[Elecraft] Diversity Reception

[Jim Brown]

On Mon,8/3/2015 1:31 PM, Bob McGraw - K4TAX wrote:
> I tend to agree with  Brian.

I do as well. It's important to realize that a large percentage of 
fading is the result of signals arriving from multiple paths, resulting 
in slightly different travel times from the source. This time difference 
results in phase shift directly proportional to the time difference and 
to the frequency.

When heard on VHF/UHF, we call this "picket fencing," and the fading is 
much more rapid because of the higher frequency/shorter wavelength. On 
the much lower bands (AM broadcast, 160M) we call it selective fading, 
and the time between peaks and nulls is much greater (proportional to 
wavelength differences). Diversity reception for this kind of fading 
depends on antennas at different locations -- the time differences, and 
thus the phase shifts, will be different at the two locations, so a fade 
(strong cancellation) at one location will usually not be a strong 
cancellation at the other.

The most critical thing about diversity is that separation, and keeping 
the two receivers close enough in frequency so that the ear/brain is not 
bothered by the difference in the note.

Another point -- It is generally a bad idea to COMBINE (sum) the outputs 
of the two receivers into a single electrical channel. Two reasons. 
First, as Bob has noted, the ear/brain is really good at choosing 
signals presented to two ears -- that's how we hear directionality, and 
it's how stereo works. Indeed, stereo works on the TIME difference of 
the sound reaching the two ears. Second, if the travel times to the two 
antennas are sufficiently different, the AUDIO signals can be enough 
different in phase and equal in amplitude that they cancel in the 
electrical channel. When the electrical signals are directed separately 
to L and R loudspeakers, there is relatively little cancellation, and 
there is none in headphones with one radio per ear.

I've summarized this from the result of decades of experience with sound 
systems, an extensive study of phychoacoustics (the science of how 
humans hear), as well as from 60 years in ham radio.

73, Jim K9YC