Topband: 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

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