[TowerTalk] 1 or 2 dB

[Jim Brown]

On 5/19/2022 6:23 AM, Lux, Jim wrote:
> I'm not so sure that it's out of reach.  yes, trying to implement it 
> with gear from 1980 would be challenging. But with more modern 
> equipment, where the "radio" is a black box controlled by a "front 
> panel" or "computer" it gets easier.

The Elecraft K3 with second RX that is the same as the main RX, and 
which can be synced with the main, allows diversity reception, and I've 
been using it since 2008.

Diversity requires an antenna for each RX, spaced as widely as practical 
from each other. It was invented in the earliest days of radio to 
counter the effect of selective fading, which is the the cancellation of 
two or more arrivals of the wavefront from the same TX that have 
followed different paths, arriving at different times. The time 
differences cause the arrivals to have a variable phase relationship 
with each other, combining algebraically to cancel or add, depending on 
the resulting phase relationships. Diversity works best when the 
antennas have the greatest spacing, so that when cancellation is 
occurring at one antenna, it is less likely to do so, or even to 
increase, at the other.

> And the diversity combining - doing it in analog is hard, but in the 
> digital domain it's much easier, and for the most part it can be done at 
> audio (or post down conversion to baseband or low IF).

As diversity has been practiced since the beginning, combination is done 
in the brain of the operator, with audio from the two receivers in 
opposing ears. That's how it's done in the K3. The result is a sort of 
spatiality to the sound, a bit like the true stereo image produced by a 
spaced pair of microphones dedicated to left and right loudspeakers.

Combining the outputs of the two receivers to a single (mono) channel is 
problematic, because the phase relationships at audio have a good chance 
of cancelling.

73, Jim K9YC