HF Propagation Calculator

HF signals travel long distances by refracting off the ionosphere's F2 layer, and the Maximum Usable Frequency (MUF) is the highest frequency that will actually bend back to Earth rather than punching through into space for a given path — go above it and your signal just disappears upward. This calculator builds an educational estimate of that critical frequency (foF2) from the Solar Flux Index you enter (higher solar activity ionizes the F2 layer more strongly), a time-of-day factor that peaks at 14:00 UTC in this model (ionization tracks the sun, with F2-layer ionization lagging into the afternoon rather than cresting right at local solar noon), and a seasonal multiplier (F2 ionization tends to run stronger in spring/fall and weaker in summer). It then applies the standard secant law, MUF = foF2 × sec(θ), where θ is the signal's angle of incidence on the ionosphere — computed geometrically from your target distance assuming a 300 km-high F2 layer — since a signal hitting the ionosphere at a shallow, oblique angle over a long path refracts back down at a higher frequency than one hitting it straight overhead.

From MUF it derives the Frequency of Optimum Traffic (FOT, conventionally 85% of MUF, a safety margin against day-to-day MUF variability) and a rough Lowest Usable Frequency (LUF, roughly MUF/3, below which D-layer absorption eats the signal), then recommends an amateur band whose frequency sits near the FOT. Hop count assumes a maximum single-hop distance geometrically limited by that same 300 km ionosphere height, with roughly 8 dB of signal loss estimated per hop. Treat every number here as a simplified daytime-model estimate for learning propagation concepts — real MUF prediction depends on full ionospheric sounding data, your specific ground track's geomagnetic latitude, and whether you're propagating via F2, E, or Es layers, none of which this simplified model captures.