Re: NEC-LIST: How do I simulate a CARDIOID Array?

> I am experiencing problems in simulating a cardioid array at
> 300MHz. The array consists of two vertical ½ lambda dipoles,
> stacked horizontally with a separation of ¼ lambda. The
> dipoles are fed with equal magnitude and 90 degrees phase
> difference.

This probably refers to 90 deg phase difference of the feedpoint
currents, not the feedpoint voltages.

Your model fixes the phase difference of the two feeding voltages. By
symmetry, for phase shifts of 0 and 180 deg, the currents will have
the same phase shift as the voltages, but for other phase differences
this is not true due to the mutual coupling of the
dipoles. Correspondingly, you get bad results for close spacing and
phase differences other than 0 and 180 deg.

There seems to be no direct way of fixing the feedpoint current
in a NEC2 calculation. The 'elementary current source' cannot
be used here; it illuminates the antenna structure with the
field emitted by a small elementary dipole and therefore belongs
to the same class of excitations as 'incident plane wave'.
There are only voltage sources for feedpoints on wires.

Here is a possible solution (computed for 300 MHz only):
Changing the excitation cards to
EX 0 1 5 0 1 0
EX 0 2 5 0 1e-6 0
FR 0 1 0 0 300 0
gives the feedpoint currents 1.48710E-02 -3.96715E-03*j and
-6.13813E-03 +9.46600E-03*j respectively, while the lines
EX 0 1 5 0 1e-6 0
EX 0 2 5 0 1 0
FR 0 1 0 0 300 0
yield currents of -6.13813E-03 +9.46600E-03*j and
1.48710E-02 -3.96715E-03*j. (A voltage of 1e-6 is used to
represent zero here; note that if the voltage given on the
excitation card is zero, it actually defaults to 1+0*j V.)

Using linear network theory, you get the matrix which maps
(I1, I2) to (U1, U2) by inverting the matrix which has the two
(I1, I2) pairs given above as its column vectors. Multiplying
the column vector (I1, I2) = (1, j) with this matrix gives
the (U1, U2) vector for a 90 deg current shift.

The final model looks like this:

CM
CE
GW 1 9 0 0 -.207 0 0 .207 .02
GW 2 9 0 .25 -.207 0 .25 .207 .02
GS 0 0 1
GE 0
EX 0 1 5 0 94.5095 9.0914
EX 0 2 5 0 39.1212 21.2747
FR 0 1 0 0 300 0
RP 0 1 360 1000 90 0 1 1
EN

The two feedpoint currents are now of equal magnitude and have
90 deg phase difference. Check if this gives the radiation pattern
you expect.

This solution is somewhat cumbersome, of course, and you would
have to repeat that process for each frequency. Maybe someone
else knows a simpler way of doing it.

Jo (dl2kcd, ve7cdl)
Received on Sat Mar 28 1998 - 12:18:55 EST