Hi James, Rik, Alberto and the group,
Yes an impressive distance.. even though the ERP was a bit above the
usual value for amateur radio on 13 cm. AMSAT-DL has applied for, and
been granted, a special permission for this experiment which expires at
the end of this year.
The *principle* was simple:
- send a 5 minute carrier on 2.45x GHz, then
- receive for 5 minutes, remove impulse noise (WLAN), run the signal
through an FFT with ~11 Hz equivalent receiver bandwidth
- add all received power spectra to calculate an average (*)
- repeat the above until the sigma (standard deviation) in the frequency
bins gets low enough to see the weak reflection in one of those bins.
- while transmitting, keep the frequency constant to one or two Hz (not
as easy as it sounds)
- while receiving, permanently compensate the Doppler shift (which a
marvellous piece of software by G3RUH did)
- during the experiment, keep the dish pointed to Venus with an accuracy
better than 0.1° (which another piece of software by G3RUH did)
- hope that the 5-kW magnetron, and the cooling system (including a
vaccum cleaner and a water cooler from a VW Polo) holds..
(*) A single FFT is not sufficient to lift the signal out of the noise.
It was between 8 and 11 dB below the noise in a 10 Hz bandwidth; the
ratio of <signal plus noise> to <noise> was between 0.3 and 0.4 dB; the
latter measured today when it wasn't raining. This was sufficient to
"see" the signal after 2 minutes, so we didn't have to average over
consecutive 5-minute-receive intervals.
Each dot of the "HI"-message lasted 5 minutes, because that's the
approximate travelling time... in other words, "QRSS300" dictated by
Venus ;-)
Details, including the "Magnetron taming" circuit by DJ4ZC will appear
in English language on www.amsat-dl.org hopefully soon. This vital part
of the system includes 3 PLL circuits, a servo motor controller for the
coarse frequency control (slow mechanical tuning, 6 MHz, on the output
waveguide), and a fast controlling loop with about 1.5 MHz (iirc) loop
bandwidth which involves 2 * 4CX1000 for the 'faster' frequency control,
which also provides the SSB modulation. In fact, the energy not "used"
for the SSB signal at a given instance is 'moved away' (spread out) by
15 kHz to both sides of the center frequency. Maybe some of these
principles can be used in class-D amplifiers for other frequencies,
which finally takes us back to LF / MF-related topics <g> .
Cheers,
Wolf DL4YHF .
James Moritz schrieb:
Dear Wolf, LF Group,
Congratulations on an excellent achievement - I expect it has broken a
few DX records! I will be interested to hear more about how it was
done, when information becomes available.
Cheers, Jim Moritz
73 de M0BMU
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