History of the Radio Receiver
The radio receiver has undergone considerable development since the first
radio signals were received. Today there is a great interest in vintage radio
receivers, valve technology and the way in which radio has developed since the
earliest days of wireless.
In 1895 Marconi demonstrated the first viable radio system, now over 100
years later the radios that are in use today bear no resemblance to the early
equipment that was used. The equipment that was used in the 19th Century was
crude and very insensitive, nowadays receivers are very sensitive and they offer
many facilities. They are also used in a wide variety of applications from
broadcast reception, through cellular telecommunications to satellite links and
much more. To be able to operate in all these diverse areas, receiver technology
has changed beyond all recognition. These developments represent the work of
many people from the earliest days of wireless right up to the modern day. Some
of these people have their names entered the technology history books, but the
majority were just ordinary engineers or radio enthusiasts who remain unknown.
Wireless beginnings
The story behind the development of the radio receiver begins with the discovery
of radio waves themselves. A brilliant Scot named Maxwell was the first person
to prove electromagnetic waves existed. However he only showed this
mathematically and he was never able to demonstrate them in a practical form.
Although many people stumbled across them and demonstrated effects that now we
know were radio waves, it was a German named Heinrich Hertz who knowingly
demonstrated these new waves which Maxwell had proved existed. He used some
spark gap equipment to transmit and receive radio or Hertzian waves as they were
first called.
Hertz used a number of variations of the basic equipment. Essentially the
transmitter consisted of a circuit in which a spark was made to jump across a
gap. A second circuit with similar dimensions but with a smaller gap was placed
within a metre or so of the first circuit. When a spark was made to jump across
the gap in the transmitter circuit, a smaller but simultaneous spark would be
seen to jump across the gap in the second.
Naturally the range of this arrangement was very limited, mainly because the
receiving circuit had to pick up a large amount of energy for the spark to jump
across the gap.
Coherers
It was soon realised that more sophisticated and sensitive methods of detecting
radio waves were needed. A device called a coherer became the basis for
reception, and remained in widespread use for about ten years. The coherer is
based around the effect that had been known since the 1850s that small particles
of dust or even metal filings stick together or cohere when an electric field is
present.
The first person to use the phenomenon to detect radio waves was a Frenchman
named Edouard Branly. He discovered that the resistance of a glass tube filled
with metal filings fell from a few megohms to a few hundred ohms when placed
close to a discharge. A short mechanical shock then restored the coherer to it
high resistance state.
Once Branly had developed the basic idea, Oliver Lodge popularised it when he
gave a lecture in 1898 in honour of Hertz who had recently died.. Lodge also
made improvements to the device. Usually the coherer was made to operate a bell
so that when a spark or discharge took place the bell rang. A self-restoring
feature was also introduced. The current flowing through the coherer was made to
operate a small tapper that restored the coherer as well as ringing the bell.
This meant at it was ready for the next discharge almost immediately.
Guglielmo Marconi
It was possibly Marconi who did more for the new technology of radio than any
other person, especially in its early days. He believed that these new waves
could be used to communicate over great distances. He also undertook many
experiments and steadily improved the distances over which signals could be
detected. He looked at the coherer and had his assistant spend many hours
experiment with different materials to find the best combinations, and in this
way he made some significant improvements.
As an indication of the way in which his developments were progressing he
managed to span the Bristol Channel, and later he managed to send a message
across the English Channel. During this experiment the signals were picked up at
his factory in Chelmsford. This was considerably further than anyone had
expected the signals could travel, and it made Marconi think that it would be
possible to span the Atlantic.
Although Marconi's company did not have the funds to support a venture of
this size, undeterred he set about the task, building stations in Britain and
America and after many difficulties he managed to make contact in December 1901.
This was an enormous achievement and it made headlines in the newspapers, but
the sensitivity of the receiver proved to be the limiting factor. This set
Ambrose Fleming, professor at University College London and consultant to
Marconi thinking about the ways in which improvements could be made.
Fleming's Valve
The idea for this next development in receiver technology found its origins with
Edison in America. He had been investigating the reasons for the short life of
light bulbs. After a short while the inside of the bulbs became blackened and he
could not find a way to prevent this. It was thought that carbon from the
filament was coating the inside of the glass. In one experiment to overcome the
problem he placed a second wire or electrode into the bulb and noticed that
current would flow between the electrodes if the negative end of a battery was
connected to the heater filament and the positive end to the additional
electrode. He also noticed if the battery was reversed, then no current flowed.
Surprisingly, Edison could not find a use for this interesting phenomenon.
Fleming who had seen the effect demonstrated by Edison wondered if it could be
used to detect radio waves. He set his assistant to set up an experiment to
discover if it could be used, and to their delight it did. He called it his
oscillation valve because it acted in the same way as water valve in only
allowing flow in one direction.
The idea behind Fleming's Oscillation Valve
Crystal detectors
While Fleming's valve was a great stride forward it would take some years before
thermionic technology was fully adopted. One of the reasons for this is that it
was expensive to manufacture and to run. It could only be powered by batteries
when used as a radio detector and batteries did not last for long because of the
power required by the filament. Batteries were also very expensive as they had
not been developed as much as they have been today.
Around this time work on other types of detectors started to be undertaken
and it resulted in what was later known as the cat's whisker. It consisted of a
crystal of a material such as galena with a small springy piece of wire brought
up against it. The detector was constructed to that the wire contact could be
moved to different points on the crystal, and thereby obtain the best point for
rectifying the signal and the best detection.
These detectors soon gained the name cat's whiskers as a result of their
construction. They were never very reliable and the whisker needed to be moved
periodically to enable it to detect the signal properly. However they were very
much cheaper than valves and gained widespread acceptance.
It is interesting to note that the cat's whisker was the first semiconductor
device. The materials that were used were semiconductors, and cat's whisker
formed a very crude point contact diode.
Triodes
Despite the success of the cat's whisker, work did not stop on the development
of thermionic technology. An American named Lee de Forest was a competitor to
Marconi and needed to develop receiver technology that did not infringe any
patents to which Marconi had access. To achieve this he devoted a large amount
of time in developing a thermionic detector which did not infringe Fleming's
patents. He took out a number of patents in the period between 1905 and 1907
covering a variety of developments that culminated in the form of the triode
valve in which there was a third electrode called a grid. He called this tube an
Audion. With the benefit of hindsight it seems amazing that the Audion was
initially used as a leaky grid detector and it took until 1911 for it to be used
as an amplifier. Once this fact had been discovered many people were quick to
exploit this fact in a variety of applications. One of the first areas in which
valves were used was in the manufacture of telephone repeaters, and although the
performance was poor, they gave significant improvement in long distance
telephone circuits.
With the discovery that triode valves could amplify signals it was soon
noticed that they would also oscillate. This was a mixed blessing. It was a
great disadvantage because these early valves were very difficult to stabilise
when used for signals above a few kilohertz. However the fact that valves could
be used as oscillators was exploited in generating signals. Previously high
frequency signals had been difficult to generate. If steady signals were
required, electromechanical techniques had to be used, and these had obvious
frequency limitations. With the use of valves it was possible to make relatively
compact electronic oscillators.
TRF Receivers
Once the triode was established as an amplifier it made a tremendous difference
to radio receiver performance as it allowed the incoming signals to be
amplified. Previously most sets used crystal detectors and even with a large
aerial, the signal levels were low. The introduction of the triode valve enabled
signals to be amplified so that more distant or weaker stations could be heard.
However in virtually all cases the valve was sued as an audio amplifier because
of the stability problems with these early devices.
To be able to achieve sufficient gain, more than one valve was needed, and as
their cost was very high, people looked at making the most efficient use of
them. One way that proved very successful was introduced in 1913 and involved
the use of positive feedback in the form of a regenerative detector. This gave
significant improvements in the levels of gain that could be achieved. The idea
These regenerative receivers proved to be very successful. The amount of
feedback could be adjusted to the point of oscillation, and this greatly
increased the gain and selectivity, enabling this type of receiver to
out-perform all other forms.
War developments
With the outbreak of the First World War, there was a great impetus to develop
wireless technology further. Both sides of the conflict recognised the benefits
it could bring, both in terms of improved communication and intelligence. One of
the first areas on which development activity was focused was the valve itself.
In these early days of thermionic technology their performance was poor. They
lacked gain, especially at high frequencies and they were prone to oscillate
when they were used at frequencies anything above a few kilohertz.
Originally it was thought that small amount of gas in the envelope was key to
their operation. However an American named Langmuir disproved this and as a
result a new generation totally evacuated "hard" valves were introduced. Not
only was the operation of valves improved by the complete evacuation, but it
also allowed the heaters to have coatings applied to them to improve their
emission. In the old "soft" valves the gasses in the envelope contaminated the
coatings making them unusable.
The other problem with valves was their susceptibility to oscillation. One of
the main reasons for this was the level of capacitance between the grid and
anode. A number of attempts were made to reduce this. H.J. Round undertook some
work on this and in 1916 he produced a number valves with the grid connection
taken out of the top of the envelope away from the anode connection. This proved
to give a major improvement, but the final solution did not come until the
1920s.
Although the TRF receiver represented a major improvement in performance over
what had been available before, it still fell short of the needs for some of the
new applications. To enable receiver technology to meet the needs placed upon it
a number of new ideas started to surface.
One of these was a new form of direct conversion receiver. Here an internal
or local oscillator was used to beat with the incoming signal to produce and
audible signal that could be amplified by an audio amplifier. Although the basic
principle of the direct conversion had been known about for many years many
considered the system was wasteful of valves because the oscillator and mixer
did not contribute to the gain of the set. Even in military circles this was a
consideration because of the size and cost of the valves and their associated
batteries.
The problem was solved by one of Britain's leading wireless engineers, a man
named H J Round. He developed a receiver he called an autodyne in which the same
valve was used as a mixer and an oscillator, Whilst the set used fewer valves it
was difficult to optimise the circuit for both the mixer and oscillator
functions. To make the next leap forward in receiver technology a new type of
set was needed.
The superhet
To solve the problem a new type of receiver known as the superhet, or supersonic
heterodyne receiver was developed. 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 and 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 superhet of today because his
receiver still retained the idea of a variable frequency filter, even though it
was at a lower intermediate frequency.
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 preset 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.
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