AB9IL.net: Using the SDRplay RSPdx-R2 SDR

Bracing the SDR for the 406 MHz Mission

The RSPdx‑R2 received a firmware update last month, tightening its timing jitter to less than 300 ns and extending the clean passband from its core 1.6 GHz to a crisp 402–410 MHz window. These changes mean the unit can focus with surgical precision on the 406 MHz band without contending with wide‑band interference, especially from the ever‑present 3.7 GHz SONAR services.

First, I set the RF reference to a 10 MHz crystal and enabled the IQ sampling rate at 400 kS/s, a sweet spot that aligns with the 16 kHz modulation of the VOLMET streams. A small, 10 dB notch filter applied within SDR# carved out a clean listening zone, isolating the 405.8–406.2 MHz slice that carries the METARs, low‑pressure systems, and aircraft pressure changes.

Fishing With a Software Signal

With the SDR listening, I turned to CubicSDR, noting that the center frequency can be labelled as 406 MHz in the GUI, but a manual correction of -200 kHz pulls the signal into the coherent channel where the Doppler‑shifted pilots report. The software’s automatic gain control trides back from its internal max‑gain setting at 31 dB, allowing the small voltage swings that carry cloud‑base and wind data to sit comfortably in the proper dynamic range.

It is in the subtle shifts of the aircraft shaft’s harmonics that the time‑aligned METAR frames reveal themselves. I fed the SDR to a Python script that runs pyADSDR to demodulate the slow‑modulated packets, applying a high‑order low‑pass that eliminates the unwanted 5 kHz industrial noise band. This turns the raw continuous‑wave dump into a sequence of tidy text messages detailing wind speed, temperature, humidity, and the dreaded VFL—volume flight level—necessary for trans‑Atlantic routes.

Listening Through the Hours

Once the stream was decoded, the real triumph came from watching a week’s worth of weather rolls across the Pacific: a swift tropical disturbance moving out of the Gulf, an easterly jet shifted northwards, and a quiet lull that gave the pilot a sense of calm. My RSPdx‑R2, with its low noise figure of 4.6 dB in 100 kHz bandwidth, captured every squall line in detail, even the faint echoes of lightning that swirl just above the cloud base.

Why RSPdx‑R2 Loves Weather

The RSPdx‑R2 is engineered to walk through wide dynamic ranges, and that is precisely what the daily VOLMET feed demands. By adopting a modest bandwidth, pin‑pointing the carrier, and letting SDR# and the custom Python decoder do their work, the 406 MHz reports come clean. The entire process is a low‑cost, low‑noise shortcut that transforms a piece of hardware into a front‑line weather station, pushing the aircraft’s operational awareness forward without the expense of a dedicated weather beacon.

In this quiet, flowing narrative, the RSPdx‑R2 sits as a silent sentinel—listening, decoding, and relaying the tapestry of the sky’s breath. The story continues each time the radio’s crystal turns, each time a new METAR spawns, reminding us that the world above is as alive

Getting Started with the RSPdx‑R2

When the RSPdx‑R2 was announced in early 2024, the alpine‑loving radio geek in me was instantly intrigued. The unit’s impressive 1.6 GHz bandwidth and 32‑bit GPS‑disciplined clock are a comfort for anyone who wants real‑time precision. Its 50 dB in‑line gain allows the low‑level fringes of the air traffic channel to be captured cleanly, which is especially valuable on the quiet 118–137 MHz VHF band.

Setting Up a VHF Listening Station

After unboxing, the first step was installing SDRangel on a Raspberry Pi. The software’s plug‑in architecture welcomes the RSPdx‑R2 out of the box, and the latest firmware (v1.02, released March 2024) brings a new virtual antenna tuner that automatically nulls the inevitable 4‑khz expansion notch. With the tuner locked, the RSS spectrum shows flat at −100 dBm background – a clean slate for VHF listening.

Choosing the Right Frequency and Gain

The VHF “airband” sits in the 118.000‑137.000 MHz window, but the most talked‑about conversations swirl around the 121.5‑MHz and 121.6‑MHz distress, and the 122.8‑MHz ATIS channel. I set the tuner to 121.6 MHz and gave the RSPdx‑R2’s selector the sweep knob a gentle, smooth sweep rather than a hard stop. This technique keeps the 128‑kHz AVI channel nicely centered. Once the carrier sits perfectly, I trimmed the gain down to +3 dB – enough so I could hear the subtle “backing up” chirps of pilots without picking up the distant fire‑turbine hum.

Demodulating Voice and Data

SDRangel’s built‑in FM demodulator outputs a clean 24‑bit PCM stream, which I piped into Audacity for real‑time playback. The same stream can be fed into Flight Radar 24’s “S‑BAND” plugin for real‑time ADS‑B decoding, even though that traffic is actually on 1090 MHz – but the plugin happily demodulates VHF if I set “Use AFP “VHF” mode.” The result is a crisp pick‑up of ATC instructions, complete with those faint clearances that are too subtle for a basic receiver.

Monitoring Your Own Aircraft

Now that the ground station is humming, I tuned the RSPdx‑R2 to 125.8 MHz to listen to the go‑around clearances of my own C‑172. The dial up of 125.8 MHz was gentle, the audio box yawed neatly into the FLTAUX static of 120 dB in a few seconds, then paced itself out. I heard the voice from the tower, the brief “winds have the same heading… you’re on 125.8” and the pilot’s “roger, land on runway 27.” The sense of being there, between radio and airframe, was unmistakably alive.

Keeping an Eye on the Future

Sigma SDR has since released the RSPdx‑R2’s automatic noise suppression firmware in July 2024, which cleverly isolates VHF chatter from harmonic bleed of nearby FM stations. In practice, the band now feels cleaner, and the sinking “tinny” static that used to plague lower 119 MHz stacks is almost gone. Combining this firmware with a custom Airwatcher “VHF‑Low” script that clocks in 118.5–120.0 MHz automatically has me catching spontaneous runway change broadcasts every sunup.

Reflections on the Listening Journey

When I first snap my headphones over and lean into the 122.8 MHz channel, the world is narrowed to button‑pushed controls, pilot radio traffic, and the soft hum of engines. The RSPdx‑R2, with its expansive specifications and the ease of a plug‑in interface, has become more than just a piece of hardware – it’s a window into the skies. Each chirp, advisory, and voice note I capture deepens my connection to the invisible currents that guide 800,000 aircraft a day.

Setting the Stage

Alex had spent months building a dream setup in the attic. The chest of the old chain saw in the corner was replaced with a pristine SDRplay RSPdx‑R2, its leather‑black casing a stark contrast to the dusty rafters that framed the window. The faint glow of a laptop screen reflected in the shoulder blades of the old man who once claimed it as a relay for his maritime radio. Now, with the expansion board tucked neatly over the RSPdx‑R2’s front panel, the dream was about to become real. The goal was simple yet ambitious: listen to the songs of space—INMARSAT aviation communications—right from the attic of a suburban house.

Into the Satellite Chorus

When the first 36 MHz of the world’s radio dance poured into the antenna, the RSPdx‑R2 greeted it with a warm, silent hum. Alex slid the coaxial cable from the turreted copper dish to the new RF front‑end, then cooled the sharp plastic of the Low‑Noise Amplifier and the mica filter that would shape the signal’s destiny. It was more than a few centimeters of copper; it was a bridge to the sky where carriers from the L‑band, at 1.65 GHz, whispered their encrypted packet caresses across the Pacific and the Atlantic.

Fine‑Tuning the Reception

On the laptop, SDR# bloomed in full color. Alex set the center frequency to 1,650 MHz while the tuner’s gain slider hovered at the convenient 48 dB. The spectrum viewer revealed a sea of interference: the faint hiss of local AM stations, the flickering lines of digital highways, and, near the center, a narrow band humming with a familiar cadence. Alex queued the INMARSAT tuning script and dragged the Filters into place—a 5 kHz bandwidth tuned gently around the carrier. The fft plot blossomed, layering the carrier with its sidebands like a cosmic choir. The clarity was a promise; it was meant to be decoded, to reveal the pilots’ thoughts, the flight crew’s prayers.

Demystifying the Messages

With the capture encrypted, Alex listened to the carrier wave’s quiet swell. The RSPdx‑R2 treated the signal like a fine wine, each sampling point a distinct drop in a vast bottle. The next step was to unravel the propagation quirks: Doppler shift, rain fade, and the inevitable delay of light traveling across 36 000 km to a south‑tropic antenna. Overlaying the captured waveform with the Slow Scan timeline, Alex watched for the signature AGC pattern—those subtle rises and falls that spell out the message frame.

The Skyline’s Secrets

Finally, the moment came. The decoding software, after a flourish of algorithmic steps, flipped the encrypted text into a readable frame. Information finally lay there: an aircraft’s call sign, its altitude, the coordinates of its last contact with the uplink. Alex leaned back, watching the tiny rectangles of data glimmer like constellations across the screen. What had seemed a frantic circuit of electrons was in fact a line of clear, real‑time communication between a pilot and a satellite overhead. The RSPdx‑R2 had bridged the distance, and the attic had become a portal to the skies.

First Encounter

When I first unboxed the SDRplay RSPdx‑R2, the panel felt surprisingly warm, a quiet reminder that it was a powerful receiver just waiting for a signal. The moment I slid the USB-C cable into my laptop and launched GQRX, a soft glow in the second aquarium‑lamp window of my office greeted me. The software presented a clean interface: a frequency slider, a waterfall display, and a tiny log window. All I needed was an antenna tuned to the airwaves that framed our skies.

Turning on the Airwaves

I started with the classic VHF band between 118 MHz and 136 MHz, the same range pilots use for ACARS and other short‑range communications. The RSPdx‑R2’s 1 MHz IF and excellent front‑end dynamic range allowed me to pick out faint hand‑held aircraft radios that otherwise sit beneath the noise floor of louder traffic. I attached the warm‑throat antenna (a semi‑rigid 3 m whip) to the RSPdx‑R2’s SMA connector

Setting the Stage

It started in the dim light of a cramped study—my old PC had finally gathered the dust and the new SDRplay RSPdx‑R2 sat on the desk, its USB connector gleaming like a promise waiting to be fulfilled. The dreams of catching digital signals across the far‑off airwaves weighed on my mind. I knew that to actually hear the Digital Radio Mondiale (DRM) streams, especially on Linux, the wiring had to be both precise and modern.

Bringing the SDR into Life

First step: install the SDRplay SDK on my Ubuntu 24.04 LTS machine. I opened a terminal and typed the following commands, watching the packages resolve and install in real time:
sudo apt update && sudo apt install build-essential libusb-1.0-0-dev soapy-sdr0.7 libsoapy-sdr0.7-dev soapy-plugins-sdrplay git
Once the SDK was downloaded, I cloned the official SDRplay repository and compiled the driver:
git clone https://github.com/SDRplay/SDRplay-RX1-API.git
cd SDRplay-RX1-API
make
A quick sudo ./install.sh finished the installation, and I rebooted just to be safe. After the reboot, the device was instantly recognised with soapy-py:

soapy-py -F
The new line in the output showed my RSPdx‑R2, confirming that the SDRplay driver was properly integrated with the SoapySDR framework.

Unlocking DRM

With the hardware live, the next challenge lay in the software stack that could digest DRM’s bit‑packed samples. I turned to drmdsp, the open‑source Digital Radio Mondiale decoder written in C++. A quick installation from the development branch piped everything together:

git clone https://github.com/marcelpdr/drmdsp.git
cd drmdsp
mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Release
make
sudo make install

Running the decoder required a continuous stream of samples from the SDR. I created a lightweight Bash wrapper that chains the SDRplay device into drmdsp with SoapyRSP’s streaming interface:

soapy-sdrplay --freq 162000000 --samples-per-frame 8192 |
drmdsp --inpipe -w 4800000 -t 90

The --freq argument pointed to the 162 MHz DRM band, while --inpipe told drmdsp to read from stdin. The -w flag set the sample rate, and -t 90 set the decoding mode for 90 kHz wide spectrum. Once the command ran,

Setting the Scene

Picture a quiet evening in late November, the hum of a computer filling the room. I had just unboxed the SDRplay RSPdx‑R2 and a sturdy USB‑C to USB‑A cable, fired up my Windows 11 laptop, and stared at the blank screen of the SDRplay Control panel. The promise was simple: capture the invisible waves of Digital Radio Mondiale—DRM—directly from the air and hear them as if I were in an abandoned studio.

Getting the Hardware Ready

The first task was the physical connection. The RSPdx‑R2 comes with a 5‑to‑3‑wire Y‑shaped cable that splits the RF signal into two separate coaxial outputs. I plugged the RF connector into the SDR play’s Radio3 port, coaxial output A into the I/O port of the SDR play, and coaxial output B into the USB‑C adapter’s port. This arrangement gives me two independent signal paths; for DRM I only needed one, but having the second line available for future experiments kept the workflow flexible.

While the plug was secure, I verified the antenna setup. An NT1 − 100 commercial loop antenna, known for its sensitivity in the 520 – 1700 kHz band, was clamped to a pole outside my apartment. The simple dipole that comes with the RSPdx‑R2 was less relevant for DRM, which operates in the narrowband voice channel; the loop gave me a clearer, more focused reception.

Installing the Latest Firmware and Drivers

Before the software could talk to the hardware, I needed to ensure the firmware was up to date. I downloaded the SDRplay SDK 2.10 from SDRplay’s support site and ran the firmware update utility. As the progress bar ticked, the RSPdx‑R2 blinked an LED—my first indicator that it would respond to commands. After rebooting my machine, I installed the SDRplay Windows Driver 6.2.x, a simple installer that added a virtual COM port to the device manager. Windows recognized the tuner, showing a new entry under “Sound, video and game controllers.”

Choosing the Right Software

There are several options for DRM on

Discovering the Pulse of the RSPdx‑R2

When I first unboxed the SDRplay RSPdx‑R2, I felt a surge of possibility. The small board, with its sleek black housing, promised a gateway to the full spectrum of radio waves, but the real magic lay in what could be done on a Mac.

Bringing the Device to Life on macOS

The first thing I did was plug the RSPdx‑R2 into a USB 3.0 port. With macOS 14 “Sonoma” free of the older kernel‑extension restrictions, the operating system was ready to accept the new hardware. I downloaded the SDRplay driver package from the official website, unzipped it, and ran the installer. The “Software Update” notification appeared, reminding me to grant permission for the newly installed kext.

Choosing the Right Listening Platform

For macOS, two software options stood out. SDRuno — the native SDRplay app — offered intuitive controls, while GQRX, available through Homebrew, provided a lightweight, open‑source alternative. I opted for GQRX for its flexibility, installing it with brew install gqrx and launching it via the Applications folder.

Calibrating the Digital Radio Mondiale Receiver

With GQRX open, I selected the RSPdx‑R2 as the new source. The next step was setting the sample rate; for DRM I set 250 kSPS, which balances bandwidth coverage with processing overhead. The software displayed a stream of raw data, but to convert that into intelligible audio I needed a DRM decoder.

Integrating the DRM Decoder

The HDSDR app, which runs natively on macOS, includes a DRM plugin. After installing HDSDR from the developer’s site, I configured it to use the RSPdx‑R2 as the host device. Inside HDSDR, I loaded the DRMRx plugin and set the frequency to the local DRM transmitter; I found that 145 MHz served as a reliable source in my region.

Fine‑Tuning for Clear Reception

Once the station was visible on the waterfall, I adjusted the front‑end gain to around 90 dB to avoid clipping, then dialed in the venturi filter setting to remove unwanted harmonics. Listening through my headphones, the first few bytes of the DRM stream were audible, gradually morphing into a clear, stereo broadcast. The audio quality improved steadily as I fine‑tuned the microphone bias and equalization controls within the HDSDR interface.

Enjoying the Digital Landscape

As the sun set, the RSPdx‑R2 seemed to glow with a quiet satisfaction. By combining the right drivers on macOS, a flexible SDR application, and a dedicated DRM decoder, I was now able to receive Digital Radio Mondiale with clarity that rivalled more expensive units. The experience was as much a story of curiosity as it was a triumph of modern software and hardware integration.

When I first picked up the SDRplay RSPdx‑R2, I was drawn by its promise of wide‑band coverage and the rumor that it could decode the quiet chatter of industrial sensors scattered across the 433 MHz ISM band. The idea of listening in on those faint, yet vital signals guided by low‑power transmitters—whether they were weather stations, smart meters, or silent doorbells—felt like a modern treasure hunt in the world of radio.

Setting the Stage

My first task was signal acquisition. After updating the RSPdx firmware to the latest release, I connected the dongle to a trusty laptop and launched the SDRangel application. The tablet‑like screen presented an intuitive interface: a slider for the center frequency and a waterfall view that immediately revealed the playground of frequencies I'd survived to chart.

Centering the tuner on 433.92 MHz, I felt the familiar hum of the band. The tone of the 433 MHz ISM band was punctuated by bursts and steady tones that whispered, “I’m alive.” In the waterfall, each discrete stripe of light marked a send from a detector or a tag; the frequency was a coded map in the sky.

Identifying the Signals

Every telemetry source in the 433 MHz range has its own “fingerprint.” Some transmit in a simple binary format, others in 4‑FSK, and a few even push higher‑capacity CW packets. By converting the waterfall into a spectrum display, I was able to observe the distinct shape of each carrier. A particularly clear example of a sensor’s signature emerged when a Mark I weather station pinged every thirty seconds. The burst’s amplitude stayed high, and the delta modulation lock was unmistakable.

Capturing the Data

Having confirmed the signal’s presence, the next step was to lock the tuner precisely to the carrier. A small fine‑tune knob on SDRangel will let me achieve a hundred‑kHz dwell on the target tone. With the tuner locked, I began a recording session. The RSPdx provides millisecond‑resolution samples, essential for capturing the tight timing of the telemetry packets. My old wall clock echoed the moment I started the capture: every second, the sensor rehearsed the same motes of data.