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This is in the form of an IC, which contains either a 2 pack or 4 pack of specialised FETs Field Effect Transistors that require no bias voltage on their gates to conduct current from source to drain. They simply rely on signal voltages to turn them partially or fully on.
The original article, which appeared in the January edition of QST magazine, authored by Bob Cutler callsign N7FKI, uses such a device, and both medium wave and short wave receiver versions are illustrated for constructors to build. Readers can access the QST website just Google a few keywords and obtain a copy of the article, free of charge.
The discovery of the germanium diode during WW2 and the subsequent discovery of the point contact transistor by Bell Labs in , rekindled the joys of simple receivers, as those components came onto the market via disposals shops and hobby stores. Now, they have all but had their day, as manufacturers and retailers once again, move away from the hobby end of the market.
If it were not for a band of very dedicated enthusiasts, the hobby part of radio may well have died a long time ago. Well, my basic knowledge of FET devices at the time of acquiring the QST design was a little rusty, to say the least, not having experimented with them for some time, so after a bit of scratching around on the net, and down at the local library, I reclued myself as to their peculiarities. Devices that are static sensitive have protection diodes on the inputs, and the ALD device certainly has those.
Basic JFETs also have a protective diode between the gate and source connections, presumably for the same reason, but neither the QST design, nor my adaptation of it, use any internal diodes as a rectifier.
The basic premise behind the QST article, is that you drive the gate of the FET with an RF voltage derived from the top of the tank circuit via C1, and the gate switches on and off very rapidly, at the resonant frequency that you are tuned to.
The source connection is tapped into the tuning coil low down, as a means of impedance matching with the headphones and acts as the anode, while the drain connects with your headphones as the cathode, to complete the detector part of the circuit. Ideally, short antenna wires work best near the top of the tuning coil, but in this case, the best position seems to be right at the bottom tap!
Another aspect of using a JFET in this manner, is the actual sound quality that you get in the headphones. The audio is very clear ans is in no way muffled, or distorted. Diodes tend to introduce various distortions, and they also exhibit very high output impedance. With this design using the 2N device, it is possible to get rid of most distortions and at the same time, use just about any kind of audio transducer that you may have on hand.
Diode detectors can also cause loading on the tank circuit if they are tapped too high up the coil windings. The JFET in this circuit is tapped way down, at around 10 turns, with the antenna lead sitting just under that on 5 turns from the grounded end of the coil.
Get some good quality, thinnish and untinned copper colour, not silvered colour multi stranded hookup wire for winding the tuning coil and a tube made of plastic, rather than cardboard. Simply fasten the wire onto one end of the rod using some tape, then wind on around 50 to 60 turns, with two taps - one at five turns from the grounded end for the antenna lead in and the other one at ten turns for the Source anode connection.
You may need to experiment with the number of turns, and the tapping points on the coil, depending on the value of VC1. To the left you can see a basic circuit diagram of my radio, based on the QST design, and to the right, a physical wiring diagram of how it all goes together.
Two alternative methods for connecting either a crystal earphone left or low impedance headphones right. Parts List C2 0. M or similar.
2N5484 - 2N5484 JFET N-Channel High Frequency Transistor