Transmitter Construction

Part of Radio

A radio transmitter generates an electrical signal at a specific frequency, amplifies it, and feeds it to an antenna where it radiates as electromagnetic waves. This article covers building transmitters from the simplest spark gap to practical AM voice transmitters.

Why Build a Transmitter

A receiver lets you listen. A transmitter lets you be heard. In a rebuilding scenario, the ability to broadcast information — weather warnings, trade announcements, emergency calls, and coordination between settlements — transforms isolated communities into a connected network. Even a low-power transmitter broadcasting on a predictable frequency and schedule becomes the backbone of regional communication.

Transmitter construction ranges from trivially simple (spark gap) to moderately complex (AM voice). You can build a functional transmitter the same day you decide to, using materials found in any workshop.

The Spark Gap Transmitter

The spark gap transmitter is the oldest and simplest radio transmitter. It produces broad-spectrum radio noise across a wide frequency range, sufficient for Morse code communication over distances of 10-100+ kilometers.

How It Works

1. A battery or dynamo charges a capacitor (Leyden jar or plate capacitor)
2. When the voltage reaches the spark gap breakdown voltage, a spark jumps the gap
3. The spark causes the capacitor and coil (inductor) to oscillate at their resonant frequency
4. This oscillation radiates from the antenna as a brief burst of radio waves
5. A telegraph key in the primary circuit controls when sparks occur

Materials

Component	Material	Purpose
Capacitor	Leyden jar or plate capacitor	Energy storage
Spark gap	Two metal balls or pointed electrodes, adjustable spacing	Switches the circuit
Inductor	10-50 turns of heavy wire on a form	Sets resonant frequency
Antenna	Long wire, cut to wavelength	Radiates signal
Key	Telegraph key	Controls transmission
Power supply	Battery bank or dynamo (500-2000V via induction coil)	Charges capacitor

Building the Spark Gap

1. Mount two brass or steel balls (15-25mm diameter) on adjustable stands
2. Set the gap to 3-5mm for initial testing
3. Connect one ball to the capacitor, the other to the inductor
4. The wider the gap, the higher the voltage needed to fire, and the louder (more powerful) each spark

High Voltage Danger

Spark gap transmitters operate at 500-5,000 volts. These voltages are lethal. Never touch any part of the circuit while powered. Use an induction coil (Ruhmkorff coil) to step up battery voltage, and keep all high-voltage connections insulated and away from accidental contact.

Tuning

The frequency of transmission is set by the inductance (L) and capacitance (C) of the circuit:

Frequency = 1 / (2 x pi x sqrt(L x C))

To change frequency:

• More turns on the coil = lower frequency
• Larger capacitor = lower frequency
• Fewer turns or smaller capacitor = higher frequency

Add a variometer (variable inductor) or variable capacitor to adjust frequency during operation.

Performance

Parameter	Typical Value
Frequency range	500 kHz - 2 MHz
Power	1-50 watts
Range (Morse code)	10-100+ km
Bandwidth	Very wide (wasteful of spectrum)
Modulation	On-off keying (Morse code only)

Continuous Wave (CW) Transmitter

A CW transmitter produces a clean, single-frequency signal instead of the spark gap’s broad noise. It uses a vacuum tube or transistor as an oscillator.

Using a Single Vacuum Tube

If you have access to a triode vacuum tube (or have built one):

1. Wire the tube as a Hartley or Colpitts oscillator
2. The tank circuit (coil + capacitor) sets the frequency
3. Feedback from the plate circuit to the grid sustains oscillation
4. Key the oscillator on and off for Morse code

Using a Crystal Oscillator

If you have a quartz crystal (from scavenged electronics):

1. Wire the crystal between grid and ground of the tube
2. The crystal forces oscillation at its precise natural frequency
3. This produces an extremely stable signal that receivers can tune to easily
4. Crystal oscillators drift very little with temperature or voltage changes

Crystal Scavenging

Old computers, watches, and electronic equipment contain quartz crystals at specific frequencies. A crystal marked “7.040” oscillates at 7.040 MHz. Collect and catalog any crystals you find — each one is a potential transmitter frequency.

AM Voice Transmitter

An AM (amplitude modulation) transmitter carries voice by varying the strength of the radio signal in sync with sound waves. This requires two stages: an oscillator (carrier) and a modulator (audio).

Block Diagram

1. Oscillator: Generates the carrier frequency (the radio wave)
2. Audio amplifier: Amplifies the microphone signal
3. Modulator: Combines the audio with the carrier, varying the carrier’s amplitude
4. Power amplifier (optional): Boosts the modulated signal
5. Antenna: Radiates the signal

Building a Simple AM Transmitter

The Oscillator Stage

1. Build a Colpitts or Hartley oscillator using one vacuum tube or transistor
2. Set the frequency using a tank circuit (coil + variable capacitor)
3. Verify the oscillator produces a clean, stable signal by listening on a separate receiver

The Modulator

1. Build a single-stage audio amplifier using another tube or transistor
2. Connect a carbon microphone (or any microphone) to the input
3. The amplifier output connects to the oscillator’s power supply or final amplifier

Modulation Methods

Method	How It Works	Difficulty	Quality
Plate modulation	Audio varies the plate voltage of the oscillator	Moderate	Good
Collector modulation	Audio varies transistor collector voltage	Moderate	Good
Base/grid modulation	Audio drives the control element	Easy	Fair
Absorption modulation	Carbon mic in the antenna circuit	Very easy	Poor but functional

The Carbon Microphone Transmitter

The absolute simplest voice transmitter for emergencies:

1. Build or tune an oscillator to your desired frequency
2. Place a carbon microphone (or a loosely packed container of carbon granules between two metal plates) in series with the antenna feed
3. Speaking into the microphone varies the resistance, which varies the antenna current, which AM-modulates the signal
4. Range: 1-10 km depending on power and antenna

Making a Carbon Microphone

Fill a small tin can with loosely packed charcoal granules (crushed charcoal, not powder). Place a metal disc on top and attach wires to the disc and the can bottom. Speaking near the can vibrates the granules, changing their resistance. This is a functional microphone for radio and telephone use.

Power Amplification

For greater range, add a power amplifier stage after the oscillator/modulator:

1. Use a larger vacuum tube or multiple transistors in parallel
2. The amplifier increases the signal power without changing its frequency or modulation
3. A 1-watt oscillator feeding a 10-watt amplifier produces 10 watts of output
4. Match the amplifier output impedance to the antenna impedance for maximum power transfer

Power vs. Range

Transmitter Power	Approximate Range (HF, Morse)	Approximate Range (AM Voice)
0.1 watt	10-50 km	5-20 km
1 watt	50-200 km	20-100 km
10 watts	200-1000 km	100-500 km
100 watts	1000-5000+ km	500-2000 km

Ranges vary enormously with frequency, time of day, antenna quality, and atmospheric conditions. HF frequencies (3-30 MHz) can bounce off the ionosphere for intercontinental communication.

Safety and Interference

RF Safety

Radio frequency energy at transmitter power levels can cause burns and tissue damage. Never touch the antenna or tank circuit while transmitting. Keep the antenna away from people. At 10+ watts, maintain at least 2 meters from the antenna during operation.

Frequency Selection

Choose your operating frequency carefully:

• Avoid frequencies used by emergency services (if any remain operational)
• Establish community-agreed frequencies for different purposes
• Use the lowest frequency that provides adequate range — lower frequencies propagate farther
• Schedule regular transmission times so listeners know when to tune in

Common Mistakes

1. No antenna tuning: A transmitter connected to an untuned antenna wastes most of its power as heat in the output stage. Always tune the antenna to resonance at the operating frequency.
2. Oscillator instability: If the frequency drifts, receivers lose the signal. Use rigid, temperature-stable coils, and crystal control if possible.
3. Over-modulation: Driving the modulator too hard distorts the audio and creates splatter (interference on adjacent frequencies). Speak at normal volume, 15-20cm from the microphone.
4. Poor grounding: The transmitter chassis, power supply negative, and antenna ground must all connect to a good earth ground. Poor grounding causes RF burns, instability, and interference with the operator’s own receiver.
5. Ignoring harmonics: Every transmitter produces signals at multiples of its operating frequency (harmonics). These waste power and cause interference. Add a low-pass filter between the transmitter and antenna to suppress harmonics.

Summary

Transmitter Construction -- At a Glance

• Spark gap: Simplest transmitter — capacitor, gap, coil, antenna. Morse code only, 10-100+ km range. High voltage hazard.
• CW transmitter: Single vacuum tube or transistor oscillator produces a clean Morse code signal at one frequency
• AM voice: Oscillator + audio modulator; carbon microphone in the antenna circuit is the simplest voice modulation method
• Power amplification extends range: 1 watt covers 50-200 km on HF Morse, 100 watts reaches intercontinental distances
• Always tune the antenna to the operating frequency for maximum efficiency
• Crystal oscillators provide the most stable frequency control from scavenged components