NEC-LIST: nec2 coupling accuracy, or my missunderstanding

I wrote a little ruby (nice interpretive language) wrapper round the C
port of the nec2 program, and began happily generating simulations. So
far so good.

(When I am finished I could offer it to anyone interested - ruby is object
oriented - you can define objecte wires, dipoles, sources, loads etc and move
them around, duplicate them, draw them , run nec2 on them and collect and
analyse the results - just generates nec cards and runs the program and eats
the output)

I am new to EM - so here comes what is probably a silly
misunderstanding:

I expected from linearity, symmetry and recipricosity) that I could write the
relationship of observed currents C(i) in a symmetrical array of similar
elements to the currents which would be obtained in each aerial alone in the
same external radiation field Y(i) as:

c(i) = y(i) + sum(i!=j, k(i,j)c(j))

or solving the matrix equation

C = (I - K)^-1 Y

where k(i,j)=k(j,i)=k(i-j)=z(i,j)/(z(i,i) + zload).

Is this nonsense - can someone please point out what the correct relationship
should be - or where to find out...

I cannot persuade nec2 to give a constant (independant of the signal direction
of Y) mixing matrix relationship between Y and C for an applied plane wave.
It works perfectly for 2 dipoles, but even 3 fail miserably. I currently use
the C port of nec2 - but the fortran gives the same currents, I checked.

Since I expect a toeplitz symmetrical mixing matrix, I can use DFT to find the
unique matrix from knowledge of Y and C - I have not used the nec2 coupling
calculation.

Y can be taken as the current when other aerials are O/C, or just the simply
calculated plane wave values at the aerial positions - it seems to make
little difference at the 0.8 lambda spacing I have used. (resonant dipoles
with matched resistive load)

I have tried increasing segments and thinning the wires to silly levels - the
mixing results are not even stable at 400 segments per 1/4 wavelength, and
never direction invariant. It is mostly the phases which change, but the
amplitudes also vary significantly.

Even in the 2 aerial case, where the coupling is signal invariant, I find that
the drive point impedance (applied voltage/same aerial current) and inferred
coupling coefficient are not stable with increasing segments : -

dipole
    wires
        0.2902m (0.242 lambda) long
        0.1mm radius (too thin due to skin effect?, but might simplify calc?)
    gap 1mm
    load 46.5 ohms, resistive
    frequency 250 MHz, wavelength 1.2m

Array:
    2 dipoles spaced by 1m
        400 segments per wire
            k 0.1161228 + 0.1388941i
            z 122.49201, 2.18
        200 segments
            k 0.1116499 + 0.1332574i
            z 115.01422, 1.98
        100 segments
            k 0.1060464 + 0.1261806i
            z 106.85743, 1.73
        50 segments
            k 0.0995596 + 0.1179915i
            z 98.71440, 1.45
        25 segments
            k 0.0926639 + 0.1093200i
            z 91.36430, 1.15
        12 segments
            k 0.0856765 + 0.1003358i
            z 84.84516, 0.81
        6 segments
            k 0.0796091 + 0.0923226i
            z 79.83157, 0.44
        3 segments
            k 0.0741984 + 0.0846942i
            z 75.66007, -0.07

help

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