[Hallicrafters] DD-1 Audio amplifier


rbethman rbethman at comcast.net
Fri Feb 24 16:21:21 EST 2012


Long before the SP-600-JX series, there was a Northern Radio Diversity 
version of the SP-600 J-11(No X), that was on a Navy '51 contract.  It 
required two receivers and also the Northern Radio LMO to keep both 
receivers in sync frequency wise.

I've got one, I've seen a few others.  One could select either the 
internal LO or the external LMO.

Bob - N0DGN

On 2/24/2012 3:50 PM, Richard Knoppow wrote:
>
>       Fascinating!  Its possible to adapt many receivers for
> diversity. You need two of the same receiver and must
> provide access to the AVC and detector load. The SP-600-JX,
> especially the JX-17, came with the necessary connections.
> The JX-17 allows the use of an external LO source which
> keeps both (or more) receivers synchronized. Its helpful if
> the overall gain can be trimmed so that one receiver does
> not dominate the combination. There are special diversity
> combiners which provide more sophisticated switching from
> one to another signal. The crude arrangement possible with
> the SP-600 simply ties the two AVC busses and detectors
> together, which ever has the loudest signal predominates but
> the other signal is still present. A true diversity combiner
> switches from one to the other.
>       The article above gives a good survey of the history of
> diversity systems but I think does not mention some other
> anti-fading techniques such as the steerable antennas
> explored by Bell Labs. These were based on the discovery
> that the vertical angle of incoming radio signals varied
> with time so that a significant improvement in reliability
> could be achieved by making an antenna whose vertical angle
> could be steered to follow it. I am not sure these "MUSA"
> antennas were very widely used but they were described in
> detail in the Bell System Technical Journal and, I think, in
> the Journal of the IRE. I don't have citations readily
> available but suspect a Google search might find some.
>
>
> --
> Richard Knoppow
>


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