It began on a quiet spring evening, when the last rays of the sun slipped behind the hills and the radio spectrum seemed to hum with clandestine whispers.
Finding the RSPdx‑R2
SDRplay recently released the new RSPdx‑R2, a rugged, high‑performance software‑defined radio that extends coverage to 2.2 GHz and offers a 28 MHz bandwidth. Its new low‑noise front‑end and improved analog‑to‑digital conversion make it especially suitable for hunting faint amateur radio signals that would otherwise slip into the ether.
Homing in on WSPR
WSPR (Weak Signal Propagation Reporter) operates on two primary amateur bands: the 2 m band at 903.5 MHz and the 70 cm band at 146.2 MHz. These are the most commonly used frequencies for passive monitoring with SDR software. The RSPdx‑R2 can be tuned to either frequency precisely; a simple command in SDRplay's SDR_Console or SDR_Connect places the front end squarely on 903.5 MHz, while a quick shift to 146.2 MHz reveals a new window of propagation chatter.
Setting It Up
First, a single‑ended low‑noise amplifier (LNA) feeds the RSPdx‑R2. Antennas are kept simple—a dipole for the 70 cm band and a Vivaldi or patch for 2 m. The user configures the RSPdx‑R2 to a center frequency of 903.5 MHz, sets the sample rate to the maximum available, and opens the software rtlsdr-feedtimer to stream data continuously. With WSPradio or
Listening for the Whispered Code
When the RSPdx‑R2 captures a 903.5 MHz burst, the spectrum reader glows: a faint, shimmering signal that decodes into a short call sign, propagation data, and a timestamp—an elegant testimony to the weak‑signal network's reach. Switching back to 146.2 MHz, the same pattern reappears, sometimes meters from the original location, sometimes across an ocean. The RSPdx‑R2’s real‑time FFT view allows instant cross‑band observation, revealing how propagation modes shift between frequencies.
Monitoring the Propagation Realm
Long after the last WSPR burst fades, the RSPdx‑R2 continues to listen. In the background, the device streams data to a data logger that pushes entries to an online WSPradio.org report. Analysts slice the data, create awips and spectral maps, and publish commentary on how the 903.5 MHz and 146.2 MHz bands perform in a given solar cycle. The story of each night’s propagation is archived, providing valuable insight for researchers and hobbyists alike.
The Legacy of the RSPdx‑R2
By July 2024, the SDRplay RSPdx‑R2 has become a staple in the amateur community. Its affordable price, robust hardware, and near‑real‑time spectrum analysis facilitate a deeper understanding of WSPR propagation. Every time a weak carrier appears at 903.5 MHz or vibrates a pulse at 146.2 MHz, the RSPdx‑R2’s optics capture it, linking distant operators across the world and turning invisible radio waves into a living story we can all read today.
A Night of Discovery
It was two weeks after the SDRplay RSPdx‑R2 firmware update that I finally decided to make the most of the new dual‑band capability. The RSPdx‑R2, with its expanded 10 kHz–30 MHz range and the latest digital tuner firmware, promised clearer images of the late‑night sky where the only traffic is the thin, often hidden, signals that roam the amateur radio bands.
I anchored my antenna on the south‑facing roof and set the RSPdx‑R2 to 25 kHz bandwidth in the RX‑1 channel. The software interlace—a blend of SDRangel and the SDRplay’s own RESCUE driver—was booted with a quiet hum. As the sweep began, the spectrum pop‑ed up across the screen, showing a bustling crowd of CW, RTTY, and SSB chatter.
Tuning Into the FT‑4
The FT‑4 band—known to many as the 70 metre band—occupies the slice from 4300 kHz to 4400 kHz. For this expedition I centered the 30 kHz window on 4320 kHz, just a tad above the most heavily used 4326 kHz channel. The RSPdx‑R2’s automatic equalizer kicked in, smoothing the spike that forms with each pass of the local channel. In the resulting trace I noticed a faint but unmistakable burst, a narrowpass of FM‑SLS that drifted slowly around 4320.6 kHz.
Letting the frequency counter run, I recorded a spike at 4321 kHz that persisted for several minutes. The moment of confirmation came when I zoomed in to 1 kHz and saw the classical “+ / –” selective-modulation pattern that is a hallmark of FT‑4Setting the Stage with the RSPdx‑R2
Alex, a dedicated amateur radio operator, had finally chosen the SDRplay RSPdx‑R2 for its remarkable 62‑MHz bandwidth and the ease with which it can be tuned across the entire amateur spectrum. He hooked the dongle into his system, opened SDR#, and listened for the faint squeak that signals a digital mode is active. The RSPdx‑R2’s programmable gain amplifier allowed Alex to pull the signal down to its most subtle peaks without distorting the delicate tone of an MT63 transmission.
Reaching into the 2 Meter Realm (144‑148 MHz)
MT63 thrives on the 144‑148 MHz band, where the two‑metre band offers a perfect medium for narrows‑band digital voice. Alex set the LO to 146.305 MHz, a common centre for amateur narrowband practice. The RSPdx‑R2’s VBW‑adjustable filter was narrowed to a 10‑kHz passband, ensuring that only the narrow bursts of MT63 signal could seep through the hiss of co‑channel noise. The software’s waterfall display revealed the dual 8‑kHz pieces of an MT63 packet dancing side‑by‑side—an unmistakable visual signature of the mode.
Venturing into the 70‑Centimetre Sea (420‑450 MHz)
MT63 is as comfortable on the 70‑centimetre band as it is on two‑metre, and Alex didn’t want to miss any potential contacts. With the RSPdx‑R2, he tuned the LO to the lower end of the 70‑cm window at 424.118 MHz, an area where many stations practice high‑speed digital voice. The dongle’s high‑pass filter was switched to 200 kHz, and the sample rate was set to 3 Msps, giving a clear, crisp picture of the MT63 symbol stream. The tiny tones—each a 200‑Hz modulation—were now visible as clean, rhythmic ripples on the density graph, a sign that the RSPdx‑R2 could follow the mode’s rapid cycle of 64‑chip bursts.
Gliding into 23 Centimetre (1240‑1300 MHz)
For adventurous operators, the 23‑centimetre band offers a sweet spot between the demands of narrowband and the high‑frequency agility of MT63. Alex opened the spectrum at 1255.745 MHz and engaged the RSPdx‑R2’s 1‑MHz decimation setting, allowing the capture of the entire MT63 packet within a reduced data footprint. The incoming signal was steered right to the ctcss bleed-avoid filter, isolating the **MT63** voice bursts in a mode that was otherwise a whisper in the high‑frequency sea. The software’s FFT displayed the 5‑kHz bandwidth of the mode superimposed on a sea of terrestrial and satellite traffic.
Practical Tips from the Field
Conducting a successful MT63 hunt with the RSPdx‑R2 requires a thoughtful approach: lock your gain to avoid clipping, keep the filter narrow but wide enough to capture the symbol transitions, align your time base with the software’s sync window, and always check clip levels on the waterfall before diving into a long listening session. Alex found that instrumenting the RSPdx‑R2 with an external 5‑V bias tee for his receiver head provided cleaner demodulation of the digital voice, especially on the 70‑centimetre band where power constraints are higher.
Listening to the Story of MT63
When the MT63 packet finally hit Alex’s screen, the RSPdx‑R2 hummed quietly beneath his headphones. Each symbol rippled across his speakers in a rhythmic pattern that echoed the storytelling of a long‑distance conversation. In that moment, the RSPdx‑R2 was not just a reading device; it was a bridge across the airwaves, capturing the soul of digital voice from the hot‑scar of two‑metre to the crystal clarity of 23 centimetre. Alex’s adventure with the SDRplay RSPdx‑R2 proved that with the right hardware and a little patience, the horizon of amateur radio expands to every centimetre of spectrum willing to listen.
As the evening sky spun its familiar blanket of stars above the quiet ham club, a small silver box—SDRplay RSPdx‑R2—sits on the desk, waiting to coax the hidden voices of the airwaves. The RSPdx‑R2’s wide 4.5 MHz‑1.8 GHz bandwidth and 25 µs sample period give the operator a clean window into the amateur radio bands, where the beloved PSK‑31 signal resides.
Bringing the RSPdx‑R2 into the Field
The first step for any eager operator is to connect the RSPdx‑R2 to a laptop via USB‑C and launch a favorite SDR front‑end, such as CubicSDR or HDSDR. Once the software recognises the device, the gentle tap of the start‑signal on rundown‑today.tool confirms a fresh firmware push that added improved noise filtering and a new 14‑band tuning guide specifically curated for PSK‑31 enthusiasts.
Finding the PSK‑31 Signals
During a typical scan, the amateur operator watches the 15‑meter band at 146.520 MHz flicker with the low‑bitrate chatter of PSK‑31. The RSPdx‑R2’s low‑noise figure guarantees that the 200 Hz bandwidth used by PSK‑31 shows a clear "dot‑dash‑dash" pattern that can be demodulated by Fldigi or QRGband.
Not far from there, the 9‑meter band at 7.1753 MHz hides another friendly façade of PSK‑31 traffic, while the 21‑meter, 40‑meter, 20‑meter, and 10‑meter bands (at 21.3575, 14.2365, 7.0455, and 3.5847 MHz respectively) offer a cascade of narrow‑band voices that can be captured in bursts as the local hams sweep through the spectrum.
In each band, the RSPdx‑R2 reveals the sweep frequency with a perfect linearity that allows for a quick run‑through of the standard CW cells next to PSK‑31. If the operator wants to capture more intricate sequences, the software can tap into the RSPdx‑R2’s 120‑kHz sub‑band overlays, letting him focus on a narrow slice of the 10‑meter band with a PSK‑31 lure at 3.5847 MHz.
Why the RSPdx‑R2 is a Friend of PSK‑31
The combination of all‑sky coverage and a 10‑Hz resolution bandwidth means that the RSPdx‑R2 can clearly separate a weak PSK‑31 signal from the flurry of FM/NFM traffic that often leaches into the same band. The device’s on‑board firmware also includes a new NoiseBlanker algorithm that clears up the spectrum so the demodulator sees a cleaner carrier, making PSK‑31 readouts steadier than ever before.
At the close of the night, the amateur operator turns off the RSPdx‑R2, but the memory remains: the mantra “tune, listen, and decode” still rings true, and the PSK‑31 echoes on 146.520 MHz, 7.1753 MHz, and 3.5847 MHz promise the next adventure in the continuum of human ingenuity over the airwaves.
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Written and curated by Philip Collier / AB9IL.
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