Here is the spec on the transmitter: -------------------------------------------------------------------------- From: "Subject: TSO-C74c, AIRBORNE ATC TRANSPONDER EQUIPMENT"
2.10 Reply Transmission Frequency. The center frequency of the reply transmission must be 1090 ±3 MHz. Note that you need some BW for the modulated signal, which is PPM at 1mbps. ------------------------------------------------------------------------------ -------------------------------------------------------- From "The Report of an Investigation into the Characteristics, Operation and Protection Requirements of Civil Aeronautical and Civil Maritime Radar Systems" 3.1.1.3.2.2 Airborne systems The airborne transmitters have a specified output power of 24 ± 3 dBW for commercial aircraft, and 18.5 dBW for light aircraft, whilst replying on a frequency of 1090 ± 3 MHz (as specified in Annexe 10 of the International Civil Aviation Organisation (ICAO) international specifications). However, in a study conducted recently in the USA, it was found that over 3% of commercial aircraft and over 9% of light aircraft failed the transmitter frequency limits . ------------------------------------------------------ Now this all gets tricky because while the transmitters that exceed the 3MHz limit have been measured, they are supposed to be calibrated. For ATC, rather than simply not recognize the planes out of spec, they build the hardware to handle +-5MHz. Basically failure is not an option. But for us hobbyists, I suppose meeting the 3MHz limit would be nice. But using 2 or 3 dongles would mean a power splitter would be needed, or use 2 or 3 antennas.
