[MRCA] Field Exercises/NVIS

[Robert Flory]

Re: German NVIS 

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