A synthetic FlexRadio-6000 emulator for testing AetherSDR — a hardware-free spectrum / waterfall / S-meter / CW test "radio" you drive from your browser.

flex-sim looks like a real FlexRadio 6000 on your network: AetherSDR discovers it, connects, and renders a live panadapter, waterfall, S-meter, TX meters and CW from a programmable signal engine. No radio required.

Pure Python 3.8+ standard library — zero dependencies. GPL-3.0.

1. Download the binary for your computer from the Releases page:
   • Windows: flex-sim-windows-x64.exe
   • Linux: flex-sim-linux-x64
   • macOS: flex-sim-macos-arm64
2. Run it on a computer that is not running AetherSDR — a spare PC, a Raspberry Pi, a NUC, a VM… anything on the same network. (Why not the same computer? It's one simple rule — see Networking. You can run it on the same machine, it just needs a couple of extra steps.)
   • Windows: double-click flex-sim-windows-x64.exe. It isn't code-signed, so Windows SmartScreen says "unrecognized app" (More info → Run anyway) and some antivirus (Norton, Defender) may flag or quarantine it — see the false-positive note below.
   • Linux / macOS: chmod +x flex-sim-linux-x64 && ./flex-sim-linux-x64

"My antivirus flagged it!" That's an expected false positive, not malware. PyInstaller bundles the Python interpreter into one .exe, and that packing trips AV heuristics — it happens to most PyInstaller apps. flex-sim is pure-stdlib Python with zero dependencies and the entire source (~1000 lines) is right here in this repo, so you can read it or run it straight from Python and skip the binary (and the warning) entirely. To use the binary anyway, restore/allow it in your AV.

3. Open AetherSDR. It should list a radio — model FLEX-6600, serial FLEXSIM00. Select it and connect.
4. Open the control panel at the address flex-sim prints on startup — http://<that-computer-ip>:8731/ — and pick a test pattern. You should see a live waterfall and S-meter.

If AetherSDR doesn't find it, tell flex-sim where AetherSDR is so it can announce itself directly:

flex-sim-windows-x64.exe --ae 192.168.1.50      # <- the IP of the PC running AetherSDR

flex-sim is a pretend radio on your network, so it needs its own IP address, separate from the computer running AetherSDR. (They both use network port 4992 — on a single IP they'd collide.)

• Easiest — run it on a different computer on the same network (spare PC, Raspberry Pi, NUC, VM). It gets its own IP automatically and AetherSDR finds it just like a real radio. Nothing else to set up. Most people should do this.
• Want it on the same computer as AetherSDR? It still needs its own IP, which Windows won't hand a second program directly. Two ways:
  • WSL (recommended on Windows): run flex-sim inside Windows Subsystem for Linux — WSL gives it its own IP. See Same-machine setup below. (This is the proven path.)
  • --port: flex-sim --port 5992 --ae <AE-ip> keeps it on the same IP but moves it off AetherSDR's port 4992.

Adding --ae <AetherSDR-IP> is always worth it — it makes flex-sim announce itself straight to AetherSDR (helps when network broadcast doesn't reach, or across subnets).

python3 flex_sim.py --ae <AetherSDR-IP>

On Windows use python if that's how Python is installed. Handy flags: --pattern carrier · --ctl-port 8731 · --version · -h.

python3 flex_sim.py --radios 3 --models FLEX-6300,FLEX-6600,FLEX-6700 --ae <AE-IP>

Runs N virtual radios that AetherSDR sees as separate rigs — a hardware-free multi-radio bench. Each gets its own IP (the --ip base, then +1, +2 …), serial (FLEXSIM00…), and model from --models (cycled). Models differ in capacity — 6300/6400 = 2 slices / 1 SCU, 6600 = 4 / 2, 6700/8600 = 8 / 2 — so a mixed rack tests single- vs multi-MCU side by side, with up to the model's slice count of stacked receivers each. On one host the extra IPs must exist on the interface first (real rigs each have their own) — see Same-machine setup.

The web rack panel (http://<flex-sim-ip>:<ctl-port>/) shows every radio as a "1U strip" — power toggle, model selector, frequency/meter readouts, and a live pattern picker — so you can power-cycle or re-model any radio without restarting. The shot above is a 10-radio rack started with:

python3 flex_sim.py --radios 10 \
  --models FLEX-6700,FLEX-6600,FLEX-6300,FLEX-8600,FLEX-6700,FLEX-6600,FLEX-6400,FLEX-6300,FLEX-6600,FLEX-6700 \
  --pattern test_card --ctl-port 8740 --ae <AE-IP>

http://<flex-sim-ip>:8731/ — pick a pattern (the hint box says what it exercises in AetherSDR), set the noise floor / signal level in dBm (with S-units), signal width and noise colour; key TX (forward-power + SWR meters); send CW (normal / full break-in, driven from AetherSDR's own CWX keyer).

noise_floor · ramp · cal_tones · carrier · cw · swept_carrier · comb · step · impulse · staircase · noise · tx_blank. The panel's hint box explains what each one exercises.

python3 loopback_test.py carrier      # mock-AetherSDR: handshake + VITA + meter checks, no real AE

Running flex-sim on the same Windows PC as AetherSDR, via WSL2 (which gives it its own IP, so no port clash):

1. Put this in %USERPROFILE%\.wslconfig so WSL's :4992 doesn't relay onto Windows:

   [wsl2]
   localhostForwarding=false

2. In WSL: python3 flex_sim.py --ae <Windows-host-IP-as-seen-from-WSL> (usually the default gateway, e.g. 172.x.x.1).
3. AetherSDR (on Windows) discovers flex-sim at WSL's own IP (e.g. 172.x.x.x).

For rack mode on one host, add the extra IPs to the interface first (one per extra radio):

sudo ip addr add 172.17.189.199/20 dev eth0   # radio 2
sudo ip addr add 172.17.189.200/20 dev eth0   # radio 3

(These are cleared when WSL restarts — re-add them after a reboot.)

On Linux, a macvlan network gives the container its own LAN IP (clean — see docker-compose.yml). On Docker Desktop for Windows/Mac, containers aren't reachable at their own IP from the host, so Docker does not solve the same-machine case there — use WSL. Docker is for a separate Linux box.

The Releases binaries are built by GitHub Actions (.github/workflows/build.yml). To build locally:

pip install pyinstaller
pyinstaller --onefile --name flex-sim flex_sim.py    # -> dist/flex-sim(.exe)

GPL-3.0-or-later — see LICENSE (matches AetherSDR's license).

Created by Nigel Fenton (G0JKN) — design, direction, and testing against live AetherSDR. Code generated by Claude (Anthropic) via Claude Code under Nigel's direction — the same AI-assisted, human-reviewed workflow AetherSDR itself uses.

Status: v0.1 (beta). Wire format reverse-engineered from AetherSDR's own decoder — see PROTOCOL.md. Design notes in DESIGN.md.