[Elecraft] Low antennas in high places

[Ron D'Eau Claire]

Stuart wrote: 

NVIS (Near Vertical Incidence Skywave) dipoles or antennas are any antenna 
operated at low elevations above ground from lying on ground to being say 
0.1 wave high, and certainly well below conventional heights for that 
antenna.  

---------------------------

The optimum height is 0.2 wavelength. It's no surprise that's the optimum
spacing for a two-element beam. The earth acts as a reflector behind the
driven element. If the spacing (height of an NVIS antenna) goes beyond 0.2
wavelength, the pattern starts to spread toward the horizon. That's why we
try to get our horizontal dipoles that will be used for DX up above that
height! One cravat there is that you don't always know the 'effective'
height. Over dry, sandy soil or rocks the effective height may be greater
than the height of the radiator above the ground. 

Lower heights work too, but as the antenna is brought closer to the earth
the overall gain drops due to ground losses. At 0.2 wavelengths, the NVIS
will show over 6 dB of gain straight up: that's equal to quadrupling your
transmitter power! 

The advantage of low dipoles on slopes for DX noted by Moxon is that the
earth behind the radiator acts as a reflector. If you have a vertical cliff,
you have a two-element beam pointing in the direction the radiator is
facing. Even more gentle slopes will produce good results because that
"vertical" lobe is actually very broad. A 20 or 30 degree slope will put a
lot of RF out near the horizon. And a BIG advantage to a cliff-side antenna
is much reduced ground losses in the far field. Most low angle radiators
lose a lot of low-angle radiation because of the ground losses around it
(unless you're at sea surrounded by salt water). Even a vertical, which
theoretically has maximum gain at zero degrees to the horizon, will show
little radiation below 20 or 25 degrees. All that radiation down lower is
converted to heat in the earth. 

Ron AC7AC