Alphasat

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Receiving the Alphasat Beacon on 39 GHz

 

After interest in millimetre wave EME is tried to receive the Q-band beacon from Alphasat at 39.402 GHz

Some data from the available beacons
 
  Table summarizes the Alphasat beacon characteristics.

Parameter  Ka-band Q-band
Frequency 19.701 GHz 39.402 GHz
Polarization linear vertical linear tilted NW-SE 45
Antenna boresight 32.5 N, 20 E 45.4 N, 9.5 E
EIRP (Effective Isotropic Radiated Power) 19.5 dBW 26.5 dBW

My initial idea to try and receive the beacon was to use a Witlink 2000 microwave link receiver and test to receive the beacon.

I tried a setup able to receive between 38.7 and 39.5 GHz so quite usable for this experiment.

After connecting the Witlink 2000 to power supply I found that the unit had an output at 141 MHz but this was only 5 MHz wide.
I tried to find at what frequency the unit was working and found fast that is was not the right frequency.
I needed to find a way to change the LO frequency but this is a software setting which I cannot handle up to now.
I have made some arrangements that I expect to be able to set the frequency of the Witlink unit within some weeks time.

After this fail I tried to disconnect the first down converter and see at what IF the expected frequency would be, a measurement using the harmonics of my 18 GHz synthesizer learned me that the IF for receiving the Alphasat beacon would be around 3410 MHz. So that was easy and I became the idea that for the time being the best way to proceed is using a available converter for 9 cm to 2 meter to get an IF of 154 MHz.

An initial test with the 18 GHz synthesizer worked out fine and I could receive the frequency of the beacon. Some more mechanical preparation was done and then I had to wait for some good weather to try life receive of Alphasat.

On 5 February 2020 weather was nice and I mounted the Witlink unit on my mast.

For receiving Alphasat I need an elevation of 27.5 degrees and an azimuth of 157.8 degrees.

With help of some nice app's on my Iphone I adjusted the dish as good as possible to the needed heading and I tried to find a signal using a spectrum analyzer, but nothing was found.

I then started turning the dish up and down and later right and left and after about 15 minutes I found a signal.

Optimalisation of azimuth and elevation took me lot longer

The receive system is the standard Witlink front end so a lot of extra losses between antenna and the pre amp.

Perhaps I will rebuild a unit special for receiving Alphasat and make some long term measurements of received signal strength

Listen to the signal in a 3 kHz bandwidth SSB filter


Interesting measurement result: 20-2-2020

During the monitoring of the Alphasat beacon I found that the received frequency was having quite some doppler on it
This doppler is caused by the Orbit of the satellite

I measured the received signal for a few days and it is obvious that the orbit is a daily repetitive pattern

I measured this using SBSpectrum, there is some ripple on the measured frequency,
this is caused by the down converter LO stability and the temperature variations from the LO

It is obvious that around 3 hr and 15 hr the orbit is returning, the satellite drifts a eight form orbit through hemisphere every day

The scale of this measurement: the vertical scale is in total 4800 Hz wide

In total I measured 4266 Hz doppler

this is much more than published and therefore it's obvious that the Alphasat is in inclined orbit due to end of life period of the satellite

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