[Hallicrafters] DD-1 Audio amplifier

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; "Duane 
Fischer, W8DBF" <dfischer at usol.com>; "Waldo Magnuson" 
<magnuson at mac.com>; <Hallicrafters at mailman.qth.net>
Sent: Friday, February 24, 2012 12:34 PM
Subject: Re: [Hallicrafters] DD-1 Audio amplifier


Here's a fascinating article on the DD-1:

http://www.radioblvd.com/DiversityDD1.html

Joe Connor


     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
Los Angeles
WB6KBL
dickburk at ix.netcom.com