[Elecraft] [K3] Diversity Reception and Antenna Directivity

[Jim Brown]

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