Passband limits?

1st Official Post

    Hi, do we have to stay within passband edges in RS44 and FO29? What I mean is, you put the downlink Frq only 2kHz above lower edge of the passband to Doppler.SQF file of SatPc32. Now when the bird is passing you downlink is below passbands lower edge.


    73 de Ismo, OH1NHW

    I tested in SatPc32 with RS44. I have Frq pair in Doppler.SQF-file 435.612Down nad 145.993Up. When only Uplink is tuned by Cat (IC-970 full duplex), downlink stays 435.612. Now when the bird has passed my Qth and going away (I'm still in footprint), the Uplink trops below lower edge of the Uplink 145.995. So, my main question is do we have to stay all the time within passband edges?


    ismo

    • Official Post

    I don't know SatPC32, I wrote my own software and had to puzzle with calculating shift-frequencies for 2m and 70cm. I know from other discussions on AMSAT-BB that SatPC32 has a function "Full Doppler". Then the frequency should stay stable on the transponder.


    Quote from OH1NHW

    So, my main question is do we have to stay all the time within passband edges?

    Yes, otherwise your signal is lost or you move in an other QSO e.g. at the passband edge.


    This is the reason, why I prefer a stable downlink on all transponder bands. But other OM's had other arguments and created other rules.

    Dear Ismo,


    you need to consider the Doppler shift.


    If you are transmitting exactly on the upper limit of the passband and the bird is approaching your QTH, it will not receive your signal correctly (within passband limits) because it is outside the passband for the bird (this might be 7 - 8 kHz on 70 cm). So you need to adjust your TX signal well below the upper limit of the passband (that frequency limit minus Doppler shift) to stay within the passband.


    If you are transmitting exactly on the lower passband limit and the bird is approaching, you should be received well inside the passband limits (your TX frequency plus Doppler shift). And you could TX on a frequency that is (a bit) lower than the lower edge of the passband - Doppler will shift it into the passband. But all this valid only if the bird is approaching!


    So the passband limits need to be considered from the view of the sat - not your TX frequency!


    Of course, vice versa if the bird departs from your QTH (negative Doppler shift). And the same consideration for the re-transmitted signal (downlink).


    So it is not important on what frequency you are transmitting (or receiving), important is that the bird will receive your signal within its passband limits. Then it will be re-transmitted on the other frequency band (within its passband limits), and again the frequency on your receiver might read beyond the passband limits.


    Does this answer your question/confusion?


    73s Oliver DL9QJ