[Test-Equipment] Kahn Units

[Sheldon Daitch]

I hadn't thought about Kahn's SSB system in years.  We had several Kahn 
units
at the Greenville VOA transmitter sites, normally used with the Gates HF-50C
50 kW HF transmitters.  Those 6 units are long gone, and I can only 
guess the
Kahn units were taken out later, as well.

I don't think we ran them as SSB with reduced carrier.  We used them as
one sideband with maybe somewhat reduced or maybe even full carrier.  It
has been well over 20 years since I worked with the Kahn units, and I just
can't remember what kind of carrier power we kept the TX at.  I am 
thinking we
ran the full carrier power.

One of the advantages of using the transmitters in that mode is, IIRC, 
we could
run the "AM" in the utility bands, for our feeder program service.  As a 
normal AM
transmitter, it was not permitted on the frequencies we used.

thanks for that memory jogger.

73
Sheldon
WA4MZZ

Richard Knoppow wrote:

>
> ----- Original Message ----- From: "Gary Schafer" 
> <garyschafer at comcast.net>
> To: "'Discussion of Electronic Test Equipment'" 
> <test-equipment at mailman.qth.net>
> Sent: Tuesday, April 22, 2008 6:35 PM
> Subject: RE: [Test-Equipment] FS: Heath IM4180 FM deviation meter
>
>
>> If you really want to get into it, high quality AM can be generated by
>> properly phasing two FM transmitters together.
>>
>> 73
>> Gary  K4FMX
>>
>    There are several patents on transmitters of this type. The system 
> is known generically as "outphasing" and a Google search will find a 
> good deal of information. One of the earliest patents was issued in 
> France to Chireix and the system is often called by that name. It was 
> used in the 1960's for a high power RCA broadcast transmitter called 
> the Ampliphase but it was not very successful in practice. Outphasing 
> modulation is currently used for some microwave transmitters.
>
>    There is a good deal of technical information about FM on the web 
> and I wrote off list to Charles with some suggestions. Again, a Google 
> search will find both simple and quite advanced treatuses on it. FM 
> seems simple but is really not very intutive. Really understanding it 
> requires some math and aspirins.
>    BTW, in theory FM sidebands extend infinitely, however, the further 
> they are from the center frequency the smaller the amplitude so, 
> beyond a certain limit, they are not significant.
>    A system of narrow band FM was used in the first Ampex video 
> recorders. Its worth looking up because it violated the theory of the 
> time indicating that the theory was wrong.
>
>    All modulation can be generated by combining amplitude and what is 
> sometimes called quadrature modulation. Quadrature modulation is a 
> mathemetical concept and does not exist physically however, it is a 
> component of the analysis of any type of modulation.
>
>    Sometime around the 1960s a fellow named Kahn wrote a number of 
> articles touting a system he had developed for using AM transmitters 
> for SSB by splitting the original signal into AM and FM components. 
> The AM component went into the audio input and the FM component was 
> used for RF drive. When everything is balanced right the AM 
> transmitter acts like a high efficiency linear amplifer. I have not 
> done a patent search but I suspect his patents will show up on a 
> Google Patents search. I believe the military used Kahn adaptors for 
> some applications.
>
>    I've used the Bessel null method of setting FM deviation on 
> satellite uplink transmitters many times. It isn't difficult but 
> requires a good narrow band detector of some sort (a spectrum analyser 
> is ideal) and an accurate knowledge of the audio frequency used for 
> modulation.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk at ix.netcom.com
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