The History Column: Appleton and Barnett’s Experiment to Measure the Height of the Ionosphere
Marconi’s first demonstration of transatlantic radio communication on 19 December 1901, from Poldhu in Cornwall, UK, to St John’s in Newfoundland, Canada, was an important landmark in our subject. However, it was evident that the propagation path was far from line-of-sight, and the exact propagation mechanism was unclear. In an entry in the Encyclopaedia Britannica in 1902. Oliver Heaviside postulated the existence of a conducting layer in the atmosphere that would reflect radio waves and allow long-range propagation, and Robert Watson-Watt proposed the name ‘ionosphere’ in 1926.
The British physicist Edward Appleton and his New Zealand-born PhD student Miles Barnett devised an elegant experiment to measure the height of the ionosphere.
This exploited a BBC broadcast transmitter located at Bournemouth on the south coast of the UK, and a receiver located at Oxford. The frequency was 770 kHz, and the dates of the experiments were 11 December 1924 and 17 February 1925
In a letter dated 2 January 1925 to Balthazar van der Pol, Appleton noted: ‘I calculate that at about 100 miles from a station (broadcasting) the low rays should be of equal amplitude & we should get fading – and we do’ [his emphasis].
The description of the experiment reports that: ‘… Capt. A.G.D. East of the BBC arranged the transmitter so that a known small change of wavelength (e.g. 5 to 10 meters) could be uniformly produced in a given time (e.g. 10 to 30 seconds) … No [amplitude] modulation of the wave took place during the experiment’.
The frequency modulation of the transmitted signal resulted in a beat note between the direct signal and the reflection from the ionosphere, proportional to the path difference, observed on a galvanometer. An example is plotted in the figure below. From this, the height of the ionosphere could be calculated, and in this case the answer was that the reflecting layer lay at a height of 80 – 90 km.
This experiment may be regarded as the first example of a passive radar, exploiting a (co-operative) broadcast transmitter, and also the first example of an FM radar. Both of these techniques are now regarded as mainstream. Appleton was awarded the Nobel Prize in Physics in 1947 for his work proving the existence of the ionosphere and measuring its height.
Later in 1925, Gregory Breit and Merle Tuve in the USA used a pulsed radar technique to make similar measurements. The configuration used was broadly similar to that used by Appleton and Barnett, with transmitter and receiver separated by a significant distance, and measuring the time difference of arrival at the receiver between the direct signal and that reflected from the ionosphere. The frequencies used were approximately 4.2 MHz (l = 71.3 m) and 7.2 MHz (l = 41.7 m).
Further information may be found in:
Appleton, E.V. and Barnett, M.A.F., ‘On some direct evidence for downward atmospheric reflection of electric rays’, Proc. Roy. Soc., Vol.109, pp261-641, December 1925.
Breit, T. and Tuve, M.A., ‘A radio method of estimating the height of the conduction layer’, Nature, vol.116, p357, 5 Sept 1925.
Authored by Hugh Griffiths
University College London