Z-matrix

> what data card do I have to add to the nec2 input file
>to get the complete Z-matrix (or something else which gets the
>strength of mutual coupling between two antennas or the ports
>of a two-port antenna)?

You probably do not need the Z matrix. It relates the amplitudes of the
spline basis functions to the fields at the segment centers with the
structure not there (open-circuit impedance matrix, so every segment is
an open port). The matrix Y=Inverse[Z] relates the fields at points on
the structure with all of the structure present to the basis function
amplitudes. However, NEC uses L/U decomposition to get the solution, so
you would have to compute the inverse and then apply a transformation as
done in subroutine TBF to convert basis function amplitudes to currents
at segment centers.

This is done for you in the currents printed in the output. Just excite
the antenna ports one at a time, and the currents on the segments
representing the other ports will give the mutual admittances. Remember
that regular segments are short-circuited ports unless you excite them
with a voltage source. However, a segment with a network or transmission
line connection (NT or TL command) is open circuited unless you
explicitly short it. One way to short circuit a network-segment port
is to put a very small voltage source on it (like 1.e-10, but anything
less than 1.e-20 will be turned into one volt by NEC). Then the current
in this small voltage source (but not the segment current) will be the
short-circuit current in the port. We forgot about shorting network
ports in NEC-2, so the maximum coupling calculation (CP command) is not
correct when networks or transmission lines are involved.

Jerry Burke
LLNL
Received on Mon Aug 07 1995 - 15:30:00 EDT