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KF5JRV > TECH 11.02.17 14:23l 69 Lines 3534 Bytes #999 (0) @ WW
BID : 10257_KF5JRV
Read: GUEST
Subj: Xtal Radio Components
Path: IW8PGT<IZ3LSV<IW2OHX<UA6ADV<CX2SA<N0KFQ<KF5JRV
Sent: 170211/1215Z @:KF5JRV.#NWAR.AR.USA.NA #:10257 BPQK6.0.13
In the heyday of the crystal radio set there were many suppliers supplying a
wide variety of the different components and other parts needed to make or
build one of these sets.
Today, technology has moved on and the components for crystal radio sets are
not as obvious, or they may be far more expensive.
Fortunately it is till possible to make or build a crystal radio set without
too much expenditure.
Crystal radio components
Although a variety of circuits can be used for a crystal radio, the simplest
form is shown in the diagram below. This uses just four components and serves
to illustrate what component parts are needed.
Obviously more complicated circuits can be used, but these have the added
complexity of requiring more components.
Crystal radios require very few parts and these are summarised below:
Coil or inductor: In the original crystal radios, this component could
take a variety of forms. The coils were made on cylinders, on small formers or
as a flat format where the wire was almost weaved on a flat circular shape with
radiating slits cut in the base material so the wire could be wound on it and
kept in place. Others coils formed the input matching transformer. Some were
wound on two separate formers and there was a mechanical arrangement to alter
the coupling between them.
For the home constructor, this component for the crystal radio can be made
relatively easily. Typically it will need to be wound on a cylindrical former.
Often card cylinders for wrapping paper or kitchen roll provide a useful and
easy t use base that is easily available. Plastic pipe can also provide a good
base as well. Other coils can be made from discarded transistor radios. Often
these radios have ferrite rod antennas, which could form the basis of the coil.
Although not in any way authentic, the circuit would work.
Tuning capacitor: In the original crystal radios, this component could be
in one of a variety of forms. Often these components had a series of vanes
alternately connected to different connections of the capacitor. The vanes
could be moved in and out, the degree of meshing altering the capacitance.
Today, it may be possible to obtain small plastic variable capacitors
intended for use within AM transistor radios. The older metal and larger
components are much more expensive these days.
Signal diode: The original crystal detectors were the cat’s whisker style
detectors described on another page. These consisted of a small piece of the
rectifier material of crystal held in a holder onto which a thin piece of wire
would be placed. This created a point contact diode an allowed the signal to be
rectified and hence detected or demodulated.
Today small glass packaged germanium signal diodes like the e.g. OA47,
OA90, OA91, IN34 etc. are often used. These diodes, rather than silicon diodes
should be used because germanium diodes have a much lower turn on voltage,
around 0.2V rather than the 0.6V of the silicon diodes. Schottky diodes an also
be used as these have a low turn on voltage as well.
Both these types of diode are widely available for a few pence. They are
much cheaper than many of the other components for the crystal set.
Headphones: The headphones used must be high impedance versions.
Originally high impedance moving coil headphones were used. Often these were
based around the earpieces used in telephone handsets, and also the headsets
were often quite uncomfortable.
73 Scott KF5JRV
KF5JRV.#NWAR.AR.USA.NA
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