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High Selectivity Crystal Radio with Short Wave
Description:
This Crystal Radio is highly selective as to separating stations and tunes in the A.M. band from 570 KHZ up to 1.7 MHZ and the short wave bands up to 2.5 MHZ. It is suggested that crystal earphones be used….. or (preferred) feed the audio output of this tuner into an audio amplifier. A good antenna of at least 50 feet is required along with a good earth ground connection. Switch 1 will allow the selection of signals from either short or long antennas to induce the proper amount of signal up into coil L1 for good selectivity and Sensitivity. Never operate this receiver during electrical storms, however, the fact that L2 is always connected to the antenna and ground, ensures that static charges are always being removed from the antenna, thus making it less likely to attract lightning.
How it Works:
Signals from the antenna are fed to antenna coil L2. These are induced up into tuning
Coil L1. Capacitor C1 and L1 tune in (resonate) at a particular station’s frequency. Diode D1 removes one half of the signal as in rectification and capacitor C2 integrates or combines the voltages to produce an audio signal. Resistor R1 provides a current path for the signals and also discharges C2. To sum all of this up…. D1, C2, and R1 change a high frequency signal into a much lower one that we humans can hear. The audible sounds are passed on to the output jack J1. This might make a good science fair project if the high frequency (RF) to low frequency (Audio) frequency conversion was addressed.
Construction:
Cut a piece of 3⁄4 inch thick wood to 6 5/8 wide by 5 1⁄2 deep. 3 of the edges may be routed with a design. The coil consists of a piece of 3 inch PVC that is 4 5/8 inches long. Start winding the #22 wire 9/16 of an inch from one end. After 12 turns, scrape off some insulation, make a twist and continue winding for another 24 turns. Now space 1 1/16 inch and start winding L1. After 17 turns, scrape off some insulation and make another twist, wind another 28 turns. Wrap masking tape over the bare copper twists and then spray several coats of clear acrylic or Poly Urethane to seal the coils. After the coil is dry, mount it to the wood base, using 1⁄2 inch tall nylon spacers. Note: a # 62 drill bit is used to drill the small holes into the PVC to anchor the wires.
The front Aluminum panel is 6 5/8 inches wide by 4 1⁄2 inches high by 1/8 inch thick. This thick aluminum allows for the countersinking of the 6-32 machine screws that go into the front of the variable capacitor. About 40 feet of #22 wire is required for L1 and 36 feet for L2. A 1⁄2 inch wide by 2 inch piece of acrylic plastic has been epoxied to the back of the tuning knob and a black line drawn with a fine line Sharpie Pen, to enhance tuning.
Trouble Shooting:
  1. Check your wiring… perhaps yours is incorrect

  2. The diode has been overheated during soldering and is damaged.
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