Radio
Why This Matters
The telegraph requires a physical wire between two points. That wire can be cut, knocked down by storms, or simply not exist between communities separated by mountains, rivers, or hostile territory. Radio eliminates the wire entirely. A radio signal travels through the air at the speed of light, reaching any receiver within range — no infrastructure between sender and receiver. A single community with a transmitter can broadcast to every settlement within tens or hundreds of kilometers simultaneously. This is how you coordinate across a region, not just between two points. Every military, maritime, aviation, and emergency service in the pre-collapse world ran on radio, and for good reason: it works when everything else fails.
What You Need
For a crystal radio receiver (no battery required):
- Copper wire: approximately 20-30 meters of thin insulated wire (magnet wire from a motor is ideal, 22-28 AWG)
- A cardboard tube, PVC pipe, or wooden dowel: about 5-8 cm diameter, 15-20 cm long (coil form)
- A diode: a germanium diode (1N34A or similar) is ideal. If unavailable, you can make a crude one (see below).
- High-impedance earpiece or headphone (2000+ ohms — a telephone earpiece or crystal earphone works best)
- Antenna wire: 10-30 meters of any wire, strung as high and long as possible
- Ground connection: a metal rod driven into damp earth, or a connection to metal water pipes
For a homemade diode (cat’s whisker detector):
- A small piece of galena (lead sulfide crystal), iron pyrite, or silicon (a piece of a silicon chip or solar cell)
- A thin wire (safety pin wire, fine copper wire) bent into a point
- A mounting base
For a simple AM transmitter:
- Transistor (NPN type such as 2N2222, 2N3904, or any general-purpose NPN — scavenged from electronics)
- Resistors: 1k-10k ohm (from circuit boards)
- Capacitors: 100pF to 0.01uF range (from circuit boards)
- Copper wire for inductor coils
- Battery: 9-12V
- Microphone: a speaker used in reverse, or a carbon microphone from an old telephone
- Antenna wire: 5-20 meters
For a spark gap transmitter (Morse code only):
- Two metal points (bolts, thick wire tips) with an adjustable 1-3 mm gap
- Induction coil or automotive ignition coil
- Battery: 6-12V
- Telegraph key
- Antenna and ground
Tools:
- Knife, pliers, screwdriver
- Soldering iron (if available — not strictly required)
- Any material for mounting components (wood, cardboard)
How Radio Works: The Short Version
Radio waves are electromagnetic waves — the same type of energy as light, but at much lower frequencies that pass through walls, trees, and clouds. Every electrical circuit that has current changing rapidly (oscillating) radiates radio waves from its wires. A radio transmitter is a circuit designed to oscillate at a specific frequency and radiate those waves intentionally through an antenna.
A radio receiver is a circuit tuned to the same frequency that converts those waves back into electrical signals you can hear.
TRANSMITTER RECEIVER
Sound -> Microphone -> Oscillator circuit Antenna captures waves
| |
Antenna radiates Tuned circuit selects
radio waves ~~~>~~~>~~~> the right frequency
|
Detector extracts
the audio signal
|
Earpiece/speaker
plays sound
Frequency and Wavelength
Radio waves are described by their frequency (how many times the wave oscillates per second) measured in Hertz (Hz):
- 1 kHz (kilohertz) = 1,000 oscillations per second
- 1 MHz (megahertz) = 1,000,000 oscillations per second
| Band | Frequency Range | Wavelength | Range | Best For |
|---|---|---|---|---|
| LF (Low Frequency) | 30-300 kHz | 10-1 km | 100s of km | Long-distance ground wave |
| MF (Medium Frequency, AM radio band) | 300 kHz - 3 MHz | 1000-100 m | 10-100s of km | Regional broadcasting |
| HF (High Frequency, shortwave) | 3-30 MHz | 100-10 m | Global (sky wave bounce) | Long-distance point-to-point |
| VHF (Very High Frequency) | 30-300 MHz | 10-1 m | Line of sight (50-100 km) | Local communication |
For a post-collapse community, the MF and HF bands are most useful: MF for regional broadcasting, HF for long-distance communication via sky wave (signals bouncing off the ionosphere).
AM vs FM
- AM (Amplitude Modulation): The transmitter varies the strength of the radio wave to encode sound. Simple to build. Vulnerable to static and interference. Used for long-range broadcasting.
- FM (Frequency Modulation): The transmitter varies the frequency slightly to encode sound. Better audio quality, less static. Harder to build. Limited to line-of-sight range.
For post-collapse purposes, AM is strongly recommended: it is far simpler to build and has much greater range.
Method 1: Crystal Radio Receiver
The crystal radio is the simplest radio receiver possible. It requires no battery or external power — it runs entirely on the energy captured from radio waves by its antenna. Crystal radios were the first practical radio receivers (early 1900s) and remain functional today. If any AM station is broadcasting within range, a crystal radio will receive it.
Building the Tuning Coil
The tuning coil (inductor) determines which frequency the radio receives. Combined with a capacitor (or the inherent capacitance of the circuit), it forms a resonant circuit that selects one frequency and rejects others.
Step 1 — Take your cardboard tube, PVC pipe, or wooden dowel (5-8 cm diameter, 15-20 cm long). This is your coil form.
Step 2 — Punch or drill two small holes near one end of the tube, about 5 mm apart, to anchor the wire.
Step 3 — Thread the end of your magnet wire through the holes, leaving a 30 cm tail for connection. This is the start of the coil.
Step 4 — Wind the wire tightly around the tube in a single neat layer, each turn touching the previous turn. Wind approximately 60-80 turns. The exact number depends on your tube diameter and the frequency range you want to receive:
- Larger diameter tube = fewer turns needed
- For the AM broadcast band (530-1700 kHz): 60-80 turns on a 5 cm diameter form using 24-26 AWG wire works well
Step 5 — Punch two more holes at the end of the winding and thread the wire through to anchor it. Leave a 30 cm tail.
Step 6 — For a tunable radio, you need a way to vary how much of the coil is in the circuit. The simplest method: do not coat the coil with insulation. Instead, scrape a thin stripe of enamel off the top of each turn of the coil (using sandpaper or a knife edge), creating a bare copper track along the top. Make a sliding contact (a stiff wire or metal strip mounted so it can slide along the top of the coil, touching different turns) to act as a tuning control. This is called a “slider” tuner.
Building the Detector
The detector separates the audio signal (the sound) from the radio-frequency carrier wave. Without it, you would hear nothing — the RF oscillation is far too fast for your ear to detect.
Option A: Scavenged germanium diode (best option)
If you can find a germanium diode (type 1N34A, OA91, or similar — from old radios, circuit boards, or electronic kits), use it. Connect it in series with the earpiece. Done.
Option B: Cat’s whisker detector (build from raw materials)
This is the original crystal radio detector, used from 1906 to the 1920s.
Step 7 — Obtain a piece of galena (lead sulfide — a naturally occurring mineral, found in lead ore deposits, silver-gray with a metallic luster). A piece about 1 cm across is sufficient. Alternatively, use iron pyrite (fool’s gold), silicon from a broken solar cell, or even a blue steel razor blade.
Step 8 — Mount the crystal on a small metal cup or base using low-melting solder, melted wax, or a mechanical clamp. Connect a wire to the metal base.
Step 9 — Bend a thin, stiff wire (safety pin wire, piano wire, or fine copper wire) into an L-shape with a sharp point at the end. Mount the base of the L on a small arm that lets you adjust where the point touches the crystal surface.
Step 10 — The point of the wire (the “cat’s whisker”) must touch the crystal surface at precisely the right spot to form a semiconductor junction that acts as a diode. This requires patience — carefully move the whisker point across the crystal surface while listening on the earpiece. When you find a “hot spot,” you will hear a radio station. The contact is delicate; vibration or bumping the radio may lose the spot.
Assembling the Circuit
Step 11 — Wire the components in this order:
ANTENNA ----+---- TUNING COIL (with slider) ----+---- DETECTOR (diode) ---- EARPIECE
| | |
+------------------------------------+------- GROUND ROD ---------+
Detailed connections:
- Antenna wire connects to the top of the tuning coil (the slider end).
- The slider provides a variable tap point on the coil.
- The bottom of the coil connects to the ground rod.
- The detector (diode or cat’s whisker) connects from the slider tap point to one terminal of the earpiece.
- The other terminal of the earpiece connects to the ground rod.
Step 12 — Erect your antenna. String a wire as long and as high as possible — ideally 10-30 meters long, 5-10 meters above the ground. A wire strung between two trees, between a roof and a pole, or along a ridge line all work. The longer and higher the antenna, the more signal it captures and the more stations you can hear.
Step 13 — Connect the ground wire. Attach a wire from the circuit’s ground point to a metal rod driven at least 1 meter into damp earth, or clamp it to a metal water pipe.
Step 14 — Put on the earpiece. Slowly move the slider along the coil. At different positions, you should hear different stations (if any AM stations are broadcasting). Adjust the cat’s whisker if using one — finding the right contact spot is the most finicky part.
Tip
Crystal radios are very quiet — they produce barely enough power to drive a sensitive earpiece. You CANNOT use a speaker; you must use a high-impedance earpiece (2000+ ohms). Old telephone receivers, crystal earphones, or piezoelectric earpieces work. Modern low-impedance headphones (8-32 ohms) will produce almost no sound.
Method 2: Spark Gap Transmitter
The spark gap transmitter is the simplest possible radio transmitter. It was used from the 1890s through the early 1920s. It sends radio waves generated by an electrical spark jumping across a gap — these waves can be detected by any AM receiver. It transmits only Morse code (on/off keying), not voice.
Warning
Spark gap transmitters are electrically noisy — they broadcast across a wide band of frequencies simultaneously. In the pre-collapse world, they were banned because they interfere with all other radio communication. In a post-collapse scenario with no radio regulation, this is not a concern. However, be aware that a spark gap transmitter will be heard on many frequencies, not just one.
Step 1 — Obtain or build an induction coil. A car ignition coil is ideal — it steps up 12V battery voltage to thousands of volts, producing a hot spark. If no ignition coil is available, wind your own: wrap 200-300 turns of thin wire (secondary) over 20-30 turns of thick wire (primary) around an iron core.
Step 2 — Build the spark gap. Mount two metal points (bolt tips, thick wire ends, or tungsten points from automotive ignition) facing each other with an adjustable gap of 1-3 mm. Mount on a non-flammable base.
Step 3 — Connect the circuit:
BATTERY(+) --> TELEGRAPH KEY --> INDUCTION COIL (primary) --> BATTERY(-)
|
INDUCTION COIL (secondary)
|
SPARK GAP ---- ANTENNA
|
GROUND ROD
Step 4 — Press the telegraph key. The induction coil steps up the voltage, a spark jumps the gap, and radio waves radiate from the antenna. Release the key — the spark stops. Short press = dot. Long press = dash.
Step 5 — Adjust the spark gap for reliable operation. Too wide: spark cannot jump, no transmission. Too narrow: spark is continuous and weak. The gap should produce a crisp, sharp spark each time the key is pressed.
Range: With a car ignition coil, a 12V battery, and a 10-meter antenna, expect a range of 5-30 km depending on terrain and receiver sensitivity.
Method 3: Simple AM Transmitter
An AM transmitter modulates voice or audio onto a carrier wave, allowing actual speech to be transmitted. This is significantly more complex than a spark gap transmitter and requires scavenged electronic components.
How AM Transmission Works
An oscillator circuit generates a steady radio-frequency (RF) carrier wave. A modulator circuit varies the amplitude (strength) of that carrier wave in step with the audio signal from a microphone. The combined signal is fed to an antenna and radiated.
Building the Oscillator
Step 1 — Build an LC oscillator. Wrap 20-30 turns of magnet wire around a 2 cm diameter form (a pen, a dowel). This is your inductor (L). Connect a small capacitor (100-470 pF) across the coil. This LC circuit resonates at a specific frequency determined by the inductance and capacitance:
Frequency (Hz) = 1 / (2 x pi x sqrt(L x C))
For the AM broadcast band (around 1 MHz), typical values are:
- Coil: 20-25 turns of 24 AWG wire on a 2 cm form
- Capacitor: 100-200 pF
Step 2 — Connect a transistor (NPN type: 2N2222, 2N3904, or similar) to sustain the oscillation. A basic Colpitts or Hartley oscillator circuit works:
Colpitts oscillator (simplified):
+9-12V
|
[1k resistor]
|
+------ Collector
|
[Transistor NPN]
|
+------ Base ----[10k resistor]----+
| |
+------ Emitter |
| |
[Coil L] |
| |
+----+----+ |
| | |
[C1] [C2] |
| | |
+---------+----- Ground |
|
+--------- (feedback to base via capacitor divider)
The exact component values depend on available parts. This is where experimentation is necessary — swap capacitor values and coil turns to hit the desired frequency.
Step 3 — Test the oscillator. Turn it on and tune a crystal radio or any AM receiver nearby. You should hear a steady tone or hum as you tune across the oscillator’s frequency.
Adding Audio Modulation
Step 4 — Connect a microphone. A simple carbon microphone (from an old telephone handset) or a small speaker used in reverse (speakers generate a voltage when sound hits them) works as a microphone.
Step 5 — Feed the microphone signal into the transmitter circuit. The simplest AM modulation method: connect the microphone in series with the battery supply to the oscillator. As sound hits the microphone, its resistance changes, varying the supply voltage to the oscillator, which varies the strength of the output signal. This is “collector modulation” and it is crude but functional.
+BATTERY --> MICROPHONE --> [oscillator power input]
Step 6 — Speak into the microphone while the transmitter is operating. On a nearby AM receiver, you should hear your voice.
Antenna and Range
Step 7 — Connect an antenna to the oscillator output. For the AM broadcast band (around 1 MHz), a quarter-wave antenna would be about 75 meters long — impractical. Instead, use the longest wire you can practically erect (10-30 meters). It will be less efficient than a full-size antenna but will still radiate.
Step 8 — Connect a ground rod to the circuit ground.
Expected range: With a single transistor and 9-12V battery, expect 100-500 meters with a short antenna. Multiple transistor stages (amplifiers) can increase this to several kilometers.
Antenna Types
| Antenna Type | Construction | Best For |
|---|---|---|
| Long wire | Single wire, as long and high as possible | General purpose, receiving and low-power transmitting |
| Dipole | Two equal-length wires extending in opposite directions from the feed point | Focused transmission, better efficiency |
| Ground plane | Vertical wire (quarter wavelength) with 3-4 horizontal radials at the base | Omnidirectional transmission, good for broadcasting |
| Loop | Rectangular or circular loop of wire, 1-3 meters across | Directional reception, noise rejection |
Dipole length calculation: Each half of the dipole should be approximately:
Length (meters) = 143 / Frequency (MHz)
Example: for 1 MHz AM → each half = 143 / 1 = 143 meters (impractical)
for 7 MHz shortwave → each half = 143 / 7 = ~20 meters (practical)
For AM broadcast frequencies, a full dipole is impractically long. Use the longest wire you can and accept reduced efficiency, or target higher frequencies (shortwave band) where practical antenna sizes work.
Improving Range and Reception
For Receivers
- Longer, higher antenna: The single most effective improvement. Every doubling of antenna height roughly doubles received signal strength.
- Better ground: Multiple ground rods, buried radial wires, or connection to large metal objects in contact with earth.
- Add an audio amplifier: A single-transistor amplifier between the detector and earpiece can increase volume dramatically, allowing speaker operation.
- Shield the radio from local noise: Keep away from electric motors, generators, and other sources of electromagnetic interference.
For Transmitters
- More power: Add transistor amplifier stages. Each stage can multiply power output by 10-50x.
- Better antenna match: The antenna works best when its electrical length matches the transmitted frequency. Use a tuning coil (variable inductor) at the antenna base to improve the match.
- Higher antenna: Elevate the antenna as high as possible. Transmission range increases significantly with antenna height.
- Directional antennas: A Yagi antenna (a driven element with parallel reflector and director elements) focuses energy in one direction, dramatically increasing range in that direction at the expense of coverage in other directions.
Common Mistakes
| Mistake | Why It’s Dangerous | What to Do Instead |
|---|---|---|
| Using low-impedance headphones with a crystal radio | Almost no sound — crystal radio output is too weak for low-impedance loads | Use 2000+ ohm earpiece (crystal earphone, old telephone receiver) |
| No ground connection | Dramatically reduced reception and transmission range | Drive a ground rod 1+ m into damp earth and connect it to the circuit |
| Antenna wire lying on the ground or very low | Almost no signal captured or radiated | String antenna as high as possible — even 3-5 m up makes a huge difference |
| Using water-based ink on coil form (it conducts) | Short circuits between coil turns | Use dry coil forms. Coat with wax or varnish if available |
| Cat’s whisker contact too heavy | Crystal surface damaged, detector stops working | Use the lightest possible contact. The whisker should just barely touch the crystal |
| Spark gap transmitter near other electronics | Wideband interference disables all nearby radio reception | Operate spark gap transmitter away from receivers. Switch to AM transmitter when possible |
| Not tuning the receiver to the transmitter frequency | No signal received, even when transmitter is working fine | Coordinate frequency. Test with transmitter and receiver in the same room first |
| Transmitting without an antenna | Most power reflected back into the transmitter, potential component damage | Always connect an antenna before transmitting. Even a short wire is better than none |
| Ignoring the ground connection for the transmitter | Reduced range, transmitter may oscillate unstably | Connect a ground rod, especially for AM band and lower frequencies |
| Expecting long range from a single transistor | One transistor at 9-12V produces milliwatts — range is limited to hundreds of meters | Add amplifier stages for longer range. Or accept the limited range and use relay stations |
What’s Next
With radio capability, your community can:
- Telecommunications — build more sophisticated communication networks, including FM, SSB, and digital modes
- Establish a regional radio network connecting multiple settlements
- Broadcast weather warnings, medical emergencies, and coordination messages
- Monitor for signals from other survivor communities — you are no longer isolated
Quick Reference Card
Radio -- At a Glance
Crystal Radio (receiver, no power needed):
- 60-80 turns magnet wire on 5 cm tube + diode + high-impedance earpiece + long antenna + ground rod
- Receives AM broadcast stations. Tune by sliding contact along coil.
- No battery required — powered by captured radio waves.
Spark Gap Transmitter (Morse code only):
- Car ignition coil + spark gap (1-3 mm) + telegraph key + antenna + ground
- Range: 5-30 km. Sends on many frequencies simultaneously.
AM Transmitter (voice):
- Transistor oscillator + microphone modulating supply voltage + antenna + ground
- Range: 100-500 m (single transistor). Add amplifier stages for more.
Antenna Rules:
- Longer = better. Higher = better. As much wire as you can, as high as you can.
- Dipole length (each half) = 143 / frequency in MHz
Frequency Bands:
- AM broadcast: 530-1700 kHz (regional, ground wave)
- Shortwave: 3-30 MHz (global, sky wave bounce)
- Higher frequency = shorter antenna needed, but shorter range (line of sight)
Key Components to Scavenge:
- Germanium diodes (1N34A) from old radios
- Transistors (2N2222, 2N3904) from any circuit board
- Variable capacitors from old radios
- Car ignition coils for spark gap transmitters
- Telephone earpieces (high impedance) for crystal radios
The #1 Rule: Antenna and ground are more important than the radio itself. A crude radio with a great antenna outperforms a great radio with a poor antenna, every time.