Radio and SDR

It is all the same thing

Radio waves, visible light, X-rays, microwaves, and gamma rays are all the same phenomenon — electromagnetic radiation. The only difference is wavelength. Radio waves have the longest wavelengths. Gamma rays have the shortest. Everything in between is part of one continuous spectrum.

James Clerk Maxwell predicted electromagnetic waves mathematically in 1865 — before anyone had detected them. Heinrich Hertz proved they were real in 1887. Guglielmo Marconi worked out how to use them to send information in 1895. Within thirty years, voices were travelling invisibly through the air across entire continents.

What nobody expected was that the universe was already full of radio signals. Stars, galaxies, pulsars, Jupiter, the Sun, and even the faint afterglow of the Big Bang itself — all broadcasting in radio frequencies, all the time, waiting for someone to listen.

Different frequencies, different uses

The radio spectrum is divided into bands by frequency. Different bands travel differently, bounce off the atmosphere differently, and carry different kinds of information.

ELF
Submarines

AM Radio
MW

Shortwave
HF

FM Radio
VHF

Aircraft & Satellites
UHF

Microwave
SHF

WiFi
5G

VHF (30–300 MHz)

FM radio, aircraft communications, weather satellites at 137 MHz. Travels line-of-sight. AwareSignal listens here for NOAA satellite passes.

UHF (300 MHz–3 GHz)

Aircraft transponders (ADS-B at 1090 MHz), mobile phones, DAB radio. AwareSignal tracks aircraft positions using ADS-B in this band.

1420 MHz — The Hydrogen Line

Neutral hydrogen emits radio waves at this exact frequency. It is the most common element in the universe. SETI uses it as the most logical frequency for interstellar contact.

HF (3–30 MHz)

Shortwave radio, amateur radio, international broadcasts. Bounces off the ionosphere so signals can travel around the Earth. Jupiter radio storms are detected here.

What SDR actually is

Traditional radios are built in hardware — physical circuits tuned to specific frequencies. Change the frequency and you need different hardware. Software-Defined Radio replaces most of that hardware with software running on a standard computer.

An SDR receiver is just a very fast analogue-to-digital converter connected to an antenna. It samples the radio signal millions of times per second and sends the raw data to the computer. Software then does everything else — filtering, demodulating, decoding — in code rather than circuits.

This means one cheap USB dongle can receive almost anything across a huge frequency range, just by changing the software. Aircraft transponders, weather satellites, FM radio, amateur radio, solar radio bursts — all from the same hardware.

What you can detect with a basic SDR setup

✈️ Aircraft positions (ADS-B at 1090 MHz) Every commercial aircraft broadcasts its position, altitude, speed, and flight number. A basic SDR and antenna can track every plane within 200–300km in real time.

🛰️ Weather satellite images (137 MHz) NOAA-15, NOAA-18, and NOAA-19 beam weather images as they pass overhead at 800km altitude. With a discone antenna and SDR software, you can decode full colour weather images of the UK and Europe.

☀️ Solar radio bursts During solar flares, the Sun produces radio bursts detectable in the HF and VHF range. These sound like a brief whoosh of static descending in frequency. AwareSignal monitors for these.

🪐 Jupiter radio storms Jupiter's powerful magnetic field creates radio storms detectable from Earth using HF radio. When Jupiter is above the horizon and a storm is active, you can detect a gas giant 600 million kilometres away with basic equipment.

📻 FM radio, DAB, amateur radio FM stations, digital radio, amateur operators, maritime communications, emergency services — all receivable with an SDR and the right antenna.

🌌 The hydrogen line (1420 MHz) With a directional antenna, an SDR can detect the radio signature of neutral hydrogen in the Milky Way — the same signal SETI uses to search for extraterrestrial intelligence.

AwareSignal — listening from Doncaster

AwareSignal is a home radio observatory built as part of AwareSTEM. It runs on two Nooelec NESDR SMArt V5 SDR dongles connected to a Scanking Discone antenna and a Jupiter V-dipole antenna with a SAWbird+ LNA (low noise amplifier) for filtering.

The system runs continuously, tracking aircraft over South Yorkshire, capturing weather satellite passes, monitoring solar activity, and logging signal data. Results feed to the public AwareSignal dashboard in real time.

It demonstrates something important: serious radio science does not require expensive equipment or a university lab. It requires curiosity, a £25 USB dongle, and an antenna you can make from wire.

View AwareSignal live data →

What you need to start

Item	What it does	Approx cost
RTL-SDR USB dongle	The receiver. Plugs into any computer USB port. Covers roughly 500 kHz to 1.75 GHz.	£25–35
Antenna	The supplied stub antenna works for FM and aircraft. A discone covers more frequencies. A dipole cut to 137 MHz receives weather satellites.	£0–30
SDR# or GQRX software	Free software to visualise and decode radio signals. SDR# for Windows, GQRX for Linux/Mac.	Free
dump1090	Free software that decodes ADS-B aircraft transponders and displays live aircraft on a map.	Free
WXtoImg or SatDump	Free software that decodes NOAA weather satellite passes into actual weather images.	Free

Total cost to start: under £35. You can be tracking aircraft and receiving satellites the same day the dongle arrives.

🔬 First experiment: track an aircraft

Plug your SDR dongle into a computer. Install dump1090 (free, takes five minutes). Point your antenna out of a window. Open dump1090 in a browser.

Within seconds you should see aircraft appearing on a map — their flight number, altitude, speed, and position, all decoded from the 1090 MHz signal their transponder is broadcasting right now.

Every dot on that map is a real aircraft, broadcasting its position to anyone with the right equipment. You just became a ground station.

⚗️ Deeper: the physics behind it

Radio waves travel at the speed of light — 299,792,458 metres per second. Frequency and wavelength are inversely related: a 137 MHz signal has a wavelength of about 2.19 metres. This matters for antenna design — an antenna works best when its length matches the wavelength of the signal you want to receive.

A quarter-wave antenna for 137 MHz (weather satellites) is about 54.7cm long. Cut a piece of wire to that length, connect it to a coax cable, point it up, and you have a functional weather satellite antenna. The physics has not changed since Hertz proved radio waves existed in 1887.

The reason SDR works is that modern chips can sample radio frequencies fast enough to capture the full signal digitally. A cheap RTL2832U chip — originally designed for digital TV reception — can sample at up to 2.4 million samples per second, which is enough to cover a 2.4 MHz slice of the radio spectrum at once.

Where this fits in the bigger story

Radio and SDR connects directly to several chapters in the AwareSTEM Story of Everything. Each one explains the science behind what AwareSignal detects every day.

📡 Radio and Signals — the chapter ⭐ First Stars — stellar radio emissions ☀️ The Sun — solar radio bursts 🤖 AI — how AwareCub uses signals 🚀 The Future — SETI and the search 🌌 Full timeline

Explore more AwareSTEM

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