Written by Steve Stollro N8GNJ
(Originally published in The Communicator, November-December 2025 issue)
In a sense, we have been living in the era of “Software-Defined Amateur Radio” for about a decade. Many of the commonly used modes today—such as FT8—are “almost entirely implemented in software.” What truly makes FT8 usable is the software running on the host. However, FT8 is more like a “modulation scheme” that must rely on traditional HF radios to transmit in single sideband (SSB) mode.
However, in the era of Software-Defined Radio (SDR), even the act of “SSB transmission” itself is controlled by software. Currently, we are only “layering” digital modes like FT8 (data) or FreeDV (digital voice) on top of SSB because SSB technology is mature and cost-effective—after all, we have been using it for nearly a century. But as of 2025, we no longer need to treat SSB as a “last resort” because SDR allows us to fully customize transmission waveforms—if you can imagine it, you can achieve it.
Old-fashioned SSB is no longer “outdated”; Polar Modulation is a brand new, more efficient SSB generation technology that has been applied in QRP Labs’ QMX/QMX+ and FlexRadio’s latest models.
The new regulations from the Federal Communications Commission (FCC) recently abolished outdated regulations regarding data modes in the HF band, retaining only a simple principle: “Maximum bandwidth of 2.8 kHz.” Therefore, as long as the mode we design meets the following minimum requirements, we can use it freely:
– Located within the amateur radio designated frequency bands
– Occupying bandwidth ≤ 2.8 kHz
– Digital modulation method “publicly disclosed”
– Using “minimum necessary power” to ensure reliable communication.
Example 1: Orthogonal Frequency Division Multiplexing (OFDM)
Orthogonal Frequency Division Multiplexing (OFDM) breaks the transmitted signal into many mutually orthogonal subcarriers, each of which can independently select modulation, power, forward error correction (FEC), and other parameters. If you have ever heard OFDM in HF amateur radio, it is because the following modes use OFDM at the audio layer:
– VARA HF
– FreeDV
– Fldigi OFDM
With SDR, we can implement OFDM directly at the RF layer, no longer treating it as “audio modulation” squeezed into SSB. The computational load of OFDM is enormous, and it can only be easily achieved today with the prevalence of high-speed computing. Twenty years ago, to run OFDM in real-time, each device would need to be equipped with a DEC VAX minicomputer!
Example 2: Frequency Hopping Spread Spectrum (FHSS)
Frequency Hopping Spread Spectrum (FHSS) is not a new concept; in 1942, Hedy Lamarr and George Antheil patented it (US Patent No. 2292387). In the 2-meter band of amateur radio, we can fully implement FHSS with extremely short “dwell times” (such as 20 milliseconds) to “find gaps” in the crowded VHF segment.
Example 3: Opulent Voice (OV)
OV is sponsored by the Open Research Institute and aims to achieve high-quality digital voice using the higher bandwidth of V/UHF. OV employs interleaved data frames, allowing users to seamlessly switch between voice and data in a single transmission, even switching packet by packet. OV is fully implemented in SDR—just flash the image onto a low-cost Analog Devices ADALM-PLUTO without any traditional “modem” hardware.
Hardware Example: open.space Digital Phased Array
At the Pacificon exhibition in October 2025, the open.space project showcased a 4×4 MIMO SDR “tile”:
– Frequency: 4.9–6.0 GHz (C band), full duplex
– Bandwidth per channel: 40 MHz, 8+8 bit I/Q
– Power per channel: 1 W
– Receiver noise figure: approximately 1.2 dB
– Using Lattice ECP5 FPGA, latency < 1 ms
This module can be used independently as an experimental SDR or combined into a large phased array for:
– Fox-hunting and Direction of Arrival (DOA) measurements
– Open-source Wi-Fi or 4G/5G small base stations
– High-definition image transmission for drones and robot communication
Even more exciting, this is a completely open-source project. Imagine using a 4×4 phased array on the rooftop of a high-rise building to achieve a 5GHz high-speed data link or directing the beam towards GEO amateur satellites for hemispherical coverage!
Conclusion: Software-defined, infinite possibilities. From vacuum tube television repair classes (1970s) to the current era of “writing a new radio at your fingertips,” we have witnessed the leap in computing power from “minicomputers” to “pocket SDRs.” At the GNU Radio conference in September 2025, more than half of the attendees were licensed amateur radio operators; there were also 7 new licensees from the on-site exam. This indicates that SDR professionals are not only willing to experiment in the amateur bands but also have the capability to take immediate action.
In the coming year, we will undoubtedly see more V/UHF software-defined transceivers, more innovative modes, and more applications that were “previously impossible.” Amateur radio is no longer an “old hobby” but a playground for infinite exploration. Are you ready to pick up GNU Radio Companion and write your own waveform?