AN EARLY COHERER RADIO RECEIVER
INCLUDING SOME THOUGHTS ON EARLY RADIO DETECTORS
A comprehensive story containing historical, educational, technical and biographical elements & opinions just for the fun of it
IntroductionI have always loved exploring into the beginnings of things. While writing my article on my first amplified and oldest surviving radio project, I started wondering about the very beginning of radio and how it evolved from the early microwave experiments of Hertz in the 1880s to the long wave and medium wave broadcasts that began to link the world just 20 years later.
detection of radio waves
It is not my intension to write anything exhaustive regarding how early radio began or to describe the brilliant insights into electromagnetism that James Clerk Maxwell formulated about the time of America's Civil War. I will not describe how Hertz and other scientists generated and detected the first radio waves, but only want to describe the first really practical radio that could receive telegraph signals without wires and over relatively great distances. I will skip the details regarding Tesla's brilliant invention of high frequency resonant circuits and the powerful spark gaps that excited them except to say that they were the key to transmitting powerful radio signals of sufficient radiated energy so as to overcome, at least to a certain degree, the profound "deafness" of the first radio detector, the coherer detector. This article then is not about early radio transmitters, but the detectors used in early radios.
Right now it might be reasonable to ask why a radio "detector" is necessary at all. Well, electromagnetic waves in the radio spectrum can not be seen or felt. None of our sense organs are equip to send or receive radio waves because these waves are way too high pitched to hear and way too long to see. Although we are surrounded by a sea of radio waves, we don't sense their presence. Even the most powerful transmitter would be useless unless there is some way to "detect" the presence of its radio waves.
To detect radio waves, you need, of all things, a detector. Not a lie detector, not a metal detector, not a smoke detector not a leak detector, but a physical device that can detect radio waves. At first, all a radio detector needed to do was to transform radio wave pulses into direct current pulses that could operate an ordinary telegraph set. Just as long distance communications began with the telegraph and the Morse Code, so too did early radio. The first radios were technically radio-telegraphs and the operators of these sets were radiotelegraphers. Even as late as the mid 1960s, a U.S. Naval Radioman had to be a radiotelegrapher and that's where I learned my Morse Code. By the way, since these telegraphs used radio waves rather than hundreds of miles of wire strung up on poles, they were called "The Wireless Telegraph" or simply "The Wireless." To operate a telegraph without the use of long wires between stations, the coherer detector, which I will describe in this article, was a pretty good start, but, as we will see, it has very serious limitations as a radio detector.
The coherer has two main faults that makes it far from an ideal radio detector. First and most serious, the coherer is an extremely "deaf" detector. By "deaf," I mean that it can not detect faint, far away signals. The other fault, not noticed at first because the broadcast technology hadn't been developed yet, is its inability to detect radio waves that have voice or music superimposed on them. It wasn't until many years after radiotelegraphy was well established, long after the Titanic broadcasted its distress messages, that practical technology was developed to superimpose undulating voice and music onto radio waves, but by then the coherer had already been obsolete for a long time. In the beginning though, the only thing a radio detector needed to do was turn radio waves into usable current pulses that could operate a telegraph sounder or be heard in an earphone and for this, the coherer was, as previously mentioned, a pretty good start. This is then is a description of the first really usable radio detector, the coherer.
The Coherer as a radio detectorThe coherer detector developed from a very interesting discovery that the French scientist Edouard Branly made around 1890. He discovered that if you put a small radio frequency current through a tube filled with metal filings, the tube's resistance goes way down and it will suddenly begin to conduct a large direct current. Monsieur Branly called his invention "the radio conductor" and in terms of sensitivity, it was a huge improvement over the way radio waves had previously been detected (by the faint light given off by a tiny spark across a gap between two wires). A British scientist named Sir Oliver Lodge quickly picked up on Monsieur Branly's device and started experimenting with it himself, but didn't like the name "radio conductor," so he called it a "coherer" because he thought the radio frequency current caused the metal filings to clump together. You see, the word 'cohere' comes from a Latin word for clump or stick together, so instead of calling Branly's device something silly like a "sticker" or a "clumper," Sir Oliver, like all good British scientists, picked a fancy Latin name for this device. You might be interested to know that when the illustrious Mr. Marconi sent a long distance radio message across the English Channel for the very first time, the message was addressed to Edouard Branly thanking him and acknowledging his "remarkable work" that enabled this thing to happen.
Monsieur Branly, Sir Oliver and other scientists soon discovered that once the filings were 'cohered' together, they stayed cohered and that severely limited the device's usefulness. I mean, what is the use of something if it turns on, but won't turn off? To overcome this, Sir Oliver, added a "tapper" circuit to shake up the filings and restore them back to their high resistance state. Guess what Sir Oliver called his invention? Give up? OK, he called it a 'de-coherer' and basically it was just a buzzer that was mechanically coupled to the coherer to shake up the coherer and keep the filings from always being stuck together. Now, if I was Sir Oliver, I would have called my invention a "dis-Lodger" Ha, ha, that's a joke. Get it? Sir Oliver Lodge's dis-Lodger. Come on, that's funny. Oh, never mind.
The problem with the buzzer is that it created its own radio frequency noise and so it had to be shielded from the antenna and coherer, usually by being put in a metal cup. This was the first known application of the principle of "RF shielding" which was to become very important in later radio design.
To this day, just why filings stick together in the presence of a radio current is still not completely understood, but the fact remains that if a pile of metal filings are loosely placed between two electrodes and no radio frequency current is present, the resistance between the electrodes is quite high. However, if a radio frequency is present, somehow the sharp edges of the metal filings microscopically "weld" together and the resistance is dramatically lower.
Details of the Coherer
A practical radio receiver of the late 19th to early 20th Centuries.
This design used familiar technologies from the telegraph industry.
Radio operators could be recruited from the ranks of existing telegraph operators.
In the diagram above you see a long wire antenna. Passing radio waves will cause a radio frequency voltage to appear on that wire. It was found that the length of the antenna is important and depends on the frequency that the spark-gap transmitter (actually a Tesla Coil with an antenna) is tuned to. As you see, in this diagram of a very early radio, there is no tuning circuit beyond the natural length of the antenna, but in the early days, there were so few stations, none was needed. Naturally this made these receivers subject to all kinds of interference ranging from electrical discharges from lightning strikes, streetcar sparks and even sparks from the buzzer and relay contacts within the radio itself. Tuned capacitor and coil arrangements soon began to appear in the antenna circuit.
Below the antenna is the coherer encapsulated in a glass tube with the air sucked out. One side of the coherer is connected to ground and the other side is connected to a very sensitive telegraph relay circuit. The circuit consists of a choke coil (to isolate the antenna and coherer from the relay coils and any noise generated there), a telegraph relay, a battery and the coherer itself. Under no-signal conditions, the current through the coherer is so little, the relay is not pulled in, but when a signal is present, the coherer conducts, current flows and the circuit is complete thereby causing the relay to pull in. When the relay pulls in, the much heavier and louder telegraphic sounder also pulls in producing an audible "CLICK" and the sounder holds in until the relay opens and when it does, the sounder make its loud "CLACK" in return.
There's a big problem once the relay pulls in though. As I mentioned earlier, normally the relay won't let go because once the filings in the coherer are 'welded' together, the current keeps flowing forever or until something shakes the coherer and causes the filings to stop cohering. To overcome the stickeyness of the filings in the coherer and to break this connection when the signal from the antenna is no longer there, a simple buzzer circuit is mechanically linked to the coherer to "shake up" the filings and cause the coherer to go back to its non conductive state. Of course, if the radio signal is still there, the filings will stay welded during the shake and the relay will stay pulled in.
How the coherer worked to allow Morse Code to be transmitted:In a normal Morse Code character --- let's say the letter 'C' --- the sound pattern is (click----clack) (also known as a dash), (click-clack) (also known as a dot), click----clack (dash), click-clack (dot) --- and that is the letter C. In other words, the relay holds the sounder in for a relatively long time, releases it, follows it by a relatively short click-clack, followed by a relatively long click---clack and ending with another relatively short click-clack. As awkward as it is to describe, this is a very, very familiar pattern that all telegraphers instantly recognize as the letter C.
A telegraph sounder receiving Morse CodeTo faithfully reproduce this pattern of clicks and clacks, the de-coherer must try to interrupt the coherer's current several times for even the shortest period click-clacks or otherwise the operator won't be able to tell short periods from long periods. For example, say that the de-coherer was operating so that the coherer was being shaken once every 10 milliseconds. If the click-clack for a short 'dot' is 50 milliseconds, the relay will be held in a maximum of 60 milliseconds (50 + the time to shake it again which may be as much as 10 milliseconds). If the click-clack of a long 'dash' is 100 milliseconds, the relay will be held in for a maximum of 110 milliseconds. The difference between dashes and dots, even if their length varies a little from ideal, is easily distinguished by a telegraph operator.
I haven't a clue what is being sent.
By the way, since these early radio receivers used standard telegraph equipment, a commonly used Telegraph Register, which recorded the Morse characters as a series of long and short lines on a strip of narrow paper, could be used instead of the sounder. This would enable the messages to be permanently recorded so they could be read later. Besides that, looking at a series of long and short lines (representing the dots and dashes), people such as myself would find it much easier to decode Morse characters than listening to all that clicking and clacking noise. Still, listening to sound and decoding the letters and numbers as they were received was the most common method in use and it was about to be improved.
A Telegraph Register for recording messages
A year or two after this, one of Mr. Bell's telephone receivers was substituted for the telegraph relay and it was discovered that the human ear could make out Morse Code even better that way and no relay was needed. From then on headphones were used instead of relays and sounders. I am pretty sure that a telephone receiver would pick up the frequency of the buzzer as it de-cohered in the presence of a radio frequency current. If that is true, the operator would hear a the dots and dashes as a series of long and short beeping sounds. From the demonstrations I have provided, I think you will agree that it is a lot easier to make out Morse Code characters if they are a series of beeps rather than if they are a series of clicks and clacks. In other words, the letter C would now sound like "beeeeeep, beep, beeeeeep, beep" which I think you will agree sounds much nicer than clicks and clacks. In addition, the human ear has a marvelous ability to hear patterns and fish out Morse characters even when a lot of noise is present.
Here the telegraph relay and sounder is replaced with a receiver developed for the new telephone industry.
The telephone receiver was much more sensitive than even the best telegraph relay and could detect
even the tiniest changes in conduction within the coherer. The sound quality was also greatly improved
making it easier to listen to.
The coherer was an important early tool because it allowed scientists and engineers to convince the world that wireless communications was possible and it enabled them to discover many important radio principles that were important to the later development of radio. Although the coherer did work and was an important first step, the truth is, it really was a very poor and a very "deaf" kind of detector that required a huge signal from a powerful, nearby transmitter. Fortunately, the early spark-gap transmitters were extremely powerful, but even so, the distances were very limited because of the coherer's deafness.
As Marconi and others worked with the limitations of the coherer, at the same time the race was on to find and build better detectors that had the sensitivity to pull in weak signals from far away. By the middle of the first decade of the 20th Century, scientists and engineers began to develop other kinds of detectors that weren't so "hard of hearing" so that ships and shore stations could communicate with each other in a regular and reliable way over hundreds of miles.
At this point I should mention that a lot of brilliant scientists, including the Indian genius, Jagadish Bose, brought the coherer to a high state of development, but ultimately all their hard work amounted to very little because it lead to a dead end. Soon detectors based on entirely differently principles and which worked so much better were developed by other scientists.
The end of the coherer as a commercial radio detector and some words about its replacementsEarly in the 20th Century, scientists working for the Marconi company developed a complex magnetic detector (the Maggie) that worked a whole lot better than any of the coherer designs. The Maggie enabled ships to communicate ship to ship and ship to shore reliably and until about 1918, the Marconi company's "Maggie" was the standard shipboard radio detector.
The Marconi Magnetic Detector
Diode detectorsAround the time of the sinking of the Titanic in 1912, the Fleming Valve diode and the De Forest Audion triode and the crystal detectors were found to be superior to the Maggie and soon afterwards those detectors began to replace the complex, expensive and very awkward "Maggie." According to reports, the very modern radio room of the RMS Titanic was equipped with all the latest radio gear. It had a Fleming Valve radio, a crystal detector radio and, of course, an official Maggie detector radio. If I'm not mistaken, all radio messages received by the Titanic while it was at sea were by means of the Fleming Valve radio, probably because it was more sensitive at picking up weak signals than the crystal detector radio and clearly superior to the "Maggie" in both ease of operation and sensitivity. Of course, the Fleming Valve radio had some serious disadvantages: (1) the tubes were very expensive (2) batteries to run the filament needed frequent changing and, (3) the tubes had hot, glowing filaments that would burn out from time to time (as the early tubes were notorious for). I'm sure large ships like the Titanic had an ample supply of spare tubes and batteries and certainly the Marconi company could afford to buy tubes and batteries by the boatload. With so much money to be made sending and receiving radio messages for wealthy passengers there on the Titanic, those extra costs were trivial compared to the good performance their Fleming Valve radio gave.
The Fleming Valve and crystal detectors
Of course, all or most of Titanic's distress messages were heard by radiomen who were using the Maggie detector.
A Fleming Valve diode tube.
The dark band around the tube is a silk thread
that was put around the tube as a strain relief for the
anode (plate) wire soldered to the side of the tube.
Upright, this tube is about 3 inches tall.
A BASIC FLEMING VALVE RADIO
Notice how similar this radio is to the crystal radio shown below.
Both use an electronic diode to turn radio signals into varying
DC waves that can be heard by the human ear.
Neither radio amplifies the signal and both load down
the tuner, but the Fleming Valve detector is more sensitive.
A BASIC CRYSTAL RADIO
The crystal and cat's whisker formed the detector which allowed
the radio signal to be heard in the phones. The crystal detector
wasn't quite as sensitive as the Fleming Valve detector, but it was
very cheap, rugged, didn't need batteries and didn't burn out.
When the Titanic sank, the closest ship was the SS Californian. She carried the complex "Maggie" as her only detector, but unfortunately her radio operator had gone to bed before the first distress signal was sent. Earlier that evening, the Californian's radio operator had tried to tell Titanic that they were stopped dead in the water because of all the icebergs nearby and that Titanic, approaching this same area, should be extra alert, but he was told to "shut up, shut up" by Titanic's radio operator. After being told to "shut up" so rudely, the Californian's radio operator did as he was told and stopped trying to call Titanic. Later that evening, on orders from the captain, the Marconi operator turned off his radio gear and went to bed.
That night, after the official Marconi radioman had gone to bed, one of the Californian's officers, a man who knew a lot about radio and who knew the Morse Code, let himself into the radio room to play with the ship's radio equipment. He knew what a distress call sounded like and he could have heard Titanic calling for help (even with a coherer because they were that close), but the officer didn't know how to get the complex "Maggie" working so he heard nothing. In later years this officer blamed himself for not insisting that the Marconi Company's operator teach him how to use the "Maggie" while they were at sea.
For reasons never completely explained, the SS Californian's captain never got his radio operator out of bed that night to find out what was going on with all the sky rockets they were seeing. Because the rockets didn't seem to be the proper "distress" color, it is likely that the captain assumed that they were midnight celebration fireworks. Perhaps too, after having their warning rebuffed so rudely, the captain was reluctant to allow his radioman and himself to be insulted again. Whatever the reason, the one ship that could have done the most good and saved the most lives, never arrived in time to save a single person.
**(see a more complete description of this incident at the bottom of this article)
As you can imagine, the sinking of this ship caused a lot of ripples -- and not all of them were on the surface of the sea. A first class ticket would have cost about $100,000 in today's money and a lot of these very rich and very important people were killed. Worse yet, a shockingly larger percentage of poor passengers were killed too, so big changes in safety procedures and restructuring of radio operations at sea followed the disaster. There were new arrangements in the way the Marconi Company's men were integrated into the ship's crew and from then on, all ships were required to have radio receivers and people who knew how to operate them. Ships at sea were required to listen for calls on specific distress frequencies at the top of every hour, night and day and the German style S-O-S replaced CQD as the accepted international distress call.
Backing up a little, not long before the sinking of the Titanic, it was discovered that both naturally occurring and man-made crystals made excellent radio detectors. Crystals of lead sulphide (galena) were the most common and most useful radio detectors. The complete detector also included a "cat's whisker" - a sharp springy wire - that was mounted so it could sweep across the crystal face to find a "hot spot" where the signal was loudest. The crystal detector has the advantage of being reasonably sensitive, rugged, very cheap (compared to an Audion tube or Fleming Valve), It can not wear out or burn out and a crystal detector requires no batteries. Besides that, there were mines that were full of tons of galena crystals, but replacement tubes were very expensive and rare. The big disadvantage of the crystal set was that the operator would loose contact and have to hunt for a new "hot spot" if the radio was moved or jarred in any way (like hitting a iceberg at full speed). I should also mention that when sending messages, the crystal had to be protected from the high energy of the transmitter or it would be destroyed -- the Fleming Valve was more rugged in this regard.
Crystal diode detector
For the reasons mentioned just now, the crystal detector became the most widely used detector up until about 1920. As far as I can tell, most of the radios used by the world's armies during World War One had crystal detectors, however many of the more sophisticated headquarters radios and ship board radios, with access to replacement tubes and batteries, used a Fleming Valve, a Audion tube or the newly available and greatly improved hard vacuum triode as "grid leak" radio detectors (all of which I shall discuss shortly). Although the vast majority of military radios were crystal sets (as mentioned), it is my understanding that the French Army sent into the field, several thousand early vacuum tube receivers too, but more will be said about them later.
After America entered the Great War, the famous radio engineer, Edwin Armstrong joined the U.S. Army Signal Corps and there he was put to work operating a huge radio consisting of dozens of improved audion tubes (Type R valves) the British were using to locate enemy radios. This radio was so huge that it took an engineer and a half dozen technicians just to operate it, but, as crude as it was, it enabled the Allies to know just where the German radios were. It was while working on these big radios and analyzing captured German radios that Armstrong developed the ideas that he would later patent as the "Super-Sonic Heterodyning Radio" or, as we say today, the superheterodyne. By the way, at the end of this article I have a link to an essay on the superheterodyne radio and how it works.
Armstrong allowed the U.S. Army to use his many radio patents without charge during the Great War, but as far as I can tell, none of the American Army's official military radios used in Europe were designed to use tubes and nobody's army or navy used Armstrong's vastly superior regenerative system. The crystal detector radio the U.S. Army used (the famous BC-14/SCR-54, which was an almost exact copy of the French military crystal radio) was considered "good enough" for the Army's purposes and we all know that "good enough" is the mortal enemy of progress.
I have recently learned that 150 one tube radios were built by Westinghouse for the U.S. Bureau of Standards during the latter part of World War One. I do not know for a fact, but I greatly suspect that they used a British 'R' valve. Apparently they were not used in Europe and I think it is unlikely that the design was based on Armstrong's regenerative detector. These radios are so rare, I can find very little technical information about them.
A diagram of the U.S. Army's crystal radio used during WW 1
This radio was very popular with early radio listeners who bought them cheap at Army Surplus stores.
By the way, the American and similar European military radios were designed to operate on what was then considered the "short wave" band, from 550 kilocycles to 1600 kilocycles. Manufacturers turned out tens of thousands of these sets for the military and at the end of the war they were eagerly bought up by radio experimenters. I don't think it is any coincidence that the tuning range of these military surplus sets exactly matched what was later to become the AM broadcast band. I think it is highly likely that the tuning range of these wartime radios pretty much defined what the AM broadcast band would become.
Lee De Forest's Audion detector tube
An improved detector, but with limitations
As already implied, the Fleming Valve was the first vacuum tube and it was developed as a radio detector, which it did very well. Obviously though, the Fleming Valve could not amplify a radio or audio signal so its use was limited. A couple of years later Dr. Lee De Forest was experimenting with Fleming Valves in an attempt to improve their performance. He noticed that if he wrapped some wire around the outside of the tubes and connected the antenna there, the detected signal seemed louder. After this he had his glass blowers put in a third element (consisting of a grid of wire) between the hot cathode and the cold anode plate inside the tube and he noticed that the resulting three element tube detected radio signals much better than any crystal or Fleming Valve radio could. De Forest's new tube contained a lot of gas in it and he thought it was the ions in the gas that made it work so he called it an Audion tube -- Aud (for audio) ion (for ionized gas). De Forest was completely wrong about how his tubes worked, but they worked anyway -- sort of. It took other, much smarter and better educated researchers to discover exactly how the Audion worked and that they could be vastly improved (for most purposes) by removing all the gases inside. They found that removing all the gasses would create a high ("hard") vacuum inside the tube so that the electrons from the hot filament could move more easily from the filament to the cold anode plate and, more importantly, the movement of electrons followed the signal on the grid much more faithfully.
A radio using a Audion tube as a "grid leak detector"
This design offered some amplification of the signal
and it didn't load the tuned circuit (to the right) nearly so much.
Similar radios using gas filled detector tubes were used
by the French military during WW1, as mentioned earlier and,
to a limited extent, by the US and British Navies.
The Audion worked well as a radio detector, but for amplifying audio (sound), they caused too much distortion and were useless in this application. To make matters worse, not all Audions were the same. Some brand new ones worked well as detectors and others worked poorly and they would change characteristics the longer they were in service so you just couldn't rely on them. All this contributed to the unpopularity of the Audion detector even though good ones were noticeably superior to diode detectors. For all these years then, the only amplifier tube available to the public was the Audion and this is the way things stood until the end of WW 1 and the explosion of radio technology in the 1920s.
An Audion Tube from 1909
The grid and filament are behind the rectangular metal plate (anode) shown in the photo.
Armstrong's regenerative detectorEdwin Armstrong invented his regenerative detector in 1911 while he was in college. It was, by far, superior to the best crystal detectors in every respect and superior to the Audion detector too, but it didn't catch on for several years. The reason the regenerative detector was so slow to catch on was because proper electron tubes hadn't been developed for it yet. It was only after WW 1 was over, when in 1921 the famous WD-11 and similar high vacuum triode tubes were finally available to the general public, that the regenerative detector became a practical device.
For a few years after the war, when early electron tubes were rare and expensive, single tube regenerative radios (similar to the one I built as a kid) were very popular. With a lot of amplification from regeneration, they didn't need any more than one tube to power a headset (or one or two more tubes if you wanted to run a loudspeaker). As simple, effective and cheap to build as regenerative detector radios were, they were not easy for the average person to adjust and what's worse, they would squeal and howl as they were tuned across the dial. Especially because of all these bothersome noises, radios with regenerative detectors gained a terrible reputation with the public and indeed, many early radio advertisements included a guaranteed promised that THEIR radio was easy to tune and absolutely would not squeal or make funny noises.
For all its faults, the regenerative detector was far superior to the crystal detector and so, by the mid 1920s, radios based on the regenerative principle began to make the crystal radio, with its cats whisker, obsolete.
Because they were difficult to tune, because of all that loud squealing, because improved tubes were rapidly evolving and were becoming available to power the more advanced Tuned Radio Frequency (TRF) radios and especially the new superheterodyne radios, the regenerative detector, like the diode detector before it, started to disappear so that by 1930, except for just a few very inexpensive models, the regenerative detector was virtually extinct in America and in most other industrial nations too. An exception to this was the simple and affordable German Volksempfanger radios who's design, manufacturing and sale was encouraged and coordinated by the Nazi government. Propaganda minister Joseph Goebbles was keenly aware of AM radio's value as an effective propaganda tool (much as the American Right Wing political and religious broadcasters value and use AM radio today). Knowing AM radio's power to influence ordinary people, the evil Doctor Goebbles used the power of his government to insure that working class people had an affordable radio so they could listen to his frequent propaganda broadcasts. In fact, a popular nickname for the Volksempfanger was "the Goebbles radio."
Armstrong discovered that adding regeneration vastly improved
the sensitivity and the ability to "tune out" interfering signals.
Notice how similar this detector is to the Audion detector. In fact
the US Supreme Court mistakenly failed to grant Armstrong
patent rights to this radio and gave it to De Forest instead.
The return of the Audion as a radio detectorThere was a rapid evolution of better and better electron tubes during the decade of the 1920s and this caused a similar rapid evolution of radio technology designed for the consumer. So-called 'Neutrodyne' and 'screen-grid tetrode tube' TRF radios appeared and in 1924, the very first superheterodyne radio (the Radiola AR812) hit the American market.
Armstrong's superheterodyne radio and many of the TRF radios did not use a regenerative detector to strip the sound off the radio waves they were tuned to so, ironically, an improved version of the Audion tube came back into universal use as a radio detector. These new tubes operated on a principle similar to the old Audion tubes in that the gas in them caused them to act as thyratrons and this allowed them to detect voice and music and to provide some additional amplification too. Unfortunately, these tubes still suffered from the same weaknesses as the older Audion tubes in that they were short lived and unreliable.
These so-called "grid leak detector" tubes were used in multi-tube radios (TRF and superheterodynes) from about 1921 to about 1928 when tube designers dropped the thyratron detector and updated an even older technology, the Fleming Valve detector. These "new" Fleming type detector tubes have a high vacuum in them and so they last a whole lot longer and are a whole lot more reliable than the thyratron detectors.
The return of the Fleming Valve as a radio detectorStarting in the late1920s, nearly every radio detected its audio by use of a "thermionic diode tube." These diode tubes, as mentioned, were actually a Fleming Valve and a typical example is shown in the left hand section of the drawing below. The other half of the tube, shown on the right, is a triode (a vastly improved Audion) and its purpose is to amplify the detected audio before sending it on to the final amplifier and the loud speaker. Thus the Fleming Valve was married to a vastly improved version of the Audion in these new so-called "2nd detector" tubes. Of course there is a huge difference between the high vacuum triode tube and the poorly performing Audion tube since Audions were unable to amplify sound without terrible distortion and therefore couldn't be used as audio amplifiers.
Soon after the Great War ended, vacuum tube technology and its manufacturing began a rapid development as AM radio became an important source of entertainment. Radio designers and manufacturers now had excellent advanced design, multi-element, high vacuum tubes to work with. These tubes were available at reasonable prices because they were made in quantity on automated assembly lines rather than being hand made, one at a time. Tubes such as AC power rectifier diodes, detector diodes, linear amplifying triodes, tetrodes, pentagrid converters, power pentodes, sharp cutoff and remote cut-off pentodes all experienced a very rapid evolution. At the same time, huge transmitting tubes that could broadcast tens of thousands of watts were also making a rapid evolution. By the early 1920s radio broadcasters were popping up all over the place and everybody was listening to the radio. By the early 1930s the superheterodyne radio was king and the newly developed beam-power tetrode tubes drove high fidelity "moving coil" loud speakers with enough watts of power so that everybody in the house could listen without having to have headsets.
Portable radios continued to use the tube as shown, but later home
radios powered by AC used a cathode that was indirectly heated by the filament.
The rise and fall of thermionic tubes as the backbone of electronics technology
By World War 2, radio communications had become extremely sophisticated with extensive world wide networks all based on vacuum tube technology. Up to about 1955, vacuum tube technology continued to expand with tubes getting smaller and at the same time they were able to do the most wonderful things with electronic signals way up into the ultra high frequency (UHF) ranges and beyond. However, after 1955, tubes began a slow decline in importance as the transistor, based on crystals of germanium and silicone, took the lead.
The return of crystal technologyFor many decades during the 20th Century, scientists studied and experimented with the secrets of crystalline structures especially those very special crystals called semi-conductors. It was found that certain impurities that found their way into crystals and put there not by nature (as in the case of the cat's whisker galena crystals), but on purpose (called "doping") effected the way the crystal conducted electricity. After many experiments and after learning much about the way crystals behaved at the atomic level, the first transistor was produced in 1947.
Of course, the first transistor was too crude to do much, but it demonstrated that crystals could be made to do everything that the vacuum tube could do and do it ever so much smaller. After this, a very rapid development of what is called "solid state electronics" began with the first commercial transistors and the first transistor radio making it to market in 1954. In fact, by 1957 I had a one transistor crystal radio that I built and played with as a kid (a Remco Radio mentioned in my "First Amplified Radio" article).
Naturally, these early "solid state" devices were very expensive and they performed very poorly at high frequencies, but as manufacturing technology became more and more efficient, the costs came down and the performance of the tiny crystal devices greatly improved until they completely took over from vacuum tubes. After the mid 1970s, nobody sold products with vacuum tubes in them anymore except for the CRTs in TVs and PCs and the magnetron tubes still used in microwave ovens.
For a long time now we have been back to using crystals, but oh what crystals. Engineers and scientists living in 1912 couldn't have imagined the kinds of crystals we have today. Crystals that allow us to read text and see pictures from all corners of the world and have them displayed on liquid CRYSTAL screens. The images on these screens are produced by fantastically powerful computer circuits that use chips of crystallized silicone. Any one of the many marvelous feats that the modern personal computer does routinely would have been considered the deepest magic just a few decades ago.
All this "solid state" magic and fantastic digital technology aside, we should never forget that the electronics age and the digital age would never have started without the vacuum tube. The technology that grew out of the coherer gave birth to electron tube technology and without the great advancements in electronics made possible by electron tube technology, we would not have the world of electronic communications and the vast digital networks of today. I really doubt that the first transistor could have been built without all the vacuum tube powered test equipment that was used there at the Bell Lab. I think we owe a lot to those who developed this early technology and we should at least remember who some of those people were. Steve Jobs made "smart phones" look pretty, but it was fellows like Branley, Lodge, Tesla, De Forest, Armstrong, Farnsworth and many others that started it all almost from scratch. If Armstrong is virtually forgotten today, I wonder how long Jobs' name will be remembered?
A challenge to experimentersOf course, for over 100 years the coherer has been hopelessly obsolete, but recently I have noticed that there are a few simple experiments using home-made coherers posted on YouTube. As interesting as some of these experiments are, they are not "real" radios because none of them (that I have seen) can be used to receive messages. Still, I'd very much like to see somebody build a complete coherer radio that would actually function as a wireless telegraph. It shouldn't be any more difficult than building a crystal radio, so why not? In addition to the fun of building such a radio, it should be very interesting and fun to see just how far away it is able to receive a signal from a calibrated transmitter of known power output. When done experimenting with such a coherer radio, a hobbyist could easily turn it into a crystal radio for listening to AM radio or it could serve as a basis for a transistorized regenerative radio. Many happy hours of experimenting with such a project could be had. Who wants to be the first?
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Other links to my radio pages that you might enjoy:
If you are interested in learning a little more about basic radio theory, please checkout
An essay on the Armstrong Superheterodyne Radio Principle
If you want to learn something about how crystal radios work, please see the story of
My Heathkit CR-1 high performance crystal radio
If you are thinking of building a crystal radio, perhaps you'd like to read the story about
My high performance crystal radio kit
Please note that my Armstrong "Crystal" Radio project, linked to below,
will greatly outperform this radio and is cheaper and easier to build.
My Armstrong regenerative radio projects
This is the story of my first regenerative radio I built when I was in the 8th grade,
My First Amplified Radio
And my first introduction to the wonders of vacuum tube technology.
Over 50 years later, I built an almost identical radio, but using very cheap and easily available components.
An Armstrong "Crystal" Radio
Amazing performance from a radio that is easier and cheaper to build than almost any crystal radio.
If you want to build a simple radio yourself, this is the one I suggest.
from "The Old Geezer Electrician"
If you are looking to build something with the same great performance of these other regenerative radios,
but looks a whole lot nicer, I would like to suggest
The Geezerola Senior radio
Another Armstrong regenerative radio story, but this one tunes shortwave,
My Regenerative Shortwave Radio.
If there is nothing on your local AM radio worth listening to, perhaps you would like to build
A low power AM transmitter
I have several other stories regarding antique and home made radios you might like to read.
Select Another Really Interesting Radio Story.
or, as a last resort, you can search for something starting at
My Home Page
** The Titanic's main radio operator told the Californian's radio operator to "shut up - shut up" when he tried to warn him about a nearby ice field that they were stuck in. This was indeed rude behavior and very well may have been a key factor in the later disaster. The reason the Californian's radio operator was told to shut up is because the Titanic's radio was being used to send and receive very lucrative personal messages back to shore (Cape Race) for wealthy customers. Titanic's radio operator didn't take the time to listen to warnings from the SS Californian (call sign MWL) because, (1) although the message originated from captain Lord, the message was worded very informally and almost as idle chit-chat between radio operators (sort of like, "say old man, guess what our ship is doing right now?"). The message was not proceeded by a short request for a break-in, but was sent directly on top of radio traffic that was being transmitted from Cape Race. Of course, the signal from Cape Race was so faint that the Californian's Maggie detector probably couldn't hear it. After breaking in, MWL should have waited for permission to proceed from MGY and, most importantly, the message should have been sent as a Masters Service Gram (MSG) addressed directly and formally from one ship's captain to another ship's captain:
(for example, "MGY DE MWL.-.-.- MSG from Captain Lord to Captain Smith .-.-.- navigation warning .-.-.- massive icebergs ahead .-.-.- visibility poor with no discernible horizon .-.-.- Californian engines stopped for the night .-.-.- advise you stop or reduce speed for safety .-.-.- MSG receipt requested -.- ").
Captain Smith was considered the world's finest captain and it is my opinion that such a man would have responded appropriately to a formal notification of deadly navigation hazards directly in his ship's course. (2) the Titanic's radiomen were busy making a lot of money (and I mean a lot of money), for the Marconi Company with personal radio traffic - after all, this was the primary reason the Titanic even had a radio on board. Besides that, the Titanic was "unsinkable" so why should the radio operator "waste" valuable air time listening to informal reports of big icebergs when even a brief interruption would have cost hundreds dollars in lost sales? Finally, there is this: for most of the previous 24 hours, the Titanic's transmitter had been off the air due to a high voltage short in a part of the apparatus that was very difficult to get to. Because of this outage, there had accumulated a huge backlog of messages that needed to be sent. Overworked as they were and exhausted from lifting the heavy electrical machinery of the transmitter, it would be understandable if Titanic's radiomen were a bit short tempered.
I have a hypothesis that it was Titanic's rude behavior that caused the Californian to be less than eager to try to contact Titanic even after seeing sky rockets around midnight. Perhaps the Californian's captain thought "These have to be celebration fireworks for some big midnight party -- just look at the color of those things -- I'm not going to get my radioman out of bed just so he can have his feelings hurt again by those rude people." Was it perhaps rude behavior coupled with a urge to make money for the Marconi company that cost the Titanic's radio operator his life and the lives of many hundreds more besides? I think there is a good chance this was a factor.
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