Vest Pocket Crystal Receiver

[smfadmin (OP)]

The picture says it all ... here is a crystal radio no bigger than a packet of cigarettes:

The Basic Circuit

Code: [Select]

Antenna
   |
   +---[L1 coil]---+---[C1 variable cap]---+
                   |                       |
                 (tap or                   |
                  slider)              [Crystal
                   |                   detector]
                   |                       |
                   +---[C2 bypass cap]---[Earphones]
                   |                       |
                  GND                     GND

Components

Antenna — long wire, ideally 10–30 metres. No antenna = very weak signal.

L1 — Coil — 50–100 turns of fine enamelled copper wire on a ferrite rod or cardboard tube. Fixed or sliding contact (variometer) in pocket sets. Forms the inductive half of the tuned circuit.

C1 — Tuning Capacitor — small variable cap, typically 0–365 pF on a thumbwheel. With L1 forms a parallel LC tank circuit. For the AM broadcast band (530–1700 kHz): f = 1/(2π√LC)

Crystal Detector — galena (PbS) crystal in a cup with a fine phosphor bronze cat's whisker wire. Forms a point-contact diode, rectifying the RF signal and leaving the audio envelope. Whisker position was critical and fiddly. Later pocket sets used a fixed germanium diode (1N34A), eliminating the fiddling.

C2 — Bypass Capacitor — 100–500 pF across the earphones. Provides a low-impedance RF path to ground so only audio reaches the earphones.

Earphones — must be high-impedance crystal or ceramic type (2,000–10,000 ohms). Low-impedance dynamic earphones won't work without a matching transformer.

Ground — water pipe or ground stake. Essential to complete the circuit.

Pocket Form Factor Trade-offs

Smaller coil = lower Q = poorer selectivity. Fixed tuning range limits reception to one or two strong local stations. Some sets used a ferrite rod loopstick antenna, adding directionality — rotating the set nulls interference from one direction.

Power Budget

All audio power comes from intercepted RF energy alone. A strong local station 10 km away might deliver a microwatt or two — enough to hear in a quiet room, nothing more.