Crystal Shortwave Radio Theory
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This block diagram shows each stage of the "EconOceanic" crystal shortwave receiver. Click on each component to read more about it.
Aerial | | Antenna coil-->Tuning coil-->Tuning capacitor-->Detector-->Output | | Earth ground
The aerial's job is to capture electromagnetic waves from all sources and directions and funnel those charges into the antenna coil. The loudness of received signals is directly proportional to the height and length of the aerial. Mount the aerial as high as possible, preferably outdoors. A single wire 100 feet long makes an ideal aerial. The "EconOceanic" design provides for two antenna connections: one 1/8-inch phone jack and a spring-loaded clip (originally intended for fastening hi-fi speaker cables).
If your antenna is mounted outdoors, you should disconnect it whenever there is any chance of lightning in your vicinity.
The Antenna coil is the primary winding of the pair of coils that make up the antenna-tuning transformer. It is a fraction (1/6) of the number of turns that make up the tuning coil. The secondary coil multiplies the electrical field created by the antenna coil.
The earth ground connection effectively extends the antenna into the earth, which contains electrical signals. Keep the ground wire as short and straight as possible. There may be some sensitivity loss if the excess ground wire is coiled into a loop. Use either a metal cold water pipe that is buried at least several feet in the ground or an electrical conduit that is similarly buried or grounded. You can usually make a satisfactory ground connection using the screw that fastens the cover plate to an electrical outlet box.
The tuning coil is the inductor which, in series with the tuning capacitor, determines the resonant frequency of the receiver according to the formula: frequency = 1/[2*pi*sqroot(L*C)]. Wind your coil carefully and space it close to the antenna coil (about 1/8" apart) to tune the specified frequencies.
The amount of coupling is directly proportional to the distance between the windings. If you slide the windings far apart, there is little coupling and a narrow resonance peak (e.g., 100 KHz). If you move the windings closer, there is more coupling and a broader resonance peak (e.g., 200 KHz). A broader resonance peak will mean less selective tuning; that is, several stations will be heard simultaneously at one tuner setting. Experiment, if you wish, with spacing your coils. For example, try 1/4-inch between the primary and secondary windings and test your design for selectivity. Have fun as you learn.
The tuning capacitor is the variable component in the "tank" circuit which comprises the coil and the cap. By varying the capacitance, you can change the resonant frequency of the receiver within the limits of the coil inductance and the total capacitance available in the variable capacitor.
In this radio, a diode serves as the detector, separating the fluctuating direct current (containing the voices and music of the broadcast) from the amplitude-modulated, radio-frequency, alternating-current carrier wave that was transmitted from the radio station antenna. The direct current generated by the detector will drive the output device.
The output device changes the electrical energy in the detector circuit into mechanical energy that moves air against our ears to create sound. For satisfactory sound output, this needs to have an input sensitivity of a few millivolts and an input impedance of a few thousand ohms, such as a pair of 2000-ohm headphones or Radio Shack's amplified speaker (part no. 277-1008). Don't try to use a permanent-magnet speaker, such as one scrounged from an old radio. Permanent-magnet speakers require more DC energy than a crystal set can generate.
Copyright 1997 Walter Heskes and Philip I. Nelson, all rights reserved. This radio construction project, including all descriptions, diagrams, photos, and the underlying electronic design, is published here for the noncommercial use of radio hobbyists. You may print and reproduce these project instructions for your personal use. Commercial use of this material is strictly forbidden.