Propagation physics

Critical Frequency: How High You Can Go Straight Up

Critical frequency is the highest frequency that an ionospheric layer reflects when a signal hits it straight up (vertical incidence). Each ionospheric layer has its own critical frequency: foE for the E layer, foF1 for the F1 layer, and foF2 for the F2 layer. The F2 layer’s critical frequency, foF2, is the one that matters most for HF radio because the F2 layer is responsible for most long-distance HF propagation.

Why it matters for HF operating

The critical frequency is the anchor for almost every HF propagation prediction. NVIS (Near Vertical Incidence Skywave) operations are gated by foF2 directly: NVIS works when foF2 is above your operating frequency. Maximum Usable Frequency (MUF) for a given path is computed from foF2 and the path geometry. When forecasters talk about the ionosphere being “soft” or “lifting,” they are usually describing foF2 movements.

Key values to know

  • foE (E layer). Typical daytime values 2 to 4 MHz; near zero at night.
  • foF1 (F1 layer). Typical daytime values 4 to 6 MHz; the F1 layer disappears at night.
  • foF2 (F2 layer). Quiet conditions 2 to 6 MHz, active solar conditions 8 to 14 MHz, persistent through the night.
  • Measurement. Critical frequencies are measured by ionosondes, ground-based instruments that sweep frequencies straight up and time the reflections.

Common misconceptions

The critical frequency is a vertical limit, not an oblique limit. A path between two points can carry frequencies well above the critical frequency because the geometry presents a shallower angle to the layer. This is why long-distance contacts work on bands that NVIS does not. Mixing the two up leads to assuming a band is dead when only the vertical-incidence path is closed.

  • foF2: the critical frequency that matters most for HF
  • F2 layer: the ionospheric layer foF2 describes
  • MUF: the oblique-incidence cousin of critical frequency
  • Ionosphere: the layered medium that produces critical frequencies
  • NVIS propagation: the mode that lives directly under foF2

SkyWave shows current foF2 on the Go screen and uses it to draw the NVIS sandwich, so you can see where critical frequency sits relative to each amateur band. See it in the app →