QRP operation over the first geostationary amateur radio satellite

by Wolfgang Buescher, DL4YHF

last updated: December 2019.

Contents

  1. Initial QRP tests in spring/summer 2019
  2. A tiny, light-weight homebrew mesh dish

1. Initial QRP tests in spring/summer 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, controlled via rotary encoder, 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 least 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...

2. A tiny, light-weight homebrew mesh dish

Because a 13 cm PA module (from Poland) spent several weeks 'in transit' over Christmas, and the trusty old Helix was a bit weak in SSB (compared with other users), it was time for yet another antenna for the uplink, with a bit more gain.
First, a mesh dish was built entirely from parts from the DIY store, just small enough to be passed through a 60 cm wide attic window. Thus, a template for a 60-cm diameter parabolic prime-focus dish with 25 cm focal length was drawn on a piece of thick cardboard (which was then spray-painted to improve stiffness).


Light-weight mesh antenna, 60 cm diameter with backfire helix feed.
Click on the image to magnify.

Formula for the parabola (snippet from a CalcEd script; for a given diameter D and focal length F; H = vertical height of the reflector, all inputs in meters):

D := 0.6       // input: diameter
F := 0.4*D     // input: focal length for given F/D (here: 0.4)
H := D^2/(16*F)
F =:  0.24     // calculated focal length for the given F/D ratio
H =: 0.09375   // calculated height or 'Depth' of the dish in meters
@def y(x)=x^2 / (4*F)  // parabola formula 
@plot( X:=0...0.31, y(X) )  // plot HALF of the parabola, X=0=center, mirror the rest
Using the cardboard template, a 'skeleton' was built from widely available aluminium material, then covered with eight sectors of aluminium 'stretch metal' (in German: Aluminium-Streckmetall / Streckgitter aus dem Baumarkt). This 'stretch metal' can be shaped into concave or convex objects, which is almost impossible with other materials (like perforated sheet metal / "Lochblech").
Since Helix antennas (including helical feeds) are easily fabricated without precision machinery, the first feed for the dish (shown above) was a bifillar Helix as described by G8HAJ (see Helix Feed for 13cm Es'Hail uplink on Graham's site, but don't miss the important note about the winding sense. For RHCP after reflection on the dish, the backfire helix must be wound like a right hand thread).
Here is the double helix used in DL4YHF's light-weight dish:


Double helix for a 13 cm 'deep dish' feed.
Click on images to magnify.

The double helix was 'threaded' (geschraubt) into two thin 'SMD IC' tubes to hold the turns of the 1.6 mm enamelled copper wire in place. Details about the balun / impedance match on the G8HAJ website - see link above.
Because with the feed in place, I couldn't pass the antenna through the attic window, the quarterwave balun now ends at a short piece of semi-rigid cable with SMA connector, so the (fragile) feed can easily be separated from the rest of the antenna.
On the very first test, the light-weight dish provided about 6 to 8 dB more gain than the 15-turn RHCP helix.
The driving level into the Chinese "8 Watt" WLAN PA could be reduced to 2 dB below the compression point, which significantly improved the signal quality, and imho was now acceptable for rag-chewing in SSB (besides CW).
For such small prime-focus dishes, the "backfire helix" in the focal point doesn't obstruct as much space as for example a POTY or a classic Helix with reflector plate would.
If you have a 1.2 meter prime focus dish (or even more), the area obstructed by the feed is neglectable, but for a 60 cm (or even smaller) prime focus dish, it's not.
So far, I'm very pleased by the performance of this antenna, and maybe I will build another one that can easily be disassembled into 4 or 8 parts (segments) for storage in a suitcase or backpack.
If the "mesh" (expanded metal, Streckgitter) was available with a smaller mesh size, such an antenna could even work nicely on 10 GHz for a combined RX/TX antenna.

Performance test on 2.4 GHz, using relative signal levels from the Goonhilly QO-100 Web SDR with 2.7 kHz RX bandwidth, USB:

  QO-100 Upper Beacon (PSK), unknown uplink ERP from Bochum: -68 dB
  60 cm Mesh Dish, 2*4 turn backfire helix feed, 3.5 watts : -72 dB


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