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John Avellone's Telescope Article
THE ANTENNA ON THE HILL
By John Avellone
The "antenna on the hill" at this year's STELLAFANE was a simple loop detector for "SPHERICS". Its function is to directly convert very-low-frequency (10 to 10,000 Hz) electromagnetic noise into acoustic sound at the same frequencies. Thus you can hear the otherwise inaudible natural atmospheric noises termed spherics. Some level of noise activity is always present. It was interesting to watch people start to walk by, only to stop in surprise and lean over to hear better, as they realized that the detector was "on" and working.
Typical "natural" sounds are: clicks, pops, and tweets (short duration impulsive signals generated by near and far lightning); whistlers (short tones descending from 10 kHz down to 10 Hz generated by near-by lightning impulses spread out in frequency as they travel along the geomagnetic field lines to the conjugate point and back); and the "dawn chorus" (half-second tones rising from 1 to 4 kHz, possibly associated with auroral-induced electrojet currents in the ionosphere). Of course, you almost always hear some 60 Hz hum from electrical lines and equipment.
The loop is easy to build. It consists of 90 feet of #22 insulated solid copper wire wound in 5 turns (1/4" apart) around a wood "X" frame. The two pieces of wood for the "X" are 6 feet long, so the wire run is about 4 feet on a side. The start and finish ends of the wire are connected to the red and white input leads of a 5 Ohm to 2 kOhm matching audio transformer (Radio Shack # RS 273-1380). The green and blue output wires from the transformer are connected to an amplifier/speaker (Radio Shack #RS277-1008A). The construction was outlined on a single-sheet plan that I distributed at STELLAFANE. All 40 copies went quickly!
I like to have the loop running nearby while observing. Its fun to hear what's going on in the ionosphere while stargazing. With different (shortwave) equipment, its also interesting to listen at 21 MHz to the radio noise roaring and surging in from Jupiter (Jovian whistlers?) while observing Io at maximum elongation.
Photos by John Avellone
(Click on the image to load a larger version.)
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Click here for a schematic
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Here are some links for the 21 MHz Jupiter signal and also for the spherics loop antenna:
LOOP ANTENNA: the book "EXPLORING LIGHT, RADIO & SOUND ENERGY" by Calvin R. Graf, 1985, TAB Books, ISBN 0-8306-1758-2
John C. Avellone
The Northern Virginia Astronomy Club (NOVAC)
The Society of Amateur Radio Astronomers (SARA)
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