A Quality Crystal Radio Set
Here is my design using fairly modern parts. With it, I came 1st in my local radio club home construction contest a few years ago, mainly because it was so LOUD.
It gives VERY good sensitivity and selectivity. In South London, it can pick up 7 MW and 4 LW stations (3 French) on 40M of aerial wire. The stronger ones can even just about drive a good small 8 ohm loudspeaker.
FEATURES
DUAL TUNED CIRCUITS for Selectivity.
VARIABLE COUPLING for Sensitivity/Selectivity.
VARIABLE INDUCTANCE also widens the tuning range.
DUAL BAND, MW and LW, plug in capacitors extend range to LW.
HIGH and LOW AERIAL IMPEDANCE option, for best matching.
AERIAL NEON gives visible protection against High Aerial Voltages.
LENS to see "CAT's WHISKER on the CRYSTAL"
R.F.C. to improve diode linearity and sensitivity performance.
TAPPED AUDIO MATCHING TRANSFORMER for optimum loudness.
THE CIRCUIT
PARTS and TIPS
BASE Use a wooden base board about 25 x 10cm. Fit rubber feet so that protrusions don't scratch.
CIRCUIT WIRING. Use tin plate strips tacked down, this is easy to solder to.
HEADPHONES. Use a coat hanger wire head band to connect 2x 40ohm rocking armature Telephone (4T) earpieces connected in series. These are the most sensitive transducers made and peak @ 3KHz. They can be connected permanently with flexible leads or with a plug and socket for comparing with other headphones.
AF TRANSFORMER. For best load matching and volume, use 7 equal windings of an old
telecomms line transformer that had 2x 600ohm windings in 4 sections each. The first 2 windings are paralleled up for low Z loads. Adjustable connection is made with a small wander crocodile clip to the linking straps. Typically 2-3 of the 7 taps gives the best
for the 80ohm headphones. Mount well to the base board as it is heavy component.
10nF CAPACITOR. Is used for blocking RF from the AF load. Theory says yes, but it is not really necessary component.
R.F.C. Approx 20 turns of enamelled copper wire (0.3mm?) on small polo mint sized (12mm dia) toroidal ferrite core. Again theory says yes as it will keep the diode conduction angle high and hence reduce the diode resistive losses. It will also makes the load on the tuned
circuit higher. But it is not really necessary component. Mount with an insulated screw through the middle.
GERMANIUM DIODE. Viewable old large glass bubble type eg an old OA91. Mount a
small plastic lens (from cardboard slide viewer) in front of the crystal with shaped paper clip wire, so that the surface of the crystal and the internal cat's whisker can be seen.
GANGED 500pF VARIABLES. Two needed, air spaced ideally. Large value gives best
tuning range, so the sections are paralleled up. Put knobs on the shafts and mount them well to the baseboard, as they get a lot of tweaking.
1n5 LW CAP. Two needed. Connected one side to ground and the other to a 2mm wander test plugs/sockets to give Long Wave coverage.
FERRITE ROD. Standard radio type 14cm long, 8mm dia. These give plenty of L adjustment and coupling variation. With the coils the ferrite gives high Q tuned circuits, many times higher than air space coils! Mount with plastic cable clip in the middle with high enough packing/washers to allow coils to be moved.
COILS. Two standard MW radio type with tags, eg about 30 turns of 0.3mm enamelled copper wire on a loose fitting paper tube with connection tags. Connect with twisted flexible leads as they will need sliding along the ferrite rod to vary the inductances and
coupling.
NEON Wire ended 80v bulb type. Used to safely and dramatically discharge the aerial in the low Z mode when static is around.
CONNECTIONS 4mm socket with screw up type for Earth and the 2 aerial options, with a swinging earth link plate, to make the high Z aerial configuration.
TUNING IN
1 Connect an earth, mains earth can be noisy, old Gas and Water pipes are better earths as long as they are not PLASTIC!
2 Connect the aerial to either the High Z or Low Z options. Short aerial wires compared to a 1/8 wave are always high Z. Higher and longer aerial wires are always better, as they pick up more RF power.
3 With the coils close together adjust the detector and aerial variable capacitors for a station of interest.
4 If the station is loud enough, but can't be separated from another one try adjustment of the 2 variable capacitors at once, also reduce the coupling by moving the coils apart and try again. Note the affect of tracking both variables, there is a best matching sweet spot for optimum loudness.
5 Adjust the auto-transformer tapping for loudest output, note that there is some treble difference as the match changes due to the RF decoupling Cap.
6 Try the other aerial impedance option and see if other stations are heard.
7 If the station is too HF or LF for the tuning range, try moving the coils more to the middle for LF, or to the far ends for HF.
8 With the LW capacitors connected, the tuning is less effective and moving the coils gives more frequency range.
73 de John @ GB7CIP
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