VLF Whistler Reception

Posted on Feb 5, 2014

Natural Radio` is simply radio noise with natural origins, mainly lightning, but also clouds of ions interacting with the earth`s magnetic field. Nearby lightning sounds like the familiar crackle heard on an ordinary AM radio but distant lightning produces distinctive `chirps. ` The frequencies of the original pulse can be spread out in time (a process called `dispersion)

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because the higher frequencies travel a little faster than the lower. The result is that the short impulse from a lightning strike in South America can sound like a chirp in Texas. Slower sweeping tones are called "whistlers" and they are a bit of a mystery. Allyson`s explanation is the best I`ve heard. When conditions are just right, numerous lightning strikes combine with numerous reflections to give an eerie chorus that sounds a bit like a flock of geese. One can also hear interesting man-made signals in the urban environment. Latitudes away from the equator are best for whistlers and urban areas are often awash in man-made signals, but they can be interesting, too. Before launching into the construction of sophisticated receivers it`s a good idea to throw together a few simple projects to get a feel for the hobby, and to see just how big a challenge the local interference poses. Portable designs will have relatively short antennas and the input impedance of the amplifier needs to be quite high to avoid attenuating the lower audio frequencies. Also, it`s a good idea to give the amplifier an inverting sense from input to output. Otherwise positive feedback from the headphone cable to the antenna can cause squealing and instability. These two requirements usually mean that either a grounded-source FET preamplifer or a dual op-amp is needed. It`s difficult to make a single op-amp inverting design with sufficient input...

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