Over on his blog VK5QI has shown how he has was able to re-purpose an old radiosonde into a wideband active L-band antenna. Radiosondes are small packages sent up with weather balloons. They contains weather sensors, GPS and altitude meters and use an antenna and radio transmitter to transmit the telemetry data back down to a ground station. With a simple radio such as an RTL-SDR and the right software, these radiosondes can be tracked and the weather data downloaded in real time. Some hobbyists such as VK5QI go further and actually chase down the weather balloons and radiosondes as they return to earth, collecting the radiosonde as a prize.
VK5QI and his friend Will decided to put some of his radiosonde collection to good use by modifying one of his RS92 radiosondes into a cheap active L-band antenna. They did this by first opening and removing unnecessary components that may interfere such as the main CPU, GPS receiver, 16 MHz oscillator, SAW filters and balun. They left the battery, LDO's, LNA's and Quadrifilar Helix GPS antenna which is tuned to the GPS L-band frequency. Finally they soldered on a coax connector to a tap point on the PCB and it was ready to use.
They then connected the new antenna to a RTL-SDR V3 and fired up GQRX. They write that their results were quite promising with several Inmarsat and Iridium signals being visible in the spectrum. VK5QI also used gr-iridium with the antenna as was able to decode some Iridium signals.