First QO 100 test tx

1st Official Post

    Hello to all,

    I have finally got all my components hooked up to form a QO 100 station, my TX path is a Pluto into a 6 dB attenuator, CN0417 feeding a SG labs v3 20watt amplifier, 3 metres of low loss eco flex 150 cable feeding a home made POTTY antenna on a 80cm dish.

    Using the test tune on SDR console, I can generate 16 Watts at the output of the rig. However, when connecting the cable and dish for a “live” test, I can only see my signal on SDR console showing about S4/S5, most of the other signals I see are S8/S9. Does this suggest something wrong with my POTTY? Any thoughts would be appreciated.

    Phil G8PJH

    I am now able to have QSO's via the satellite, but I am only using Pluto to Analog Devices CN-0417 preamp to POTI, so very low power. I get reports varying from S2 to S7, but this is using a 1.8 metre prime focus dish. I was amazed to be heard at all via SSB. Initially I tried an AM carrier and looked for it on the Goonhilly web SDR. I was surprised to see it, and reasonably strongly, too.


    But I have since been told that I should be much stronger, I don't see how though, I took painstaking trouble setting up the dish, my only doubt is I set the POTI aperture exactly the same distance from the centre of the dish as it was on the original manufacturers set up. So it's the same dish to aperture of the feedhorn as the original LNB was. I am having to presume that is still correct...?


    I was also told a dish of this size should show a reception of the middle beacon much stronger than I see here, too. I am sure they said -30 to -25 dBm, which sounds very high. I am seeing about --50 dBm at best, but I have no idea if the SDR Console "meter" needs calibrating, or even if it can be. The one in HDSDR I use for HF is calibrated against an in calibration signal generator, so I know that's pretty accurate for HF.


    I have spoken to a few saying they are running 200mW and I hear them +20, but when I look at their set ups on QRZ they have a 20 Watt PA in the chain ;) If 200 mW is giving a 59+20 they might as well sell the PA :)


    I managed to temporarily knacker the Pluto somehow, but to my amazement brought it back to life with the recovery tools. It was whilst I was installing v0.33 firmware, not sure what occurred, but it lives to fight again!


    I have now locked the Pluto to a 40MHz output from a Leo Bodnar GPSDO, and the Bodnar's other output channel locks the LNB to 25MHz, that all went well EXCEPT when I first removed the xtal oscillator form the Pluto's PCB and soldered a coax and SMA bulkhead connector, the Pluto stopped working. I thought I'd killed it somehow, but I had been very careful. Eventually I probed the incoming reference signal pad with my `scope and there was nothing there at all. Turned out my Chinese cable with pre attached SMA, sold as Teflon, was NOT Teflon at all, and had melted internally when I soldered it, shorting inner to braid. I threw it away and up made up my own assembly, with good Teflon coax and a branded SMA, and all was well.


    All good fun...



    I'd be curious as to what strength people hear the middle beacon on various sized dishes.


    Good luck Phil!

    Hi Phil,


    When I upgraded from 0.80 to 1.20 mtr dish I had to change the lens, difference in RX was huge, couple of dBs, DATV beacon was 4 dB different just because of getting the right lens.


    Of course that lens is for RX, not your TX issue.


    As trick from PE1RXJ to test the quality of the dish is simple, but with amazing results; when he compared 5 dishes or so: aim the dish at the sun, so need a sunny day, spray the dish with water, hold a piece of white paper at the point of the lnb. Some dishes create a nice bright spot on the paper, means focus, other illuminate the entire paper, not much focus/gain.


    And so there are many many variables.

    Keep going, its all about experimenting, and learning!


    Henk PA3GUO

    Quote from G8PJH

    Hi Chris, You seem to be making more progress than me! How did the Pluto software external standard command go?

    I seem to be having problems with my POTY, vswr of 2.7 to 1.

    Did you buy your POTY fully assembled and tested?

    I did not need any command, I just made sure the firmware did not contain an offset from 40MHz. I did find a very comprehensive web page showing nearly all the commands, I think I bookmarked it, if I did I'll e-mail it to you.


    My POTY (I keep referring to them as a POTI for some reason...) came as a kit, I just checked dimensions and soldered it together.


    I did see the feed in the below photo on the QRZ page of DJ8EI, which to me as an engineer was like looking at some page 3 bird to others ;) I an=m curious as to whether it uses an LNB a all? It sure looks a nice bit of kit,


    I have no obvious means of measuring SWR unless I fit the SG Labs amp and measure on the voltage output connections for forward and reflected power.


    Now, what has confused me, and God knows, it's not hard to do that, is the diagram showing the feed point distance for the POTY, and how that might change with no lens. The second drawing, described as "phase Centre". Having a prime focus dish several people told me to not use a lens. Indeed, adding one dropped the RX signal substantially. I intend experimenting with moving the POTY entrance hole nearer and further away from the dish.

    • Official Post

    2E0ILY your feed is not a standard POTY but one with a horn on the 10 GHz part, so get rid of all lenses. The horn is taking care of the illumination angle.


    G8PJH VSWR of 2.7 to 1 is really bad (return loss of around 7dB). While would result in 80% of the power radiated, it is very well possible that with such a mismatch, the output stage of the PA does not deliver its rated power (power gain is depending on proper matching). Typical return losses for POTY feeds are 17dB (VSWR 1.3). I assume you measure the output power with a attenuator and power meter so this will be close to perfect 50 Ohms but for some reason the POTY currently isn't.

    2E0ILY sorry Chris, getting confused. Forget you had the Rev B Pluto, which doesn’t have the switchable external standard.Well done for replacing the internal standard with an external feed, wouldn’t mind seeing a photo of that.


    DH2VA. Appreciate that 2.7 is not good. Going to take the poty off the dish and check indoors with a nano vna. Also going to measure cable where it enters the poty, as there is a small chance rain water may have got into the cable/connector.

    The photo is not of my feed, but one I saw on QRZ and envied the engineering in it.


    Quote from DH2VA

    2E0ILY your feed is not a standard POTY but one with a horn on the 10 GHz part, so get rid of all lenses. The horn is taking care of the illumination angle.


    G8PJH VSWR of 2.7 to 1 is really bad (return loss of around 7dB). While would result in 80% of the power radiated, it is very well possible that with such a mismatch, the output stage of the PA does not deliver its rated power (power gain is depending on proper matching). Typical return losses for POTY feeds are 17dB (VSWR 1.3). I assume you measure the output power with a attenuator and power meter so this will be close to perfect 50 Ohms but for some reason the POTY currently isn't.