[Milsurplus] Re: German NVIS

[Robert Flory]

The rollbar cage antenna pictured and described on     
http://www.tactical-link.com/WWII_NVIS.htm

would radiate with predominately vertical polarization and be unsuitable
for sending a signal straight up.  It is rather similar in geometry to an
antenna that I built for 80 meters once.  Picture a U on its side.  break
the bottom of the U and put a feedline there.  The bottom of the U is a
small fraction of a wavelength, say .1 wavelength.  The now horizontal
parts of the U on its side are .2 wavelength each.  The currents in the
horizontal parts are out of phase and spaced rather closely, so they will
largely cancel, leaving the radiation from the vertical part(bottom of the
U) to do most of the radiating.  Basically a short vertical dipole, with
the horizontal parts providing loading.

In the case of the rollbar cage antenna on the vehicle, the feedpoint is a
current maximum, and the bar it attaches to would be the radiator.  The
rest of the structure is mostly loading.

There is good antenna modeling software out there by K6STI, that divides
antennas up into little pieces and numerically adds up the vector sum of
the contributions of all the little pieces in all directions and makes nice
displays of patterns and gives feedpoint impedances etc.  

Rob Flory
Recovering Physicist