be considerable for longer and
fatter radiators. There's more information on this topic at
http://www.eznec.com/Amateur/Articles/Current_Dist.pdf.

Roy Lewallen

Phil Ede wrote:
> Hi
>
> I have been modelling broadcast antennas for a number of years and have
> recently upgraded to NEC4. I have run into a small problem modelling arrays
> and would appreciate some guidance.
>
> 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..
>
> Phil Ede
>

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