Topband: Diversity Reception on 160

[Tom Rauch]

> With SSB and CW, which use a heterodyne detector that mixes the signal
> with a local oscillator, the phase of the audio output at the detector
> is a direct function of the phase of the rf signal at the antenna.  If
> you try to combine the audio from the two receivers, there will be
> phase cancellations, so that adjustment of rf phase becomes extremely
> critical, much like adjusting the phases of the noise and signal
> antennas on the MFJ or similar noise canceller.  Has anyone figured
> out a way around this difficulty?

That's an excellent understanding and description of the problem Don.

I'm not sure how that would be satisfied using two separate VCO's 
that are "dial locked", as happens in the FT1000 when one VFO dial 
adjusts two totally different synthesizers by using the same digital 
"word". Phase problems would also be compounded when the filters are 
not exactly identical.

I track my R4C's (that are driven from common oscillators for carrier 
and injection) by looking at them on a spectrum display and 
overlapping the passbands. Of course I hand-select filters that are 
as similar as possible in shape and phase delays.

The saving grace in systems where little attention is paid to phase 
detail is that our ears, listening in stereo, really don't detect 
phase well (if at all). While it is better to have everything the 
same, we can get away with some surprisingly large phase 
imperfections. 

In my own experience, which certainly does not mean it applies to 
everyone, I find exact phase and identical passband phase 
distribution are clearly worth it on noise floor signals. The effect 
I hear is that noise has an unusual hollow resonance, and a musical 
difference in that hollow rushing sound is a very weak signal. It is 
very interesting to listen to the effect with smooth rushing band 
noise in winter as I switch from close spaced to wide spaced 
antennas. Close spaced (under two wavelengths or so) antennas do not 
help me any significant amount in stereo. 

Of course they can help if I combine them in mono. Unfortunately 
combining them in  mono means I have to ride a receiver phase 
control constantly, to make up for phase shift in the signal at both 
antennas.

It is an odd effect to be able to totally null a signal out while 
adjusting the receiver filter tracking only the smallest amount, and 
add 1/4 wl of transmission line in an antenna feedline and have it 
reappear!!

73, Tom W8JI
W8JI@contesting.com