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KF5JRV > TECH     23.02.17 13:16l 47 Lines 8406 Bytes #999 (0) @ WW
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Subj: Superheterodyne History
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Sent: 170223/1215Z 10971@KF5JRV.#NWAR.AR.USA.NA BPQK6.0.13

The history of the superhet radio can be traced back many years before it was actually first invented.

The invention of the superheterodyne radio did not cause a revolution within radio or wireless technology. Indeed it took many years before it became widely accepted.

However the invention of the superheterodyne radio formed a major milestone in the advancement of radio technology, as these radios provided a far more sophisticated topology on which high performance sets could be designed and developed. However when the superhet was first invented, the requirement was not there - only when many more radio transmitting stations started to become active was the need for the superhet realised.

Superhet beginnings

Apart from the discovery of radio itself, possibly one of the first major milestones in the invention of the superheterodyne radio was the discovery of the action of beat notes with radio frequencies.

In 1901, Canadian engineer, R A Fessenden discovered that improvements in wireless signalling using Morse could be made by transmitting signals in such a way that their combined effect produced the required audio signal at the receiver. To achieve this he transmitted two signals that differed in frequency by a small amount. In this way, a beat note was produced when the signals were received.

The idea was a head of its time and remained dormant for a few tears. Then in 1910 some transmission tests were carried out between two American cruisers and the radio operator noticed that the received signal strength greatly increased when the ship's transmitter was in use. This occurred even though the difference in frequency between the transmitter and the received signal was above the audible range.

Further investigations took place and a far more sensitive heterodyne receiver was developed. The heterodyne receiver showed itself to be far more efficient than the other methods used at the time. In 1913 tests were carried out between Arlington Virginia and the Naval ship, Salem and a range of 6400 miles was achieved. During these tests the heterodyne method of reception not only proved to be more sensitive, but it was also far superior under bad atmospheric conditions.

The next major steps were taken as a result of the First World War. Wireless technology was starting to be used increasingly by both sides, and new methods of improving gain as well as improving the selectivity were needed.

Lucien Levy's conversion radio

The first step in this development was taken by a Frenchman named Lucien Levy. He was investigating ways in which receiver selectivity could be improved. In doing this he devised a system whereby the signals were converted down to a lower frequency where the filter bandwidths could be made narrower.

A further advantage was that the gain of valves was considerably greater at the lower frequencies after used after the frequency conversion, and there were fewer problems with the circuits bursting into oscillation. The idea was very successful, and even though it did not totally eliminate interference as Levy had hoped it was a considerable improvement over previous receivers. However it was not the superheterodyne receiver of today because his one still retained the idea of a variable frequency filter, even though it was at a lower intermediate frequency.
Armstrong's superheterodyne radio

The idea for developing a receiver with a fixed intermediate frequency amplifier and filter is credited to Edwin Armstrong. Working for the American Expeditionary Force in Europe, Armstrong thought that if the incoming signals were mixed with a variable frequency oscillator, a low frequency fix tuned amplifier could be used. Like Levy's idea this would enable the valves to operate at a lower frequency where they would be more efficient. It also meant that a fix tuned amplifier could be used and this would be capable of providing much greater degrees of selectivity than a variable one. This is because several stages could be cascaded relatively easily and the tuning preset before use.

Armstrong's original receiver consisted of a total of eight valves. In the set the signal was converted from its incoming frequency down to a fixed intermediate frequency stage. By altering the frequency of the local oscillator the frequency of the received signal was changed. The low intermediate frequency stage allowed greater levels of gain, as in the case of Levy's set because the low frequencies allowed greater levels of gain and stability. Also having a fixed frequency intermediate stage allowed the filters to be more selective. Several tuned circuits could be cascaded to improve selectivity, and being on a fixed frequency they did not all need to be changed in line with one another. The filters could be pre-set and left correctly tuned.

The new superhet gave an impressive performance, but its development came at the end of the war. With its use of a large number of valves it was only viable for use in specialist applications, many of which were no longer required after the cessation of hostilities. Accordingly Armstrong's discovery was rarely used for a number of years. Interestingly, Armstrong was not the only person working on the idea of a superhet. Meissner in Germany took out a patent for the idea six months before Armstrong, but as Meissner did not prove the idea in practice and did not build a superhet radio, the idea is credited to Armstrong.
Superhet gains acceptance

It took many years before the superhet was widely used. In the early 1920s there were few stations that could be heard and the superior performance of the superhet was not required, especially at the considerable cost of all the valves that were used. However as the 1920s passed more stations came on the air, especially in America and the need for the selectivity provided by the superhet became more apparent. As a result the superhet started to be used. A number of developments in valve technology also helped. Originally all valves were directly heated, and this meant that each valve required a separate filament supply. The introduction of the indirectly heated valve meant that an alternating supply could be used. This was because the heaters different connection to the cathode. This allowed the bias conditions to be fulfilled even if the filaments were connected in series or parallel with one another.

The use of superhets by domestic users forced further developments to be made. Originally these sets were very cumbersome and sometimes difficult to use. Ganged tuning capacitors were also introduced to enable the local oscillator and the radio frequency stages to be tuned by one control. Many other refinements were added enabling them to be made more cheaply and easier to use.

The need for the increased performance of the superhet was first felt in America and by the late 1920s most sets were superhets. However in Europe the number of broadcast stations did not start to rise as rapidly until later. Even so by the mid-1930s virtually all sets in Europe as well were using the superhet principle.

Valve technology improved again with the introduction of further electrodes into the envelope. In 1926 the tetrode valve was introduced. This valve had a second grid placed between the normal control grid and the anode. This had the effect of reducing the capacitance causing the feedback, and enabled valves to operate far more reliably than they had done before. Three years later the tetrode was improved by the introduction of the pentode. In this type of valve, yet another grid called the suppressor grid was introduced. This overcame a discontinuity in the curve of the tetrode, and enabled further improvements in their performance.

Further refinements

In 1939 hostilities arose and again it gave a new impetus to radio development Although the superhet was well established by this time, the performance of radios in terms of selectivity, sensitivity and frequency coverage was improved as a result of the need to meet ever more exacting requirements for the war effort. During this time a number of classic communications receivers were designed. Some like the AR88 or HRO are still sought by enthusiasts today and although they are relatively large by today's standards, they can still give a good account of themselves under current crowded band conditions.

73 Scott KF5JRV
KF5JRV.#NWAR.AR.USA.NA


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