Radio
Communicate with no wire between you.
Everything for this step
- Field guide
- The electromagnetic spectrum → 2 sections Physics → 12 sections Computer science → 6 sections Electronics and computer hardware → 19 sections
- Builds
- None yet.
- Reference sheets
- 1 sheet in The electromagnetic spectrum
- Sciences it rests on
- Physics
- Track
- Signals, level 2 of 2
Why this matters
Wireless reaches ships, aircraft, remote settlements and everyone at once. It is also the technology that most cheaply re-links a scattered society: one transmitter and many simple receivers gives a shared informational world without building any infrastructure between them.
The shape of it
- Start with a crystal set. A galena crystal and a fine wire form a point-contact rectifier that needs no power at all. It receives, and building one teaches resonance, antennas and tuning for almost nothing.
- Then spark-gap transmission — crude, wideband, and the historical starting point. It works, and it interferes with everything, so move past it as soon as you can.
- Then vacuum tubes. A heated cathode, a grid, an anode in an evacuated glass envelope, giving amplification and clean oscillation. This depends directly on your glass and vacuum capability, and it is the beginning of electronics.
- Build a good antenna and a good earth. Most receiving and transmitting performance is antenna, not circuitry. This is the cheapest place to improve a station.
- Choose frequency by purpose: long and medium wave follow the ground and are reliable locally; shortwave refracts off the ionosphere and can cross continents with modest power, especially at night.
- Coordinate frequencies deliberately. The spectrum is a shared, finite resource and it is destroyed by uncoordinated use. Allocate it before there is a conflict.
- Use it for what matters: weather and disaster warning, medical consultation at distance, coordination between settlements, and broadcast education. A single radio teacher can reach a region no school could.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Build a crystal receiver first
It needs no power at all and teaches resonance, antennas and tuning for almost nothing.
- Antenna: a long wire, 20–30 m, as high as you can get it, on insulators.
- Earth: a metal rod or pipe driven into permanently damp ground.
- Tuned circuit: a coil and a variable capacitor. It resonates at f = 1 ÷ (2π√(LC)); change the capacitor to tune.
- Detector: a galena crystal and a fine wire — a cat's whisker — searching for a sensitive spot.
- High-impedance headphones, around 2,000 ohms.
2 · Get the antenna and earth right before blaming the set
Most receiving and transmitting performance is antenna, not circuitry. This is the cheapest place to improve a station.
Wavelength in metres = 300 ÷ frequency in megahertz. A half-wave dipole for a frequency is about 143 ÷ frequency (MHz) metres long overall — about 7.15 m for 20 MHz. Height matters: get it clear of buildings and trees. Check the earth connection after dry spells, and use heavy wire for it.
3 · Keep antennas clear of power lines, and ground the station
Antennas, masts and wires touching overhead power lines have killed many people putting up radio stations.
Never erect an antenna where it could fall onto, or be blown against, a power line — allow more than its own length plus a margin of clearance. Fit a lightning arrester and an earth at the point the feed enters the building, and disconnect the antenna during thunderstorms.
4 · Pass through spark transmission quickly
Spark-gap transmitters — a high-voltage spark exciting a tuned circuit and antenna — work, and were the historical start. They radiate across a very wide band and interfere with everything.
Use spark only to prove a link, for as short a time as possible, and replace it with tube oscillators as soon as electronics allows. Spark transmitters left in service block a whole band for every other station.
5 · Treat transmitter voltages as lethal
Tube transmitters run at hundreds to thousands of volts, and capacitors hold that charge after power is off. High-power radio-frequency current burns deep into tissue at an antenna or feedline.
- Interlocked covers that cut power when opened.
- Discharge every capacitor with an insulated shorting stick before touching anything.
- One hand in your pocket when working near live high voltage.
- Nobody touches an antenna or feedline while the transmitter is keyed.
6 · Choose frequency by purpose and time of day
- Long and medium wave — below about 3 MHz — follow the ground and are reliable locally, day and night.
- Shortwave — about 3–30 MHz — refracts off the ionosphere and can cross continents with modest power. Lower shortwave frequencies work better at night, higher ones by day.
Keep a log of which frequencies reached which stations at which hours, and build a schedule from it.
7 · Coordinate the spectrum, and use radio for what matters
The spectrum is a shared, finite resource, destroyed by uncoordinated use. Keep a register of assigned frequencies, stations and hours, set in published rule, before there is a conflict.
Then use it for what matters: weather and disaster warning, medical consultation at distance, coordination between settlements, and broadcast education — a single radio teacher can reach a region no school could. Build receivers before transmitters, so there is someone to hear.