Posts by DH2VA

  • MY QO100 SETUP

    Doing absolute dbm measurements is all but trivial. A good indication of noise performance is to wave your hand in front of the feed. A good system should have a noise temperature of ca. 150K (give or take) when pointed to the sky, your hand has 300K. Easily detectable.

  • QO-100 setup with AMSAT-DL parts, will this work?

    Quote from DO1OHB

    1. RF band pass filter is always a good item but the CN0417 have one inside so it would be better to have a band-pass behind the last PA (but then you need a stronger one) - I have the same point and not sure how to fix.

    Just one remark on the filtering in general: a bandpass filter is required to get rid of any spurious emissions of the SDR (and there are plenty). You can (and should, see later) add this at a low power stage. Follow-up stages will then only generate additional emissions because of their non-linearity. Driving them -3dB (or even -6dB) from max helps a lot but even when driven hard, a normal bandpass will not help you here. The IM3 (3rd order intermodulation) will be directly next to the signal itself, so it will pass though the filter anyways. Another possible additional generated emission would be the 1st harmonic (at 4.8 GHz) but as the TX feed will likely have little to no matching there the amount of radiated power on 4.8 GHz will be low. This can easily be fixed with a lowpass if required and these are much more easy to build for higher power levels.

    The best 13cm bandpasses for hamradio (as far as know) are the interdigital filters made by DK2DB (check his shop) based on a design by my old man DL3NQ in the 1970s. They are rather big (ca. 30cm) and can handle a few dozen W of power but have a BW of 25MHz. So Q-factor of ca. 100 but unsuitable for removing IM3 of course..

  • solved-Leo Bodnar GPSDO Output waveform

    Welcome Henry !


    (this is the english section of the forum, so I reply in english that other can profit from this thread as well)


    I don't have this particular GPSDO but to me the pulseshape looks quite nice. Check again with a 50 ohm termination because I think that is what the Leo Bodnar GPSDO expects. Generally, rectangular signals for these low frequencies are better suited (some PLL chips even require them instead of sine wave inputs).

  • Adalm Pluto & GPSDO

    be careful: your calculation assumes 50 Ohms load. There is no such termination in the Pluto so you have to provide the 50 Ohm resistor yourself. Otherwise you might end up with a significantly higher Vpp value.

  • SDR Console Keyboard Shortcuts

    For the first question: you can change the redefine keyboard shortcuts via the small 'eject'(?) symbol next to the undo arrow in the top symbol bar.


    Then go to 'weitere Befehle' and 'Tastenkombinationen anpassen'. Select under Transmit->TX your desired key.


    For changing the double Ctrl-Key frequency popup, I haven't figured this out yet..

  • What TX amplifier do you plan to use?

    Has anybody thought about controlling the gate bias voltage startup? First apply drain, then with some delay gate? You could actually use this method as PTT.. This obviously works only for LDMOS, GaAs FETs are a different beast..

  • Polarization Losses on QO-100 Uplink

    Quote from G0MJW

    Achim did manage it but I don't know exactly how.

    If you refer to the QO100 antenna: none of my business.. For me up and until last year, my trusty G3RUH patch was the gold standard. Even most helix antennas are not 100% circular (had a nice chat with Kent WA5VJB about that). And all measurements of DJ7GP show that actually measuring it is far from trivial (as any precise measurement).

  • Polarization Losses on QO-100 Uplink

    Quote from sv8qg

    Either the propagation path changed (that’s impossible)

    Very much possible. See TEC (total electron content) and Faraday rotation (which is negligible at 11GHz, but present at 2.4GHz). Check out NASA Jet Propulsion Lab's excellent documentation: https://descanso.jpl.nasa.gov/propagation/1108/1108.html


    The QO-100 S-band RX antenna is good in terms of circularity but not perfect. So you will see it as shown by Rasto in his excellent paper when you rotate a linear uplink antenna. As the uplink propagation path changes with time, the angle of maximum/minimum signal will move as well. You could do extensive measurements here and write a very interesting paper about S-band Faraday rotation.

  • Coax Cable

    My personal rule of thumb is 1dB max between PA and feed.. (and don't forget the 0.1dB per connector). So for 5m on 2.4 GHz (0.8dB cable attenuation), I would therefore use Hyperflex 5 (42.5dB/100m @2.4G) up to 1.9m and for longer runs Hyperflex 10 (21.8dB/100m @2.4G), so about 4m max. But as HB9EKO says, it will always be a tradeoff. Your mileage will vary..

    Most important: don't try to save money on the connectors, always use the approved connectors for any coax cable.

  • Obstacle in RF path close to dish - Effects? Alternatives?

    Just make sure you tie everything very well together. Such a dish has quite some wind load and when the thing falls off the temporary testing table your neighbors/landlords won't be exactly happy.

    Even if somebody had a similar issue already, the wavelength is only 13cm and moving the overall geometry by a quarter of it can drastically change everything. So it will be very hard to compare and even harder to simulate. You are correct though: this is what hamradio is all about, best of luck!