A Radio Primer
© March 1998-2013, America Patriot
(This article is not simply radio theory. It is actually a story about childhood discoveries. However, the article will give some basic radio theory skills and teach you something about how to build a simple, but useful radio from almost nothing. AP-March 1998, changes, updates, modifications and error correction 1998-2013)
I became interested in radio when I was about 5 years old. I used to visit my great grandmother deep in the mountains of Kentucky as a little boy. She had this absolutely magnificent, old (even then) radio. It was huge and sat on a pedestal. She had exactly one electrical socket in the entire cabin in which she lived and into that socket was plugged that old radio. When I visited her one time and spent the night sleeping on the floor of the old log cabin, I asked her before bed time to turn on the radio.
She had this elaborate tuning scheme where she fiddled with several knobs and eventually tuned in a station that was playing “The Grand Ole Opry” show. I listened for a while and then went off to bed, but not before I asked her how the voices got in the box. Back in those days, we didn’t own a television, and the only radio I can recall before that one was in the car in which my father drove us from Detroit, to Kentucky. She smiled at me and said there was a little man in the radio. I laughed and said there wasn’t. She reached up on top and unlatched the lid of the big wooden box and the light went out. The only other light was from the “coal-oil lamp” (kerosene for those of you that don’t speak Kentuckian!) and she reached inside the radio. I was so small I remember it being way over my head. She pulled a little doll of some sort from within. He had huge, coal-black eyes and a big smiley mouth and she held him up for me to see.
“That is the man that makes the music…” and of course I interrupted, telling her it wasn’t either. So, my mother and great grandmother sent me off to sleep before I could protest much more. Anyway, that was the beginning of my reason for wanting to understand radios. I KNEW in my heart my great grandmother wouldn’t fib to me, but, I also knew that little man couldn’t have been doing all those voices and songs by himself inside that little box! I tried several times to look inside that box, but gave up eventually. I couldn’t reach it.
When I was about 8 or 9 years old, another grandmother, my father’s mom, brought me a Christmas present when she visited us in Michigan. The box I opened contained a handful of wire, some plastic parts, and some metal pieces, along with an earphone and a little glass thing with wires coming out of the ends. The box said “Build your own Radio!” I thought, “Yeah… right…”.
I’d already been looking inside of radios and televisions by that time, and knew there was no WAY this thing would work. Where were the tubes! Where were the batteries or the electric plug? How about a speaker!? Anyway, that evening, after things were settled down and I could sneak off to my room with my new toys, I kept being drawn to that box. Finally, I started reading the instructions. I learned it was what was called a “Crystal Radio”, and wasn’t quite like “real crystal radios”, but would be just as good. If you have stuck with me this far you will see that I was interested in radios and communications from a very early age. And if you continue to stay with me for a few more pages, I promise a little entertainment, some actual learning and a skill you will never regret knowing. That is building a simple little radio set that you can use to pick up stations from all over the US, Canada or your region of the planet.
About the time I finished reading the instructions on how to put this thing together, I began to realize that the people that had put the kit together were dead serious and were promising me that it would pick up AM radio stations. I still couldn’t believe it, but set to building the thing.
There were instructions for how to wind a coil of wire, and they were very precise. Also, there was a long metal rod, a little longer than the length of the coil, and a brass ball with a hole in it. It was supposed to slide along the brass rod, and after being mounted inside of the plastic “radio box” the coil would mount inside. Some metal clips held things in place. The two ends of the wire had to be cleaned off to make an electrical contact and the length of the coil itself where the brass ball was supposed to touch also had to be cleaned of the enamel insulation on the “magnet wire”. Eventually, I got to the part where I snapped the pieces in place, the wires connected like they showed in the manual and the little glass thing with two wires I discovered was called a “Crystal Diode” (ah… hence the name ‘crystal radio’, I thought). Actually, I learned much later that wasn’t the origin of the name, but for my eight year old mind, I was already seriously stretching electronic theory!
Well, that night, I finished the radio and just before I was going to put the headset to my ears, my mother called in and told me it was time to get to sleep. I reluctantly put the thing away on my dresser and went off to say good night to the adults and my siblings (some of whom had toys with batteries and were already running them down).
The next day, I basically forgot about the radio until later that evening. I remember playing GI Joe with my brothers for several hours that day. When I did remember the radio, I quickly forgot about GI Joe and my brothers and ran into the room where the radio was and set up out onto the desk my brother and I shared. I hooked up the earphones and started listening. I ran that little brass ball (connected to the plastic push-lever that moved it) back and forth a million times and heard… absolutely nothing. Not a sound escaped from the earphones. I thought they might be broken, so I kept messing around with them until I ended up breaking one of the earphones. Mad at myself, I grabbed the instructions once more and started reading. I read them over and over. I was missing something.
The thing I was missing was the back page of the manual. It was only about 8 or 10 pages, but the very last page, the one I needed was missing. After digging through all of the stuff piled in the bedroom floor, I found the page I was missing. It was talking about an “Antenna”. Not only that, it was talking about something called “The Ground”. I couldn’t see how the “Ground” was going to help me get this stupid box working. There were no batteries and I’d broken one of the earphones already. This called for some special work. The antenna was supposed to be a wire, at least forty feet, stretched from my house to a tree. The only tree I had was about fifteen feet outside my window, and it was winter, AND it was wet and cold outside! So, I did the only thing a kid can do with the odds against him… I went and found my Father and asked him to explain this stupid thing to me.
Now, my father was, of course, the smartest man in the whole universe. Even Superman wouldn’t have messed with my dad in those days. He was big, about 6’2″ and weighed in at about 200 pounds. He was a Marine in the Korean War, knew karate and drove a truck when he went to work… a BIG truck. SO he MUST know something about this thing. Well, he didn’t. In fact, he did help me string a wire out the window to the tree, but didn’t know anymore about how a radio would work without batteries than I did. Then he smiled and told me to “read the instructions”. ACK! I was going to have to do this myself!
That evening, I finally got the antenna wired up, connected to the radio and the broken earphone repaired (my father DID know how to use a soldering iron). I sat down in front of the radio and began wiggling that lever back and forth. NOTHING! I was getting mad and I was going to write a letter to these people that got kids hopes up! I had, by some coincidence, attached a piece of wire to “The Ground” clip, still not quite understanding what driving a pipe into the dirt outside was going to accomplish. A some point, after becoming frustrated, I managed to kick the wire over against the heater vent on the baseboard. I remember them in that house, because they were shiny silver and didn’t match the wooden floors, or the color of the walls. I hear a “CRACKLE” in the ear phone.
This time I hooked the wire to the vent, not really caring this time if it worked. I was already thinking of ways that GI Joe could use the thing as a command post. He would appreciate the wires more than I did. I plopped the earphones that were attached to a wire head-thingy over my head and for the last time, started pushing the little level up and down the dial. When I reached one side of the limit I could push the lever, music filled my ears. It was music from a radio play. Some applause and laughter and then the actors began speaking. I listened to the program for the better part of ten minutes before I realized this contraption was working! I ran and got my entire family, who by this time thought I was headed for the loony with wires all over the bedroom, and earphones on my head, and NO BATTERIES. Then my dad listened. He told me, “See, I knew you could get it working” and he wandered out of the room. No one else really appreciated what I learned that day, except me, and perhaps my father.
The station I heard was CKLW, in Windsor, Canada. That station was probably a good 25 or 30 miles from my house in Detroit, Michigan. A couple of weeks later, I went to the library and checked out books on radio, AM radio specifically, because my little “crystal” set actually was an Amplitude Modulated radio receiver. Those books caused me to continue with electronics into High School and on into the military a good many years later. I began to experiment with my little radio receiver after that and after talking someone into letting me shovel snow from their walk (I made 3 dollars!) I convinced my dad to take me to Olson Electronics where I bought a bundle of diodes, some “magnet wire” and some little boards with holes in them so I could build my own radios. While I was there I saw something I had never seen, a Citizens Band Radio. The CB actually would become my second transmitter many years later, but, that is another story. I built several radios from scratch, and mostly, they all worked.
Basically, after I learned all I could about AM radios, I read other books on the theory of how radios and televisions functioned. This little radio, the crystal radio, turned into a hobby I kept all my life. Every so often I will throw together the parts to make one, use it for awhile, and usually give it away to someone who’s never seen one before. It’s been a few years now, and the last one I built went to a student at the technical school at which I was teaching. Those of you that have read thus far are asking, “When is this guy going to shut up and give us some real instructions?” Well… in the next chapter. Go get a cold drink (or a warm one if it is winter time where you live) and take a break for a bit. When you come back, be prepared to read some slightly technical material. I promise not to make it too technical for those of you that don’t know anything at all about radios. See you in a few minutes.
A Simple “Crystal” Radio
Welcome back to radio theory 101. First of all, let’s talk about the ‘theory’ behind radio signals, as it will help you to understand more about why you are going to be winding coils, and stringing wires high in the air above your head.
In physics, a radio signal is considered to be a ‘wave’. A wave is simply an electromagnetic waveform. An example that you could see would be dropping a pebble into a calm pond. The ripples you see are waves. Another example would be to take a rope, about fifteen or twenty feet long and tie it to a doorknob or other stationary object. Now step back to about two-thirds the length of the rope and begin to whip the loose end up and down. The motion will carry over in the rope, and create a wave that moves toward the stationary tie point of the rope. Basically, that is a wave. Sound also travels the same way, as waveforms, but sound waves are much slower than radio waves.
Radio waves travel at the speed of light, which just happens to be approximately 300,000,000 meters per second. That is 300 MILLION METERS in one second. In another measurement, light travels at about 186,000 miles per second. For anyone that doesn’t know, in US measurements, one meter is equal to 39.47 inches, or just over a yard. Incidentally,. Sound waves travel at about 1130 feet per second at sea level and a little slower above sea level (at my altitude, which is 6520 feet above sea level, it travels on a given day at about 1138 feet per second, depending on the temperature).
Radio and light waves travel at about the same speed, no matter if it is in air, or in a vacuum, like space. A quick reality check is to think back, if you remember the moon landings. There was about a 3 second delay from the time you heard Houston Control end a transmission, and the beginning of the transmission from the astronauts on the moon. The reason is, the moon is just far enough away most of the time for light (or radio waves) leaving the surface of the Earth to get to the Moon, 1.5 seconds later. Round trip, it takes a full 3 seconds or more if the Moon is a little further away!
Since a radio wave is really an electromagnetic waveform, it has other properties. For instance, it can be used to induce current into wires. Since that is a whole different class on physics, we won’t delve too deeply here. Suffice it to say that when a radio wave cuts across or passes over a wire, a current is induced into that wire. This current is the essential part of the radio wave we need to “grab” from the air to collect the information or intelligence on the radio signal. Intelligence can be information, music, pictures (like television) or even occasionally, news reports! (Sometimes there isn’t ANY intelligence in news reports!) Our basic “Signal capturing device” is nothing more than an ANTENNA! A simple piece of wire will usually work for this.
Once we have that radio signal captured on the wire, we have to give it some place to go. Something I mentioned in my story was the idea that “The Ground” had something to do with that radio working. Though I didn’t realize it when I connected that wire to the heater vents, I was ‘grounding’ my receiver. I understand it now, but didn’t then.
Electrical signals (like the one we have induced into our antenna) need some place to go. The Earth, that is the planet Earth, and the earth or dirt beneath our feet will actually conduct signals. In fact, if I were to build an antenna 100 feet tall, and leave it standing in the air, and made sure to bury the end closest to the earth, electrical currents from radio signals would be induced all day (and night) long, and travel down the antenna to the earth where they would… simply, for our purposes, dissipate or vanish. What we need to do now, is to put a machine or some kind of tool in the middle, between the antenna and the ground, to do some work for us. It must decipher that electrical signal and change in into something that we can use – in this case sound.
The next few pages, I will describe a simple radio that should work just about anywhere in the world. You should be able to build this simple device for very little cost, unless you want to spend more to make it look pretty or something.
The radio itself will need some things, and then you will need to make an antenna to make this contraption work. Most of these items can be obtained at Radio Shack, or some similar electronics store. There are places to mail order some parts from, but that probably won’t be necessary unless you live so far away from town that it isn’t worth the trip in!
You will need several meters of something called Magnet Wire. Magnet wire is thin copper wire, covered with enamel paint. It is measured in ‘gauge’. What you want is something that is not as thin as human hair, but not too thick either. It will be approximately 22 or 24 gauge. If you buy it, it will probably come on a roll and be a couple of hundred feet and I have no idea what it might cost these days. Barring buying it, you can obtain magnet wire from the inside of a transformer. Just about any kind will do, so the wall warts that you use to power telephones, tape recorders, scanners and such. These are usually called “AC Adaptors”. Problem is you don’t want to tear one up, if it works. A few people have broken things like that at garage sales and so on. Look around.
You will have to CAREFULLY disassemble the plastic case to get the actual transformer out of the inside. I typically use a hammer. :) Just crack the case and pull it apart and remove the heavy metal thing inside. That is the transformer. If you can see reddish colored wire going around and around the inside of the heavy metal part, then you have hit pay dirt! Unwind all the wire and put it around a wooden dowel or something similar, you will need most of it. Once you collect the wire, you will need a coil form upon which to wind the wire.
My favorite tube is easily obtainable from right around most homes. It is just the right length and width to make a coil that will allow you to tune in local AM radio stations. That tube can be found in most bathrooms, just before you toss the trash out… a (ahem…) toilet paper cardboard tube. It is 4.5 inches long and 1.75 inches in diameter. When you begin to wind the wire, you will need to lay it from one end and tape it or some other way secure it to the tube. Carefully laying the wire about ½ inch from one side, begin winding the wire neatly, and not overlapping anything. When you are finished, you should have a nice, neat coil that goes around and around the tube, leaving about œ inch of space on either end of the tube. The coil, if you measure will be 3.5 inches wide along the tube. You will need to make sure the wire is tight around the tube so that it does not move at all. Once it is, tightly secure it at both ends and make sure you leave a few inches of wire at either end. Figure 1 shows you what it should look like.
If you look at the picture above (Fig. 1) you can see a basic idea of the way the coil should be set up. Once you have the coil built, you might want to find a board or something upon which to mount it.
Next we will want to obtain some parts for our earphone. What we require for this to work is something called a “High Impedance” earphone. At the time of this writing, I know of no places to purchase the correct parts, but I know of a source that is tried and true. If you take any standard telephone handset apart (I don’t mean the new-fangled electronic telephones, but the older, 1950-1970 telephone units), you will find that the earphone portion will unscrew. By unscrewing this plastic cover and removing the metal part inside you will have exactly what we need to make an earphone. If you look at figure 2, you will see what the front of the device looks like. Figure 3 is a little larger picture of the backside of the object, showing that there are two screws. This is where your wires connect to the radio. Some of these devices had the wires soldered on. This is ok, if you know how to solder (another handy skill to learn ANYWAY). Set this aside until we are ready to put it together.
The next thing we will need is a part from Radio Shack or some other electronics supply store. Radio Shacks are usually all over the place and I know they have them in many countries besides the US. If I remember correctly, they are called Tandy Stores in Australia. Not sure about the UK and Europe however. The device you are looking for is called a ‘diode’. In the past, before semiconductor devices were invented, the “crystal” of the “crystal radio” was ACTUALLY a rock that contained small crystals of gallium. The “crystal” you are going to use is made of a semiconductor material called Germanium and perhaps some gallium-arsenide. No matter, these devices are so inexpensive; you can probably pick up about a dozen for a buck. Figure 4 shows a representation of what a glass encased germanium diode looks like. The colored bands are not important, but they are there in the picture to show you what you might see. They might be different colors, or only one band. However, one end will be marked in some manner. Incidentally, that end that is marked is called the “cathode” and the other end is called the “anode”.
Note that even though there are some technical aspects of how a diode works, we won’t go too deeply into the theory right now. Suffice it to say that the diode “detects” the audio signal from the radio signal, turning it into something that can be used to power the earphone.
So far, we have three important parts; the earphone, the coil and a diode. We also need a few more things. We will need some hookup wire; any kind will do, as long as it isn’t too thick. You will require a mounting board for this radio as well, an antenna (about 40-60 feet of wire will work), some wire to make a ground connection and, we will need a ‘tuner’ of some sort. Your ground should be a cold water pipe, as close to where you wish to set up the radio. An antenna should be strung outside, if possible and as high away from the ground as possible. A length of wire will suffice, with one end terminating at the point where it will attach to the crystal radio.
The idea of a tuner is to change the size of the coil that you have made. Rather than unwind the wire to make it shorter or longer, we are going to make a mechanical device that will allow us to make the coil think it is longer or shorter. Figure 5 shows the details, but essentially, you will need something that will slide back and forth across the already assembled coil. In figure 5 you will see the assembled device. A metal slider is mounted with a screw to the board and bent so that it will touch the coil. It can be made from any stiff sort of metal, even a tin can cut with tin-snips, or a wire mounted to the underside of a Popsicle stick would be ok. If you slide it back and forth you will see it creates a small arc across the coil. What you will need to do next is tricky, but must be done. Get some fine grit sand paper (00 if you have it should work) and lightly but CAREFULLY sand the insulation from the wire along the arc that the metal strip will eventually contact. What you are making is the actual tuner for the radio.
By adjusting the location of this slider, you will be able to change the frequency of the radio by changing the “inductance” of the coil. If the coil is correctly built, and the diode is the correct type, AND you get the antenna, the ground and the earphone all in place, you will hear AM radio stations in the range of about 500 kHz to 1600 kHz, or the AM Broadcast band!
The Theory of the Crystal Radio
I’ve already given you most of the theory behind this little device. Basically, a radio signal hits the antenna, and looking for a place to “go to rest”, it tries to go to ground. Mother Nature is good about trying to reach equilibrium, so the signal will seek to neutralize itself, hence (a simple explanation, granted but correct for our purposes) it tries to go to the ground connection.
The coil sets the frequency. Technically, there probably would be a tunable capacitor instead, but those are harder to find than the simple tools we have here. By adjusting the coil’s physical size (that is, moving the slider back and forth) you can change the amount of inductance, which in turn determines the operating frequency of the device. The interesting part is, that the larger the coil, the LOWER the operating frequency, and the smaller the coil, the HIGHER the frequency
The diode “detects” the Amplitude Modulated signal that is riding on the Radio Frequency CARRIER wave. That is, the radio wave itself. The audio signal, voice or music, has been superimposed over the radio signal by a method called “modulation”. If you had a way to see the signal, it might look like figure 6.
Once the diode ‘sees’ the AM signal coming in, it will cut in half the top signal, giving you the bottom half of the above picture. What you are seeing (ok, hearing!) is the actual audio signal as it was originally produced at the radio station. An FM signal is quite different and can not be reproduced using this exact method of detection. It is possible to do something called “slope detection” with an AM receiver, but it is not really practical with this sort of radio. An actual “schematic” follows in Figure 7.
Now, one last item of interest, let’s assume you do not have access to diodes, or crystals from rocks. What to do, what to do? Well, if you can find some old, blued razor blades there are tiny crystals of gallium produced on the surface of these blades in the manufacturing process. These crystals are deposited on the surface of the blade when they are turned blue. I realize they are hard to come by in this US these days, but there are places they can still be found. However, by itself, it won’t work. You will need a tiny piece of a wooden pencil, a safety pin and in place of the diode; this little device WILL serve as the detector. Please, see figure 8 for the details.
I hope this gets you started on the road to radio reception without the aid of batteries, electricity and with very little equipment or cost. I must warn you of several things. First of all, this is not that easy to make work and patience will be a necessity. Secondly, when you get it working, it will surprise and amaze you that something like this will actually work! Lastly, you might get hooked on playing with toys like this and wish to experiment. There are many books already published on such things, but this sort of information is very difficult to find in the library these days. Perhaps, someone is trying to keep us from knowing this kind of information, or worse, the skills to do such things have died with the older people who came up with it. In either case, this is my attempt to pass on this incredible and important, as well as useful information. I suggest you do the same with it.
To one and all – 73!