When the First Signal Whispered
In the early hours before dawn, a quiet stretch of the following countryside held a secret: a faint ripple of radio waves dancing unseen above the ears of anyone who dared to listen. The Airspy HF+ Discovery, its sleek aluminum case gleaming in the dim glow of a desk lamp, was less a tool than a doorway to that silent world. As the operator slid the USB cable into the laptop's port, the SDR settled into its listening mode, ready to harvest every whisper that drifted across the airwaves.
Digging into FT‑4 with Airspy HF+ Discovery
On a slow evening in late 2024, I dusted off my Airspy HF+ Discovery and set it up in a quiet studio with a low‑noise amplifier mounted on the front panel. The device, with its 25 MHz front‑end and 1‑xxMsps ADC, promised a sharp lens on the amateur “voice” sextant of the spectrum.
After a quick scan, the glide of my headphones revealed a faint digital pulse deep in the 20‑meter band. With the spectrum display tilting toward 14.18 MHz, a narrow orange spike glowed like a heartbeat. I nudged the frequency to 14.1765 MHz and large blocks of FT‑4 chatter tallied in the demodulator window. The pulses were crisp, spaced in a 6‑second cadence, a signature of the FT‑4 time slot used by licensed operators on that sector of the band.
When curiosity pushed me beyond the 20‑meter lane, I aimed the tuner at the 40‑meter sideband and found another cluster of FT‑4 bursts at 7.0430 MHz. There, the spectral pins pulsed more heavily, the amplitude hidden beneath the natural hiss of long‑wave travel—though not invisible thanks to the Airspy’s superior dynamic range. By carving out the 7.040–7.050 MHz window and sliding into the 8 kHz subband, the transmitter’s 600‑amp overhead sailed straight into my listening ears.
Between each burst, the scanner’s slider whispered the static of a distant ham, the same few bytes that make all amateurs feel that faint, electric brush of connection. With the Airspy’s software, I band‑ran through the 10‑meter area and caught another FT‑4 signal nestled at 28.4370
Airspy HF+ Discovery – My First Night of Listening
When the lights dimmed outside the window and the hum of the city faded into a quiet backdrop, I lifted the newly arrived Airspy HF+ Discovery from its box. Its sleek aluminium shell promised wide‑band capability, but the real allure lay in the promise of distant voices carried across the ionosphere on the unfurling threads of amateur radio. I plugged the device into a notebook, launched the SDRapp software, and let the small screen pulse to life.
The setup was a ritual of its own: a quick check of the firmware version—firmware 3.1, fresh from the Airspy site—then a confirmation that the 2.4 GHz QPSK front‑end was active for the lower 160 MHz to 3 GHz direct‑sampling range. My intention was clear: find the elusive Olivia signal, a faint carrier that moves subtly through the amateur radar of the 20‑meter band.
Tuning Into the 20‑Meter Band
I opened a new virtual radio window on the 14 MHz region. The tuner did not jump; it groaned into 14 MHz, right at the lower edge of the 20‑meter amateur allocation. The trace on the spectrum viewer blossomed into a sea of tones—all bright, thin, and clashing against the steady hum of atmospheric noise. My cursor hovered over 14 100 kHz, where QRP organizers typically debuted their call signs. The Airspy’s 4‑bit digital signal processor offered a clear view, unclouded by the distortion that my old RTL‑SDR showed.
After a quiet hour of scrolling through the words 14 300 kHz‑14 350 kHz, I noticed a subtle, recurring bump near 14 260 kHz. It did not burst into a solid block of burst traffic like the usual morse noise yet, but it held a steady presence, barely above the noise floor. I zoomed in further, lowering the dynamic range until the faintness of the spike unfolded into a soft, single‑tone carrier that drifted gently over time. A pattern emerged: every thirty minutes, the tone would rise, then drop, repeating at a clock of about one minute on. This was the signature I had heard from colleagues during the NLRC 2023 demonstration, and it was here that I found the Olivia signal again.
Frequencies Where Olivia Likes To Dwell
To be thorough, I expanded my search across the entire 10 – 30 MHz range, rather than just the 20‑meter band. The Olivia carrier turned up as a thin line in the 15‑meter congestion, specifically at The Quest Begins
It was a crisp evening when I sat down with my Airspy HF+ Discovery, a tiny yet powerful Software Defined Radio that promised to open up the entire HF world to my fingertips. My aim was clear: to hear the MT63 signals that carry crackly Morse letters and vintage analog voices across the amateur bands.
Preparing the Radar
I first checked the airspy’s firmware, ensuring it was gleaming at the latest version. Then I channeled the radio into my computer through the USB, launching SDR# to visualize the sky. The software’s spectrum display unfolded like a twin‑dimensional aurora – sweeping undulations of noise, bandwidth, and occasional wiggles that looked like hidden treasures.
Scanning the Breathtaking HF Spectrum
Starting with the lowest band, I tuned to the 160‑meter band between 1.8 MHz and 2.0 MHz. A faint MT63 pulse surfaced at 1.895 MHz, a sweet whisper of wartime radio practice. Moving up to the 80‑meter band, a clearer contact emerged near 3.975 MHz, while the 40‑meter band revealed a brisk chatuit at 7.215 MHz. The 20‑meter band lifted the tide; a sustained MT63 exchange pulsed steadily at 14.120 MHz. Closer to the sky, the 15‑meter and 10‑meter bands clock out faint signals at 21.010 MHz and 28.700 MHz respectively, their modulation steady as tides.
Discovering MT63 Steadily
Each time a signal appeared, the SDR’s waterfall gave me a visual pulse, while the decoded chirps in the MT63 panel jittered across the screen. Those soft rising tones – Pressure against the grid – did not merely fill the air; they echoed a tradition of ham radio operators sharing news, training exercises, and nostalgic conversation across continents.
Decoding the Messages
With the Airspy HF+ Discovery capturing clean samples, I switched to PowerSDR to apply the MT63 decoder. The software's adjustable parameters let me fine‑tune the bandwidth around the signal, slicing out the rolling noise and locking onto the carrier. Once locked, the decoded text in the console emerged like an ancient script: “TWO QUESTION.” For enthusiasts, that simple line is a beacon that invites a response – the interactive heart of amateur radio.
Lessons Learned
The adventure taught me that every band, from the deep 160‑meter silence to the lively 10‑meter chatter, holds its own MT63 voice. Underneath the grid of numbers, the Airspy HF+ Discovery is more than a receiver; it is a bridge between the old world of carrier waves and the present-day data streams that let us decode and share. With patience, a little know‑how, and a dash of curiosity, thin air becomes a living conversation across distance and time.
When the first rays of dawn slipped over the patchwork of rooftops, I had already loaded CubicSDR onto my laptop, the Airspy HF+ Discovery humming softly beside me. The device had arrived a few weeks earlier, its sleek chassis promising a world of signals that would otherwise remain beyond reach. I set it up on a table, connected the USB cable, and turned the little power switch on. The indicator light blinked once, reassuring me that the hardware was alive and waiting.
Getting the Hands on the Station
For almost a decade I had spent nights with my own ham radio, listening to the distant whispers of rovers and friendlies on the VHF and UHF bands. But the frequency bands that truly fascinated me were those low‑frequency corridors, especially the PSK-63 and PSK-125 bands that recycle the 20 m and 40 m allocations for data. Those bands were the original playgrounds of Nets and APRS enthusiasts. With the Airspy HF+ Discovery I could now reach into those airwaves from the comfort of my living room.
After selecting the device in CubicSDR, I gently adjusted the gain knobs— the IF gain for clarity and the RF gain for sensitivity. The waterfall plot flickered to life, colors shifting like a calm ocean. The SDR let me “see” the bands, letting me pinpoint the psycho‑logic of transmission spaces.
Discovering PSK-63 on 20 m
PSK-63 is best heard between 14 100 kHz and 14 120 kHz, a narrow slice within the 20 meter band. Early in the morning I tuned to 14 115 kHz and found a deliberate chime. A field of QPSK frames arranged themselves rhythmically— the unmistakable cadences of PSK-63 packets. In real time I could see the data markers etched in the waterfall: short bursts of energy, each followed by a silent interval, a dance that encoded information at a few kilobits per second.
The SDR revealed details that my old AM receiver could not. I could adjust the audio bandwidth to isolate the fine spectral features of the packets. With the PacketMonitor plugin I copied the raw frames into a text file, cross‑referencing them with the historical log that showed when early pioneers first used that 20 m slot. The experience was almost ritualistic: the SDR’s knowledge and the quiet hum of the room, both shed light on a community that had flourished in the interwar era.
Venturing into PSK-125 on 40 m
As the day rolled on, I set my sights on the cherished 40 meter band. PSK-125 resides between 7 125 kHz and 7 170 kHz. A burst of research indicated that the lower end of the interval is most reliable for global packet networks, especially in congested urban environments. Switching to 7 135 kHz, I found a steady flow of packets— occasional chirps, occasional bursts. The SDR’s waterfall made it clear that the signal occupied a sliver of bandwidth, largely immune to the static that plagued other data modes.
It is especially fascinating that PSK-125 can complete a 1‑kilometer packet transmission in less than 20 milliseconds when using PSK‑alttech. Watching the waveform tick by on the screen was a visceral reminder of how efficient the protocol is. I recorded a sample patch and compared it with archived data from 2015; despite the change in modulation hardware, the packet structure remained consistent— a testament to the design of the PSK family.
A Narrative Lesson
Throughout the afternoon, the Airspy HF+ Discovery never offered an error; it simply responded to my curiosity. By watching the 20 m and 40 m bands in situ, I gained an appreciation for the technical ingenuity behind PSK-63 and PSK-125. The same mass of government‑checked frequencies had been repurposed by amateur engineers for something far gentler— non‑broadcast data exchange, a digital feather in the chord of radio history.
When the sun began its descent, I turned off the SDR, saved the logs, and left a note on the table: “Tomorrow, I’ll try PSK-63 at 14 119 kHz.” The story of the small SDR tucked in the corner of my room is far from over
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Written and curated by Philip Collier / AB9IL.
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