Recently, we came across SimTX, an open-beta multiplayer shortwave radio simulator. Everyone shares the same simulated band, so whatever one user transmits from their virtual station can be picked up by anyone else, simulated propagation permitting. The website suggests running a pirate station, making your own numbers station, or just tuning around and listening.

For realism, the ionosphere is based on IRI-2020 climatology with live ionosonde data assimilated on top, groundwave and noise follow ITU-R models, and every link passes through a Watterson fading channel with Doppler, multipath and storms. Users can transmit from a mic, an audio file or raw IQ from GNU Radio in modes including AM, SSB, CW, NFM, WFM, WSPR and FT8, and can even launch and track their own satellites.

To receive, the SimTX app runs a local rtl_tcp bridge, so SDR++ or any other rtl_tcp-capable SDR program can tune the simulated band as if it were an RTL-SDR, without any real radio hardware. The app runs on Windows, macOS, and Linux, and accounts are created via Discord. The software does not appear to be open source. The demo video below shows a song broadcast from a simulated Amsterdam and received 1145 km away in Budapest.

This may be useful for playing around with and seeing what different TX/RX variables and propagation effects do to signal reception.