[Elecraft] Diversity Reception

[Edward R Cole]

Interesting statements from  both Bob-K4TAX and Jim-K9YC.  In both 
cases they were addressing modes that one "hears" such as CW or voice 
(SSB/AM/FM).  I remember the commercial systems that used satellite 
Rx and "voted" the best SNR signal to be passed to the audio 
channel.  And I also agree with the notion of listening to each 
channel of a two RX diversity receivers sent to one's ears giving a 
stereo "spacial" effect which the brain quickly adapts to.

However it is not always bad for the two diversity channels to be 
combined for best signal.  The example is digital signals which the 
human brain cannot process.  The example was given of using two 
antennas of differing polarity for HF.  The two signals received will 
have orthogonal information which a computer program can process 
(very similar to how I and Q signals are processed).

The phase angle can be recovered and the sum of the two signals 
combined for best SNR signal.  MAP65 does exactly that when fed by 
two diversity Rx tied to the same LO so both phase and intensity info 
reflect the polarity of the received signal.  That's how my 2m 
adaptive polarity receiving system works (and the K3 + KRX3 are 
central in my diversity system).

I know of CW eme operators who use dual-pol diversity Rx and listen 
to the signal using the A/B setup cited.  As the signal changes 
polarity it seems to move around the room (stereo effect).

At present there are two dual-pol diversity receiving systems 
available for eme:
1.  My system that uses the K3 as central Rx, and
2.  The IQ+ which is a 2m dual channel SDR with direct conversion to baseband.

I believe I am the only one in the world currently using the K3 in 
this application (mainly due to lack of dual-channel transverters; I 
have a custom-made DEMI L144-28HP-DRX).

So many ways to enjoy diversity reception!

73, Ed - KL7UW
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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


73, Ed - KL7UW
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