IONOS HF/VHF/UHF Channel Simulator

The Amateur Radio Safety Foundation introduces the IONOS Channel Simulator, an audio-processing, 1-4 path (ray) simulator that models common HF and VHF/UHF propagation channels. It is intended to accelerate modem and protocol design, analysis, optimization and comparison, but has other uses as well. It allows off-air laboratory testing and evaluation of radio protocols and modems using statistically-standardized channel characteristics that would be almost impossible to achieve with over-the-air RF testing. Automation features allow scripting for multiple test runs under automatic control for huge time and labor savings. Since no transceivers are needed, the device may also be used as an inexpensive operator training station for exercising and teaching digital mode software. The simulator is based on the well-documented Watterson model [1] used by many laboratory grade instruments costing many thousands of dollars.

The simulator also performs an experimental BUSY DETECT function for research purposes. It provides a testbed for a much improved computer-controlled busy channel detection capability for fully automated stations. In this role, the device performs accurate and rapid spectrum analysis over a specified frequency band of interest. Optimization of the speed and accuracy of the busy detector algorithms continues. This function is not intended for manual user control so is totally controlled by serial commands sent between the device and a host computer. Appropriate computer software is required to participate with the Winlink Team in Busy Detect research.


The IONOS Channel Simulator is currently in a limited Beta release for testing and feedback from interested developers and hams. If you have an interest in joining our testing program, or simply want to know more about this low-cost instrument, please inquire by email to Rick Muething, KN6KB.

Technical Specifications

Firmware Rev 1.8, May 15, 2020

  • Description: IONOS SIM is a self-contained audio DSP based ionospheric simulator based on the Watterson Model [1]. It uses the Teensy 4.0 CPU (ARM Cortex-M7 processor at 600 MHz) and Teensy Audio DSP Library. The MIT open source licensed software (see appendix) was developed on the Arduino platform.
  • Primary Use: The development, characterization, maintenance and comparison of HF and VHF/UHF modems and protocols including “sound card” modems and protocols.
  • Channels Modeled: WGN (White Gaussian Noise: Spread 0 Hz, Delay:0ms);, MPG (CCIR MultiPath Good) Spread .1 Hz, Delay .5ms ; MPM (CCIR MultiPath Moderate) Spread .5Hz, Delay 1ms; , MPP (CCIR MultiPath Poor) Spread 1 Hz, Delay 2ms; MPD (MultiPath Disturbed) Spread 2.5 Hz, Delay 5ms; Flat Fading 0-40 dB Fade, (Rate .1 to 100 Hz); Static Freq offset +/- 200 Hz; Slow FM deviation +/-.1 to 100 Hz, FM Rate .1 to 100 Hz.
  • Multipath: (2,3 or 4 channels) Delay update rate: 32x spread above. (CCIR/ITU recommendation)
  • Audio Paths: WGN: 1 path. Multipath: selectable 2,3 or 4 channels (rays). Input to Output Delay: ~4-16 ms.
  • Connections Modeled: 1 Simulator: Half duplex, both sides, symmetric channel models.
  • Two Simulators: Full Duplex both sides: symmetric or asymmetric channel models.
  • Bandwidths: 3KHzBW:40Hz - 3.3 KHz, 6KHzBW:40Hz - 6.3 KHz. Ripple Noise modeling: WGN filtered to 3 KHz or 6 KHz bandwidth. S:N -40 to +40 dB, 1 dB steps.
  • Crest Factor Measurement: Input signal Crest Factor is computed and displayed in WGN Mode.
  • Inputs: Ch1, Ch2 via standard Stereo 1/8” (3.5 mm) jack. AC coupled. Input protection. Impedance > 50K Ohms. Input gain control .1 to 100 adjusts internal level to 300 – 1700 mv p-p.
  • Outputs: Ch1, Ch2 via standard Stereo 1/8” (3.5 mm) jack. AC Coupled. Short circuit protection. Impedance 150 Ohms. Output level control sets output level 100 to 2000 mv p-p.
  • Power Required: Nominal 5 v (internally reg. to 3.3 V) via micro USB connector. ~ 200 ma.
  • Display: 320 x 240 pixel color TFT for Text and Graphics. 2.4”x 1.8” (60mm x 45mm)
  • Operating and Display Modes: Self-test and calibration verification; Set modes and parameters; Spectrum Display (3 & 6 KHz), ~60 dB dynamic range; Multipath dynamic IQ (2,3, or 4 channel) delay plot.
  • User Parameter and control: Two Incremental encoders (Mode, Parameter) each with push toggle functions. Parameter default values can be saved to EEPROM. USB serial ASCII commands for automated operation.
  • Size: 6.1” (155 mm) L x 4.7” (120 mm) W x 1.4” (36 mm) H (excluding knobs).
  • Built-In Test: Built in Test (TEST3K, TEST6K) Tests and confirms Input to output continuity, Functions and Calibration accuracy +/- .5 dB in WGN mode across 3000 Hz or 6000 Hz spectrum.
  • Alternate Functions: Integrated Busy Channel Detector (experimental). Uses only serial USB ASCII command interface with host program.
  • Firmware License: MIT Open Source Initiative.

[1] Watterson, C.C., J.R. Juroshek, & W.D. Bensema. 1970 Experimental confirmation of an HF channel model IEEE Transaction of Communication. Technology. Vol COM-18. Pp 792-803 Dec 1970

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