There is another VERY important thing that is being missed by this discussion. A VERY large component of fading is due to multipath -- that is, the arrival of more two or more wavefronts that travel different paths, and thus have slightly different travel times. This produces a phase shift which varies with frequency, position, and the path. When the two wavefronts are precisely in phase and equal in amplitude, they add by 6dB; when they are precisely 180 degrees out of phase and equal in amplitude, they cancel. At any other value of phase difference, there is partial addition or partial cancellation. And, of course, the more nearly equal the two wavefronts, the deeper the cancellation.
This is WELL recognized as multipath on VHF and UHF, but few hams realize that the same thing is happening on the lower bands. Think about it -- the phase shift is a direct function of frequency, so the very fast "picket fencing" at VHF/UHF of a mobile station or the flutter of a signal reflected by a moving aircraft becomes MUCH slower fading on the lower bands. A fading period on the order of a minute or two is not uncommon on 160M. When there is multipath on HF and MF circuits, the paths (most?) often differ by their vertical arrival angle. This suggests that one might expect a small time offset between horizontal antennas at different heights based on their vertical patterns and the vertical arrival angle of multiple wavefronts favored by the directivity of one antenna or the other. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
