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How Did Karl Jansky Measure Galactic Radio Emission at 20.5 MHz When the Earth’s Ionosphere Reflects Radio Waves at This Frequency?

-- | February 8, 2016

Question: If 20MHz waves are ‘bounced’ by the ionosphere, how is it that Karl Jansky detected 20.5MHz waves coming from the galactic centre in 1932? Also, what sort of receiver would he have used? — Brendan

Answer: I had to consult with my colleague Ken Kellermann, who is an expert on Karl Jansky’s work, to find the answer to your first question. The thickness of the ionosphere increases and decreases in response to the Sun’s activity, being thicker when the Sun is at sunspot maximum and thinner at sunspot minimum. The thickness of the ionosphere also affects how good a radio reflector it is. The thicker the atmosphere, the better job the ionosphere does at reflecting radio waves. It turns out that Jansky made his measurements at a time when the Sun was at a sunspot, or activity, minimum. The answer, then, to your first question is plain and simple “dumb luck”! If Jansky had not been making his measurements, where were designed to search for sources of noise in transatlantic communications, when the Sun was at sunspot minimum, he may never have detected what he called “cosmic static”, or emission from astronomical objects at radio wavelengths. As for the answer to your second question, he used a (by today’s standards) simple shortwave receiver, a sketch of which you can see in an article by John Kraus on Jansky’s measurements.

Jeff Mangum

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