24GHz EME Reception Results
Having had reasonable success with our 10ft dish receiving the 24GHz signals from AO-40, we temporarily dismantled the AO-40 equipment and put the transverter back on 24192, in readiness for a 24GHz EME test organised by Barry VE4MA and Al W5LUA on 10 October. [These two stations were the first to complete the first 24GHz EME QSO a few months ago, and they had decided to run some more tests and invite stations to listen].
We also built a new feed for the dish, based on the VE4MA choke ring feed, but fed with waveguide. The feed is identical to that used for AO-40, except that no polariser was used and the conical horn was replaced with a choke ring.
The hours before the EME test were spent optimising the position of the feed for best sun noise. The position was quite critical - with the feed a few mm away from optimum the sun noise dropped by a dB!. After an hour or so no further improvement could be obtained and we ended up with 13dB of sun noise (at an elevation of 19 degrees with a thin overcast).
Our dish mount does not allow elevations above 34.5 degrees, and we had to wait until 5 minutes before the test to see what level of moon noise could be achieved. The moon appeared right on queue, and once it was fully into the beam we measured ~ 1.8dB. This is about the same as we see on 10GHz, and tracking thereafter was very easy, just keeping the moon noise peaked!
With G4KGC on the receiver and G3WDG handling the tracking, we were pleased to find VE4MA's signal quite easily, a few seconds into his transmit period. During the 30 minute sked, we copied both stations, VE4MA at "O" level and W5LUA at "M" level, and heard them complete their second QSO of the morning.
We watched their signals on the computer also using the AE4JY AO40rcv software, which has an audio spectrum analyser display with waterfall. The capture below is VE4MA's signal, and clearly shows the libration spreading widening the signal (a clear tone would show up as a narrow line).
The corresponding audio can be downloaded from here (860kB). The signal can be heard being tuned from HF to LF, followed by a more or less constant pitch carrier and then (in cw) W5LUA de VE4MA (repeated) followed by MMMM..
Both Barry and Al run about 80W output on 24GHz (from TWTs). It is not clear why so much power is needed on 24GHz - the link budget indicates that a few watts should be sufficient. Sun noise/moon noise suggests that the additional atmospheric path loss is not that high. Perhaps it is something to do with antenna beamwidths being smaller than the moon, or the effects of the libration spreading making the signals sound weaker. Anyone got a spare QRO TWT for loan??!