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
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.
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.