How to build an AM radio transmitter

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How to Build a Low-Power AM Radio Transmitter (Prepper Starter Guide)

Read First: Safety & Legality

Transmitting radio signals is regulated. In the U.S., very low-power AM operation without a license falls under FCC Part 15 and has strict limits on input power and antenna length. Build for learning and bench testing, and operate only within your local laws. If in doubt, listen only or use a dummy load on the bench while you learn. We link primary references below so you can verify details for your location.

What You’ll Learn

This guide gives you a practical on-ramp to AM (amplitude modulation) transmitters suitable for hobby/education and ultra-low-power neighborhood audio links. You’ll see how a carrier is generated, how audio modulates that carrier, and how a simple antenna system radiates the signal. We’ll outline three beginner-friendly circuits, then walk step-by-step through the simplest one. Finally, you’ll get troubleshooting tips, safe test methods, and upgrade paths for cleaner audio and better stability.

Core Concepts in 90 Seconds

AM in one sentence: take a steady RF carrier in the AM band (~530–1700 kHz) and vary its amplitude with your audio signal so a nearby AM radio can demodulate it. The building blocks are: (1) a stable RF oscillator (the “carrier”), (2) a modulator that lets audio ride on the carrier’s amplitude, (3) a power source, and (4) an antenna plus a decent ground/counterpoise. Keep leads short, filter harmonics, and start with a nearby AM receiver for tuning.

Legal Baseline (U.S. Part 15 Snapshot)

For unlicensed AM in the U.S., Part 15 commonly referenced limits include: ≤100 mW input power to the final RF stage and a total length ≤3 meters for the transmission line + antenna + ground lead. Emissions outside the AM band must be well suppressed. That’s why we emphasize low-power circuits, short antennas, filtering, and local reception. If you need more range or flexibility, pursue appropriate licensing or operate on amateur bands with a ham license.

Three Beginner-Friendly AM Transmitter Paths

1) Crystal-Oscillator + Audio-Transformer (Simplest)

This classic educational design uses a fixed-frequency crystal oscillator module (~1.0–1.2 MHz) to generate the carrier, then places an audio transformer in series with its supply to “wiggle” the amplitude with your music/voice. It’s inexpensive, quick to build on perfboard, and great for learning. Downsides: modest audio quality, limited modulation depth, more harmonics—so add simple output filtering and keep your test range short.

2) Single-Transistor LC Oscillator with Supply/Bias Modulation

Here the carrier comes from a transistor LC tank (coil + capacitor). You inject audio by varying the transistor’s supply or bias so the RF amplitude follows your audio. It’s more “radio-like” than a crystal module, tunable across parts of the band with a variable capacitor, and can sound better if you filter harmonics. It requires careful layout and stable power to avoid drift.

3) LM386 (Audio Amp) Driving an RF Stage

A popular variation uses an LM386 as a small audio power amp to modulate the RF stage (often by supply modulation). It can give stronger, cleaner modulation if you manage gain and avoid overdriving. Pair with a stable RF oscillator (crystal or LC), add a low-pass or band-pass filter, and you get a nice step-up from the bare-bones beginner build.

Bill of Materials (Starter Build)

  • Crystal oscillator module around 1.0–1.2288 MHz (5V or the voltage your module specifies)
  • Audio transformer (e.g., 1 kΩ : 8 Ω) for modulation
  • Audio source (phone with volume low, or a small mic preamp)
  • Supply (battery pack or clean DC bench supply matching oscillator spec)
  • Prototype board, hookup wire, small electrolytic and ceramic capacitors (for decoupling and filtering)
  • Optional: small RF choke and capacitor for a simple low-pass output filter
  • Antenna wire (short, per legal limits) and a ground/counterpoise (short, per legal limits)

Step-by-Step: Crystal-Oscillator + Transformer Build

  1. Layout & power: Mount the crystal oscillator on perfboard. Decouple its supply with a 0.1 µF ceramic close to the pins and a 10 µF electrolytic nearby to keep it stable.
  2. Insert the modulator: Put the audio transformer primary in series with the oscillator’s positive supply line. Feed your audio source into the transformer secondary (through a coupling capacitor if needed). Start with very low audio volume.
  3. RF out & antenna: Take the oscillator RF output to a short antenna lead. Keep the total of antenna + feed + ground lead within your local legal limit (e.g., Part 15 commonly referenced total ≤3 m). Add a small low-pass (e.g., RF choke then capacitor to ground) to reduce harmonics.
  4. Ground/counterpoise: Provide a short ground/counterpoise connection. On a bench, a small sheet of foil or metal tray tied to circuit ground can help—keep it short to stay compliant.
  5. First test: Place an AM radio a few feet away. Tune around the oscillator frequency until you hear a strong carrier (quiet “hiss” drop). Bring up the audio volume gently until you hear clear modulation without distortion.

Tuning & Troubleshooting

  • No carrier heard: Verify oscillator supply polarity/voltage and decoupling. Try another AM radio and sweep the band slowly—some modules aren’t exact round numbers.
  • Distorted audio: Turn down the audio source, keep modulation below clipping. Add a simple RC low-pass (e.g., 0.01–0.047 µF) on the audio line to tame harsh highs.
  • Weak range: Short, neat wiring; solid decoupling; ensure antenna and ground leads are within allowed limits but not needlessly shorter than they must be. Small output filter can help by cleaning the signal so the receiver’s AGC behaves better.
  • Drifty frequency (LC builds): Use stable capacitors, shield from airflow/heat, and consider a crystal-controlled carrier for beginners.

Upgrades for Cleaner, More Capable Builds

  • Filtering: Add a proper low-pass or band-pass output network to suppress harmonics and reduce interference.
  • Better modulation: Explore balanced/square-law/diode-ring modulators for improved linearity and audio quality.
  • Antenna matching (within limits): With very short antennas, a small matching network can improve radiation efficiency—tune carefully and re-check compliance.
  • Shielding & enclosures: Metal project boxes and feedthrough capacitors tame stray coupling and hum.

Bench-Only Option (When You Can’t Radiate)

If you need to test audio/modulation without radiating, terminate the RF into a small “dummy antenna”/resistive-capacitive load and monitor with an RF probe or a nearby radio using a very short capacitive “sniff” lead. This lets you practice safely and legally while you refine your circuit and audio chain.

Authoritative References & Learning Links

Prepper Notes (Great Plains Context)

AM has two practical advantages here: it’s easy to build with common parts, and nearby receivers are everywhere (old radios still work). The tradeoff is range—especially with legally short antennas—so think hyper-local: property, campsite, or convoy spacing. For resilience, print your schematics, stock a few spare crystals, and keep a small parts kit with audio transformers, RF chokes, and capacitors. Practice now so you know exactly how far your clean, compliant signal reaches.

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