The longwave and mediumwave bands hold a legendary, almost nostalgic status for many electronics engineers, radio hobbyists, and collectors. However, while physical high-power transmitters are increasingly falling silent across Europe, the band is coming back to life on the laboratory workbench. The AMWaveSynth ecosystem represents a remarkable open-source project that goes far beyond simple modulators. It combines mathematical ITU wave propagation models, hardware acceleration on FPGAs, and network-based cockpit systems into a fully-fledged tactical simulator for radio direction finding and historical reception experiments.

1. The Synthesis Base: AMWaveSynth in Detail

The AMWaveSynth forms the foundation of the entire project. Designed as a pure software modulator, it generates a freely definable landscape of parallel, AM-modulated radio stations. At its core is an NCO (Numerically Controlled Oscillator) pipeline. The system receives the modulation signals as PCM audio samples via separate UDP ports (starting at port 1234 and up) and uses them to calculate a RF spectrum. The resulting signal is provided as a continuous data stream. This data stream can be transmitted directly into the actual ether using low-cost Software-Defined Radio (SDR) hardware, such as the famous VGA-chip-based osmo-fl2k SDRs.

2. The Group’s SDR Hardware Revolution

To transmit the calculated data stream into the real ether, early experiments often utilized repurposed VGA adapters (such as osmo-fl2k). However, the radiolab81 group has since gone a step further and developed dedicated, open-source SDR transmission solutions:

smiSDR (github.com/radiolab81/smisdr): Utilizes the Secondary Memory Interface (SMI) of the Raspberry Pi for high-performance, wideband RF transmissions.

parlioSDR (github.com/radiolab81/parlioSDR): An SDR concept based on the parallel interface (Parallel IO) of the ESP32-P4 microcontroller.

3. The Time Machine: WRTH and Historical Landscapes

One of the most fascinating application areas of the system is its use as a genuine "time machine." The AMWaveSynth setup makes it possible to bring historical editions of the World Radio TV Handbook (WRTH) to life. Instead of transmitting merely fictional test tones, users can import the exact frequency allocations, transmitter powers (kW), and geographical coordinates of entire countries or continents from all decades. When combined with corresponding contemporary audio recordings, a perfect, historically accurate RF replica of the European or American ether of that era is created within the laboratory.

4. The Physics Engine: ITU-Standard Fading

However, a static radio landscape would be sterile. The AMWaveSynthPropagationSimulator add-on acts as a mathematical control entity, simulating real-time atmospheric conditions between the historical transmitter coordinates and a freely selectable receiver (RX) position. The engine calculates the groundwave according to ITU-R P.368-10 (including frequency-dependent earth attenuation) and the skywave according to ITU-R P.1147-2 (depending on ionospheric reflection, season, and solar position). As soon as the sun sets in the simulation, the attenuation values change. The simulator sends commands to the spectrum generator for each transmitter. The result is astonishing: as twilight sets in within the simulation, the skywave amplitudes increase—on the connected tube radio or SDR receiver, you experience the exact same atmospheric fading known from real-world practice. For those who find the reception still too perfect, the AMWaveSynthPropagationSimulator can even simulate passing thunderstorm zones.

5. Virtual Road Trips with the MovableRXLocationPlayer

A special extension of the simulator is the MovableRXLocationPlayer plugin. It allows the ingestion of standardized GPS tracks. The plugin simulates the movement of the receiver along this route at an adjustable speed. As the virtual vehicle crosses international borders or approaches and moves away from transmitter masts, the propagation engine recalculates the signal levels in the background. With the actual test radio sitting on the workbench and a glance at the digital map, you can hear the typical shifting and fading of the radio stations as if you were actually taking a road trip along the legendary Route 66 or across Europe.

6. The Tactical Level: Direction-Finding Training

The latest evolutionary stage transforms the project into a multi-user-capable training environment for Radio Direction Finding (RDF). 

The student launches two applications: DFscope serves as a graphical "glass cockpit" console. It loads the antenna characteristics specified by the instructor (e.g., a ferrite loop antenna from an .MSI or .ANT file) and visualizes them as a rotatable polar plot. When the student turns the digital direction-finding handwheel, the app reports the orientation back to the simulator. The simulator then attenuates the SDR's RF signal in real time. The student searches for the signal minimum (null steering) on the physical radio receiver—exactly like traditional direction finding in the field.

7. The FPGA Proof of Concept: The Ultimate Integration

Although the PC-based software implementation is heavily optimized, the sister project FPGA_AMWaveSynth represents the ultimate technical showcase. The goal here is not to overcome CPU limitations, but rather the fascination of casting the entire DSP pipeline autonomously into hardware. The architecture, written in Verilog, intelligently splits the load: a central sine ROM serves parallel multi-channel NCOs. An ESP32 microcontroller acts as a bridge between the PC control interface and the FPGA's high-speed register bus. This setup impressively demonstrates how powerful and extremely low-latency modern hardware synthesis can operate within a minimal footprint.


The AMWaveSynth ecosystem is a prime example of modern SDR development. It bridges the gap between pure software simulation, historical preservation, and real-world hardware practice. For universities, amateur radio clubs, and makers, it provides a playground to bring radio physics and historical radio landscapes to life

https://github.com/radiolab81/AMWaveSynth
https://github.com/radiolab81/AMWaveSynthPropagationSimulator