Sideband Asymmetry in Ionospheric Cross Modulation

We study the phenomenon of ionospheric cross modulation between LF/MF broadcast stations using modern reception techniques, allowing us to see details that were not seen with the analog receiving techniques from the 1940s. We show experimentally that the sidebands of the transferred modulation are often asymmetric, in amplitude and/or phase. We give qualitative and quantitative explanations for this asymmetry, based on taking contributions from outside the first Fresnel zone into account. The asymmetry is found to be strongest for longer paths, lower carrier frequency, and disturbing transmitter closer to the receiver.