NEC-LIST: NEC-List Digest, Vol 5, Issue 11 Which excitation should I use ?

Hi

Thanks for some excellent and very helpful responses, especially the advice from Roy Lewallan, which I am now studying.
The question of which excitation I should use, actually follows an earlier question I raised for Lambda/4 monopoles lambda/4 apart,
see http://www.devilsfoot.com/Nec/Archive2003/0279.html

In this, Roy followed with the comment:

"Ironically, the pattern of an array with elements near
anti-resonance (e.g., monopoles near a half wavelength high), is better
when the elements are end fed with voltages that are equal in magnitude
and in quadrature than when end fed with quadrature currents"

I am now looking for an explanation for this effect, and have the following observations

a) When electrically short antennas are brought together into an array, their individual electrical aperture must diminish,
even before there is any significant nearfield overlap,

b) When long antennas are brought in close proximity their nearfields begin to overlap so they will interact,

c) In using NEC to obtain an embedded pattern of an element in a phased array, by surrounding a single
excited elements with a number of terminated elements, I have found that quite a few elements are required before the lobes merge into a single main lobe.
 I therefore think of neighbouring elements towards the edge of an array and in close proximity, as having a parasitic function, even when excited,

d) The relative phase between the excitation applied at the drive point of any element near the edge of an array and the field
incident on it from its neighbours will not necessarily be the same for each element.

Could these factors be working together somehow to effect the difference in current distributions ?

With two electrically long antennas in close proximity, which we are primarily concerned with here, could there be two modes at work,
a differential mode, which on its own would produce the desired pattern, and a common mode
 which has a monopole pattern, which then fills the null ?

Phil Ede

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