= Appleton-Barnett radio interference experiment
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On 11 December 1924, <Edward Victor Appleton> and <Miles Aylmer Fulton Barnett> perform an experiment: the Bournemouth broadcasting <radio transmitter> slowly changes <wavelength>; an Oxford <radio receiver> records alternating maxima and minima from <ground wave>-<skywave> <wave interference>. The inferred equivalent height is about $90\,\mathrm{km}$. A separate comparison of simultaneous loop and vertical <antenna> signals tests the downward arrival direction. The <E layer> is established first; the higher <F layer> is found in winter 1926–27. These dates describe different findings.<The Ionosphere by Edward Victor Appleton>[Edward V. Appleton, The ionosphere, Nobel lecture, 12 December 1947, printed pp. 80–81, Fig. 1, Eq. (1), and the wavelength-change and antenna-comparison paragraphs]{ref}
Why change the <wavelength> deliberately? Ordinary fading alone could arise from changing emission, receiver drift or a moving propagation path. A controlled sweep predicts a specific progression of <phase difference> between two arrivals. The directional <antenna> comparison supplies an independent kind of evidence for an overhead contribution. Agreement supports an atmospheric return; fitting a distance alone would not establish a unique <electron density> profile. In the model below, comparable received amplitudes make the <interference fringes> easier to see. A practical repetition would also calibrate the <frequency> sweep, track <radio transmitter> output and receiver gain, and compare repeated sweeps with fixed-<frequency> intervals to detect time drift. These are controls inferred from the measurement model, rather than additional procedures attributed to the historical account.
The <radio receiver> does not need to resolve two pulses to detect two paths. If one path takes longer, the two contributions reaching the receiver during a slow <frequency> sweep carry slightly different emission <frequencies> and can produce a slow <beat> in the detected amplitude. At strictly fixed <frequency> with stationary paths, the arrivals instead have a fixed <phase difference>: there is no continuing <beat> merely because one path is longer. This distinction connects the historical observation to <ionospheric interference fringe spacing>.
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