An experimental beacon on 144MHz has been deployed for evaluation. The beacon is designed to permit observation of aircraft enhancement propagation by way of a 500+s unmodulated carrier in each 600s cycle. Ident is by very slow Morse code. Necessary bandwidth (ITU-R SM.1138-3) is just under 5Hz, requiring 5Hz receiver bandwidth for decoding under weak signal conditions.
- frequency: 144.245MHz, 144.244MHz USB dial freq, 144.245MHZ dial frequency in CW mode on modern transceivers (accuracy should be within 200Hz);
- power: 20W EIRP (current details: https://vkspotter.com/?action=beacon-item&bid=332), NSW, horizontally polarised, antenna is 6m AGL;
- modulation: ~10 minute cycle uses A1 Morse modulation (OOK) QRSS1 (1s dits) callsign (VK2OMD) followed by key down for the rest of the cycle.
The oscillator on the keyer can have accuracy as bad as 1000ppm, and a power interruption would cause it to restart at a random time, so the modulation pattern is not syncronised to the wall clock.
The narrow band modulation means it can be decoded in 1Hz receiver bandwidth, allowing decoding with packages such as SpectrumLab some 20dB or more lower than by ear.
Above is a screenshot from SpectrumLab, albeit a relatively strong signal where S/N in 2kHz is about 0dB… but as can be seen from the plots, there is around 30dB of margin left. A settings file for SpectrumLab is linked below.
Under aircraft enhancement, Doppler shift will be seen, possibly multipath reception. The transmitter is located under the Melbourne-Sydney flight path, so may often be subject to aircraft enhancement (when / if the aircraft fly again).
The transmitter is a Tait T800 @ 2W out with smbk keyer, antenna is a 4 element Yagi at 6.2m height.
The transmitter uses a TCXO reference, so should be reasonably accurate over time.