Posts by DH2VA

  • Lunar Gateway

    TLDR: having the possibility to use a high gain antenna on the gateway compared to QO-100 global beam compensates for the 20dB more path loss. It will be +- identical in terms of ground equipment.. more details to follow in one of the next journals :)

  • Uplink with WiFi Panel Antenna 20dBi

    Nope, really easy. Check the WebSDR on your mobile phone or laptop while finetuning the dish. The better the initial alignment with compass, the quicker you are done.

    There is a catch though: I would not radiate full power when standing next to it, not even with 2W. Microwave energy is not really good for organic tissue, eyeballs are most affected. Call me paranoid..

  • Frequency Stability of the QO-100 NB Transponder

    Quote from G8UGD

    I do wonder how to check stability of any osc for both accuracy/drift over a period of time, is there an ultra-stable signal on HF that one can lock and compare to or any software designed for the measurement one can get gold of easily?

    No. We receive here DCF77 from Germany and due to variations in the propagation, its stability is only of the order of 1E-9 on short timescales.

  • Frequency Stability of the QO-100 NB Transponder

    G8UGD it's hard to give a clear answer but if you think about how the oven internally works, it might give a better understanding what can be done from outside to help the temperature controller.

    Theoretically to keep the crystal oscillator at any given temperature, the controller would heat if the target is too cold and cool if the target is too hot. Heating is easy but for cooling the control loop requires that the energy has to go somewhere (no active cooling) so we need some way out.

    The prime reason for anybody to isolate his OCXO is to separate the temperature variation from the outside from the device. That unfortunately means that heat cannot go out anymore as well thus changing the control loop time constants for both heating and cooling. It probably will still work (operating temp according to datasheet up to 70 degC) but the sweet spot is 25degC.

    The reason for DF6JB's proposal to use thick aluminum walls is to keep the thermal conductivity to the ambient (aluminium being a good heat conductor) but averaging out any short term effects like opening the window or even the door. The total energy change will be the same with or without thick walls but the slope will be softened by the thermal capacity of the aluminum. Ideally, the slope will be slow enough so the OCXO temperature controller can fully compensate for it without the user noticing. Without this thermal capacity the temp controller might be just too slow and not be able to catch up resulting in a larger delta-T at the crystal and hence a delta-F.


    Does this answer your question?

  • Frequency Stability of the QO-100 NB Transponder

    pe1hzg fully true. The example given above is a best-case scenario as TX and RX drift are closely correlated. If I listen to somebody else's signal the two drifts are uncorrelated and the worst case can happen (according to Murphy's law, it will..) with both drift having opposite signs and the result would be the sum of the two. So something of the order of 1.2 Hz/sec.

    Again, this is for a 1E-10 oscillator at constant ambient temperature. If you open the window in the shack to let in some fresh air, hell will break loose. I have about half a dozen GPSDOs of different grades sitting here in my lab and you can clearly notice it.. Ulrich Bangert has described in his article linked above that he has placed his OCXO in a aluminum vault with 20mm (!) wall thickness. This is quite some kilograms of mass and therefore acts as a thermal lowpass. Do not try to pack the OCXO in an insulating box (styrofoam) as the oven will need a certain heat dissipation to actually work. The aluminium vault is actually great as the thermal conductivity is low but the thermal mass is high.. my best double oven HP10811 is placed in such a vault and then on top equipped with its own UPS to keep it running even in case of a power failure. Uptime 8 years..

  • Frequency Stability of the QO-100 NB Transponder

    I have heard several reports about OMs struggling to reach the advertised speeds for the higher VARA speed levels. As people were suggesting the stability of the transponder LO frequency could be a problem I decided to investigate the required stability parameters a bit more.


    According to the specification by the developer (Jose, EA5HVK) VARA is based on an OFDM with modulations between BPSK and up to 32QAM for the higher speed levels. See also his blog post for more details. He also states in his VARA specification document that short term frequency stability for both TX and RX should be <0.5 Hz/sec.


    Many OMs have their ground setup fully locked to a 10 MHz GPSDO. While this seems perfect at first sight, I would like to remind everybody about the difference between stability and accuracy. Ulrich Bangert (DF6JB, SK) has written an excellent article about it (german only). A very popular GPSDO used in the QO100 community is the Leo Bodnar GPSDO. Several measurements suggest, that the stability at 1 second intervals is of the order of 1E-10:


    http://leobodnar.com/files/Mic…eobodnar%20GPSDO_rev2.pdf

    http://leobodnar.com/files/Inf…Frequency%20Reference.pdf


    1E-10 (@ 1s) is quite a respectable number for such a low cost device, but is it enough for the VARA requirements on QO100?


    At 1E-10 any carrier generated at 2400 MHz will have an instability of 0.24 Hz in 1 sec. The downlink is on 10.5 GHz and assuming a perfectly stable transponder LO, this results in a downlink drift of 0.24 Hz here as well. However, the LO for the RX chain is also locked to the reference and should therefore drift by 1.05 Hz. The difference of the two (1.05-0.24=0.81 Hz) would be visible on the downlink as drift in a 1 sec time interval. As this is larger than the required 0.5 Hz a degradation in performance is to be expected.


    Disclaimer: I cannot and will not give any technical details of the QO100 transponder due to NDA. But I think it is obvious that any space-qualified transponder oscillator will have very good stability values so for the sake of simplicity we assume it is non-significant compared to the numbers above.


    The solution would be now to use a more stable (not necessarily accurate!) reference oscillator. James Miller G3RUH has an example with (now obsolete) hardware. In a few words good stability on short timescales requires (among other things) good thermal stability of the 10 MHz master oscillator. On short terms the GPS control servo is (should be) non-effective and a good free-running OCXO might be even better (again: stability vs. accuracy). This probably means something like a HP10811 (which many still regard as one of the best references available for hamradio) or even one of the double oven HP10811 versions used in the Z3801. They reach 1E-11 and below. But they also need to be kept running 24/7 as letting them reach thermal equilibrium may take a few hours..



    73s Achim, DH2VA

  • Pigtail cable dB comparison

    RG-316 has more loss (slightly thinner) but higher temp range compared to RG-174. And don't forget about connector interface losses which are non-neglectable in relation when having short jumpers.

    That said: for a fixed installation, probably UT141 (semi rigid) is best.

  • Under-performance of EP-AB003 WiFi amplifier

    Any amplifier driven too hard (making it non linear) in the RF path will shown this very behaviour (generating 3rd order intermodulation or IM3 for short). You just see them in the example as the low audio are the most prominent contents.

    For those transmitting some kind of waterfall text it is more easily seen as ghost images below and above the letters as the main letter itself is sent with constant levels across frequencies (compared to SSB voice).

  • MY QO100 SETUP

    SV1BDS 60 and 80cm should be different in SNR by a bit more than 1 dB only (compared to their 2.5dB difference in gain) as you are already limited by transponder noise. So your pics looks ok to me.

  • Thank you for shutting down QO-100!

    operators using QO100 do know

    • where to point the antenna
    • which frequencies to use
    • which power is required


    Unless you have very selective reading, it is hard to miss the information about power levels. I don't buy this being anything else but ignorance.

  • Thank you for shutting down QO-100!

    First of all an apology for the title to the vast majority of users which follow the rules on using the QO-100 transponders. Your activity both as operators but also as hamradio experimenters, builders and ambassadors for our hobby is our fuel to continue working on future amateur radio satellites.


    The following lines are addressed to the few individuals (YOU) which constantly ignore the rules set up by AMSAT-DL. I am writing 'ignoring' on purpose. Not 'bending' or 'challenging' but 'ignoring'. After more than half a year the rules should be obvious to all users. Working QO-100 is by no means an easy feat and we can certainly rule out 'accidental' transmissions to the satellite. Still not a single week passes without me observing your mindless and selfish behavior on the transponder: transmitting outside its authorized frequency limits, exceeding the defined power levels limits and using excessive bandwidth (to mention only a few). Although I will not play the 'shame and blame' game (others are much better at this and I thank those for doing it for me!) you should note that your behavior is gravely endangering the future of amateur radio satellites and our status as 'hitchhiker-payload' in particular! The rules are in place for very good reasons and while you might not agree with them: here they are. I cannot and will not justify them individually (last but not least due to NDA restrictions) and I do not think AMSAT-DL has to. QO-100 is a very generous gift by the satellite operator and a few very supportive persons and we should treat this gift with decency and care. It is NOT a service and you have NO rights on using it. If you do not agree with the rules, I would recommend you to go somewhere else and let others enjoy the gift offered to us. As a reminder, amateur radio via satellites is a low-power mode and there is absolutely no justification to use more power than required to use the transponder. Your constant misbehavior sheds a very bad light on the amateur radio community. It will eventually lead to temporary or permanent stop of QO-100 operations and subsequently kill any future efforts to get similar payloads flying on future commercial satellites. Not only this is asocial behavior against all well behaving hamradio operators but it is a punch in the face of all volunteers of the Phase-4A project which have dedicated considerable time of their lives in the past years to make this happen.


    Thank you for that.