Grantville Gazette, Volume IX

by Eric Flint

  * * *

  Two years before, the pages of books in the Ring of Fire had started flapping like butterfly wings. Now storms were brewing in far away Moscow. Economic storms, technical storms . . . to be followed, perhaps, by political storms.

  FACT:

  Radio in 1632, Part 3

  by Rick Boatright

  In our two previous discussions of telecommunications in the 1632 series, we focused on radio communications uniquely available to up-timers ("Radio in the 1632 Universe," Grantville Gazette, Volume One) , and to wired communications ("So You Want to do Telecommunications in 1633," Grantville Gazette, Volume Two). In this article we will discuss radio options available to down-timers both for transmitters and receivers. This will require a brief discussion of radio theory, which we will restrict to no more than one equation.

  As discussed in Grantville Gazette, Volume One, up-timers are generously supplied with radio equipment. Various categories include CB radios, FRS handhelds, commercial FM radios for utility and police work, ham radios in a variety of bands and "down-time built from up-time parts" radios including the transmitter for the Voice of America. As well, there are custom military and diplomatic "cw" radios.

  Down-timers want radios for a variety of reasons; people anywhere near Grantville want to listen to the Voice of America radio station. Starting in early 1635, its sister station, the Voice of Luther in Magdeburg will go on the air. People, especially governments and military types further from Grantville, want the magic instantaneous communications over great distances that radio gives you.

  How can they do either of those things without tubes, or transistors, or the other things we think of when we say "electronics"?

  The magic word is "detector."

  Take a telephone receiver. In a telephone circuit. current flows through the handset and is stronger or weaker according to the signal sent by the telephone transmitter. So the diaphragm in the receiver moves in and out, attracted to the electromagnet more or less depending on there being more or less electricity flowing through it.

  If we have an antenna attached to a handset and then to ground, and we put the antenna near an AM radio transmitter, the transmitter makes its radio signal stronger or weaker, just like the telephone current is stronger or weaker in response to varying sounds. That radio energy is received by the antenna, passed through the receiver, and into the ground. We ought to be able to hear it. But we hear nothing . . . why? It's because the radio waves are going BACK AND FORTH, positive and negative, many, many thousands of times per second. The oscillations are so fast that the diaphragm of the receiver just sits there. Nothing happens.

  We need a way to DETECT the RF currents. The first detectors used in commercial radios were called "coherers." Edward Branly developed the first workable ones. Imagine a glass tube filled with sharply cut nickel and silver shavings. Hook a battery and an earphone to the tube. The high resistance of the shavings prevents electricity from flowing through the tube, and you hear nothing. But, if we also hook the tube up to our antenna and ground, and there is a strong radio signal in the area, the electric field from the radio signal lowers the resistance of the shavings, and they conduct suddenly. You then hear a CLICK in the earphone as electricity suddenly runs in. If you rig a small hammer to tap the tube when electricity runs through it, this re-arranges the shavings, and you hear another click as the current goes away. If the radio signal is turned on and off (as it is, when you're transmitting Morse code) then you hear a series of clicks in the ear piece each time the radio turns on. You also can listen to the clicking of the hammer against the glass tube (until it shatters.)

  Tesla is famous for inventing a coherer which ROTATED instead of being hit with a hammer. This was considered a big step forward.

  But coherers require strong signals, they can ONLY receive Morse code, and only that sent slowly enough to allow the coherer time to reset. For about ten years in the real world, they were the best we had. But they sucked, and everyone knew it.

  What was needed was a way to keep the electricity from running through the earphone in BOTH DIRECTIONS. If you could cause the signal from the antenna to be "rectified" and only take the POSITIVE or the NEGATIVE half of the signal, the little pulses would "add up," each one pushing the diaphragm of the earphone a little further out instead of causing it to wiggle back and forth too fast to hear.

  Many people attempted to make rectifiers. The most successful was a sharp platinum wire placed JUST BARELY into a pool of weak acid. When the electricity went one way, tiny bubbles formed insulating the tip, and when the electricity went the other way, the electricity would flow normally. It was fussy, and worked well only for wizards, required constant adjustment and so forth, but it did work. Then, in 1906 Greenleaf Whittier Pickard patented a solid state rectifying detector. Pickard (whose name is almost unknown in the real time line, and who deserves far more acclaim than he gets) discovered that a variety of materials—if prepared just right—would rectify a signal. They would conduct electricity easily in one direction while having a resistance hundreds or thousands of times higher in the other direction. Additional benefits included that the crystal detector had no moving parts, no liquids and no glass tube.

  The situation down-time is similar to the situation in the US around 1920. In the autumn of 1920, in Pittsburgh, PA, station KDKA went on the air 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. Many of them cost more than $2,000 USD (in 2006 equivalent dollars), and less affluent families could not afford to have one. However, in 1922 the U.S. Bureau of Standards publication Construction and Operation of a Simple Homemade Radio Receiving Outfit showed how almost any family having a family member handy with simple tools could make a radio. It became an immediate best seller. More than any other system, this design was responsible for bringing radio to the general public.

  Similarly, in 1632 the publication How To Make and Use a Crystal Radio will give anyone with sufficient patience everything they need to know to build a simple crystal radio receiver. The parts list which must be purchased is astonishingly short.

  A coil of very fine wire (for making the earphone), about 2 ounces.

  A very small magnet (for making the earphone)

  An iron nail

  A coil of larger wire for the tuning coil and antenna, about 1 pound.

  A piece of lead ore (galena) the size of your little fingernail (the crystal)

  Locally sourced materials would include thread, glue, a disk of parchment for the earpiece drumhead, a wooden cup, a board, and a coil form (toilet paper tube) wound from paper and glue to a size drawn in the printed instructions. (The traditional up-time forms are toilet paper rolls and oatmeal boxes. It is presumed that down-time hand-made toilet paper tubes will be fabricated.)

  Home made earphones are possible, as demonstrated by thousands of hobbyists, but commercially prepared earphones especially sensitive piezo-electric sets will be available inexpensively quite early (See "Dr Phil's Aeolian Transformers" in Grantville Gazette, Volume Six).

  The circuit's sensitivity and tuning ability is improved by the presence of a capacitor, which will require the purchase of a sheet of copper foil. Beyond that, mere careful measuring and careful assembly will almost certainly result in a working radio.

  Purpose built coil forms, ceramic forms for wrapping toilet paper tubes, purpose built glass and ceramic antenna insulators, lightning arrestors, pass-through tubes for walls, and such will be available for purchase from electronics dealers soon after VOA goes on the air. But even the poorest village should be able to afford a single crystal radio, the cup forming the earpiece being passed from hand-to-hand as programs change.

  Crystal sets as described have several disadvantages. The largest is that only a single person can listen at a time. Al
so, you can't rig multiple receivers to a single antenna, so a village wanting to have several people listen at once would end up with a forest of copper hanging over their heads.

  The solution is an amplifier to make the weak signal strong enough to hear. There are several amplifier designs available which do not require tubes or transistors—which is good, since we don't have any to spare. They are all horrible compared to the cheapest single transistor amp, but they will allow a group of people in a quiet room to hear a radio at the same time.

  The simplest amplifier is to place the earphone at the base of a trumpet. If you think about the classic Victrola record player with its flower-like horn, the weak thready sounds coming from the needle were transformed into a room-filling sound by expanding the waves in a hyperbolic horn. This can be done with any earphone and a properly built trumpet. The same company which makes record players will doubtless produce adaptors for earphones.

  A more robust solution is the "telephonic amplifier." Take an earphone, glue a telephone microphone to it, and use a powerful battery to power the output into a telephone receiver. A stack of four telephone circuits like that can amplify a crystal radio eight to ten times, the final output going into a loudspeaker or into the base of a hyperbolic horn.

  The best non-electronic down-time amp is the selenium photo-voltaic amp. Take a thin foil of selenium (which you will have to find, purify and stretch into thin foils. The more light that hits it, the less its resistance to electricity. Hook a battery and loudspeaker up to the selenium foil (Okay, it's not that simple but it can be made at home if you have selenium.) and then, use a mirror moved by your crystal earphone to focus more or less light on the selenium. A very substantial amplification is possible. Alexander Graham Bell first patented this as a "photophone."

  So, almost immediately after the Ring of Fire, every village in the area has gotten a broadsheet with instructions about building a radio, there are radio companies making deluxe sets and amplifiers, and it is the rare tavern which doesn't AT LEAST have a "radio room" for listening to VOA and probably is trying to save up for a "Real Up Time Radio."

  That takes care of the receiving part. What about transmitting?

  Sadly, transmitting radio is MUCH harder than receiving it. To receive a signal you only have to catch that tiny, tiny bit of a signal that hits your antenna. Sure, your antenna may be 100 feet long strung from house to house, but still, all you're catching is that little itsy bit that hits your antenna. On the other hand, the transmitter had to fill all of space with that much power. Your itsy bit, your neighbors itsy bit, the bit over there by the horse corral that doesn't HAVE an antenna in it, the space down by the creek, and so on. You're catching a snowflake's worth of power, but the transmitter must create the snowstorm.

  Also, we have to decide what KIND of transmitter we want. Do we need a transmitter to send voice and other sounds, or is it enough to tap out Morse code and send messages? Code transmitters are much easier than voice transmitters, and we'll talk about those first.

  Consider a bell. You tap a bell with a hammer, and the bell "rings" for a while. The harder you tap, the louder the initial sound, and the longer the ringing lasts, but eventually the energy from the tap dissipates, partly by heating the bell, and partly by transferring energy to the air making sound. You can send Morse code by tapping a bell. The size of the bell controls the loudness and the pitch or note of the bell. A big bell is louder. A big bell sounds a lower note. You can raise and lower the note of a bell of a given size within limits by making the shell thicker or thinner, and by using stiffer or more flexible materials, but nothing you do is going to make a two inch bell sound like it is a three foot bell.

  Radio waves, (and light) are waves with two parts. There is an "electric field" wave, and a "magnetic field" wave. They are related and create each other. When you wave a magnet over a coil of wire, it makes electricity move in the coil. If you move electricity through a coil, it makes a magnetic field. Radio waves are electric and magnetic waves creating and supporting each other as they travel through space. They oscillate up and down, back and forth, similar to sound waves in air. If you take a coil of wire (which you remember causes a magnetic field to be created when it has electricity running through it) and a capacitor—a device for storing electric fields—and hook them together, they form a "resonate circuit" which will ring, just like a bell rings when hit. If you put a pulse of electricity into the circuit, it goes round and round the coil, and makes a magnetic field, then it gets stored in the capacitor, and as the magnetic field collapses it causes another pulse going the other way which bleeds out the capacitor and charges it the other way. Bigger capacitors, and bigger coils change the resonate frequency of the circuit just as bigger walls and bigger diameters change the pitch of the bell. (If you're looking for an analogy for the stiffness of the material, I'm really stretching an analogy beyond all limits here, but you could think of the tightness or diameter of the coil as an equivalent and I wouldn't be upset. This is an analogy, all right? I'm not giving you equations, be happy.)

  Back to the bell. If you graph the loudness of the bell after you hit it, the loudness decays away fast at first and then slower and slower . . . the graph tapers to a point, shaped sort of like a ski jump. Similarly if you hit a resonate circuit with an electric pulse, it rings with RF, tapering in a very similar shape. If you hook the circuit up to an antenna, the RF is sent out into space, just like the sound is sent from the bell. It's very loud at first, and then quickly (in much less than a second) the radio energy is used up in heating the coil and transmitting radio waves out into space.

  Just like the bell, the harder we hit the circuit, the louder it rings (until we hit it hard enough to make it melt). So, to make a Morse code transmitter, we put up an antenna, we build a big, strong resonate circuit out of a coil and a capacitor, and we connect the coil to a powerful electric power source very briefly so that the circuit "rings" . . . we do that over and over, each "tap" of electricity making a ringing, and we can send Morse code by controlling the timing of the taps. More electricity corresponds to "harder taps."

  I can hear it now. "What about the sparks? Don't you need sparks?" That would be "yes and no." The function of the spark gap is to cause there to be a very high resistance in the circuit which allows the capacitor to charge. When the capacitor is charged "enough," the sparking voltage of the gap is reached, and the spark gap "sparks." This causes there to be a lower resistance in the circuit causing the capacitor to discharge. The discharge through the conducting spark takes the form of a damped oscillation, at the frequency determined by the resonant frequency of the circuit. If you could make a switch that did the same thing WITHOUT sparking, the system would still make radio waves. The spark gap acts as a voltage dependent switch. STOPPING the spark is as important as starting it. The eventual solution was a "rotary" spark gap that broke the spark by pulling the contacts apart as a central disk rotated, and then lined them up again as the disk rotated the next stud into line with an unmoving contact.

  So, the parts list for our down-time Morse code transmitter is considerably longer than our crystal radio. We will need:

  An antenna cut for the frequency we want to transmit on.

  A coil wound from heavy wire with careful spacing, thick enough to be self-supporting in air because we don't want anything shorting out the coils.

  A large capacitor to match the coil. Stacks of glass interlayered with gold foil—or better, sheets of mica interlayered with gold foil.

  A large copper rod or copper plated iron rod driven into the ground.

  A source of electricity—a bank of the same type of batteries we use to run the telegraph will do. We will need a large number of batteries (perhaps as many as one hundred gallon sized batteries) to make a powerful signal.

  A "spark coil." This is a pair of coils wound around a common center. The first coil, which is attached to the batteries, has only a few loops (four or five). The second coil, attached to the circuit,
has MANY loops, dozens certainly, so that our transmitting voltage is several thousand volts.

  A switch that turns the electricity to the coil on and off. Each pulse through the buzzer makes a pulse into the resonate circuit. A door-bell buzzer like arrangement is fine, but it must be scaled up to handle heavier currents.

  A Morse code key that allows us to turn the electricity to the buzzer on and off as we need to transmit.

  A rotary spark gap.

  A small electric or other motor to rotate the spark gap.

  Everything must be very heavily constructed. The high voltages are dangerous and will eventually break down almost any insulation. If the capacitor breaks down, the entire device is likely to melt. Smaller units, with the same basic design can be made semi-portable, but to achieve any distance you need large currents and high voltages. The problem is that the spark transmitter spreads its power over a broad band of radio frequencies. A typical spark transmitter has an efficiency forty to one hundred times less than a modern CW transmitter, so a one-watt portable battery operated CW transmitter made by up-timers would be equivalent to a 100 watt massive spark station in terms of how far away it can be received. Spark stations handling multiple kilowatts of signal were very common prior to the development of tubes. In addition to all that, they are loud, dangerous and give off large quantities of ozone which can damage the operators lungs.