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Saturday, February 27, 2016

Sputnik Replica Transmitter, an "Error" in the Sputnik Schematic, and Why 20.005 MHz?


Mark K6HX pointed me to very interesting Hackaday article on Frank PA3CNO's Sputnik transmitter replica.  As blog readers will recall, we went through a period of Sputnik-mania a few years ago:  http://soldersmoke.blogspot.com/search?q=sputnik  Chief Designer Comrade Mikhail Rainey AA1TJ sent me some of the Russian tubes (like those pictured above). 

The Hackaday article pointed to our post reporting that Oleg RV3GM had found the schematic:
http://soldersmoke.blogspot.com/2013/04/sputnik-schematic-found.html   Stefan reports that PA3CNO found "an error" in the original Soviet schematic:
http://www.radio.cc/post/Franks-power-supply-for-sputnik    A mistake you say?   HAH, I say!  Hah!  This must have been part of a sinister commie plot to prevent the capitalist imperialists from ever being able to reproduce the glorious transmitter of the Soviet people.  They almost succeeded. 

Just kidding.  

In the course of looking through our old Sputnik posts, I came across a question I posted:

I have a question: OK so the crafty Soviets picked 20.005 MHz for some good reasons: Being so close to the WWV freq, it would be easy for hams and SWLs to find it with precision. In the November/December 2007 issue of "Break In" (from NZ -- thanks Jonathan-san!) ZL3DW notes that this frequency selection would allow a receiver set to exactly 20 MHz to "produce an audio tone plus or minus the Doppler shift without ever going through zero beat." But zero beat with what? Most of the receivers out there would not have had BFOs, right? So the Soviets wouldn't have been using ordinary CW, right? Were they using AM, with the beeps produced by an audio oscillator modulating the carrier? 

Was their diabolical plan to use WWV as the BFO for those using ordinary AM SW receivers?   If so, a 5 kHz separation from WWV seems to be too much right?  Especially when the Doppler shift on approach would push the frequency up a bit. Maybe they just chose this freq to make it easy for listeners to find -- just a bit above WWV.  Comrade Rainey surmised that they were keying the PA stage -- the oscillator "backwave" was at times audible on the ground.

What do you think Comrades?
DSW and 73.

1 comment:

  1. And yet, there was a time when when some stereo receivers would include a 10KHz notch filter so you couldn't have a problem from the next AM broadcast station over, 10KHz away.

    So 5KHz doesn't seem too bad. It has the advantage that if you didn't have a good receiver, the WWV signal would present not only an accurate frequency, but a stable one. That was a trick in the early days of SSB, front end injection, since if you could build a stable oscillator (or use a BC-221 frequency meter), injecting it at the signal frequency meant that even the receiver's local oscillator drift didn't come into play (other than needed to retune it if the signal got out of the passband. The beat between the desired signal and the injected frequency would be constant so long as both were stable.

    Though, most receivers that tuned 20MHz did have a BFO. Living room radios that happened to tune the shortwave bands tended to stop about 15 or so MHz (a band tended to be a 2:1 tuning range, so they left off the top half of the "shortwave band". Less happening up there, and it's harder to reduce images, and a receiver would generally be less stable the higher up you went. Even "communication receivers" like the Hallicrafters S-38 had bad sensitivity, bad image rejection and less stable operation on the top band.

    And yes, WWV would make it easy to find SPutnik, though I don't know if that figured into the USSR's plans.

    Read up on the Kettering Group, based at an elementary school in Britain, they started tracking satellites early on, and later became a reliable source of new satellites in orbit.

    Michael

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