NEC-LIST: Slotted cylinder

Stanley,

Your model of the slot antenna in a cylinder looks OK. I tried running
it, but added the command

RP0,46,37,1002,0.,0.,2.,5.,

to compute the average gain. It is only necessary to integrate theta
from 0 to 90 degrees and phi from 0 to 180 degrees since the pattern is
symmetric outside this sector. The average gain was 1.037 with NEC-4
and 1.048 with NEC-2, both in single precision. The value should be
1.0, but the 4 percent error looks good for this type of model. There
is no need to model both sides of the surface. The solution of the EFIE
will yield the sum of the interior and exterior currents. Within the
limits of model accuracy, the interior currents will only radiate to the
interior and the exterior currents will radiate to the exterior, so the
radiation pattern and input impedance, from the current through the feed
wire, should be correct. It is possible to separate the interior and
exterior currents with a combined EFIE and MFIE equation or by modeling
separate surfaces, but this would not be advisable except with a code
designed to handle the singularities in the fields for two surfaces very
close together. The advantage of separating the interior and exterior
currents is smaller numerical "leakage" of fields from the interior to
the exterior, but this is not important in your problem, since there is
lots of intentional leakage.

A possible reason for your poor agreement between NEC and the
eigenfunction expansion could be radiation from the ends of the finite
cylinder, although end radiation should be negligible with a cylinder
length of 3 wavelengths. The circumferential slot would tend to excite
the TM modes with a lower cutoff than the TE01 excited by the axial
slot. You said that the axial slot in a thiner cylinder gave good
agreement. It might be interesting to try modeling a closed cylinder
excited by an axial dipole at its center. Then varying the length of
the cylinder might show the importance of radiation from the ends and
leakage through the grid.

Jerry Burke
LLNL
Received on Tue May 06 1997 - 10:08:31 EDT