While I was measuring a harmonic content of the signal from an oscillator in HF range, I tuned the great Kenwood TH-F7 TRX to VHF band. The receiver mode was set to USB and the frequency band to 108-138 MHz, where any jamming signal should not appear (air traffic control voice and data communications reside there).
At 126MHz appeared some strange sounding signals. I listened a while then connected the TRX to PC audio input. Spectral analysis revealed following spectrograms.
Fig. 1. Spectrogram #1: Antenna 10meter horizontal dipole, RX tuned to 125.998MHz
Fig. 2. Spectrogram #2: Antenna 2m vertical GP unipole, RX tuned to 125.998MHz
The signal record is here: 125998kHz-11m-ant01-3sig (mp3, 250kB).
There are many signals of this characteristic at the same base frequency and some of them are just active sweeping while others idle. The pattern of the frequency sweep is dynamic in time and every signal has different variable timing and frequency track. Also, the frequency change is not monotonous (steps look like a dotted curve on the spectrogram where the change is steep).
It should be noted that the signals exhibit no fading in amplitude, no doppler shift and signal strength is quite low (20dB S/N max. estimate) with any tested antenna. I could receive them almost everywhere (locator JN89) in the urban areas and outside as well. One of the possible sources are the BTS (GSM cell phone transceivers).
Sometimes in the free time I’ll take a directional antenna and try to locate the source.
Or is there anybody who knows the truth? Please drop me an email or comment.