AM Transmitter
Part of Radio
An AM (amplitude modulation) transmitter impresses audio information onto a radio carrier wave by varying the wave’s amplitude — the foundation of broadcast radio communication across distances of tens to hundreds of kilometers.
Why This Matters
A radio receiver lets you listen. A transmitter lets you be heard. The ability to transmit voice and information over radio waves, without wires and across terrain that would stop any messenger, transforms a community’s communication capability. A settlement with a working AM transmitter can coordinate with distant communities, broadcast emergency information, support supply chain coordination, and maintain social cohesion across a dispersed population.
AM transmission is historically the first practical broadcast technology, and for good reason: the demodulation (reception) requires only a diode and a capacitor — a crystal radio. This means that with one transmitter, you can reach everyone in range who has built a simple crystal set, which requires no batteries or power supply at all. The asymmetry is powerful: one powered, complex transmitter serves dozens of simple, passive receivers. This matches the resource distribution of rebuilding communities.
Understanding AM transmission means understanding oscillator circuits, modulation, antenna theory, and the power requirements for useful range. This article covers the functional design of a simple AM transmitter using vacuum tube technology — the technology available before transistors, and the technology you can build from scratch.
AM Modulation Explained
A radio carrier wave is a high-frequency oscillating electromagnetic field — typically 550-1600 kHz (kilohertz) for the standard AM broadcast band. This frequency is too high to carry audio directly (human hearing stops at about 20 kHz; radio starts in the hundreds of kHz).
Amplitude modulation works by varying the strength (amplitude) of the carrier wave in proportion to the audio signal:
- When the audio signal is at maximum positive, the carrier amplitude increases to maximum
- When the audio signal is at zero (silence), the carrier runs at constant amplitude
- When the audio signal is at maximum negative, the carrier amplitude decreases to minimum
The receiver extracts this amplitude variation (using a detector/demodulator circuit) and converts it back to the audio waveform, which drives a speaker.
Mathematical description: AM signal = A_c × [1 + m × cos(2π × f_audio × t)] × cos(2π × f_carrier × t)
Where m is the modulation index (0 to 1; m=1 is 100% modulation). Over-modulation (m > 1) causes the carrier amplitude to go to zero at the negative peaks, creating harsh distortion and splatter of energy into adjacent frequencies.
Block Diagram of an AM Transmitter
A complete AM transmitter requires four functional stages:
1. Oscillator: Generates a stable carrier frequency. A tuned LC circuit (inductor-capacitor resonator) with a vacuum tube amplifier maintaining oscillations produces a continuous sine wave at the desired frequency. Crystal control gives much better frequency stability than a plain LC oscillator.
2. Buffer amplifier: Isolates the oscillator from the output stages. Without a buffer, connecting the antenna and modulator stages loads the oscillator, pulling its frequency and causing instability.
3. Modulator stage: Combines the carrier with the audio signal to produce the amplitude-modulated wave. The audio signal controls the power supply voltage to the final amplifier stage (high-level modulation), or directly modulates the grid voltage in an earlier stage (low-level modulation).
4. Power amplifier and antenna: Amplifies the modulated signal to the required power level and drives the antenna. The antenna radiates the signal as electromagnetic waves.
5. Audio chain: Microphone → preamplifier → audio power amplifier → modulator. This chain converts sound to electrical signal with sufficient power to modulate the carrier.
The Crystal Oscillator
For a practical radio transmitter, a stable carrier frequency is essential. Even small frequency drifts (a few kilohertz) cause the transmitter’s signal to drift off the frequency where receivers are tuned.
LC oscillator (basic, not crystal-controlled):
- A resonant circuit: an inductor (coil of wire on a ferrite or air core) and a variable capacitor (parallel plates with adjustable spacing) in parallel
- The resonant frequency: f = 1/(2π√(LC))
- A vacuum tube configured as a Hartley or Colpitts oscillator feeds energy back into the LC circuit to maintain oscillation
- Adjusting the capacitor changes frequency
Crystal control: A piezoelectric crystal (quartz) vibrates mechanically at a precise frequency when an electrical signal is applied. Used in a crystal oscillator circuit, it constrains the oscillator to the crystal’s resonant frequency with exceptional stability — drift measured in parts per million per degree Celsius rather than the hundreds of Hz per degree of a plain LC oscillator.
For a rebuilding scenario, quartz crystals must be cut and ground to precise dimensions — a specialized skill requiring precision tools. However, an LC oscillator with a good-quality variable capacitor and a temperature-stable inductor is adequate for short-range communication where receivers can be retuned to follow slight drift.
Vacuum Tube Power Amplifier
The final stage must deliver enough power to the antenna to achieve useful range.
Triode transmitter tube: A triode (see the vacuum tubes article) in common cathode configuration:
- Grid: receives the modulated RF signal from earlier stages
- Plate: connected to tuned load (tank circuit) and power supply
- Cathode: reference point
Tank circuit: An LC circuit tuned to the carrier frequency, connected between the plate and the positive high-voltage supply. This circuit stores energy and produces a clean sine wave output even when the tube’s current is pulsed (class C operation). Class C operation (tube conducts for less than half the cycle) is far more efficient than class A, typically 70-80% efficiency versus 25-35%.
Typical component values for a medium-wave (1 MHz) transmitter:
- Tank inductor: 25 microhenry (25 turns of heavy copper wire on a 40 mm form)
- Tank capacitor: 1000 pF variable (for tuning) in parallel with 500 pF fixed
- Plate voltage: 200-400 V DC
- Plate current at resonance: 50-200 mA depending on power level
Output power: Power delivered to the antenna = 0.5 × I_plate_max × plate_voltage_swing. A transmitter with 300V plate supply and 100 mA plate current delivers approximately 7-15 watts of RF power — adequate for 10-50 km range depending on antenna, frequency, and conditions.
AM Modulation Circuit
High-level AM modulation (the practical approach for simple construction):
The audio power amplifier’s output transformer secondary winding is connected in series with the final RF amplifier’s plate supply voltage. The audio signal adds to or subtracts from the plate voltage of the RF amplifier, varying its output power in proportion to the audio — producing AM modulation.
Audio power requirements: For 100% AM modulation (the full audio range), the audio power must equal half the DC power input of the modulated stage. If the RF final stage draws 20 watts DC, the audio modulator must supply 10 watts of audio. This is a significant requirement — a good quality microphone and several stages of amplification are needed.
Low-level modulation alternative: Modulate an early, lower-power stage and then amplify. Simpler audio stages needed, but requires linear amplification of the modulated signal through all subsequent stages (cannot use efficient class C). Net efficiency is lower but the design is simpler.
Antenna Design
The antenna converts electrical oscillations into radiated electromagnetic waves. Antenna efficiency has more impact on effective range than transmitter power.
Quarter-wave vertical antenna: At 1 MHz (medium wave), one wavelength is 300 meters. A quarter-wave vertical antenna is 75 meters tall — impractical. At 3 MHz, the quarter-wave length is 25 meters — more manageable.
For practical operation at medium wave frequencies with antennas of manageable height, loading coils (inductors placed at the base of the antenna) electrically lengthen a shorter physical antenna. A 15-20 meter vertical wire antenna with a base loading coil can operate efficiently on medium wave frequencies.
Ground system: A vertical antenna needs an efficient ground return. Bury 8-16 copper or steel wires (radials) radiating outward from the antenna base, each at least 30 meters long, just below the surface. This simulates an infinite ground plane. Without a ground system, the transmitter efficiency is severely compromised.
Antenna tuning: The antenna and transmitter must be matched in impedance. An antenna coupling unit (a variable inductor and capacitor network) between the transmitter and antenna allows adjustment for maximum power transfer.
Power Supply Requirements
The transmitter requires both low-voltage supplies (for filaments and low-level stages) and a high-voltage supply (for the plate circuit).
High-voltage power supply:
- Transformer: Steps up 12-24 VAC (from a generator or battery charger) to 300-500 VAC
- Rectifier: Vacuum tube rectifiers (Type 5Y3, 6X5) convert AC to pulsating DC
- Filter: A series of large capacitors (10-50 microfarad, rated above the supply voltage) and series inductors smooth the pulsating DC to relatively clean DC
High-voltage safety
The high-voltage supply in a vacuum tube transmitter (200-500 volts DC) can deliver lethal shocks. The capacitors store enough charge to be dangerous even after power is removed. Never touch the high-voltage section without discharging the capacitors (through a high-value resistor) and waiting 30 seconds. Treat all points in the plate circuit as live even when the power switch is off.
Expected Range
Range depends on power, frequency, time of day, and terrain:
| Power | Frequency | Daytime Range | Night Range |
|---|---|---|---|
| 5 W | 1000 kHz | 20-40 km | 100-500 km (skywave) |
| 10 W | 1000 kHz | 30-60 km | 200-1000 km |
| 25 W | 3 MHz | 30-80 km | 400-2000 km |
Skywave propagation (night) occurs when radio waves reflect off the ionosphere. At medium wave frequencies (550-1600 kHz), nighttime skywave can carry signals thousands of kilometers. Daytime ground wave propagation follows the earth’s curvature to shorter distances.
For a rebuilding community, a 5-10 watt transmitter at 1 MHz provides reliable communication within 30-50 km by day (local community coordination) and unpredictable but often very long-range communication at night (contact with distant settlements). This capability, built from vacuum tubes, wire, and basic components, represents a major expansion of a community’s information and coordination reach.