Learn RF & SDR — from newbie to expert

A guided path from complete newcomer to confident GopherTrunk operator. Start with what a radio wave actually is, learn how software-defined radios turn the airwaves into numbers, then follow the signal all the way to decoded digital voice — and put it to work in GopherTrunk.

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Radio can feel like a wall of jargon — dBm, IQ, FSK, control channels, vocoders. It isn’t. Every one of those ideas is simple once it’s explained in order, and that’s exactly what this path does. We start with what a radio wave is and build, one short lesson at a time, until you can take a bare antenna and a $30 dongle and decode digital voice off a trunked radio system with GopherTrunk.

Who this is for. Total newcomers welcome — no math, electronics, or licence required. If you already know some RF, use the module list to jump straight to the gaps. Every lesson is self-contained and cross-linked, so you can read top-to-bottom or hop around.

How the path works. Six modules take you from physics to practice. The first modules are vendor-neutral RF and SDR theory — knowledge that applies to any radio. The later modules connect each idea directly to running GopherTrunk, so you finish knowing not just what a constellation diagram is, but how to use the one in GopherTrunk to fix a signal that won’t lock. Mark lessons complete as you go — your progress is saved in your browser. New here? Start with lesson 1: What is a radio wave?

Module 1 — RF Fundamentals

The physics every operator leans on: waves, frequency, power, antennas, and how signals travel.

  1. What is a radio wave? Frequency, wavelength, amplitude, and the electromagnetic spectrum — the one idea everything else builds on. beginner 9 min
  2. Frequency, bands & the spectrum Hz to GHz, the HF/VHF/UHF bands, and how spectrum is carved up into band plans. beginner 8 min
  3. Decibels & signal power dB, dBm, and dBFS demystified — gain, path loss, the noise floor, and why radio lives in logarithms. beginner 10 min
  4. Antennas 101 Resonance, polarization, gain, SWR, and the connectors that join your antenna to your SDR. beginner 9 min
  5. How signals travel Line of sight, multipath, fading, and why where you put the antenna matters more than the radio. beginner 8 min

Module 2 — Signals & Modulation

How information rides on a carrier — from analog AM/FM to the digital constellations GopherTrunk decodes.

  1. Anatomy of a signal Carrier, bandwidth, and the difference between a spectrum view and a waterfall. beginner 7 min
  2. Analog modulation — AM, FM, SSB The three classic ways to put a voice on a carrier, and where you still hear them today. beginner 8 min
  3. Digital modulation & constellations FSK, PSK, QAM, and C4FM — how bits become symbols, and how to read a constellation and eye diagram. intermediate 12 min
  4. Symbols, baud & bitrate Why a 4800-baud C4FM signal carries 9600 bits per second, and what that means for capture. intermediate 7 min

Module 3 — Software-Defined Radio

What an SDR actually is, how it samples the air into IQ data, and how to set it up without overloading it.

  1. What is software-defined radio? The core idea — move the radio into software — and why one cheap dongle can decode dozens of systems. beginner 9 min
  2. How an SDR receiver works Antenna to tuner to ADC — the signal chain that turns radio waves into a stream of samples. intermediate 9 min
  3. IQ data & complex signals Why SDRs output pairs of numbers, what "I" and "Q" mean, and how they capture both amplitude and phase. intermediate 9 min
  4. Sample rate, bandwidth & Nyquist How much spectrum you can see at once, the Nyquist limit, and the bandwidth/CPU trade-off. intermediate 8 min
  5. Gain, AGC & avoiding overload Set gain too low and you lose weak signals; too high and you drown in distortion. Finding the sweet spot. intermediate 8 min
  6. SDR hardware — RTL-SDR, HackRF, Airspy Picking a dongle for the job and how each one maps to GopherTrunk's supported hardware. beginner 9 min

Module 4 — DSP Essentials

The digital signal processing that lives between IQ samples and decoded bits — explained without the heavy math.

  1. The FFT & reading a waterfall How the Fourier transform turns samples into a spectrum, and how to read GopherTrunk's waterfall. intermediate 9 min
  2. Filtering & decimation Zooming in on one channel, throwing away the rest, and keeping the CPU happy. intermediate 8 min
  3. The demodulation pipeline The stages every signal passes through: tune, filter, demodulate, recover symbols, decode. advanced 9 min
  4. Clock recovery & symbol timing How a receiver finds the beat of a digital signal so it samples each symbol at the right instant. advanced 8 min

Module 5 — Digital Voice & Trunked Radio

From vocoded voice to control channels — the systems GopherTrunk was built to follow.

  1. Analog vs. digital voice Why public-safety radio went digital, and what a vocoder gives up to fit a voice in a few kbps. intermediate 8 min
  2. Vocoders — IMBE & AMBE+2 The speech codecs behind P25 and DMR, and how GopherTrunk turns their frames back into audio. advanced 8 min
  3. What is trunked radio? Control channels, voice channels, and affiliation — how a fleet shares a handful of frequencies. intermediate 11 min
  4. The digital protocol landscape P25, DMR, NXDN, TETRA, Motorola, EDACS, and friends — who uses what, and how to tell them apart. intermediate 10 min
  5. Encryption & what you can decode Why some talkgroups are silent, what GopherTrunk can and can't do, and how to spot encryption. intermediate 7 min
  6. Other signals you'll meet Paging, AIS, ADS-B, and APRS — the non-trunked signals GopherTrunk can also decode. beginner 8 min

Module 6 — Putting It Together with GopherTrunk

Everything you've learned, applied: from a bare antenna to decoded calls, and how to fix it when it won't lock.

  1. From antenna to audio The complete GopherTrunk signal path, tracing one call from RF in the air to a WAV on disk. intermediate 11 min
  2. Finding & identifying systems Using Hunt and online databases to discover what's on the air near you and add it to GopherTrunk. intermediate 9 min
  3. Tuning for a clean lock Reading the constellation, eye diagram, and symbol scope to dial in a marginal signal. advanced 9 min
  4. Calibration & troubleshooting PPM and voice calibration, plus a checklist for the usual reasons a control channel won't decode. advanced 9 min
  5. Legal & ethical monitoring Knowing what's legal to receive where you live, and the etiquette of the scanner hobby. beginner 6 min
  1. Glossary of RF & SDR terms Plain-language definitions for every term in the learning path, cross-linked to the lessons.