GPR antenna simulation

I am trying to simulate a GSSI 300 MHz ground penetrating radar antenna
using NEC-2. The antenna consists of a copper bowtie dipole, approximately
18" in length and 10" wide, with an over-laying copper ground shield
connected to the bowtie ends with discrete 220 ohm resistors. These
structures are simulated using a wire grid with resistive loading in the
wires connecting the dipole ends to the shield. Each antenna leaf is
approximated by a grid of 21 wires with an average of 5 segments per wire.
The shield is approximated with a similar density of wires and segments. In
total, the antenna model consists of 506 segments. Symmetrical sources are
applied at the dipole input terminals. The NEC simulation does not involve
any groundplanes or dielectric bodies. (GSSI specifies the 300 MHz frequency
in free-space).

I ran a series of simulations at a couple of selected pattern angles at
frequencies starting from 10 MHz to 1GHz in 10 MHz steps (this upper
frequency range is perhaps a little optimistic, but the spectral content of
the input pulse is not significant above about 600 MHz). From measurements,
the input waveform to the antenna appears to be a gaussian-like pulse of
approx 3ns duration. I multiply the fourier transform of this input waveform
by the frequency response of the antenna to get what I thought would be the
emitted farfield waveform of the antenna..... only it doesn't look like the
waveform we get from measurements. The waveform of the NEC simulation seems
to have a lower center frequency and has much more ringing (measured
waveforms show a monocycle sinusoid with a small amound of ringing).
Additionally, the shielding does not seem to provide much suppression (in
simulation) of the emitted radiation above the antenna, whereas in practice
the shielding seems to be quite effective.

Any suggestions?

Neil Chamberlain,
ECE, South Dakota School of Mines and Technology.
Received on Mon Aug 12 1996 - 18:58:00 EDT