CRYSTAL SET CORNER

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The Crystal Set 1/2

PART 1/2

I intend to write a series of articles on crystal sets used on the broadcast band, I will assume the reader has a basic knowledge of resonant circuits, along with frequency and wavelength. If you don’t follow please ask or check in a suitable book.

Broadcast frequencies are say 535khz to 1600khz with corresponding wavelengths of 560 metres to 187 metres. BC stations use a vertical antenna which creates vertically polarised waves and are best received (under ideal conditions) by a vertical antenna. The aim is to get your antenna as high as possible to get as strong a signal as possible. Because of the wavelengths involved your antenna will be shorter than one quarter of a wavelength, and will therefore look like a capacitor plus some resistance. The earliest crystal sets resonated the aerial with an inductor, e.g. a coil of wire with fixed taps or sliding contacts. This is called “loading the antenna”. By varying the number of coil turns the antenna resonates at different frequencies.

The energy in the resonate circuit is then tapped by a second circuit which includes a detector and earphones.

The tap from the detector circuit is adjusted so that it is possible to hear the required station while rejecting others close to it in frequency.

Remember a crystal set is a compromise between sensitivity (strength of station) and selectivity (ability to separate stations). The aerial tap affects the sensitivity mostly and the detector tap the selectivity. They will interact, you may need to move both taps when you want to select different stations. For a practical circuit, wind 135 to 150 turns of 26 gauge wire (0.4mm, approx. 25gns in weight) on a 200mm length of 50mm PVC plumbing pipe. The size of pipe, wire or number of turns is not critical, start by experimenting. Tap the coil every 10 turns from one end and every 5 from the other. This will allow coarse and fine adjustments, another idea is to wind the coil on a cone rather than a cylinder, also you can use sliding contacts to do the tapping. Use a germanium diode and high impedance headphones of 2000 ohms or higher.

The performance of the crystal set will depend on every part of the set. The most critical are the aerial and earth, together they determine the energy fed in to the rest of the circuit. The higher vertically the antenna, the stronger the stations, but there is a limit to how high you can put them. It is possible to make the antenna appear to be higher than it actually is.

(a) Horizontal wires in L or T configuration can be added to the top of the antenna, this is called a 'capacitance top-hat'
(b) The aerial can be loaded as in below (right), this is called ‘bottom loading’. A coil can instead be placed at the top of the antenna as below (left), this is called ‘top loading’, however this is difficult to adjust or tune to different frequencies.
(c) The resistance of the aerial can be reduced, instead of a single strand or one stranded wire a number of parallel separated wires can be used. This is called a ‘cage’ or ‘sausage’ aerial. Typically six wires are used in a hexagon configuration using insulated spacers. The wires should be the same length over the total antenna.

Now lets improve on our crystal set design

The aerial is ‘bottom’ loaded by means of the variable aerial tap, until the aerial circuit resonates at the station required To give high sensitivity and some selectivity the circuit formed by the coil, taped at about 80 turns and a single gang 415 pF capacitor is tuned to the required station. To improve selectivity the antenna circuit and capacitor controlled circuit are now two ‘close-coupled’ circuits tuned to the same frequency. The detector circuit consisting of a germanium diode (OA85, OA91 etc.) and 2000 ohm earphones is tapped at about 25 turns. The detector is now matched to the tuned circuits for best selectivity.

Peter Hadgraft

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