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Vintage Radio

Prior to the 1930s, virtually all domestic broadcast receivers used TRF circuits. One exception was the 1925 RCA 26 portable which was one of the very first domestic superhets. It used some truly innovative technology for the era.

By Rodney Champness, VK3UG

Until recently, I’d always thought that "portable" radios (if you could call them that) were an innovation of the mid to late 1930s. However, at the HRSA’s 25th Anniversary celebrations last year, I was amazed when I saw Mike Osborne’s 1925 RCA 26 portable. Not only is it a fully-working concern but it also uses a superheterodyne circuit.

Why was this so remarkable? Well, superheterodyne receivers didn’t become common in Australia until the mid-1930s. This means that, at the time, this set was a truly innovative design that was at the leading edge of technology.

The RCA 26 was also one of the earliest, commercially-made portable radios, although manufacturing portables was not particularly difficult at the time. By contrast, designing a workable superheterodyne receiver wasn’t particularly easy in 1925, as the valves that were then available were not very suitable for the task of frequency conversion. In fact, the design could be quite critical if the set was to operate at all.

That situation improved in the early 1930s with the development of the 2A7 and similar converter type valves. These new valves proved to be quite tolerant of circuit design inadequacies, making the design and manufacture of superhet receivers much easier.

Superhet principles

Before the 1930s, most sets employed TRF (tuned radio frequency) circuits. However, these had their shortcomings and superhet designs quickly took over when suitable valves became available.

The superhet (or superheterodyne) principle was developed during World War 1 by Major Edwin Armstrong of the US Army. Armstrong was a prolific radio inventor who also developed other radio techniques, including regeneration, super regeneration and frequency modulation (FM).

Basically, the superhet was developed because during WW1, the allies needed direction finding (DF) receivers that could receive the extremely weak spark transmissions used by the Germans in Europe. Apparently, tuned radio frequency (TRF) receivers could not be made sensitive enough or stable enough for this task, so an alternative technique had to be found.

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