As we previously reported, on this date 75 years ago, April 22, 1942, the last civilian radio receivers rolled off the assembly lines.  For the duration of the war, no more radios or phonographs would be produced, the nation’s industrial output instead being devoted to the war effort.

The photo here shows one of the last RCA radios to be produced before the deadline.  This ad, which appeared in the May 1942 issue of Radio Retailing, shows RCA Victrola No. 17,199,547, which was the last set to be produced at RCA’s Camden plant, on April 7, 1942.

With no more new sets available, the nation’s radio servicemen were acutely aware that they would bear the full responsibility of keeping the nation informed and entertained by keeping existing sets running.

Seventy-five years ago this month, the April 1942 issue of Radio Craft showed the defense-minded electronics hobbyist or serviceman how to put together this aircraft detector. According to the magazine, the detector would be of particular interest to civilian defense units around the country. Cost of parts was set at $50, and the detector used readily available parts. In tests, it was able to pick up conversations at three blocks, and the sound of a bomber ten miles away.

Detail of horn-microphone assembly.

The “ear” consisted of an old phonograph horn with a sensitive microphone mounted at the base. The four-tube battery operated amplifier employed three 1H5GT and one 1G4G tubes and allowed the operator to scan the skies for approaching planes with headphones.

During World War II, Amateur Radio was off the air for the duration. However, unlike the situation in the First World War, hams were allowed to keep their equipment, and there were no restrictions on listening. Hams were eager to communicate, and with radio unavailable, they were eager to explore other possibilities. Starting with the March 1942 issue of QST, each issue included an experimenters’ section, which discussed many of the possibilities. Among the possibilities were carrier-current radio over power lines, and modulated light beams.

The first ideas started to trickle in 75 years ago this month in the April 1942 issue, and some of the first experiments focused on ground current communications. Leslie C. Merrill, W1NEI reported his preliminary results with the schematic shown above. The receiver consisted of the audio section of his receiver, and the transmitter consisted of the venerable spark coil from a Model T.

From the transmitter, the signal was fed to ground rods three feet apart. At the receiving end, the ground rods were only two feet apart. Despite the close spacing, he was able to copy the signal fifty feet away.

Merrill reported that he lived out in the country, and had the possibility to space ground rods 3/8 mile apart. By extrapolating his initial results, he speculated that a range of 30 miles might be possible with a similarly equipped station at the other end.

We’ve previously covered similar ideas.  In 1940, Popular Mechanics carried plans for a similar setup with a range of about 75 feet.  And in the First World War, the Signal Corps had a field buzzer that could be configured with a similar setup.  And a 1957 “Quist Quiz” showed a similar setup using a telephone, and even noted that old timers would be familiar with such a hookup.