Posts by DH2VA

    I also think at these quoted levels (0.5-1 dB) ionospheric scintillation will play a role. Expect this to become larger and more frequent when the sun becomes active again.

    Hi Martin!

    23.2 Hz seems a lot to me on 10 MHz, I would have expected maybe 10x less. But this would still leave you with 2.xx kHz offset on X-Band. The latter I would not even complain about in a second, but 23 Hz deviation on 10 MHz sounds strange.. DB2OS : any info from Stefan about that?

    About frequency offset: I don't know this particular circuit but I assume the reference frequency generated by the GPS engine is an internal clock signal which is programmable. Hence the required 1Hz ('few Hz') offset on the 10 or 40 MHz level.

    Afaik the Leo Bodnar unit has the same 'problems' and the solution to offset a tiny bit on the reference level to reduce jitter is known also there.

    It all depends on the requirement.. if you need some reference frequency for your counter to display the correct number you go for accuracy. If you need something for narrowband ops you go for stability.

    Can it be solved? Sure.. but then you need to correct the OCXO with a GPS engine and run it with really long time constants (1000's of seconds!). Then you would need to shield all of this against temperature variations (1/10 degC is noticable!) which needs a lot of volume and mass.

    Can it be solved for the price listed in the AMSAT-DL shop: no. A suitable OCXO itself starts at 100 Eur alone. The OCXO on the board would likely not be suitable for GPS-control due to its instability.

    For those who want to know more about this very interesting topic (how to build a GPS stabilised oscillator) I can recommend two sources:

    Uli Bangert DF6JB (SK): http://ulrich-bangert.de/AMSAT-Journal.pdf

    And from the former AMSAT-NA president Rick Hambly W2GPS: https://gpstime.com/files/TOW/tow-time2015.pdf


    73s Achim


    PS: and that's why I believe only very few frequency 'measurements' made of the QO-100 beacons.

    Please take into account that the MER beacon is generated on the ground in Qatar, so any weather there (yes, they do have weather!) will influence everybody's MER.

    You have to be a bit careful about the max. current which USB is capable. With enumeration, this is 5V/500mA (which at 12V is only 200mA) and this is ok. Without enumeration USB will only deliver 100mA and after conversion to 12V, there is not enough power for an LNB. If depends on how the USB device is recognized.

    DL9SEC Du meinst den Zinnmeniskus innen am Rohr zwischen Patch und Reflektor? Das kann die Anpassung beeinflussen... aber die schlecht gelötete SMA Buchse macht das sicher. Das Lotbild vom Reflektor sieht im Bild vom DK1MHW auch ganz gut aus.

    Desweiteren denke ich dass das Kabel (RG-174?) fuer die Frequenz zu schlecht und allgemein viel zu lang ist. Wenn schon lang, dann dick (Ich hab hier Aircell 7 fuer 1m).. oder eben kurz (10cm) und Semi-Rigid (UT-141).

    Wenn da wirklich ein Spalt ist, wird das das Problem sein.. ist bei 2.4 GHz nicht förderlich für die Anpassung. dass /muss/ plan und buendig rundherum *wasserdicht* verloetet werden. Das ist aufgrund der thermischen Masse nicht einfach, aber notwendig.

    double glazed widow, and possibly interference with the lobes from the window's metal frame

    Rather certainly than 'possibly'. I suggest you move the antenna outside and be prepared for a loud signal. Anything (!) else except for air will eat on the 10 GHz downlink. That in particular includes water (rain, fog, heavy clouds, trees).

    I take the liberty to link to Daniel's excellent development of a medium speed data modem, which is tailored to the QO100 NB transponder (rather than using a system developed for HF) . Based on some very good reasoning and design choices, he manages to get 6000 bps through 2.7 kHz of bandwidth while sticking to beacon level power and therefore adhering to the rules of operation.


    https://destevez.net/2019/12/f…nd-data-modem-for-qo-100/


    Bear in mind, this is ongoing work and the higher data framing levels are still missing. It does show however what is technically feasible.

    (Sorry fuer das Englisch oben.. wir sind ja hier im deutschen Teil)

    Das ueblicherweise beim 3D Druck verwendete PLA ist als Ausgangsmaterial nicht geeignet. Diverse Versuche haben bestaetigt, dass eine 3D gedruckte Linse sogar schlechter ist als ein offener Hohlleiter (von einer richtigen Linse ganz zu schweigen). Es koennte am epsilon liegen oder am Tangens-Delta (Verlustfaktor) auf 10 GHz, was ohne umfangreiche Messungen nicht zu eruieren ist. Die einfachste Moeglichkeit ist wie bereits geschrieben eine Linse von einem anderen LNB zu kannibalisieren.

    One should also mention that not only the dielectric constant of the polymer is important (to make the lens act as a lens) but also it's absorption at 10 GHz. Otherwise, the lens will be an attenuator and you could use as well some wet cloth.

    Most negative lens experiences are caused by using improper polymers. Stick with Rexollite or Teflon (which is good to lathe if you do it carefully).