NEC-LIST, Whip antenna

Re: Gerry van Dooren, modelling of whip antenna

Don't know how relevant it is, but I have tried modeling a whip with a
base loading coil as a test of NEC for helices. The whip had a fixed
height of 32 cm over an infinite PEC plane (GE1, GN1). The coil had a
diameter of 1.1 cm and 10 turns with 10 segments per turn. A short
vertical wire was connected from the ground to the base of the coil for
the source. The length of the coil was varied from 1 cm to 8 cm, while
adjusting the length of the straight wire above it to keep whip height
constant. Wire radius was 0.01 cm.

The input impedance of the whip without loading coil at 150 MHz was
Z0=11.25 - j249.3 ohms. This impedance was subtracted from the impedance
with the coil (Zc) to determine the coil impedance, and the corresponding
inductance Im(Zc - Z0)/omega was compared with the ideal solenoid
inductance from a handbook formula. The results are shown below. At
the closest turn spacings the inductance determined by NEC is close to
the ideal value, but the average gain, which should be 2.0, is not so
good due to close-wire problems. For larger turn spacings the average
gain gets better. The inductance differs more from the ideal value,
probably because the coil is getting to be to large a fraction of a
wavelength for the ideal solenoid formula to apply.

Once one is satisfied that a coil provides the proper inductance it is
obviously better enter the inductance on a LD command in NEC, at least
for short coils that are just inductive loads.

length (cm) Rin Xin Avg. Gain L NEC L handbook
   8. 10.77 -46.20 1.93 0.21 0.14
   7. 10.93 -36.27 1.91 0.23 0.16
   6. 11.19 -22.86 1.87 0.24 0.18
   5. 11.62 -4.25 1.83 0.26 0.22
   4. 12.33 23.96 1.77 0.29 0.26
   3. 13.59 72.53 1.68 0.34 0.34
   2. 16.21 174.20 1.56 0.450 0.476
   1. 24.04 487.6 1.35 0.782 0.795

Jerry Burke
LLNL
Received on Sun Nov 24 1996 - 19:51:00 EST