Topband: Re: diversity rx

[Tom Norris]

Works well with RTTY too! There were several multi-rx diversity
systems out there using the R-390/390A that were just *for*
that. Dual diversity was used with the GRC-26, GRC-129, FRR-3,
FRR-33, 38, 39,41 and others. The FRR-34 used *three* R-390's
for triple diversity. I was using a full duplex, dual diversity
receive Harris station in the 90's as part of the FEMA FNARS
system - it used a Frederick RF modem that received its input
from 2 Harris RF-590A receivers that were fed from a crossed
dipole array. Larger point to point systems typically used a pair
rhombics spaced wavelengths apart for "space diversity"
reception.

HAL still sells the ST-8000 rf modem (TU) for use in
dual diversity data systems. Harris, Racal and others
offer similar models. While most of the data is gone
from HF (compared to years ago) many of the wideband
digital systems that are out there now still rely on
diversity reception.

But.... while all this is well and good, I don't see any
real need for diversity systems for CW, nor do I
have I found much in the way of historical info on
such systems. Nearly all are digital data based.

I do use a pair of R-390A's here at home with
a crossed dipole for AM when I want to listen
to shortwave broadcast without fading. Not
much to listen to in that area these days either,
so it is only used every now and again for 75 and
160 AM reception. (And a bit of AM broadcast)

>
>Today's Topics:
>
>    1. RE: diversity rx (Donald Chester)
>
>
>>I read the great article on the Hallicrafters DD-1 diversity receiver.
>>Apparently they did not have agc systems fast enough to respond to CW
>>and they had to do some strange stuff to get CW diversity reception.
>>
>
>The diversity receiver would work only on AM.  On CW (or SSB) the 
>phase of the audio output at the detector at each channel is a 
>direct function of the rf phase of the incoming signal, so the two 
>signals mix together and produce additions and cancellations that 
>completely nullify the diversity action.  Even when tuned to the 
>same signal, the rf phases of the incoming signal from two different 
>receiving antennas and separate tuned circuits in the rf stages of 
>each receiver, will not be the same.  On AM, the phase of the audio 
>output at the detector is unrelated to the phase of the incoming rf, 
>so the audio outputs at the two detector stages can be combined and 
>the agc action of the stronger of the two signals will reduce the 
>gain of both channels simultaneously so that the channel with the 
>stronger signal dominates.  The agc lines of both receivers are tied 
>together and the audio outputs are combined and fed into a common 
>audio output amplifier.
>
>The R-390 or R-390A have provisions to be similarly set up for 
>diversity reception, using two receivers with the added 
>inconvenience of having to dial up the same frequency on both 
>receivers.  The old Hallicrafters diversity receiver used a common 
>HFO to feed both 1st detectors, so there was only one tuning dial.
>
>The difficulty in receiving diversity cw on those receivers does not 
>lie with the AGC.
>
>Don K4KYV
>