File:Dowding & Rogers 24 tube radio receiver.jpg

English: A powerful radio receiver using 24 vacuum tubes, built by British scientists Dowding and Rogers in 1924 to try to detect radio signals from a putative alien civilization on the planet Mars. A conventional radio of the time is shown at left for comparison. This was a tuned radio frequency (TRF) receiver, and was said to be the most powerful receiver in the world at the time. The low gain of the early triode tubes required many stages to achieve sufficient amplification. The signal passed along the line of tubes, being amplified by each one, until at the end it was fed into a detector tube which rectified the radio signal, extracting the audio signal, which was applied to a speaker. The tubes were mounted around the periphery of the large board in order to keep the output widely separated from the input, to prevent coupling and feedback of signal from later stages into earlier ones, which could cause oscillation, creating howling out of the speaker. Parasitic oscillation was a big problem in many-stage TRF receivers like this.

Since the turn of the 20th century, the influential US astronomer Percivel Lowell had popularized the idea that civilized life existed on Mars, and during Mars's close approach to Earth in 1924 many radio organizations, such as the US Army Signal Corps, had projects to listen for Martian radio transmissions. No transmissions were detected with this set.

Caption:"This 24 tube radio receiver, designed by Messrs. Dowding and Rogers, English radio experts, was constructed for the purpose of attempting to pick up any radio signals sent from the planet Mars. It measures 3 feet square and an ordinary 2-tube set is shown beside it to give an idea of size comparison. It can be lifted by two men only with difficulty.

Alterations to image: cropped out surrounding images and caption, converted to greyscale.