Receiving aviation radio with an SDR stick: hardware, software and antennas

A USB stick for a few euros, a wire antenna and a laptop – that is all it takes to see on screen what is going on between 118 and 137 MHz. Software defined radio, SDR for short, has made radio reception more accessible than ever. If you want to receive aviation radio with an SDR stick, though, you quickly run into two questions: which technology really works well? And what are you actually allowed to do with it in Germany? This guide explains stick, antenna and software step by step – and honestly sorts out where the legal situation sets limits for the technology.

The short answer

Technically, an RTL-SDR stick without a built-in 1090 MHz filter, an antenna for the VHF band (such as a quarter-wave ground plane of a good half meter) and software that can demodulate amplitude modulation are enough – for example SDR++ on a computer or RTLSDR-Airband on a Raspberry Pi. Legally, in Germany: owning and operating the receiver is allowed, listening to aviation radio without authorization is not. You can still build and test the technology legally – with FM broadcast, amateur radio or ADS-B.

How an SDR receiver works

A classic scanner has a dedicated circuit for each task: filter, demodulator, squelch. An SDR stick does almost none of that in hardware. It converts a slice of the spectrum to an intermediate frequency, digitizes it and sends the raw data (I/Q samples) over USB to the computer. Everything else is done by the software – it decides which frequency you hear, with which bandwidth and which modulation.

This has two consequences. First, you see a whole frequency range at once as a spectrum and waterfall. With RTL-SDR sticks the stably usable bandwidth is around 2 to 3 MHz – enough to survey a piece of the airband at once. Second, the quality depends strongly on the chain in front: antenna, cable, filter and the right gain setting make more difference than the model of the stick.

Civil aviation voice radio runs in the VHF band between 117.975 and 137 MHz in amplitude modulation. AM has a practical reason: if two stations transmit at the same time, you hear both superimposed instead of only the stronger one – a safety advantage in radio traffic. In Europe, new frequencies are assigned in the narrow 8.33 kHz spacing; why the displayed channel number does not always match the real frequency is explained in the article 8.33 kHz channel spacing in aviation radio. Which services sit where in the band you can find under Understanding aviation radio frequencies.

The hardware: which stick fits

Almost all entry-level sticks are based on the RTL2832U chip with a tuner of the R820T family or a successor. This combination covers the VHF band without trouble. What matters for narrowband AM radio is different from ADS-B:

Feature	Why it matters for VHF voice radio
Stable clock (TCXO)	If the frequency drifts, a narrow AM channel wanders out of the filter – with ADS-B that is hardly noticeable, with voice radio immediately
Metal case	shields interference from the computer and dissipates heat
SMA antenna jack	more robust and more common than the old MCX or coax connectors of cheap DVB-T sticks
No built-in 1090 MHz filter	pure ADS-B sticks with a band filter strongly attenuate everything outside 1090 MHz
Bias tee (optional)	powers a preamplifier at the antenna – usually not needed for VHF

A widespread all-rounder is the RTL-SDR Blog V4. According to the manufacturer it has a software-switchable bias tee with 4.5 volts and needs updated drivers – with old RTL-SDR drivers it receives nothing at all or on wrong frequencies. The sticks of Nooelec’s NESDR line are similarly solid. A detailed comparison of the models, including with a view to ADS-B, is provided by the article SDR sticks compared.

Important if you already run an ADS-B station: a FlightAware Pro Stick Plus has a built-in band filter for the 1090 MHz range and is therefore unsuitable for the VHF band. And even an all-round stick cannot process ADS-B on 1090 MHz and radio around 130 MHz at the same time – the ranges are too far apart. For both you need two sticks.

If you would rather have a ready-made device without a computer, the article Buying an aviation radio scanner lists the alternatives – the legal situation is the same.

The antenna: the biggest lever

The small telescopic or magnetic-base antenna from many stick kits is a compromise for everything and good at nothing. Because the airband has a manageable wavelength, you can get far here with very simple means.

You calculate the wavelength as 300 divided by the frequency in MHz. For the middle of the band around 127 MHz that gives about 2.36 meters, and a quarter of that is a good 59 centimeters. With a small velocity factor for the wire, you end up in practice at about 55 to 59 centimeters per element.

Antenna type	Construction	Strength	Weakness
Telescopic or stub antenna	from the stick kit	ready to use immediately	low performance, usually the wrong length
Quarter-wave ground plane	vertical radiator plus three to four sloping radials	easy to build yourself, tuned to the band	optimal for only one frequency range
Dipole	two quarter-wave elements	inexpensive, also for indoors	more prone to interference without a balun
Discone	disc above cone	very broadband, also hears other bands	large, for a single band no better than a ground plane

For the location the same applies as with ADS-B: as high and clear as possible, with little metal and concrete nearby. VHF signals propagate largely quasi-optically. That lets you reach aircraft at cruising altitude over long distances, while ground stations at the airport can often only be reached with a clear line of sight. The guide Finding the best antenna location transfers many mounting principles directly to VHF – including notes on lightning protection for outdoor antennas.

The good news about the cable: at 130 MHz the losses are much lower than at 1090 MHz. A decent, not-too-thin coax cable is enough; preamplifiers at the antenna rarely help in the VHF band and tend to make overload worse.

The software: desktop or continuous operation

For software there are two approaches that complement each other well.

Desktop programs with a waterfall are ideal for understanding the technology. You see the spectrum, click on a signal, choose modulation and bandwidth and hear immediately what changes. Widespread is SDR++, an open-source program for Windows, Linux and macOS that supports not only RTL-SDR sticks but also Airspy, HackRF and further hardware. There are also classics such as SDR# (Windows) and Gqrx (Linux and macOS).

Headless decoders run in the background without a screen. The best-known project for AM voice radio is RTLSDR-Airband. In multichannel mode it demodulates several AM or NFM channels at once as long as they fit into the stick’s bandwidth, and delivers a separate audio stream for each channel. Alternatively, in scan mode it jumps between frequencies like a classic scanner. According to the project wiki it can output MP3 files, Icecast streams, PulseAudio and UDP. Besides RTL-SDR sticks it supports further hardware via the SoapySDR library and runs on Linux, for example on the Raspberry Pi, as well as on macOS. The streaming functions are the technical foundation of many online feeds – in Germany, however, section 5 TDDDG prohibits passing on aviation radio, so a stream of your own is not an option here. Why this also applies to recordings in videos is shown by the article Publishing aviation radio in videos.

Software	Platform	Strength	Typical use
SDR++	Windows, Linux, macOS	waterfall, many demodulators, multiple VFOs	learning, experimenting, signal analysis
SDR#	Windows	large plugin selection	desktop reception
Gqrx	Linux, macOS	lean, good for beginners	desktop reception
RTLSDR-Airband	Linux (including Raspberry Pi), macOS	several AM channels in parallel, without a screen	continuous operation, recording

Setting up the receive chain – tested legally

The following steps show how to set up stick and software cleanly. Signals you may legally receive in Germany are good for testing: FM broadcast is intended for the general public, amateur radio for radio amateurs. Both show you whether antenna, drivers and settings work.

With these six steps your receiver is technically prepared. What you then tune in to is your responsibility – in Germany, aviation radio without authorization is not part of it.

Typical problems and their causes

Symptom	Likely cause	What helps
Music or snatches of speech in the middle of the VHF band	overload from strong FM stations	lower the gain, use an FM notch filter
Signal visible but only noise audible	wrong mode (FM instead of AM)	switch to AM
Signals wander in the waterfall	frequency drift from warming up	stick with TCXO, metal case, ppm correction
No reception at all	missing or outdated driver, 1090 MHz filter stick	update the driver, check the stick
Strong noise floor	interference from USB, power supply or computer	USB extension, ferrite cores, stick away from the computer
Speech distorted or choppy	bandwidth too narrow or squelch too high	open the filter, lower the squelch

Most problems therefore have to do with the chain, not with the stick itself. If you proceed systematically – first drivers, then antenna, then gain, then filter – you usually find the cause in a few minutes.

Legal alternatives for spotters in Germany

If what you want is to know what is happening at the spot, you don’t need radio. The data that really helps spotters is available legally:

• ADS-B: The same stick receives the aircraft’s position reports on 1090 MHz. How to build a station of your own from it is shown in the guide Building an ADS-B receiver yourself, and using it at the fence is explained in Flight tracking with ADS-B for spotters. On the legal situation of receiving and sharing ADS-B data there is a dedicated article.
• Weather and runway in use: METAR, TAF and wind data (Reading METAR and TAF) tell you which runway is in use – without listening in at all.
• Understand radio instead of listening in: You learn how controllers and pilots communicate with learning material on radiotelephony phraseology and on the ICAO spelling alphabet. If you are serious, get a radiotelephony certificate.
• Online offerings: What webstreams are about and why they are not automatically unproblematic legally for listeners in Germany is covered in the articles Listening to aviation radio online and Aviation radio apps and webstreams.

If you also want to use the stick for data signals, you should know that for data radio such as ACARS the same question about the intended recipients applies. What ACARS is technically is explained in the article ACARS explained.

Conclusion

An SDR stick is one of the cheapest ways to understand radio technology hands-on. For the VHF airband you need an all-round stick without an ADS-B band filter, a tuned antenna of a good half meter, AM-capable software such as SDR++ or RTLSDR-Airband and some patience when setting gain, frequency correction and squelch. Antenna and location make the biggest difference, and the most common problem is overload from FM broadcast. Legally, one thing stays clear in Germany: the technology is free, but listening to and passing on aviation radio without authorization is not.