Re: NEC-LIST: Beverage antenna

G'day!

I'm surprised that there is still any activity at the international
AM/SSB maritime emergency frequency.

A NEC2 model for a Beverage antenna is quite small and simple - a
handful of segments over a ground plane. But you'll need some care in
the implementation.

For the azimuth performance, you will want to have a good
discrimination between forward and backward traveling waves, by
applying the correct source and load resistances. You should check the
variations in radiation pattern, by the depth of the front/back null,
rather than entirely rely on NEC impedance data you may have deduced
from current data in a run made with perfect voltage or current
sources. A starting value for Zs, Zl should be about equal to the
characteristic impedance Zo of the line over a perfect ground. (I
think that you should check the actual impedances upon construction
for the best results. The image of Zl at the driving point should be
about Zl)

The ends would be connected to ground (with one end terminating at Zo)
with a single segment (it would probably be quite short relative to
lambda), in which the source and the load would be inserted.

The fun would probably start when you consider the elevation radiation
angle, which, for the Beverage antenna, depends on ground
conductivity.

It's quite easy to specify a PEC (perfectly electric conductor) ground
plane using the GX "Reflection in coordinate planes" card. But you
really want to use the "Ground Parameters" GN card which specifies a
real ground, with the Sommerfeld/Norton method.

In this case, a set of Sommerfeld should be computed beforehand for the
ground resistivity using the "SOMNEC" utility, which are read upon
NEC2 execution.

If you're attempting to design a switchable pattern antenna using two
parallel wires, I'd discourage you from attempting to model two
closely spaced parallel wires with NEC2, and their associated
circuitry. It would be simpler, and perhaps more reliable, to analyze
both wires as single one, in a common mode, and then use transmission
line and circuit theory to do the front/back discrimination part.

Utility wires, which are a curse in AM/MW broadcasting, would also
affect your design. I'd take a look at ~25kV distribution circuits
located within the first few hundred meters. The shorter wavelength,
and the horizontal orientation of the antenna, would, IMHO, make them
more of concern than at MW.

That's all for now.

Alexandre
Montreal
Received on Tue Apr 21 1998 - 21:50:12 EDT