Eavesdrop on virtually the entire radio spectrum — for 30 bucks!
Not long ago, enthusiasts with an urge to listen to radio broadcasts had to make hard choices about what equipment to buy based on their interests. “Multi-band” radios provided circuitry for a selected set of frequencies, usually requiring crystals specific to each band. A basic one might receive AM, FM, shortwave, and NOAA weather radio; for more than that, you’d be looking at serious cash.
But today, a little USB dongle costing around $30 can receive AM, FM, shortwave (SW), amateur/ham HF, VHF, UHF, SHF, maritime, aviation, EMS, satellite, NOAA, HD radio, trunked radio, P25, GPS, APRS, radar, HAARP, CW, TV, GSM, pagers, baby monitors, DMR, D-STAR, PSK, RTTY, SSTV, and pretty much anything else broadcasting from 500kHz up to 1.75GHz. More expensive models can extend that range and even transmit!
Keep reading to learn more about this game-changing technology.
From Analog to Digital: A Capsule History of SDR
Sending and receiving radio waves has traditionally been the province of analog electronics. Tuning, detecting, oscillating, mixing, filtering, (de)modulating, and amplifying — the basic functions of a radio — involved crystals, capacitors, inductors, tubes, transistors, and other electronic components. These components can be assembled into gloriously complicated circuits to make use of the radio frequency (RF) spectrum. But analog radios have constraints, most importantly limitations around how much of the spectrum they can operate in. All but the most elaborate analog radios have a very restricted frequency range or include multiple sets of components to allow operation in different bands.
Starting in the 1970s, various U.S. government research groups began experimenting with using software signal-processing techniques to replace hardware components. Digital signal processing (DSP) math and code advanced rapidly in Defense Department labs and universities. The tremendous power and flexibility of software became extremely appealing as it provided a way to avoid many of the limitations of hardware. Research and development in the 1990s of software radio, a term later eclipsed by software defined radio (SDR), rapidly spread out of government into commercial efforts, showing up in high-end car radios by 1997.
With the introduction of the European digital TV standard known as Digital Video Broadcasting – Terrestrial (DVB-T) in 1997, manufacturers such as Realtek and others began producing low-cost integrated circuits like the RTL2382U, capable of decoding DVB-T signals in the 174MHz–786MHz range and packaged in small USB dongles. By 2010–2012, Eric Fry and Antti Palosaari had determined that the RTL2382 was usable as a general SDR, and Steve Markgraf and colleagues at the Osmocom project had released rtl-sdr software that allowed Linux, Windows, and Mac users to receive and decode radio signals from an astonishing range of 500kHz–2.2GHz, depending on which chip variant is used.
How Do SDRs Work?
The heart of an SDR is the analog-to-digital converter (ADC) found in a demodulator chip like the RTL2382U, where the analog electrical signal that the radio antenna receives is converted to 1s and 0s, similar to the way a signal from a microphone is digitized. A digital tuner chip, like the R820T, simply captures a selected frequency range from the antenna and sends it along to the demodulator. Once the signal can be numerically represented, the power of math can be applied via DSP software to filter, decode, and process the data into targets of interest.
In practice, real-world issues force some compromises. For example, since the analog signals may be very faint, amplification is often necessary. Amplifiers bring their own issues in filtration, noise, dynamic range, and other considerations. Bandpass filters are often used to constrain these problems, but limit the range of frequencies received (Figure A
). Many SDR designs have multiple analog input channels, each with optimized amplification and filtration to provide consistent results over the broadest range.
DSP software has been the engine at the heart of the most amazing advances in technology since Bell Labs put the first T-1 network into service in 1962. By shedding the constraints of physical properties that limit analog circuit construction, it is possible to manipulate audio, video, seismological, medical — any signal with changes that can be measured over time — with a degree of precision and flexibility analog designers can only dream of.
We Want the Airwaves!
Check out this amazing list of things to do with your SDR, with links at RTL-SDR.com. Not all may be legal in your country. Please be responsible.
• Use as a police radio scanner.
• Listen to EMS/ambulance/fire communications.
• Listen to aircraft traffic control conversations.
• Track aircraft positions with ADS-B decoding.
• Decode aircraft ACARS short messages.
• Scan trunking radio conversations.
• Decode unencrypted digital voice transmissions such as P25/DMR/D-STAR.
• Track maritime boat positions with AIS decoding.
• Decode POCSAG/FLEX pager traffic.
• Scan for cordless phones and baby monitors.
• Track and receive meteorological agency-launched weather balloon data.
• Track your own self-launched high altitude balloon for payload recovery.
• Receive wireless temperature and power meter sensors.
• Listen to VHF amateur radio.
• Decode ham radio APRS packets.
• Watch analog broadcast TV.
• Sniff GSM signals.
• Use rtl-sdr on your Android device as a portable radio scanner.
• Receive GPS signals and decode them.
• Use rtl-sdr as a spectrum analyzer.
• Receive NOAA weather satellite images.
• Listen to satellites.
• Radio astronomy.
• Monitor meteor scatter.
• Listen to FM radio, and decode RDS information.
• Listen to DAB broadcast radio.
• Listen to and decode HD-Radio (NRSC5).
• Use rtl-sdr as a panadapter for your traditional hardware radio.
• Decode taxi mobile data terminal signals.
• Use rtl-sdr as a high-quality entropy source for random number generation.
• Use rtl-sdr as a noise figure indicator.
• Reverse-engineer unknown protocols.
• Triangulate the source of a signal.
• Search for RF noise sources.
• Characterize RF filters and measure antenna SWR.
• Decode Inmarsat STD-C EGC geosynchronous satellites.
• Listen to the ISS (International Space Station).
Furthermore, with an upconverter or V3 RTL-SDR dongle to receive HF signals, you can:
• Listen to amateur radio hams on SSB with LSB/USB modulation.
• Decode digital amateur radio ham communications such as CW/PSK/RTTY/SSTV.
• Receive HF weatherfax.
• Receive Digital Radio Mondiale (DRM) shortwave.
• Listen to international shortwave radio.
• Look for radar signals like over the horizon (OTH) radar, and HAARP signals.
Getting Started in SDR
The minimal equipment needed to start experimenting with SDR consists of an antenna, an SDR peripheral, and a computer. The RTL-SDR USB dongle, generally available for around $30, is the most popular starting rig for new SDR users. There are plenty of more advanced SDRs available at a wide range of price points. Some provide a broader frequency range, offer transmitting capabilities, or offer more advanced circuitry. But the humble RTL-SDR is an awesome place to start.
While many SDRs are able to decode signals in the lowest end of the spectrum (varying according to the SDR, usually below 300MHz) via direct sampling or direct conversion, this is often at reduced clarity. As an alternative, many SDR owners will purchase an inexpensive upconverter — a device that converts signals below the SDR’s reception range, often down to “DC” (0Hz), up into the SDR’s normal range,
More advanced SDR units offer users with more than casual listening needs extra capabilities. In the sub-$200 range, Airspy offers a receiver with advanced DSP filtering (always great for cutting through noise). In the sub-$400 range, the HackRF by Great Scott Gadgets is designed as an RF experimentation and prototyping platform with a 1MHz to 6GHz operating range and the ability to transmit.
Owning more than one SDR is a joy. I personally dedicate two RTL-SDR units to a P25 trunked radio decoder for EMS and police scanning (Figure 
B), while using a HackRF for experimenting and as a panadapter for my HF radios.
Antenna Considerations
As is true with most radio applications, the biggest difference in experience often has more to do with the antenna than the radio. While all antennas are optimized for specific frequency ranges, receiving antennas are more forgiving than transmitting antennas.
SDR Software
As with many modern devices, the hardware is the generalist and the software is the specialist. SDR software takes the broadband signal provided by the SDR device and displays, filters, and decodes it into usable signals. Digital radio protocols have become so ubiquitous that, without software, much of the spectrum’s interesting activity would be unintelligible. Whether it’s mapping ADS-B location signals from airplanes, following frequency-hopping trunk systems, decoding digital FM broadcasts, functioning as a spectrum analyzer, or identifying satellites currently overhead, SDR software gloriously explodes the SDR hardware’s utility.
Most new SDR users will scroll the waterfall display around the frequency range to hunt for things to listen to. Online tutorials for pursuing specialty interests like trunked police radio, aviation tracking, or satellite telemetry provide a next-level set of activities.
It’s hard these days to find a laptop, desktop, Raspberry Pi, tablet, or phone that isn’t capable of running some version of SDR software. Windows has the broadest set of apps, though Linux, Android, MacOS, and iOS have powerful tools as well. SDRangel and CubicSDR are two of the few cross-platform (Windows, Mac, Linux) apps.
SDR Fast Start
So you bought an RTL-SDR USB dongle with a simple telescoping antenna. You now have a world of signals available to you, but many are digitally encoded, many are trunked (switching programmatically from frequency to frequency), many require a good outdoor antenna at height, and many are just infrequent and require long periods of waiting before someone does something to listen to. So what can you do to just hear something and know your setup is working?
Like many peripherals, your first step is to get the proper driver for your device and operating system. A great writeup that details getting Airspy’s SDR# (SDRSharp) running on Windows and Linux. For Macs, Adafruit has a good writeup for CubicSDR.
Before you start pulling coax, installing lightning arrestors, and setting up antenna masts, the easiest thing you can listen to with a simple antenna on your desk is the ever-present FM commercial radio spectrum. Ranging from 88MHz–108MHz, using wideband frequency modulation (WFM), these broadcasts are available nearly everywhere and powerful enough to receive with the most basic antenna setup.
Set your SDR’s tuner mode to WFM, then scroll around your spectrum display in the range of 88MHz and up. On your waterfall frequency display, you’ll see bright bands of FM activity wherever there’s a station your antenna can pick up. Now click around on these bands to demodulate the signal and hear it as audio!
While there are lots of ways to receive FM broadcasts, listening to it via SDR is a great gateway experience that will start you off on your listening journey. As you add more versatile antennas you’ll use these same steps to explore shortwave, amateur (ham), and other new bands on the radio spectrum.
Using Online SDRs
If buying and configuring hardware and software isn’t on your list, you can still check out the SDR experience with just an internet-connected phone, tablet, or computer! Thanks to WebSDR server software written by Pieter-Tjerk de Boer, PA3FWM, numerous sites around the world have set up web-based access to their SDRs and antennas and offer free access to use them. WebSDR.org hosts a listing of sites with SDR servers running. You can click on a site and tune the SDR to the band you’re interested in, see a waterfall display of the band, choose the tuner mode (AM, FM, SW, etc.), filters, and other controls just as if you were operating a radio or SDR directly.
Most of these servers support between 30 and 80 concurrent users and are limited to a specific set of bands. The band limitation is generally an aspect of the connected antennas. The majority of the WebSDR servers trend toward supporting the HF (high frequency, about 3MHz–30MHz) Amateur and Shortwave bands, but they are increasingly adding UHF, VHF, and other frequency ranges.
WebSDR servers are even useful to folks with their own radio or SDR setups. Remote online SDRs provide the ability to compare your reception locally to reception from other locations or using alternate antenna setups. Many hams also use WebSDR servers in different parts of the country to listen to their own broadcasts and hear how they’re “getting out.”
Conclusion
SDRs are rapidly becoming the dominant radio technology of the 21st century. Substituting math for circuitry allows for such tremendous flexibility that we can expect to see SDRs integrating into a wide range of applications and devices. Given the low cost of entry, makers with an interest in the radio spectrum can’t go wrong experimenting with this exciting technology.
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