Dear LF Group,
I received broadcasts at 0830 and 1230utc from SAQ yesterday,
using a tuned 3m loop and RA1792 receiver with 300Hz bandwidth.
Both broadcasts were RST 599 QRN. The atmospheric noise level
at 17.2kHz was rather high, but did not really affect copy.
The 136kHz band was quiet; I heard SA6Q at RST 569 shortly after
the 0830 broadcast, but had no reply to my calls; it seemed they
were having some difficulty with reception, although an excellent
signal here.
I also tried 14035kHz, where SA6Q were RST 599, but got no reply
there also - however, my 20m antenna is currently very poor.
I was also able to successfully receive SAQ with a homebrew
electro-mechanical receiver. As far as I know, the Alexanderson
alternator at SAQ is currently the only operating radio station with
an electro-mechanical transmitter that does not rely on valves or
semiconductors. For some time I thought it would be fun to make a
VLF receiver based on similar principles, also without any valves
or semiconductors, to receive the SAQ broadcasts. At first, I
thought this would involve some difficult mechanical engineering,
but somewhat suprisingly I was eventually able to make such a
receiver using parts from the junk box.
It is basically a direct conversion receiver. It has a 2 pole passive
preselector with RF bandwidth of about 800Hz, which also
impedance matches the antenna to the mixer. The mixer uses
saturating ferrite cores, in a way not unlike the SAQ keying circuit.
The BFO signal saturates the cores at both the positive and
negative peaks of it's waveform, and so the core windings present
an inductance that varies at twice the BFO frequency, in series
with the signal path.
The BFO signal is produced by a small high frequency alternator.
This is actually a 200 step/revolution stepper motor, salvaged from
a scrap hard disk drive. When driven as a generator, it produces a
reasonable sine wave - each winding produces a signal in phase
quadrature with the other - with 100 cycles for each revolution. So
as an HF alternator, it produces quite high frequencies when
rotating at moderate speed. Driven at 4800 rpm by a DC servo
motor, it generates a couple of watts at 8kHz, which effectively
gives a 16kHz BFO signal, due to the frequency doubling action of
the mixer. This "oscillator" is connected to the rest of the circuit
with long cables, because of the mechanical noise it produces!
The resulting audio beat frequency at 1.2kHz is low pass filtered,
and applied to a pair of 1944 vintage 'DLR no.5' headphones. With
the two earpieces in parallel, and a series capacitor of 880nF,
these are series resonant at 1.2kHz and close to 50 ohms
impedance. They are suprisingly sensitive - an audio signal -
100dBm (2.2uV into 50 ohms) is just about audible in a quiet room.
So although the RX has no amplification, in fact considerable loss,
a 30uV signal at 17.2kHz from a signal generator can be detected
at the mixer input.
The SAQ broadcasts on Sunday were received using this RX with
my 8m high, 40m long inverted L antenna, which produced a
comfortably audible signal in the headphones. In fact, the limit on
sensitivity was the QRN, which was quite strong. Later, I re-tuned
the RX to 18.2kHz, and was able to copy the VTX3 Indian navy
station, although this was considerably weaker than SAQ.
I also recorded the SAQ brodcasts on tape; listening to them, the
main defect of the electro-mechanical receiver is poor frequency
stability - the motor currently has no feedback speed contol, and so
the "oscillator" frequency varies by about +/- 1% - not a problem as
far as copying goes, but does sound a bit wobbly!
As far as I know, this is probably the only receiver of it's type in
existence, and perhaps the only entirely electro-mechanical radio
transmission and reception since the 1920s - but I would be very
interested to hear if anyone knows of anything similar.
Cheers, Jim Moritz
73 de M0BMU
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