Pedro Lilienfeld

Early on in my professional career, perhaps in the 1960s, I became aware of a characteristic effect of lightning discharges on AM band radio reception: these discharges caused distinctively explosive crackling or crashing noises, when a radio set was tuned between stations on the AM band. I thought of a very simple “transistor radio” type of pocket-sized receiver that would serve as an alarm for an approaching thunderstorm over a range of about 20 or 30 miles and more. I found that these signals were especially noticeable when tuning the radio to the low end of the AM band, e.g. around 530 kHz. Transmission at such relatively low frequencies is principally by ground wave electromagnetic conduction which does not depend on the time of day, i.e., is independent of the ionospheric conditions which govern shortwave reception.

Given that the majority of these transistor radios incorporate a ferrite antenna for AM reception, in order to maximize the sensitivity and thus the range of detection of the lighting discharges, the device should be oriented horizontally, i.e., its normal use position, and perpendicularly to the assumed direction from where the thunderstorm may be approaching.

When listening at the low end of the AM band, at a frequency devoid of any station, and in the absence of a thunderstorm, the perceived noise tends to be a continuous hiss clearly distinguished from the lightning engendered crashing transients.

It is noteworthy that lightning detectors based on the above described principle are now commercially available for around $100. However, small transistor radios which can perform the same function, can now be obtained for about $10 and can, of course, also be used for general reception including FM transmissions.