Re: NEC-LIST: Low angle radiation?

Dana,

Your note caught my attention ---- *unexpected* results for a very
simple model, a half wave dipole --- but I must say rather unique
environmental conditions.

So I modelled a resonant horizontal dipole, poor ground (see below),
frequency 7.2 MHz, height 10.4 m, with your radial cliff model (height
- 15m, radius 30m). I calculate a gain 6.09 dBi (according to
NEC-4D), launch angle 19 degrees.

And, I must say that the elevation pattern looks peculiar, as does the
azimutal pattern.

To continue the exercise I modelled a vertical half wave resonant
dipole, lower end height 10.4 m. With finite ground in front of a
vertically polarized antenna you cannot realize high gain --- and, if
the antenna is too high, so that the low angle lobe drops below
10-degrees, the ground in front of the antenna starts "eating up" the
low angle lobe, and higher lobes have greater gain. This antenna on
your cliff is on the verge to get into this difficulty --- depending
on ground conductivity.

I do not know what you mean by very poor ground, but for horizontal
polarization the exact value does not matter much: for poor ground (1
mS/m, 3) the gain of my vertical dipole (according to NEC-4D) is 3.66
dBi, launch angle 13.6 degrees); for average ground (5 mS/m, 13) the
launch angle decreases and the low angle gain decreases, 11.1 degrees
and 1.09 dBi. For this latter case the gain is 2.27 dBi at a take off
angle of 27 degrees.

This seems to be in accord with what I would expect.

I have asked myself, and others, in particular Ed Miller, the
question: how does an antenna radiate? Radiation results from the
acceleration of charge. Where does this happen? Near the ends of the
antenna, since here the velocity of a current element comes to zero,
and accelerates very quickly again to its velocity on the center part
of the antenna. So the ends of a dipole are important. The
expectation that "maximum radiation results from the parts of the
dipole where current is a maximum" is not true. Model a drooping
dipole, centre higher than the ends --- and reverse the droop; ends
higher than the centre. The latter gives this the most gain.

So where am I going in this discussion? I am not quite sure. But,
consider your dipole set back from the edge of a radial cliff, the
result we get must have something to do with the idealization of the
model, a sharp 90 degree drop, and radiation from the ends of the
antenna???

73, Jack, VE2CV

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Wed Feb 16 2000 - 14:38:06 EST