[R-390] Kahn SSB Converter

Thu May 8 04:11:25 EDT 2008

Harold,

US Army STRATCOM was a big user of ISB equipment up into
the 1970s, for long haul communications.

ISB, independent side band, only means the upper sideband
and the lower sideband are transmitted, but normally do
not carry the same audio.

A few ISB transmitters used or considered by STRATCOM
included the AN/FRT-52, -54, -57, -76 and -77. I suspect
there are others, but for the most part, I believe the
Army is out of the ISB business. The Navy is probably
the biggest user of HF transmitters, maybe even more than
the USAF, especially higher power units.

If memory serves me correctly, the CV-157
is an ISB demodulator, with two audio outputs.. Note the
R-390 series radios have a 16 kHz bandwidth selection, no
doubt for use on ISB circuits, running four voice-frequency comms
circuits.

ISB is almost always a point-to-point communications
system, fixed station configuration, and probably very
little in the way of two way communications as we think
of it in the typical radio world.

VOA used ISB in a one way circuit configuration, as
program feeder service, with one program on USB and
another program on LSB. Most other users of ISB used
the systems for point-to-point telephone communications,
with either two or four voice-frequency wide circuits.
Obviously, this required transmitters and receivers at
each end of the circuit.

Leonard Kahn was a pioneer in alternative transmitter
techniques, especially in the world of AM stereo and audio
processing. Kahn has an interesting history and a Google
search on his name brings up quite a bit of reading material.

I am going to go out on a limb, because I don't know
specifically how the subject Kahn SSB converter works,
but I don't think it is anything truly special. It is
simply an ISB converter, probably IF driven, and when
the receiver is tuned to an ISB transmitter frequency,
demodulates both USB and LSB and feeds each to an
audio channel.  Again, without looking over the manual
for the unit, I don't know if it specifically requires a pilot
carrier or not.   Going back to the CV-157 converter,
it does use a pilot carrier to drive the mechanical AFC
circuit, a phase comparator driving a motor which turns
a variable capacitor.  If the receiver or the transmitter
drifts in frequency, the comparator detects a shift, and
drives the motor in the proper direction to turn the
capacitor, putting the receiver system back on frequency.

Kahn did make equipment designed to allow an AM
transmitter to operate as an SSB transmitter with full
or near full carrier. As we know, the ideal SSB
transmitter has no residual carrier and no sideband
energy in the unwanted sideband. Kahn's adaptor allowed
an AM transmitter to transmit what was essentially one
side band with full carrier. The power amplifier was
still operated at a Class C amplifier with no modification.

Way back in the dark ages, when VOA was running the feeder
service, we had three Kahn or copycat units used with the
Gates HF-50C, 50 kW transmitters at each of the two North
Carolina sites. It was my understanding with the Kahn units,
we could operate these transmitters in the utility frequency
bands, for program service, rather than using the regular HF
broadcast frequenies for feeder service   These Kahn and
Gates transmitter combinations weren't quite as handy as the
ISB transmitters, since we could only run one program at
a time, vice two with the real ISB transmitters.

Bear in mind, this was in the days before we really got into
satellite circuit program distribution to the overseas
transmitter stations, and the few overseas
cable circuits we did have were quite expensive. The major
programs did go on the cable circuits, but many programs
used only HF relay to get the programs to the overseas
stations.

I hope this adds a little to the discussion.

73
Sheldon
WA4MZZ

Harold Hairston wrote:

> All very interesting! Is this type equipment still in operation? What 
> type of communications wold utalize this mode. Who was/is Kahn? I 
> assume the era was 60's - 70's??? I have been around the loop a couple 
> of times, including Army Sig Corps service and never heard of the mode 
> or Mr. Kahn. Maybe too new for me.
>
> Harold
>
> ----- Original Message ----- From: "Bob Camp" <ham at cq.nu>
> To: "rbethman" <rbethman at comcast.net>
> Cc: <R-390 at mailman.qth.net>
> Sent: Wednesday, May 07, 2008 7:45 AM
> Subject: Re: [R-390] Kahn SSB Converter
>
>
> Hi
>
> Lots of people use this kind of stuff.
>
> The Airphone system started out with a Kahn SSB setup using a -20 db
> suppressed carrier at UHF. The FCC rejected it as not being spectrally
> efficient enough. Had to go back and put a pilot tone in the middle of
> the audio to make them happy. Thank goodness it didn't change my end
> of it though ....
>
> Bob
>
> On May 7, 2008, at 7:35 AM, rbethman wrote:
>
>> Sheldon,
>>
>> Thanks for the correction.
>>
>> I was trying to convey that the normal every day Amateur Radio  
>> Operator is NOT equipped to work this mode.
>>
>> Now that is NOT to say that you can *hear* some interesting 
>> communications!  Just DON'T repeat what you hear to a third party!
>>
>> They put the Electronic Communications Privacy Act into effect  
>> starting in 1986.  They KEEP modifying it.
>>
>> Net result - It IS a Federal offense to repeat to a NON-Participant  
>> of the communications to a NON participant - EVEN if you DO monitor  it.
>>
>> Bloody rules!
>>
>> Bob - N0DGN
>>
>> Sheldon Daitch wrote:
>>
>>> With all due respect, ISB was not unique to the US military.
>>>
>>> The VOA ran ISB transmitters at both Greenville transmitter sites,
>>> and I am almost sure the same for Delano and perhaps Dixon, as
>>> well.  I never worked at Delano but my notes indicate #9 and #10
>>> at Delano were Continental 617As
>>>
>>> When we were still in the HF relay operations, there were three
>>> transmitters at both Greenville sites running ISB for our program  
>>> feeds.
>>> Transmitters #10 and #11 at both Greenville sites were TMC GPT-40s
>>> and #9 at both sites were Continental 617As.   All the exciters at 
>>> Greenville were
>>> TMC MMX-2 units, capable of running ISB and were the wide band
>>> audio options, with 5-6 kHz wide filters.  Don't hold me to the exact
>>> bandwidth, as I don't have a TMC manual handy.
>>>
>>> The TMCs at Greenville originally used one of the tubed versions of 
>>> TMC's
>>> ISB exciter systems.  The equipment was in the rack in 1979, when I 
>>> started
>>> working there, but the MMX-2 was the operational exciter.  Last  
>>> time I was
>>> in Greenville, 2005, maybe, all the old tube tube exciter units  
>>> were gone
>>> with only the MMX-2s in place.
>>>
>>> 73
>>> Sheldon
>>> WA4MZZ
>>>
>>>
>>>
>>>
>>> rbethman wrote:
>>>
>>>> It uses nuvistors.
>>>>
>>>> It is NOT a "NOT" a plain Jane SSB converter.  Its purpose was to 
>>>> receive ISB, (Independent SideBand),  a unique method used by the  
>>>> U.S. Military.
>>>>
>>>> Bob - N0DGN
>>>>
>>>>
>>>
>>
>> -- 
>> Bob Bethman/NØDGN/BC-610/Northern Radio SP-600 Manassas, VA -  
>> MultiBand Dipoles
>>
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