Thecrystal radio receiver (also known as a crystal set) is a very simple kind of radio receiver. It needs no battery or power source except the power received from radio waves by a long outdoor wire antenna.
Introduction
Simple crystal radios are often made with a few hand made parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones. Because crystal radios are passive radio receivers, they are technically distinct in many respects from ordinary radios containing active powered amplifiers. This is because they must receive and preserve as much electrical power as possible from the antenna and convert it to sound power whereas ordinary radios amplify the weak electrical energy "signal" from the radio wave. Today making and operating crystal radios is a popular hobby for many reasons, including:
- Historical and nostalgic significance
- The astonishing results one can get from its utter simplicity
- The challenge of receiving weak distant signals without amplification
Crystal radios can be designed to receive almost any radio frequency since there is no fundamental limit on the frequencies they will receive. The most common crystal radios are designed for the AM Broadcast Band and the 49-meter international short wave band, partly because the radio waves are stronger in those bands. Early radios commonly received spark signals as low as 20 kHz and below. Although crystal radios are designed to detect AM, they also frequently detect FM fairly well which is in the 100 MHz range.
Groups of enthusiasts and a number of web sites are devoted to their construction. Regular contests are held comparing the performance of various designs with each other. Reportedly, modern diodes, ultra-thin litz wire inductors, and low loss capacitors yield performance far beyond that of the original receivers.
How it works
A crystal radio receives programs broadcast from radio stations. Radio stations use sound waves to modulate the amplitude of radio waves and transmit them from their antennas. Radio waves from all stations in range of the receiver travel across the crystal radio antenna and induce electric current between the antenna wire and the ground wire. A tuner circuit is used to select the radio-frequency energy from just one station. The crystal detector (such as a Cat's whisker detector or diode) recovers the original sound frequency current from the radio frequency carrier. The earphones then convert the recovered audio-frequency current back into sound.
History
Crystal radio was invented by a long, partly obscure chain of discoveries in the late 1800s that gradually evolved into more and more practical radio receivers in the early 1900s; and constitutes the origin of the field of electronics. The earliest practical use of crystal radio was to receive Morse code radio signals transmitted by early amateur radio experimenters using very powerful spark-gap transmitters. As electronics evolved, the ability to send voice signals by radio caused a technological explosion in the years around 1920 that evolved into today's radio broadcasting industry.
Early years
Early radio telegraphy used spark gap and arc transmitters as well as high-frequency alternators running at radio frequencies. At first a primitive detector called a Branley Coherer was used to indicate the presence (or absence) of a radio signal. However, these lacked the sensitivity to convert weak signals.
In the early 1900s, various researchers discovered that certain metallic minerals, such as galena, could be used to detect signals. In 1901, Sir Jagadish Chadra Bose filed for a US patent for "A Device for Detecting Electrical Disturbances" that mentioned the use of a galena crystal; this was granted in 1904, #755840.
However, his work, and the patent, went somewhat unnoticed in the western scientific world, as on August 30, 1906, Greenleaf Whittier Pickard filed a patent for a silicon crystal detector, which was granted on November 20, 1906. Pickard's detector was revolutionary in that he found that a fine pointed wire known as a "cat's whisker", in delicate contact with a mineral produced the best semiconductor effect. A crystal detector includes a crystal, a special thin wire that contacts the crystal and the stand that holds the components in place.
A modern Crystal Set
The most common crystal used is a small piece of galena; pyrites was also often used, as it was a more easily adjusted and stable mineral, and quite sufficient for urban signal strengths. Several other minerals also performed well as detectors. Another benefit of crystals was that they could demodulate amplitude modulated signals. This mode was used in radiotelephones and to broadcast voice and music for a public audience. Crystal sets represented an inexpensive and technologically simple method of receiving these signals at a time when the embryonic radio broadcasting industry was beginning to grow.
In 1922 the (then named) U.S. Bureau of Standards released a publication entitled, Construction and Operation of a Simple Homemade Radio Receiving Outfit. This article showed how almost any family having a member handy with simple tools could make a radio and tune in to weather, crop prices, time, news and the opera. This design was significant in bringing radio to the general public. NBS followed that with more selective two-circuit version Construction and Operation of a Two-Circuit Radio Receiving Equipment With Crystal Detector that was published the same year and is still frequently built by enthusiasts today.
1920s and 1930s
In the beginning of the 20th century, radios were only for some people who considered it as a hobby. Radios were not accessible to the public, so they built their own radios by themselves with wires wrapped around baseball bats, boxes to form receivers, the transmitters were made from glass and iron, and the speakers they built were from newspapers wrapped in a cone shape.
Still, some historians consider the Autumn of 1920 to be the beginning of radio broadcasting for entertainment purposes. Pittsburgh, PA, station KDKA, owned by Westinghouse, received its license from the United States Department of Commerce just in time to broadcast the Harding-Cox presidential election returns. In addition to reporting on special events, broadcasts to farmers of crop price reports were an important public service, in the early days of radio.
In 1921, factory-made radios were very expensive. When compared to the dollar value of today, some would have cost around $2,000 USD. Since less affluent families could not afford to own one, newspapers and magazines carried articles on how to build a crystal radio with common household items. To minimize the cost, many of the plans suggested winding the tuning coil on empty pasteboard containers such as oatmeal boxes, which became a common foundation for homemade radios.
Non-electric amplification
- As gas lighting and kerosene lamps were widely used before the adoption of electric power, their flame was used for sound amplification. A ceramic cone with a pinhole on its tip was inserted in the middle of the flame, and an earphone unit was attached to the cone's open bottom and sealed air-tight. This acted like a little pump, modulating the fire by periodically sucking away the combustible mixture at negative half-wave, and injecting it back on positive.
- Air pump amplification was first used in pathephones, where a pump was driven by the same spring motor as a turntable. A needle-sized air pipe was placed near a sound membrane, which acted as an air valve and modulated the air flow, amplifying the sound. This method was easily converted for crystal radio, either in a dedicated device or just by putting a "pumped" pathephone's needle on an earpiece's membrane instead of a gramophone record.
Valveless amplifier
"Carbon amplifier" consisting of a carbon microphone and an electromagnetic earpiece sharing a common membrane and case. This was used in the telephone industry and in hearing aids nearly since the invention of both components and long before vacuum tubes. This could be readily bought or handcrafted from surplus telephone parts for use with a crystal radio. Unlike vacuum tubes, it could run with only a flashlight or car battery and had an almost infinite lifetime.
Cristadyne
In the early 1920s Russia, devastated by civil war, young scientist Oleg Losev was experimenting with applying voltage biases to various kinds of crystals, with purpose to refine the reception. The result was astonishing - with a zincyte (zinc oxide) crystal he gained amplification. This was negative resistance phenomenon, decades before the tunnel diode. After the first experiments, he built regenerative and superheterodyne receivers, and even transmitters. However, this discovery was not supported by authorities and soon forgotten and no device was produced in mass quantity beyond a few examples for research.
The USSR registered all radio receivers until 1962, and typewriters and copy machines until its demise. Crystadine was produced in primitive conditions; it can be made in a rural forge - unlike vacuum tubes and modern semiconductor devices. Oleg Losev died 1943 in Leningrad.
1940s
When Allied troops were halted near Anzio, Italy during the spring of 1944, personal portable radios were strictly prohibited, as the Germans had radio detecting equipment that could detect the local oscillator signal of superheterodyne receivers. Some resourceful GIs found that a crude crystal set could be made from a coil made of salvaged wire, a rusty razor blade and a pencil lead for a diode. By lightly touching the pencil lead to spots of blue on the blade, or to spots of rust, they formed what is called a point contact diode and the rectified signal could be heard on headphones or crystal ear pieces. The idea spread across the beachhead, to other parts of the war, and to popular civilian culture.
The sets were dubbed "foxhole receivers" by the popular press, and they became part of the folklore of World War II.
In some Nazi occupied countries there were widespread confiscations of radio sets from the civilian population. This led to particularly determined listeners building their own "clandestine receivers" which frequently amounted to little more than a basic crystal set. However anyone doing so risked imprisonment or even death if caught and in most parts of Europe the signals from the BBC (or other allied stations) were not strong enough to be received on such a set. However there were places such as the Channel Islands where it was possible.
Later years
While it never regained the popularity and general use that it enjoyed at its beginnings, the circuit is still used. The Boy Scouts (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the '50s and '60s, and many children with an interest in electronics built one.
Building crystal radios was a craze in the 1920s, and again in the 1950s. Recently, hobbyists have started designing and building sophisticated examples of the instruments. Much effort goes into the visual appearance of these sets as well as their performance, and some outstanding examples can be found. Annual crystal radio DX contests and building contests allow these set owners to compete with each other and form a community of interest in the subject.
Attempts at recovering RF carrier power
A crystal radio tuned to a strong local transmitter can be used just as a power source for a second amplified (often a power-efficient regenerative) receiver for distant stations that cannot be heard with a plain crystal radio.
There is long history of less successful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave rectifiers. As AM signals have a modulation factor of only 30% by voltage at peak, no more than 9% of received signal power (P = U2 / R) is actual audio information, and 91% is just rectified DC voltage. Given that the audio signal is unlikely to be at peak all the time, the ratio of energy is, in practice, even greater.
Considerable effort was made to convert this DC voltage into sound energy. Some earlier attempts include a one-transistor amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection. This history continues now with designs as elaborate as "inverted two-wave switching power unit"[and bridge amplifiers.
Construction and operation
Importance of grounding
The long wire type antennas often used with crystal radios are Monopole antennas. To receive signals from this type of antenna, a ground reference is needed to provide a place for the antenna signal electricity to flow into and out of. Because crystal radios have no other source of power than the electrical power they receive from the antenna, the grounds for crystal radios must be much better than those used by amplified radios.
The ground provides a good electrical conductor to complete the circuit for the electrical signal induced by the radio wave between the antenna and ground. The importance of this is easy to overlook by those familiar with amplified radios. Amplified radios use energy (voltage) detectors and as such do not need to take much raw power from the antenna and need little or no physical ground. Crystal radios rely on power detection and need to encourage as much antenna current as possible to flow. This requires effective grounding.
The Naive Circuit
An impractical crystal radio circuit illustrated here is often naively proposed to tune the medium wave AM broadcast band with a tuner made of a fixed parallel coil and variable capacitor tank circuit with the antenna and ground connected across it. There are many practical crystal radio circuits, but connecting both the antenna and a variable capacitor across a fixed coil like this makes tuning the whole two octave AM Broadcast Band impractical.
The reason for this is technical. Most crystal radio antennas are about 60 feet long and 20 feet high to be effective, and act something like a 250 to 300 pF capacitor (actually antennas have capacitance, resistance and inductance, but these are mostly capacitive.) If we connect a typical 250 pF antenna to the top of a tank that uses a coil of more than about 75 μH we cannot tune above about 1400 kHz at all. We must drop the size of the fixed coil below 75 μH to have any chance of tuning the top of the band (around 1600 kHz or 1710 kHz.) Even with a 70 μH coil, we need a 1000 pF variable capacitor to tune near the bottom of the band around 540 kHz. But now we need that same variable capacitor to vary down to about 4 pF to reach the top of the band at 1710 kHz. That is a variation from 100 % to down to about 0.4%.
It is impractical to implement an air variable capacitor that can achieve a variation range that even approaches that. In practice, stray capacitance of air variable capacitors limits the range from 100 % to about 5%. Other kinds of variable capacitors are seldom used for crystal radios because of their excessive losses. Consequently, knowledgeable designers do not suggest this circuit.
The circuit works adequately for receiving at a single frequency. The tuning limitation mentioned above could be overcome by making the coil variable e.g. by providing a number of taps along the windings so that choosing a different tap gives a step change in frequency.
Crystal Radio Terms
A crystal radio is the simplest kind of radio. Most radios you buy use complicated electronics to make a strong copy of the sound. A crystal radio is a simple kind of radio that just picks up the wave and changes it straight into sound. It does not use separate power or batteries to make a stronger copy of the sound. It gets all of its power only from the radio wave.
Radio waves are invisible waves of electricity and magnetism. Each radio station sends out radio waves. They travel out from the station something like water waves travel out from a splash in a pond. Water waves travel slow, about 10 miles each hour. Radio waves travel very fast, at 186,000 miles each second.
The tuner separates one radio station from all the others. Different radio stations send out waves that have different space between them. A station at "600 kHz" on the tuning dial sends out radio waves with twice as much space between them as one at "1200 kHz". Also, a station at 1200 kHz sends out twice as many waves per second as one at "600 kHz. The number of waves per second is called frequency. The tuner uses the radio station frequency to separate stations and tune in only the station you want. The tuner uses resonance to make the radio sensitive to just one frequency at a time.
KHz is short for kilohertz. It is the numbers we see on the radio tuning dial. This is the way radio stations are separated. When radio first started, before 1920, tuning dial numbers gave the distance between waves. Engineers call this distance the wavelength. Today it is how many waves hit the antenna in a second. When the waves are closer together, like for a station at 1200 kHz (kiloHertz), there are more waves per second crossing the antenna. When the waves are far apart, there is a longer time between waves. Today the tuning dial numbers are waves per second. Engineers call this number the frequency. Today's AM broadcast band has the frequencies between 535 kHz and 1610 kHz.
The antenna wire picks up the radio wave electricity. A crystal radio needs a long antenna wire. Big antennas pick up more radio wave electricity. The antenna wire is just any electric wire that goes from the radio high up in the air. Longer than 80 feet is good. It works better outside and high up. Higher than 20 feet high is good. A good crystal radio antenna can be a small copper wire going out of a window and up high in a tree. Short antennas in the house work a little bit. Antennas near power lines are dangerous and do not work well.
The ground wire connects the radio to the dirt. The dirt conducts electricity enough to give the antenna electricity some place to flow to. The ground wire helps the antenna get more power from the radio wave. A ground wire goes from the radio to something metal that goes into the dirt. The metal thing that touches dirt is called ground.
Ground is a metal thing that connects to something big, like the world. Dirt is a good ground. It gives the antenna electricity a place to flow into and out of. A good ground is a metal pipe several feet down in the dirt outside. Cold water pipes are good. They go in the ground on one end. Your body acts like a ground a little bit. Your body is too small and does not work well.
Radio wave electricity is electricity that radio waves make in the antenna wire. Radio waves hit the antenna something like ocean waves hit the shore. Ocean waves make water rush up and back with each wave. Radio waves make electricity flow up and down in the antenna wire like that. Radio wave electricity flows back and forth about a million times each second. It changes back and forth faster for a shorter distance between the waves. Engineers call this Radio Frequency electricity, or RF.
The detector changes the radio wave electricity into sound electricity. Radio stations make the radio waves get stronger and weaker as the sound changes. The strength of the radio waves copy the sound vibrations. The detector changes the back and forth radio wave electricity into one way sound electricity. When the radio wave is strong, it makes strong sound electricity. When the radio wave is weak, it makes weak sound electricity. This makes sound electricity that copies the sound vibrations.
The detector works by letting electricity flow one way but not the other. Normal wires let electricity flow both ways.
When radio first started, inventors found rocks that work for detectors. They were crystal rocks, like galena, pyrite and lots of others. That is where the name Crystal Radio came from. Now detectors are made with wires on them. Engineers call them diodes.
The earphone makes sound you can hear out of the sound electricity. The earphone connects the sound electricity to an electromagnet. The electromagnet pulls on a thin metal plate that can move. The electromagnet makes the plate vibrate and make sound. When we hold the earphone to our ear, we can hear the radio. The sound is not very loud. That is because the radio gets all its power from the radio wave. The wave does not have much power. You need to add more power from a battery or plug to make it louder. That takes complicated electronics.
Resonance happens in electric circuits and in mechanical things. It is easier to understand in mechanical things first. If hang a small weight on a string one about 9 inches long (about 23 cm) it will swing back and forth one time each second. If you try to speed it up of slow it down, you can't. It swings at just one frequency, which is one cycle per second. That is resonance. The string and weight is resonant at one cycle per second. To make it slow down by half, make the string twice as long. The length of the string changes the resonant frequency. You can say that the length of the string "tunes" the resonant frequency.
For electricity, a coil and capacitor make a resonant circuit. The capacitor plates get an electric charge from other parts in the radio. That charge flows through the coil. As it does, it builds up a magnetic field in the coil. When all the charge is gone from the capacitor, the magnetic field makes the electricity keep on flowing a little. This charges the capacitor plates the opposite way. As the opposite charge builds up on the capacitor plates, it finally stops the charge flow in the coil. Then the charge in the capacitor plates makes electricity flow the opposite way through the coil. That builds up a magnetic field in the opposite direction. The charge swings back and forth between the coil and capacitor at one certain frequency. That is the resonant frequency of the coil and capacitor.
Frequency is measured in cycles per second, and also in Hertz, abbreviated Hz. and in kHz. and MHz.
A coil is a length of electric wire wrapped around and around to help it make a strong magnetic field. There are many forms for a coil. When other parts in the radio make electricity flow through the wire, the coil builds up a magnetic field. This makes the electricity want to keep flowing even after the cause is taken away. The effect is measured in Henrys, or Micro Henries.
A capacitor is two metal plates separated by an insulator. A wire is connected to each plate. A capacitor will hold electric charge something like a rechargeable battery. When something puts positive electric charges on one plate and negative electric charges on the other plate, the capacitor holds that charge. It holds the charge because there is no electrical conductor path inside the capacitor. If there is an electrical conductor path outside the capacitor, the charges in the capacitor will flow around the path as electricity and the charge will be lost.
For crystal radios, most outside antennas act like capacitors. The antenna wire is one 'plate' and the ground beneath it acts like the other 'plate'. That is how a coil can resonate with an antenna to tune a crystal radio. This lets some crystal radio circuits tune with only a coil and no capacitor. The antenna acts as the capacitor while it also picks up the radio wave signal.