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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.^[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.

Edward Victor Appleton investigated the ionosphere by measuring how radio signals return from the upper atmosphere. Comparing direct and returning radio waves makes their path difference experimentally accessible and constrains the effective height of the reflecting region.

The response changes with frequency because the atmosphere's free electrons alter electromagnetic propagation. Scanning frequency and observing the returning signals helps distinguish ionized layers and their changing conditions. His Nobel lecture connects these measurements with the physical basis of long-distance radio propagation.^[ref]

The E layer is the lower main region of the ionosphere, below the F layer. Its free-electron population is supplied largely by solar ultraviolet photoionization and reduced by recombination. Thus its ordinary daytime ionization rises after sunrise and weakens at night, although sporadic enhancements can occur. Its electron population changes the refractive index experienced by a radio wave.

The Appleton-Barnett radio interference experiment supplied direct evidence for this returning path. On 11 December 1924, Edward Victor Appleton and M. A. F. Barnett varied the transmitted frequency at Bournemouth and observed alternating signal maxima and minima at Oxford. Interference between a ground wave and a skywave gave their path difference, implying an equivalent return height about $90\mathrm{km}$. Before dawn, reduced E-region ionization later allowed penetration to the higher F layer.^[ref]

This is a frequency-dependent propagation region, rather than a permanent reflecting material wall. The experiment measures an equivalent path height; refraction, collision losses and wave polarization matter when interpreting it in detail.

A physicist whose work with Edward Victor Appleton tested the return of radio waves from the upper atmosphere. Their experiments supplied evidence for reflecting ionized layers and distinguished returns from different heights. The Appleton-Barnett radio interference experiment thus connects an observed ground-level interference signal to the structure of the ionosphere, explaining how radio communication can reach beyond the direct line of sight.^[ref]

Awarded to Edward Victor Appleton for upper-atmosphere investigations, especially the Appleton layer. The ionosphere connects this work to radio propagation.^[ref]