Adventures with QO-100 and a PlutoSDR

Discussion in 'Satellite and Space Communications' started by M0KNC, May 30, 2019.

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  1. M0KNC

    M0KNC Ham Member QRZ Page

    I now have a working full duplex QO-100 system using a PlutoSDR transmitting and receiving via a single dish and the Pluto at the same time. It is a system in development (read as strung out across the desk!) but is currently useable. This is how I got there…

    I had already set up a super simple system for receiving the narrow band transponder for the geostationary satellite QO-100 (Es’hail-2). It consists of my 80cm offset (steerable) dish I use for TV, and an RTL-SDR connected to the “loop out” of my sat TV receiver. I just pointed the dish, set up a transponder in the TV box and the signals were there, around 739 Mhz (10.489675 Downlink freq − 9.75 LO frequency of the LNB = 0.739675 GHz = 739.675 MHz) . Then, using SDRConsole I was able to set a receiver offset and then “lock” the software to the beacon ( ) and there was a useable receiver system, with very little effort!

    That got me to thinking if I could get a transmitting system going with little money and minimum effort too… I have never had any experience with 2.4Ghz (13cm) systems. Lots of chatting and digging around on the interwebs and I came up with the what I wanted for my “lazy ham’s setup” kit. It consisted of:

    1) An SDR transmitter – I had settled on a HackRF but ended up with a PlutoSDR for practical reasons (the Linux machine I was going to use blew up!) - the Pluto was about £150

    2) An RF Amplifier – the SDR output is 7Dbm (0.005 of a watt!) – I ordered a Chinese Wifi amp for about £35

    3) An Antenna to transmit – I decided on a simple helix, as it easy to make ( but I have no test equipment to optimise its match) – I also found a 3d printable template that is designed to attach to the front of the LNB on the dish ( ) but there are LOTS of ways of making a helix antenna on line.

    4) A cable to connect the Amp to the Helix – I bought a premade 5m cable from amazon that uses 5d-fb 50 Ohm cable (about 0.3db a meter loss at 2.4Ghz) this cost about £20

    So I built the helix, double sided sticky taped it to the LNB (I know it is too far forward) connected it all together, fired up SDRConsole (I was receiving on a different computer at this stage) tuned into the TP passband then hit TUNE, and … nothing…

    After checking everything again, I fired it up again and saw nothing... then I zoomed in and adjusted SDR console on the RX to exaggerate the levels, and There it was! So weak I could hardly detect it, but It was working…

    So now I knew that I had some signal on 2.4Ghz, why so insanely weak? I took the cable out of the equation and connected the PA directly to the helix (in the drizzling rain!) and saw no improvement (and I got wet!).

    So, after some determined head scratching I realised that the issue was the (modified to stay in TX mode) Chinese wifi PA is not being driven with enough power. So off to Amazon…

    I bought a (very) cheap linear preamp, that was supposed to give 22db gain at 2.4Ghz, which needed 5v and powered it separately with a USB adapter and plugged it in between the Pluto and the PA, crossed my fingers and hit TUNE! .. There I was a carrier about 12db over the noise on the TP on QA-100, a signal sent more that 44,000 miles round trip. At this stage I had a very “rough” SSB QSO with a German station, as I was very weak on SSB and drifting all over the place (because the Pluto has no TCXO).

    I powered it down and then adjusted some connections etc. and then, when I fired up the preamp again from the USB adapter… the magic smoke escaped from the pre-amp! … Bummer. A surface mount inductor had decide it didn’t want to live anymore, and fried… I realised that I had made a mistake, as I measured the potential between the negative on the USB adapter and the earth on my 13.8v supply for the shack and there was a few volts difference. I should not have used that power to supply the preamp!

    So I went back to Amazon, and bought different (the more robust SPF5189Z module) Pre-amp , and a box of 6 buck converters so I could power things from the same supply.

    When that arrived, I set a buck converter to 5v, and connected it all up and hit tune, and it worked again, but I think the SPF5189Z has less gain than the one I was using, so I was even weaker, and certainly not usable for voice on SSB, but I did send lots of digital modes (and SSTV) and received it all OK on my receiver (with drift).

    So, I got a second SPF5189Z and daisy changed it and now I get a carrier that is generally around 20db above the noise floor of the TP as I see it on my receiver here. Useable! I then set up the Pluto in full duplex and moved the RX to the same machine and here I am now.

    Things to do when I can:

    a) Sweep the helix and adjust it properly!

    b) Add a GPS disciplined Oscillator to the Pluto (more on this below)

    c) Work out how much power I am getting to the helix (I am guessing it is not more than around 500mW.

    d) Add a filter, as the amps I am using are broadband (although I have looked around for spurious transmissions and have not found them thank goodness)

    e) Put it all in a box, and get all the lose wires etc. off my desk!

    Pluto SDR and TX drift.

    The Pluto is missing a decent TCXO, so as the temperature changes the transmitter drifts in the wind. This is very pronounced at 2.4ghz (drift does not seem to be a problem with the receiver that I have noticed with mine BTW).

    Through extensive trial and error, I have discovered that I can minimise this drift on mine by setting it in TX mode with the drive set to 100% (this is in SDRConsole) and leaving the input method muted. Occasionally hitting the “tune” button and seeing if it is drifting still. – after around 30 minutes it will stop drifting (downward in my case) and stay pretty steady.. if you turn the TX off, it will go all over the place again, so you leave it in TX mode and use the mute on the input as a “PTT” so to speak. I realise the danger of sending unwanted transmissions from the computer here, but in my tests, as long as it is in SSB mode, there is NO output (obviously in FM or AM it will be transmitting all the time, so this would not work as a workaround).

    I have also noted that if I adjust the “drive” down from 100% it will cause the signal to drift up in frequency, even after it has “warmed up”.

    The real solution to this is to replace the 40Mhz local oscillator with something more stable… Being all surface mount means that this is not too easy for us mere humans to just connect a TCXO (which you can easily do with a HackRF BTW) . However I have been told that if you cut a single track and attach a wire to it, you can inject a LO from an external source. The best source in this case would be a GPS Disciplined Oscillator (GPSdo) that would make it rock solid in any temperature. A GPSdo is about £99 and requires a GPS antenna, but is probably worth it. I intend to try this modification when I can (so much for a warrantee on the Pluto!)

    I will continue to play and see what I can do, but keep an eye open for me testing digital modes etc. on the repeater in the sky, QO-100!

    73’s Neil M0KNC
    K6CLS, W0PV, WE4B and 4 others like this.
  2. KC9SGV

    KC9SGV Ham Member QRZ Page

    Thanks !
    Keep it coming.
    Read somewhere that SDRConsole will stable the TXO with software alone.
    Maybe a different block diagram ? (I.e. Separate receiver)
    No hardware mods needed ?
    Maybe this is just a setting in SDRConsole ?
    Last edited: May 30, 2019
  3. M0KNC

    M0KNC Ham Member QRZ Page

    I think what you are hearing about is the fact that SDR console can lock the RX frequency against the beacon ( which I am doing). That ha s NO effect upon the TX ..
  4. AI3V

    AI3V Ham Member QRZ Page

    If you had to purchase everything new, (including computer etc, ) what do you estimate the total cost at?

    M0KNC likes this.
  5. M0KNC

    M0KNC Ham Member QRZ Page

    erg. taht is hard.. in the UK..
    Laptop , £600
    Dish maybe £100 with a mount
    LNB £10
    cables £30
    Pluto £150
    preamps £13 each
    8W AMp £40
    Wire for the Helix £10
    ... Just guesses..
    AI3V likes this.
  6. AI3V

    AI3V Ham Member QRZ Page

    M0KNC likes this.
  7. M0KNC

    M0KNC Ham Member QRZ Page

    yes, that is a good resource! You will often find me on the IRC chat that is displayed on the page too.
  8. W0PV

    W0PV Ham Member QRZ Page

    Absolutely BRILLIANT Neil !! Carry on.

    So glad to see someone piece together a tightly budgeted yet perfectly workable ground station solution for a GEO sat like QO-100. Nothing too extravagant needed, just some clever engineering. More fuel on the building fire that hams in other parts of the world (ie, ITU Region 2) need to heat up an effort to catch up.

    73, John, WØPV
    M0KNC likes this.
  9. M0KNC

    M0KNC Ham Member QRZ Page

    thanks heaps John.
    Whilst I appreciate heavily engineered systems, I like to play with the minimum setups :)
    Neil M0KNC

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