Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers four crystal radio designs in ascending complexity: the classic galena (lead sulfide) detector set, a modern germanium diode version, a tuned radio frequency (TRF) crystal set with antenna impedance matching, and a headphone transformer for improved audio output. Crystal radios require no power supply — they operate entirely on the energy intercepted by the antenna.

Frequency range: AM broadcast band (530–1700 kHz) with the classic design; extended to shortwave (3–30 MHz) with the TRF design using smaller coils.

Chapter 2 — Theory of Operation

2-1 Tuned Circuit

A parallel LC tank circuit resonates at the desired station frequency. At resonance, the circuit impedance is maximum (Z = L/(R×C), where R is coil resistance); off-resonant signals see low impedance and are bypassed. Selectivity is determined by Q = ωL/R; higher Q means sharper tuning but also higher loss. For AM broadcast: L ≈ 250 µH, C = 15–365 pF (variable).

2-2 Detector Diode

The detector diode (galena crystal, 1N34A germanium, or BAT42 Schottky) rectifies the AM carrier, demodulating the audio. A bypass capacitor (100 pF to 1 µF depending on audio frequency range) filters the RF carrier, leaving the audio envelope. Germanium diodes have lower forward voltage (∼0.2V) than silicon; they work with weaker signals. Galena (natural PbS crystal) historically achieved the highest sensitivity when point-contacting the sweet spot on the crystal.

2-3 Antenna Coupling

The antenna couples RF energy into the tuned circuit. Over-coupling loads the tank circuit, reducing Q and selectivity. Under-coupling reduces signal level. Optimal coupling: the antenna resonant impedance is tapped partway down the coil (inductive coupling) to match the low antenna impedance (∼100–300Ω) to the high tank circuit impedance (∼50–200 kΩ).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Classic Coil on Oatmeal Box

1. Wind 90 turns of #28 AWG enameled copper wire on a 90 mm OD cardboard tube (oatmeal box). Wind in a single layer, close-wound. Mark the center tap at turn 45 for antenna connection point.
2. Scrape enamel from the wire at 10-turn intervals (turns 10, 20, ..., 80) and solder short leads. These form taps for the antenna coupling adjustment.
3. Connect a variable air capacitor (10–365 pF) in parallel with the full winding. The capacitor shaft is the tuning control.
4. Connect the galena crystal or 1N34A diode in series with the headphones across the tank circuit. Cathode toward the high-impedance (ungrounded) end of the tank.

4-2 Headphone Impedance Transformer

1. Wind an audio transformer on a laminated iron core: primary 2000 turns #38 AWG (matches high crystal set impedance); secondary 60 turns #26 AWG (matches 32Ω headphone).
2. Turns ratio: n = sqrt(Z_primary / Z_secondary) = sqrt(2000/32) = 7.9:1.
3. Actual winding: 2000:250 turns is a practical compromise for ease of winding (ratio = 8:1 = 9 dB power gain vs. direct connection).

Chapter 5 — Operating Procedures

1. Connect a long outdoor wire antenna (30 m or more) and a good ground (cold water pipe, driven rod, or extensive counterpoise).
2. Put on high-impedance crystal earphones (or low-impedance via transformer). Rotate the tuning capacitor slowly from maximum capacitance to minimum. Stations appear as increases in audio level at their resonant settings.
3. Adjust antenna coupling tap for best audio level without degrading selectivity. Experiment: more coupling gives more volume but reduces the ability to separate adjacent stations.
4. For galena detector: use the cat’s whisker to probe different spots on the crystal surface. The sensitive spot is small — a light touch is more effective than pressure.

Chapter 6 — Calibration

1. Calibrate the tuning dial: note the capacitor setting when each known station is tuned. Mark a scale with station frequencies. This converts the dial to a frequency scale using the known stations as references.
2. Estimate Q: measure the −3 dB bandwidth of the tank circuit at 1 MHz with a signal generator and detector. Q = f / BW. Target: Q ≥100 for good selectivity (BW ≤10 kHz).

Chapter 7 — Verification and Acceptance

1. Receive at least three local AM broadcast stations, each distinctly tunable by the variable capacitor.
2. Adjacent station rejection: tune to one station and verify that the next station 10 kHz away is at least 20 dB weaker. (Measure with CYD ADC or compare audio levels subjectively.)
3. Log: date, antenna length and ground type, stations received, Q measurement result, detector type used.

Appendix A — Resonant Frequency Formula

f0 = 1 / (2π × sqrt(L × C))

AM broadcast coverage:
  At C_max = 365 pF, f0 = 1/(2π×sqrt(250e-6 × 365e-12)) = 527 kHz
  At C_min = 10 pF,  f0 = 1/(2π×sqrt(250e-6 × 10e-12))  = 3.18 MHz
  (Reduce L to 50 μH for higher frequency coverage)

Appendix B — Crystal Detector Sensitivity Comparison