QRP operation over the first geostationary amateur radio satellite

by Wolfgang Buescher, DL4YHF

last updated: July 2019.

When Es’hail-2 / AMSAT Phase 4-A / QO-100 was successfully launched Quatar in early 2019, everyone was in a hurry to get QRV on the new satellite. So was I, and as a QRP and homebrew enthusiast, the mode of choice was CW (Morse code). For an initial test, I used a cheap chinese ADF4351 evaluation board (17 Euros), and a hastily written PIC firmware to program the frequency, and directly drive a multiplexed 7-segment LED display. The eval board was mounted on a surplus 10 MHz OCXO, which is almost as stable as a GPSDO:


ADF4351 board (left), PIC with display (top), dummyload and amp (right).
Click on the image to magnify.

This kind of worked (with "on-off keying" the driver stage, a SKY65162), but the loop filter on the ADF4351 was far from being optimal (actually, what was populated on the board didn't have much in common with the component values shown in the schematics).
Instead of digging in deeper with the ADF4351, I bought a Taiwanese BU-500 transmit converter (configured for 70 cm IF input) at the Ham Radio fair in Friedrichshafen, so I was "theoretically" QRV in SSB now, too.


Inside the BU-500 (432 -> 2400 MHz TX converter).
More on the modifications inside this little box further below.
Click on the image to magnify.

The main problem was none of the windows in my flat had a clear view to the satellite; so the first RX experiments were made outdoors with the usual satellite TV dishes, a modified PLL LNB, and an SDRplay. The first successfull QSO was made in CW with a tiny double-quad antenna held out of the window on a glass-fibre mast (pointing directly towards the satellite, not illuminating the satellite dish).


Linear polarized double-quad antenna with copper-clad PCB as reflector

Even with only 1.5 watts into the above antenna (and again, no dish) from the BU-500 running "barefoot", the signal was audible, but it didn't attract too much attention when calling CQ. For a bit more punch, but still QRP (about S7 on the Goonhilly web SDR) the double-quad was replaced by a 15-turn Helix antenna temporarily mounted on the opened window. Sure this was an interesting sight for the neighbours.. but to my suprise, this RHCP Helix was even ok for a few SSB contacts.


15-turn RHCP Helix tied to the opened window for QO-100 uplink.
Chinese "8 Watt" (in reality 4 Watt) 2.4 GHz amplifier just below the antenna.
Click on the image to magnify.

To avoid the cable loss from 7 meters of Aircell 7 (ca. 4 dB on 2.4 GHz) between the shack and the "antenna window", the BU-500 (in the shack) now feeds a cheap 'WLAN booster' over the long cable. With 1 Watt from the TX converter, less than 0.5 Watt arrive at the input of this amplifier, slightly overdriving it (which is ok for CW, but not for SSB). With 0.5 Watts on the input, mine delivered 4.5 Watts output at best.


Inner guts of the Chinese "8 Watt" (in reality 4 Watt) 2.4 GHz amplifier.
Click on the image to magnify.

This amplifier was modified for 'permanent transmit' by feeding some DC into the RF detector (with 10 kOhm from the internal 6 V supply), and a 'phantom' DC power supply over the coax cable. A similar 'phantom power' injector was also built inside the BU-500, so all that needs to be connected between TX converter (shack) and the antenna is the coax cable, which (at the moment) carries the 12 V DC, and the 2.4 GHz transmit signal.


BU-500 with 12 V DC feed via coax, for the external PA.
Click on the image to magnify.

Maybe in future, the coax between shack and antenna will also carry a 10 MHz reference for the LNB, and the 739 MHz IF signal from the LNB to the SDR (or 739 -> 144 MHz converter). Not sure if the crossover network would still fit inside the PA enclosure, but the goal is to keep everything as small as possible for portable operation.
Inside the BU-500 (near the mixer's input), I installed two anti-parallel Schottky diodes (1N5711) to protect the mixer in case of "operator error" (i.e. forgetting the 12 dB attenuator between the FT-817 and the 432 MHz IF input). Under normal conditions, the voltage across these diodes is way below 400 mV (when they would start to conduct).


BU-500 with extra mixer protection (BAS70-04, dual Schottky diode).
Click on the image to magnify.

Accidentally transmitting into the BU-500 with 3.5 Watts would instantly kill the mixer (RFFC2071) as well as the input attenuator (which is rated for about 200 mW; the absolute maximum rating for the mixer's input power is +15 dBm, according to the RFFC2072 datasheet).

In July 2019, the "Camping dish" was still only used for reception. The next step was inspired by the POTY (Patch Of The Year), but I had problems getting the two dips in the SWR plot - all I could see on the analyser was a broad SWR minimum around 2400 MHz, and a narrow dip at 1900 MHz which was obviously caused by the reflector's resonance.
Also, the RX performance on 10 GHz was at leat 3 dB worse than the original LNB in the focus of the 40-cm "Camping dish". With an 80-cm, or even a 1-meter dish this would matter too much, but for a small dish you don't want to lose a single dB of RX sensitivity.
But mounting a larger dish temporarily wasn't an option (to operate from home), so the story continues...

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