Thursday, July 19, 2012

Ground Station Hardware

The Ground station hardware has been finished. It consists of a 20 dbm RF-modul (RFM22b) mounted on an old Wifi-PCI-card, a controlling board (FEZ Panda) and a 11 dbi Yagi. Design and calculations for the Yagi are made by Michael using VK5DJ's YAGI CALCULATOR.


  1. I have a concern with your communications link. At the 35-40KM altitude possible with a good balloon launch the radio link only works because of the great receive sensitivity of the RFM22b modules. That said, you may be susceptible to noise, which would raise the noise floor higher reducing the receive sensitivity dramatically, -90dBm or worse, making your receive margin around 3-4dBm. This would not be sufficient to maintain a link. The calculator I used for these calculations is available at

    Two solutions: The first is if you could slide over to the G3 band (869.400-869.750) you may run up to 500mW estimated radiated power (ERP). You could use a transmitter amplifier like this one ( to boost the signal to the legal maximum, just ensure that you take into account the gain of the antenna to stay within the 500mW limits. The receive side is excellent on the RFM22b, so I wouldn't change it.

    The second solution, since you do not know which way the antenna will face as the plane will drop, is to use a omnidirectional circular polarized antenna on the plane, and use a X-yagi or helical circular polarized antenna for your ground station. You could also use an omnidirectional design like the eggbeater antenna used for ham radio satellite work on your ground station, though this would lower the gain a little (not to worry though. You'll have plenty of signal with that amplifier.

    I hope this helps. I am looking forward to seeing news of your flight!

    Aaron Harper

  2. Hi Aaron,
    thx for your comments. Unfortunately i’m not a radio guy, I just took the numbers from the datasheet and did the math. With -121 dBm sensitivity the result is just fine. Do you think -90dBm is more realistic in real life? Where did you get this number from? What about decreasing data bandwidth?

    500mW sounds fine, but it’s only 7db more and 5 times more power consumption for tx on the plane. Furthermore the base station with RFM22b + 10 dbi Yagi already exceeds 500mW ERP.

    An omnidirectional antenna at the ground would be nice. But the eggbeater antenna has approximately 5 db less than a high gain Yagi. So that is no option, it will eat up all the link budget the amplifier would add.

    I thought about circular polarized antennas. But in that case (in opposite to real time applications like FPV) a fluctuating link isn’t highly critical. Additionally for sub-GHz rages high gain helical antennas are not that easy to construct. And for the plane circular polarized antennas with ~1dbi are too bulky for that little plane.

    Best Regards Nicolas