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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
