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AN
EARLY COHERER RADIO RECEIVER
CIRCA 1898 INCLUDING SOME THOUGHTS ON EARLY RADIO DETECTORS A comprehensive story containing historical, educational, technical and biographical elements & opinions just for the fun of it by John Fuhring Introduction
I 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, but a radio detector. 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 detector
The name 'coherer' comes from a very
interesting
discovery that the French scientist Edouard Branly made around 1890.
He discovered that a
radio frequency current tends to make sharp metal filings clump or
"cohere" together. By the way, the word 'cohere' comes from a
Latin word for clump or stick together, so instead of calling his
device something silly like a "sticker" or a "clumper," Monsieur
Branly, like all good scientists, picked a fancy Latin name for his
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 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, a British scientist, Sir Oliver Lodge, added a "tapper" circuit to shake the filings. 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 keep the filings from always being stuck together. 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 Code
To 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. A year or two later 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. In fact, the telephone receiver would pick up the frequency of the buzzer and listening to beeps is a lot easier to decode a character than 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 tones and fish out a weak signal from a lot of noise.  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 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. These principles included the use of tuned circuits between the antenna and the detector to "tune out" other radio stations operating nearby and to cut down on the other electrical noises that are out there. 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. 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. As Marconi and others discovered the limitations of the coherer, the race was on to find and build better detectors that had the sensitivity to pull in weak signals from far away. Early in 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. The
end of the coherer as a commercial radio detector and some words about its
replacements
Early 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 for several years, the Marconi company's "Maggie" was the
standard radio detector used aboard all
large ships.
 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 should stop too, 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. The "Maggies" were soon replaced, 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, people who knew how to operate them and they were to listen for distress calls at the top of every hour, night and day. The German S-O-S call replaced CQD as the accepted international distress call.
Backing up a little, not long before the sinking of the Titanic, it was discovered that naturally occurring lead sulphide (galena) crystals made excellent radio detectors when used with a "cat's whisker" - a sharp springy wire - that could sweep across the crystal face to find a "hot spot." The crystal detector has the advantage of being reasonably sensitive, rugged, very cheap (compared to an Audion tube or Fleming Valve), never wear out or burn out and they require no batteries. Besides that, replacement tubes were very expensive and nearly impossible to get, something the crystal radio user didn't have to worry about. The big disadvantage of the crystal set was that the "cat's whisker" could be jarred by the firing of a ship's guns or any heavy vibration and a new "hot spot" would have to be found. 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 or the newly available and greatly improved hard vacuum triode as "grid leak" detectors. It is my understanding that the French Army bought and sent into the field, several thousand vacuum tube receivers during the war.  A radio using a triode 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) so much. After America entered the Great War, the famous radio engineer, Edwin Armstrong joined the U.S. Signal Corps and he was appointed to set up communications for the Paris Peace Conference of 1919. As far as I can tell, none of our official military radios in use at that time were designed to use Armstrong's vastly improved regenerative system, but Armstrong did allow the U.S. Signal Corps to use his other radio patents without charge while he was in the Service.  Armstrong discovered that adding regeneration vastly improved the sensitivity and the ability to "tune out" interferring signals. Notice how similar this detector is to the one above. In fact the US Supreme Court mistakenly failed to grant Armstrong patent rights to this radio and gave it to De Forest instead. The
perfection of the high vacuum tube as a radio detector
For a few years after the war, when
electron tubes
were rare and expensive and short-lived, the single tube regenerative
radio (similar to the one I built) was very popular. With a
lot
of amplification from
regeneration, they worked on the principle of "grid rectification" as
did the early Audions. Soon after the war, vacuum tube
technology
and 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 assembly lines rather than
being hand made, one at a time. Tubes such as AC power
rectifier
diodes, linear amplifying triodes, pentagrid converters and remote
cut-off
pentodes experienced a very rapid evolution. By the late
1920s
everybody was listening to the radio and the newly developed beam-power
pentode tubes
drove their loud speakers with enough watts of power so that everybody
in the
house could listen without having to have headsets.Ironically, as the powerful and sensitive Tuned Radio Frequency (TRF) and Armstrong's Superhetrodyne radios flooded the market and the popularity of listening to the radio swept the civilized world, the Fleming Valve came back into universal use as a detector. After the mid-1920s, nearly every radio detected its audio by use of a "thermionic diode tube." These diode tubes were actually a Fleming Valve and a typical example is shown below as the left hand section of the tube. The other half shown 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.  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. Back to crystals as radio detectors A
challenge to experimenters
Of 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? The
End
Having arrived this far, obviously you have a superior attention span and reading ability that far exceeds that of the majority of web users. I highly value the opinion of people such as yourself, so I ask you to briefly tell me: Did
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Please visit my guest book and tell me. If you like my web page, tell your friends, if you don't like my web page, tell me. Other links to my radio pages that you might enjoy: If you liked the coherer story and want to read how I built an Armstrong regenerative tube radio when I was in the 8th grade,  The story of my First Amplified Radio. If you are interested in learning a little about basic radio theory, please checkout  An essay on the Armstrong Superhetrodyne Radio Principle If you are thinking of building a crystal radio, perhaps you'd like to read about  My high performance crystal radio kit If there is nothing on your local AM radio worth listening to, perhaps you would like to build A low power AM transmitter If you'd like to read the story of my latest Armstrong regenerative radio that tunes shortwave and uses an FET for the detector,  My Latest Radio. 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 (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. Return to the article. |
