KickSat Ground Station

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I am writing this at about two weeks’ before the Kickstat Sprites are launched (day of creation – 25 February 2014).

The Sprites each have an rf power of 10 mW and will only operate in sunlight. Characteristically they have a 60 kHz bandwidth and so narrow band receivers are not any use to receive them Therefore, the Rx of choice is a software defined receiver. The frequency is 437.240 MHz.

In this example I am using a Fun Cube Dongle Pro Plus. The antenna is a 70 cm helix from WiMo, and the mast preamplifier is the AMSAT-NA broadband one.

General technique

I do not have an az/el rotator and have always adopted the technique of predicting where the azimuth will be where the elevation is at its highest – some texts call this the Apex – and pointing the antenna at this point in the sky, knowing that the signal will be at its maximum there. The downside of this is that the doppler is at its maximum at this point.

With some orbits I use a variation of this technique – knowing my beamwidth is say 30 degrees I’ll align it with the path of an approaching satellite and the bird will “fly” into or across my beamwidth. This offers reduced doppler, but weaker signals.

It works for me.

Practical example using $50SAT

$50sat is a tiny satellite with a tx power of 100mW. Its transmitter is nominally on 437.5 MHz.  So it’s a good stepping stone to the Sprites.

In Figure 1 below I extract from the excellent primer by Martin Davidoff K2UBC, The Satellite Experimenter’s Handbook. This is his diagram showing a simple waterprioofing. As you can see, we had a rain shower just before the reception reported below.

Figure 1: Using an open container to protect from water

I used the AMSAT-NA broadband amplifier and enclosed it in an inverted plastic glass. I also sealed the joints on the box with bathroom sealant.

Figure 2: Practical implementation

I brought the 12 volt connectors out onto the garden table and fitted an ordinary 12 v battery here. This one was surplus from a burglar alarm.

Figure 2: battery to power the preamp

The next step was to plan the observation. I used a web source for this. It predicted that the “Apex” would be at an elavation of 74 degrees, at azimuth 349 degrees,  11:14 UTC today.

Figure 3: Web prediction

So I pointed my antenna to this point in the sky. I have a 30 degree beamwidth so it didn’t have to be that exact.

Figure 4: Guesstimate of elevation

Now to the shack and running SDR# with the dongle installed. On the left you can see that the waterfall is brighter when I connected the preamplifier outside. You might also notice some lines – this is noise. It’s wise to establish what is noise before the satellite comes over. In some locations, you’ll find that pagers or taxi radios will interfere with your reception of the weak signals from satellites. in that case you’ll need a stage of filtering BEFORE the preamp, and the whole filter-and-preamp combo in a metal box (Faraday cage).

Figure 5: Amplifier on

Then it’s  question of sitting back and waiting for the satellite to overfly. Figure 6 shows a data burst from $50dollar sat at exactly the time of maximum elevation. Notice that some of the burst is yellow, some even red, indicating higher strength. And – notice that the frequency in the display isn’t the nominal frequency of the satellite. The reason for this is that I hadn’t calibrated the dongle beforehand.

Figure 6: Successful reception of 100 mW satellite $50dollarsat

Figure 7. QSL card from contact

Inexpensive RF amplifier

I have adapted a Chinese CATV amplifier for possible use in the Kickstart project.

NOTE: IT IS DANGEROUS TO USE MAINS ELECTRICITY OUTSIDE – DO NOT DO IT!

DISCONNECT BEFORE OPENING THE CASE!

So because it was disconnected from everything I opened the case, and this is what I found inside:

There is just a simple transformer and dide matrix (four diodes). The input sockets were F sockets as used in satellite TV installations. The attraction for this is that it’s relatively easy to use rigid cable at up to 1.4 GHz (the amp is rated up to 800 MHz).

It contans a single 2SC3355 transistor which might go up to about 1 Ghz.

So I unsoldered the transformer and removed the dides so that it would just take a battery.

At first  I also took off the F connectors intending to fit BNC or N or whatever.

Looking at the datasheet the transistor didn’t like voltages above 12 volts. The box rated the amp at 1 watt at 220v AC so I thought, well, the current’s going to be very small. So I decided to power it from a 9v block battery, the sort that powers up a smoke alarm, costs £1 for one alkaline battery in Poundland.

Hey! The LED lights up! But it was getting late so I put two F connectors back and screwed it back together.

The importance of this? I can now strap it to a handheld antenna with velcro tape (yes, a bag of it costs just £1 from Poundland) with the 9v battery AND IT’S SAFE.

You can see similar-looking CATV amplifiers from China at under  a fiver on ebay. I haven’t opened one up but you can see the principle.

Disclaimer: MOD AT YOUR OWN RISK. DON’T MESS WITH MAINS ELECTRICS.