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Monday, 29 August 2016

Doppler, Frequency Inversion, What's all that then?


As I fiddle, I learn. That's the basic idea anyhow.

So, I've been making a few QSOs through these amateur satellite thingamagigs.

There are a series of Japanese birds called the XW-2 series. Their frequency allocation is as follows:

Now, the number on the far left top of that image is wrong and should read 435.000.

I just had a QSO through XW-2C, the linear transponder (the bit that receives speech on the uplink and re-transmits it on the downlink) are actualy at the following frequency ranges:

Uplink:       435,150 -> 435,170
Downlink:  145,815 -> 145,795

Now, the observant amongst you will note that the Downlink frequency I have listed goes from high to low and the Uplink from low to high. Thats because it's a linear transponder with Spectrum Inversion. So as I tune in the 20KHz passband of the satellite I have to tune +ve in the Uplink and -ve in the downlink (or the other way round).

So when the Uplink is at 435,150, the Downlink is at 145, 815 and when Up is at 435,170 Down is at 145,795. Thats all before you add in the complexity that the Uplink is in LSB and the Downlink in USB. And then theres the further complexity of Doppler.

Now, I like to think of things very simply. If you can imagine a Sine Wave shape, with the right hand end fixed and the left hand end moving left, it is quite easy to see that the frequency is being stretched and is therefore decreasing. Similarly, if the left hand end of our imaginary sine wave was being moved to the right, the wave is being squashed and the frequency is increasing.

Therefore as the fixed point (my antenna) and the satellite (moving) get closer together the frequency that I need to tune to receive the Downlink and transmit to find the Uplink are both increasing, similarly when the bird is travelling away from me, the frequency is decreasing. The effect of this change is dependent on the actual frequency itself, so the higher the frequency the more pronounced the shift needed.

Luckily the Sat32PC software calculates the Doppler for me, but I hope the rather simplified explanation above helps explain whats actually going on.

Fun egh?

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