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Understanding and Operating Your Crystal Radio – Jeffrey La Favre – March 28, 2020 – Page 1 Understanding and Operating your Crystal Radio – Jeffrey La Favre – March 28, 2020 – Page 1 Your crystal radio receiver is designed to tune stations on the AM broadcast band (radio frequencies of 540 to 1610 kHz). You will be limited in the number of stations you can hear when operating the radio without amplification. Keep in mind that a crystal radio operates ONLY on the power received from the radio station transmitter. Therefore, your ability to hear a station will depend to a great extent on the distance from the radio transmitter to your house and the amount of power the station uses to broadcast their signal. Another important factor will be the antenna you use with your radio. Most of our club members live in Geauga County. As far as I know, there is no radio station broadcasting on the AM band in our county (at least not a powerful station). From my home in Chesterland, the strongest station tuned on my crystal radio is WTAM. This station broadcasts with 50,000 watts of power on a frequency of 1100 kHz. The radio transmitter is located at 8200 Snowville Road, Brecksville, Ohio. The distance from the transmitter to Chesterland is about 23 miles (21 miles to Auburn Twp and 31 miles to Chardon). Figure 1 is an aerial view of the WTAM transmitting antenna, which is 480 feet tall. As radio waves travel away from the transmitter, the power density of the radio energy varies inversely with the square of the distance. What does that mean? We can write this as a proportion: 1 powerdensity ∝ distance2 Let me give some examples. The power density of a radio wave at a distance of 10 miles from the transmitter is: Figure 1 WTAM transmitting antenna 1 1 powerdensity ∝ = =0.01 10× 10 100 The power density at 20 miles is: 1 1 powerdensity ∝ = =0.0025 20× 20 400 Therefore, the power density at 20 miles is 1/4th the power density at a distance of 10 miles. At a distance of 40 miles, the power density will be 1/16th the power density at 10 miles. All other things being equal, if your crystal radio is located 40 miles from the transmitter, the signal strength delivered to the radio will be 1/16th what it would be if located 10 miles from the transmitter. This is why you will have a better chance of hearing a station, the closer you are to its transmitter. Of course the amount of power used in transmitting makes a difference. If two stations are the same distance away, but one transmits with 1,000 watts of power and another transmits with 50,000 watts, the 50,000 watt station will be considerably louder than the one with 1,000 watts. 1 Understanding and Operating your Crystal Radio – Jeffrey La Favre – March 28, 2020 – Page 2 Antenna for the Crystal Radio The antenna you connect to your crystal radio will determine to a great extent the number of stations you can hear. It is doubtful that an indoor antenna will be sufficient unless you live within a few miles of an AM radio transmitter. Provided that you have permission from your parents, you can install an outdoor antenna. You should ask your parents for assistance in erecting the antenna. This can be a temporary antenna, put up only when you are using the radio, or a more permanent installation. Again, you should ask your parents which kind of installation they will allow. Some people think that antennas are not attractive and you must keep this in mind. The antenna that I use for my crystal radio is of the long wire type (14 gauge, stranded and insulated). It is permanently installed. I drilled a hole through the window frame to allow the antenna wire to exit from the GEAR workshop to the outside. I installed a hook on the edge of the roof and with an insulator, attached the wire antenna. The wire length from window to the edge of the roof is 11 feet. Then the wire runs to a tree 50 feet from the roof. The attachment to the tree is 18 feet above the ground. The wire then runs to another tree 61 feet from the first tree and also 18 feet above the ground. At the second tree the wire goes through a pulley and then down to the ground where it is attached to a weight. The total length of the antenna wire, from the window to the end is 140 feet (you will also need additional length inside the house to reach the radio). The pulley is used as part of the antenna tensioning system. When the wind moves the tree branches, the wire moves through the pulley to prevent breakage of the wire. The weight at the end of the wire provides the tensioning of the antenna so that it does not dip excessively toward the ground between its supports. This antenna works well and I can hear four or five stations during the day. When you work 18 feet above the ground, there is danger of a serious or fatal injury from a fall. If you want to erect an antenna like the one I have described, DO NOT ATTEMPT THIS ON YOUR OWN. PLEASE HAVE YOUR PARENTS HELP YOU. I used a ladder to attach the antenna wire to the trees. This kind of work can be very dangerous and you must use proper safety procedures. The job takes a minimum of two people, one to hold the ladder while the other person climbs the ladder. The ladder should be secured with ropes so that it will not slip off the tree. It is also a good idea to use a safety harness that connects the climber to the ladder in case they slip on a ladder rung. A safer procedure, although possibly more frustrating, is to throw a light line over a tree branch from the ground. Attach a weight to the end of a string and attempt to throw the weight over a branch. If that does not work you might try using a slingshot. It would be a good idea to wear a hard hat or bicycle helmet while doing this in case the thrown weight comes back down on your head! If you have a very long pole, you might use that as well to hoist the wire over a branch. These methods are preferred over the ladder since they are safer. After you are successful in getting a string line over a tree branch, connect the string to your antenna wire and pull the string to hoist the antenna wire in place. Ground Connection for the Crystal Radio In addition to the antenna, a connection to ground is required. The ground connection is actually part of the antenna system. For the antenna I have described, I used a grounding rod, which you can find in the electrical department at a hardware store. The rod is 8 feet long and all but a few inches are driven into the ground with a sledge hammer. Make sure there are no pipes or underground electrical wires in the area where you install the grounding rod. It is best to install the rod near where the antenna wire exits 2 Understanding and Operating your Crystal Radio – Jeffrey La Favre – March 28, 2020 – Page 3 the house. The rod is copper-clad steel. A special bronze clamp is used to attach a ground wire to the rod. The other end of this ground wire is connected to the ground connection (black wire) of the antenna coil of the radio. The grounding rod must be in moist soil to work well. Of course it is difficult to know how moist your soil is 8 feet below the surface. But if your antenna does not seem to be working very well, you may need to water the soil around the grounding rod. If you are not sure you will use your radio very much, you may decide to erect a shorter outdoor antenna. I erected a temporary long wire antenna, 11 feet from window to edge of roof, 50 feet to a tree branch that was 7 feet off the ground, then ran the wire to the ground and tied to a weight (total of 68 feet of wire from the window to the end of the wire). For this antenna I did not use a grounding rod. Instead, I connected the ground wire of the antenna coil to the ground system of the house electrical wiring. This can be done by connecting a wire to the screw of a faceplate for an electrical receptacle – see Figure 2 (DO NOT PUSH THE WIRE INTO ONE OF THE RECEPTACLE SLOTS AS THIS COULD RESULT IN A FATAL ELECTRIC SHOCK). Using the shorter antenna and ground to faceplate, I could only hear station WTAM 1100. After you have completed the audio amplifier for Figure 2 ground wire connected your crystal radio, you will be able to hear more stations with the shorter to faceplate antenna. However, if you have an interest in hearing distant stations, the longer antenna with grounding rod will be better. When there is a chance of an electrical storm in your area, it is advisable to disconnect your antenna at a location outside your house. While lighting strikes on an antenna like yours are rare, it is better to be safe and prevent the very high current and voltage from entering your house. It is best if your exterior antenna is connected to ground outside the house when not in use.
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