It could take all day and then some to adequately explain the very simple ‘foxhole radio’ drawn above. The assumption must be made that the reader has some fundamental understanding of EMR (Electromagnetic Radiation) or the discussion must start from there. After explaining that radio ‘waves’ have both a magnetic and electrical component, discussion might lead to the wave’s inversely proportional features of frequency and wavelength as these undulate in a sinusoidal radio signal. At some point electromagnetic propagation, attenuation, reflection and refraction should be mentioned. Resonance and oscillation are crucial topics in the understanding of how radios work. This little post can not hope to do the overall topic of radio justice, nor can it even with brevity adequately describe all the workings of this homemade foxhole radio. What will be done instead is that a few pertinent tidbits of information will be tossed out piecemeal. Hopefully this approach will entice the casual reader to learn more.
A century ago many youngsters built their own crystal radios and spark gap transmitters from scraps laying about the house or farm. Those youngsters gained knowledge of science and technology in the process. Today’s youngsters seem enthralled with the little battery powered full-duplex radios that we call cell phones. How many though understand the details of how or why cell phones work, or could build their own? As archaic as crystal radios and primitive transmitters may be, they might still have some practical applications.
These two schematics of the previous drawing differ by the inclusion of a capacitor. The schematic on the right would provide better radio reception because a resonator (a capacitor / inductor oscillator or tank circuit) is created. The coil of wire acts an inductor that stores and releases the magnetic component of radio energy. The capacitor stores and releases the electronic component of radio energy. Oscillation is a repetitive variation or change of energy, back and fourth between two physical states. Like a pendulum swings back and forth, radio energy gathered from the antenna bounces back and forth between inductor and capacitor and resonate frequencies or harmonics are built up. While a radio’s antenna can be hit by hundreds or thousands of signals from different radio sources at once, the only frequency that will be selected or amplified would be the one that matches the resonator.
* The amplification from resonance if often likened to a kid playing in a bathtub. If he flails his arms about in the water randomly, he creates many bumpy small waves. If however he continually and rhythmically moves his arms back and forth in the same direction, he builds up a resonant wave that will soon slosh over the edges of the tub.
A tank circuit or LC circuit is a simple parallel resonant circuit that employs an inductor (L) and a capacitor (C). An inductor (like a coil of wire) stores energy in a magnetic field while a capacitor stores energy in an electrostatic field. The resonant frequency of the circuit will be determined only by the size of the inductor (L) and capacitor (C) unless one of the components is variable. In a tank circuit the energy introduced will slosh back and forth between components but slowly diminish due to resistance in the wire. The oscillation will diminish sooner if a resistor (R) is introduced to the circuit.
The foxhole radio is actually a WWII era variation of the earlier crystal radio that might have been used in the trenches of WWI. What distinguishes it as a foxhole radio is its choice of detector, which in this case consists of a blued razorblade and a rusty safety pen or probe of pencil graphite. A piece of blued hacksaw blade would work just as well. Gun bluing is a chemical coating applied to bare ferrous metal to prevent rust. The oxidization creates a semi-conductive surface. The graphite in pencil lead is conductive and different hardness have different resistances.
* The name “foxhole” radio comes from the fact that many American GIs during calm waiting periods in WWII, built their own radios to listen to music. Infantry often even slept in foxholes. There was good radio music to be had in those days, even if some of it came from the enemy and was laced with propaganda and psychological warfare. These radios could be constructed from scraps found about the battlefield. If they for instance, needed a capacitor but couldn’t find one from a broken piece of equipment, then they could just make one by folding a foil and paper backed gum wrapper. Manufactured portable radios were taboo for soldiers because these could give away their position to the enemy. Although many “dough-boys” 20 years earlier in WWI undoubtedly knew how to build crystal radio sets, there was less incentive to do so. Almost all radio traffic in the European theater (then) was in Morse Code until the very end of the war when isolated voice and music broadcast made their first debuts.
The detector is the device that detects EMR modulation. The first successful detectors were the “coherer” (a glass tube filled with metal filings) and the “barretter” which dipped a platinum cored silver wire into a little cup of acid. Detectors grouped within the crystal radio domain share the trait of being semi-conductive and act as a rectifying diode (which means that they allow current to flow only one way). *Some crystals are piezoelectric too; meaning they can produce an electrical charge if mechanical stress is applied, or conversely they will vibrate or generate mechanical strain if stimulated with an electrical charge. The best crystal to use was a little rock of galena (lead sulfide ore). A small piece of curled wire called a “cat’s whisker” was gently probed about the many faceted surface of the crystal to find a ‘hotspot’ where a signal would come in strong. Since good galena crystals could be hard to find iron pyrite, quartz, bornite, molybdenite, silicone, tellurium or zincite crystals were sometimes substituted. Tarnished copper pennies, blued and or rusted hacksaw blades or razor blades, rusty needles and germanium diodes (stripped from other equipment) also made useful detectors. The first vacuum tubes used for radio rectification appeared in 1906 but these were rare and costly.
The stylized symbol for a diode is taken from the ‘cat’s whisker’ / crystal radios of yesteryear. Current flow is understood to be in the opposite direction that the arrow seems to point.
* An amplitude modulated radio signal can’t be converted into sound by an earphone directly. By using a detector or diode, one side (one half) of a carrier wave along its axis of travel is separated from its counterpart. Rectifying the induced current from a wave in such a fashion results in a pulsing direct current, whose amplitude varies with the audio signal. That pulsing direct current (DC) makes the solenoid attached to a diaphragm in the speaker move or the piezoelectric crystal glued to a diaphragm in an earphone move; which pushes air and makes sound.
Crystal radios rely solely upon the very weak energy produced when an electromagnetic radio wave induces a current in the antenna. Sound can only be heard through a low impedance ear plug or very sensitive earphones. The only real difference between a crystal radio and an AM radio is that the latter has an amplifying circuit to drive a speaker. Crystal radios are only useful for AM (Amplitude Modulation) or some similarly modulated shortwave bands. FM carrier waves provide no changes in amplitude for demodulators (a detector circuit) to detect.
Today’s AM broadcasts (in N. America) are so weak that the maximum reception for a typical crystal radio would be about 25 miles. AM radio has been in decline for some time now due to several reasons. Due to the long wavelength of the present AM band, signals can propagate thousands of miles around the earth because they are reflected or refracted by the ionosphere. To avoid interference this has led to a progressive reduction (especially at night) in the amount of power that AM broadcast stations are allowed to use. In N. America (part of ITU region 2) AM frequencies 535 kHz to 1,075 kHz have corresponding wavelengths that are between 1,826 ft and 909 ft long. By contrast, FM (Frequency Modulation) broadcast signals from the same region operate at frequencies between 88 and 108 MHz. That means that our FM signals have considerably shorter wavelengths (between 11’ and 9’ long) and as a result are not returned by the atmosphere. FM signals are ‘line of sight’; are blocked by obstructions like mountains or tall buildings and have a limited range of about 50 miles due to the curvature of the earth.
* For comparison of wavelength, the magnetrons inside most household microwave ovens produce radio waves that are 12.2 cm (4.75 inches long). Many microwave frequencies could have been chosen as they would cook food as well, but the ISM 2.45 GHz (S-band) was chosen worldwide because of its common availability and non-effect upon glass, ceramic and most plastics. Many molecules like fats, sugars and water are electric dipoles, meaning they are positive at one end and a negative on the other. As these molecules continually attempt to align themselves with the alternating electrical field of the microwave, they move and generate heat. The electromagnetic field in the oven (a Faraday cage) effectively reverses its polarity 2.45 billion times a second.
* The speed of light in a vacuum is 185,000 miles/sec, or for metric the accurate speed is usually rounded up to an easy to remember 300 million m/sec. When given a frequency, the wavelength can easily be calculated, or vice-versa. Since c = f w (frequency x wavelength) the speed of light can be divided by the known, to find the unknown variable.
Analogue AM radio broadcast suffer from increased electronic noise in the form of fluorescent lights, compact fluorescent lamp (CFL) bulbs, the electrical grid, power-line networking (Internet over house wiring), LED traffic signals, computers, LED computer monitors and most home or office devices that possess electronic controllers. The FCC has treated legacy AM radio like an unwanted stepchild for the last 50 years and this neglect has assisted in AM’s decline and loss of listeners. There is talk of digitizing AM or of moving AM broadcast from the LF band up into some of the VLF band recently vacated by analogue TV channels. It is theoretically possible to build a crystal radio capable of receiving almost any particular radio frequency band. If analogue amplitude modulated voice or music was to be broadcast in some random band then the information could be understood on its tailored crystal radio counterpart. If however the information was in digital format then a crystal or foxhole radio would produce only gibberish for a listener.
History and Trivia:
The first radios or “wirelesses” date back to the 1890’s. The first wireless message to reach across the Atlantic Ocean was sent in 1901. That Morse code message is still traveling through outer space somewhere and the signal is extremely weak by now. By 1903 regular transatlantic news transmissions were being conducted by the British financed, Marconi Company. On Christmas Eve of 1906 the first human voice and music was successfully transmitted over the airwaves (from Massachusetts). The first Boy Scout handbook proper was printed in 1911 and included instructions on the construction and operation of both crystal radio and spark gap transmitters. From a house in San Jose, Ca. occasional voice and music broadcast were being transmitted between 1912 and 1913.
In the spring of 1912 the 825 foot long, 46,328 gross ton Titanic sank and 1,517 people lost their lives. Radio was still very much in its infancy at the time and was not a requirement for ships at sea, but was still a novelty. Amateur spark gap radio operators on land however were blamed for interference causing the lack of response from other ships to Titanic’s distress calls. In fact most wireless operators on 28 other ships in the N. Atlantic that day, worked only during normal business hours and were not monitoring their equipment as Titanic sank. Add to this confusion the fact that Germany’s Telefunken Company and British -Marconi Wireless Telegraph Signal Company were bitter rivals and ignored each others broadcast anyway. In a knee-jerk reaction Congress showed favoritism to commercial radio companies and the US Navy with the passage of the Wireless Act of 1912, which booted amateur radio operators from common or useful wavelengths. When WWI began both England and Germany nationalized all radio spectrum and equipment. By executive order President Wilson effectively did the same with America’s entry into the war in 1917. All radio equipment in the nation (except that of the Army or Navy) was outlawed, destroyed or confiscated. It immediately became illegal to listen to any radio and treasonous to posses or operate a transmitter.
A source of many good old ‘Crystal Radio Plans and Circuits’
“The Titanic Tragedy – Sunk between CQD and SOS”
UNITED STATES EARLY RADIO HISTORY
IEEE Global History Network – on Radio
2003 NTIA chart of U.S. Frequency Allocations (this PDF has a useful “EM” spectrum chart near the bottom that the newer 2011 version lacks).
Following the layout above one could probably reproduce a spark gap transmitter using old car parts like the automotive ignition coil for example. A small transmitter like this might be capable of sending signals a good 50-60 miles. Commercial and naval spark gap transmitters grew to be much larger, more capable and more dangerous to be around. These transmitters create a broad spectrum of noise, which is the main reason that amateurs were prevented from using them after 1912. Use of one today would bring the FCC or similar authority crashing into your residence to stop its transmissions. The U.S. Navy retained its obsolete, high voltage, fire hazardous ‘rotary’ spark gap transmitters on its ships at sea, throughout WWII. The primitive transmitters served as communications backup an as a potential means to jamb enemy communications.
* Cell phone communications are far from being secure or private and far from being as dependable as some might believe them to be in certain emergencies. All it takes is a good wind from a hurricane, cyclone or typhoon (3 words for the same thing) to knock down a few cell-towers, or for a coronal mass ejection (CME) or electromagnetic pulse weapon (EMP) of sufficient proportion – to render cell phones completely useless. Discussion of cell phone technology’s numerous drawbacks will have to wait for another time however.