by Eric Flint
Also, spark stations use up bandwidth. Each one sends out a very wide signal so far fewer can "fit" in a given piece of spectrum.
There is one situation where this can be an advantage. If you are wanting to jam reception of an up-time signal, a spark transmitter close to the receiver can effectively "splatter" over the band and block the transmission.
So, spark transmitters suck. But they suck LESS than pony express riders or building hundreds of miles of telegraph. They will be used, and "king spark" will have his day until down-time tubes come into production. Once that happens, just as it did up-time, spark will be legislated out of existence.
Of course, in addition to the other disadvantages, spark is only good for sending Morse code. No one is going to curl up by the fireside to listen to the evening news via Morse code. Reaching the mass audience requires transmitting voice. Spark transmitters just can't do that; we need something else.
There are two candidates for "something else": the Poulsen arc and the Fessenden/Alexanderson alternator.
To discuss the Poulsen arc, let's go back to Ol' Sparky. Remember how it works? We present a high voltage to the capacitor, it charges, eventually it is charged, the current rushes out of the capacitor through the spark and the circuit "rings." What if instead, we treated the spark a little differently? Instead of pulsing the current into the circuit, use a very high voltage DC current. As long as the capacitor is charging, there isn't enough voltage to spark the spark. But once the capacitor is charged, the spark goes, and the capacitor drains . . . but this lets the capacitor start charging again, which pulls voltage off the spark, and the spark stops. (Aside to the electronics types. No, I'm not going to talk about negative resistance and LC circuits.)
It is very easy to make a singing arc like that in audio frequencies, but when you try to re-design the system to run in radio frequencies, problems begin to appear. Residual ions stripped of the electrons by the high temperature of the arc "hang around" between the poles of the spark and make the stopping voltage unpredictable, as well as the starting voltage. So, as you try to increase the frequency, the spark's start and stop "jitters" and you can't get a reliable signal. Much above audio frequencies, it doesn't work.
In 1902, a Dane, Vlademar Poulsen realized that he could use a magnetic field to "sweep" the ions out of the way, and if he used a hydrogen atmosphere instead of air in the gap, the ions would be light and easy to remove. Poulsen was able to get his arc up into radio frequencies.
Poulsen arcs are big, messy, complicated devices with moving parts and plumbing. They require a constant supply of high voltage DC current produced by a big generator, generally run by an electric motor, which is ITSELF run by another generator which is run by a steam engine. They require a continuous supply of hydrogen gas (or you can use vaporized kerosene, but it isn't as good). They need large rotating graphite electrodes, water cooled copper electrodes, a bronze chamber to hold in the hydrogen around the arc, and big "sweeping" magnets around the bronze chamber to remove the offending ions. All this, to get a radio signal.
But how do you modulate it? How do you take the signal and make it talk? The most common solution was to put six carbon microphones between the arc and ground. Speaking into the mike would vary the resistance and that would change the amount of current flowing into the ground and into the antenna. If you want to play a recording instead of talking live, you will need to set a speaker in front of each microphone. Actually, that would probably be better for the announcer too, since it would allow him to be at a distance removed from the arc.
The other something else was developed in 1903 by a Canadian, Reginald Fessenden, working with an engineer from General Electric, Ernst Alexanderson. By 1916, Fessenden and Alexanderson had developed a mechanism which allowed reliable voice transmission across the Atlantic. How? Let's go back to first principles.
Take a coil of wire. Attach the coil to a meter. Nothing happens, the coil just sits there. Now, get a magnet. Stick the magnet into the coil. As the magnet goes in, the magnetic field of the magnet pushes on the electrons in the coil and they are shoved around the wire. The meter flicks to the right a little as long as the magnet is moving in.
Now, pull the magnet out. The magnetic field is pushing on the electrons the other way and they are shoved around the wire in the opposite direction, and the meter flicks to the left. It you put the magnet just outside the coil, and wave it from side to side, the same thing happens, as you get closer to the coil, the meter flicks right, as you get further away, the meter flicks left. If the magnet sits still, no matter how big the coil, no matter how strong the magnet, nothing happens. The magnetic field, and the electrons, just sit there.
Now . . . you can make electro-magnets much stronger than any permanent magnet. So, if you replace the bar magnet with an electromagnet, you can make BIG pulses of electricity. This is what is done in an alternator. There is a spinning coil, and a static coil and as they spin the electricity pulses back and forth. Neato! If we could spin them fast enough, we would get radio waves The problem is, the LOWEST frequency radio waves that work well for voice are at 100,000 cycles per second. If you think about trying to spin a large coil of wire 100,000 times per second, you'll realize just how hard it would be. High power router motors spin as fast as 24,000 RPM, but that's only 4000 revolutions per second. If the router motor is 3 inches across, the outer edge is moving at 214 miles per hour, and is experiencing a pull of 513 times the force of gravity.
Take that same motor, and try to use it as an alternator, and spin it up to 100,000 revolutions per second, and the outer edge is moving 5300 miles per hour (far above earth escape velocity, much faster than a speeding bullet) and the outer edge of the coil is being pulled with 321,000 times the force of gravity. The wire will simply fly apart long before. The fastest spinning man-made object (a carving tool similar to a dentists drill) turns at 450,000 rpm, or 7500 revolutions per second, thirteen times slower than we need for radio. The router WOULD give us reasonable power, since it handles 5 hp, around 3700 watts. But it just won't work.
What to do?
Let's go back to our two coils. One is a powerful electromagnet, with a DC current running through it. The other is a coil. They're just sitting there. Nothing happens. Now, place a hunk of iron between the magnet and the coil. As the iron comes into the field, the field seen by the coil decreases, and the meter flicks left. As the iron is pulled out, the field increases, and the meter flicks right. Cool! Of course, we have to build a strong, strong mechanism to spin the iron into and out of the field, and we have to cool it, since passing in and out of a magnetic field like that will heat metal like crazy. But still, we can spin a hunk of iron instead of spinning delicate coils and wires. Cool!
(I am oversimplifying here. Don't shoot me.) Take a big strong iron disk. Drill a series of holes around the edge and fill them with bronze. Now place the disk so that the bronze holes are lined up in the space between the magnet at the coil. Spin the disk. As the bronze window comes between the coil and the magnet, magnetic fields "get through." As the bronze window moves away, the iron interferes with the magnetic field, and the magnetic field "does not get through." We now have an alternator in which the coil and the magnet do not move. The disk can be built VERY strong, encased in vacuum and water cooled by pipes running through the center. Take a disk 4 feet across, put windows every half inch around the edge and you have 300 windows. Rotate the disk 330 times per second or 20,000 rpm, and you'll get 100,000 waves per second of RF power.
This isn't EASY, but it turns out to be within the ability of 1902 mechanical engineering. (Well, not really, they spun it one fourth that fast and used two frequency doublers. But you really don't want to know about frequency doublers. )
Use multiple pairs of magnets and coils spaced equally around the disk to increase your power.
Modulate it the same way you did the Poulsen arc, with six microphones in series between the transmitter and ground.
The Fessenden/Alexa
nderson alternator was the mechanism for radiotelephone prior to the invention of tubes. One working station remains in service in 2006 in Sweden. It has been named as a world heritage site and is run on special anniversaries.
So, there you have it, radio for everyone else. At least until the research teams manage to start building tubes again. But that, as they say, is another story.
References:
I could give you a long list of reference sites, but frankly the easiest is simply to visit Wikipedia at http://www.wikipedia.com and search for crystal radio, Poulsen arc, and Fessenden. The explanations there are good, and their reference links are constantly updated to working sites.
The Sound of Mica
by Iver P. Cooper
It is the year 1634, and the Voice of America is on the air. Since the VOA is an AM (amplitude modulation) radio station, speech and music are encoded as fluctuations in the amplitude (intensity) of a radio-frequency carrier wave. The radio waves, emanating from the Great Stone Radio Tower, spread across the German countryside, and enter the long receiving antennae of hundreds, perhaps thousands, of makeshift crystal radio sets.
There, they set the free electrons of the antenna into motion, dancing back and forth in response to the reversals of the electromagnetic field. That is an alternating current, and it is "rectified" into a direct current (flowing in one direction only) by a device called a diode. It is the crystal, probably a galena (lead sulphide) crystal, which serves as the diode.
A capacitor (a device stores and discharges electrical energy) filters out the carrier signal, passing only the electrically encoded audio signal. Finally, an earphone transduces that signal into sound.
The Voice of America is not the only broadcaster, and if you want to hear it, and not the Voice of Luther, you need a tuning circuit. The tuning circuit has both an inductor (a coil) and a second capacitor. (At least one of these must be variable for tuning to be possible.)
Capacitors (also called "condensers") are one of the most basic of electronic components. Their most fundamental electrical characteristic is their capacitance (ability to store electrical energy).
So how do you make a capacitor? The simplest one consists of two parallel conductive plates, and an intervening "dielectric." You can actually use a stack of plates, not just two, but the conductive and dielectric layers will alternate. One wire will be connected to the "odd-numbered" plates, and a second wire to the "even-numbered" ones.
It turns out that mica is probably the best dielectric material which is likely to be available in the years immediately following the Ring of Fire (RoF).
* * *
The Great Stone Radio Tower was built to trick the other European powers into thinking that long-distance radio requires massive antennas. ("Radio in the 1632 Universe, Grantville Gazette, Volume One). This bit of maskirovka was successful for only a limited time. By March 1634, the Cavrianis had figured out that the Venetian Embassy was in radio contact with Grantville. (1634: The Galileo Affair, Chap. 27). It is only a matter of time before the French and other governments realize the American capabilities, if only by inference from the celerity with which the USE acts.
Those powers will quickly appreciate the advantages which would accrue to them if they, too, had radio communications for diplomatic and military purposes. We can expect that collecting information on electronics in general, and radios in particular, is going to be a fairly high priority for the multitude of spies in Grantville and Magdeburg.
The knowledge of how to build crystal radio receivers is being widely disseminated ("Waves of Change," Granville Gazette, Volume Nine), so the focus will be on finding out how to build suitable transmitters. Details appear in another article in this issue ("Radio 3: Grantville Gazette, Volume Nine), but the simplest transmitter would be of the "spark gap" type. This can send Morse code, but not music or speech.
Now it turns out that any spark gap transmitter will need at least one capacitor that can handle high voltages. To survive the high voltages, the capacitors must use mica as the dielectric.
Mica is less critical insofar as receiver capacitors are concerned, but a receiver employing mica will have greater sensitivity than one using an alternative dielectric. That is important if you are on the fringe of the broadcast area.
* * *
While spark gap transmitters will initially be used by foreign governments, it is only a matter of time before the knowledge of how to make them is passed on to others, such as merchants. The Cavrianis used the American radio to advantage in the futures market, and their counterparts will be quick to perceive the benefits of acquiring their own radio capabilities.
* * *
As the ability to receive a radio broadcast spreads, other political groups—some hostile to the USE—will want to make sure that they can speak to the radio audience. And there will be other broadcasters, whose interests are economic rather than political.
To actually "speak," you need a radio transmitter which can simulate a continuous wave. The Voice of America's transmitter is a rebuilt, high-powered, "ham" radio outfit, while the Voice of Luther will broadcast from a "Fessenden Alternator" constructed with down-time materials ("Radio in the 1632 Universe," Grantville Gazette, Volume One). I assume that the Fessenden Alternator will initially be beyond the capacity of down-timers lacking direct USE technical assistance.
The most likely alternative would be a variation on the Poulsen arc transmitter, which combined an arc lamp, a coil, and a capacitor. Like the capacitor of the spark gap transmitter, this one needs to endure high voltages.
* * *
Of course, some folks won't want to broadcast themselves, but will be keen on jamming the transmissions of others. Capacitors can be used in radio jammers, too. ("Little Jammer Boys," Grantville Gazette, Volume Nine).
* * *
It is time now to take a closer look at why mica is so desirable for capacitor construction. Mica, to begin with, is an insulator. All insulators can be used as dielectrics, but they differ in terms of their ability, per unit thickness, to separate charges (and thereby store energy). The measure of that ability is the dielectric constant. The dielectric constant of mica is about 4–9; of common materials, only glass is superior (about 5–10).
Another important characteristic is dielectric strength. If too great a voltage is applied across the plates of a capacitor, the current will force its way through (this is called "breakdown") and may arc-weld the plates together. Higher voltages can be tolerated if the dielectric layer is made thicker, but that reduces capacitance. Hence, high voltage capacitors are usually made of materials with a high dielectric strength (a measure of the ability of a material, per unit thickness, to resist arcing).
Mica has superior dielectric strength (5,000 kV/inch, versus 2,000–3,000 for glass). So a thin mica capacitor can resist a high voltage. Having a high dielectric strength is particularly important if you are constructing a transmitter capacitor.
When a voltage is applied to a capacitor, charges build up on the plates, but some of the electrical energy is lost as heat. Ideally, the capacitor has a low dissipation factor. The dissipation factor for mica is .0003–.0004 for mica, versus .01–.05 for soda lime glass. (Eccosorb)
Thermal stability (how much does the capacitance change if the temperature changes?) is also of interest if the capacitor is being used outdoors or in unusual environments. For mica, "Capacitance will change only -2% at -54°C, and to +3% at +125°C. " (McCloskey)
Mica has other great properties, too. It splits readily into very thin, flat sheets, which are flexible, heat-resistant, chemically inert, and, in some cases, transparent. The latter property led to its use in house windows in Russia ("muscovy glass") and in oven windows in the United States ("isinglass").
Jason Cole of the University of Waterloo writes, "When it comes to modern technology, sheet muscovite is an indispensable resource. It is used in almost every electronic device sold today as an insulator. Its high resistance to the passage of electrici
ty and heat are so great that no substitute, artificial or natural, have proved to be economically suitable to replace it. No other mineral has better cleavage, flexibility or elasticity. It is possible to roll a sheet of muscovite less than 0.1mm thick into a cylinder 6mm thick and its elasticity would enable the sheet to flatten out again quite easily. Sheet mica is just as important to the electrical and electronic industries as copper wire and now ranks as one of the essential minerals of modern life."
Mica
So what is mica, exactly? It is a group of aluminum silicate minerals. The two most important species for the electronics market are muscovite mica and phlogopite mica.
Muscovite micas are divided into the ruby and green varieties, based on color. The term "ruby muscovite" includes the clear forms.
Phlogopite micas are "rarely found as colorless transparent sheets"; they are sometimes called "amber" micas. Rouse (352) says that they can't be used for capacitors because their power loss is usually over 1%, whereas the maximum permissible losss is 0.04%. They tended to be used in OTL mostly for high-temperature applications. Cole says that "Muscovite mica cannot be used in temperatures that exceed 550 degrees Celsius, whereas, phlogopite can be utilized at temperatures up to 1000 degrees Celsius."