Ionosondes and foF2

An ionosonde is a vertical radar that transmits radio pulses upward and measures the echoes reflected by the ionosphere. By varying the frequency, it determines at what height and intensity different frequencies are reflected.

The result is an ionogram: a graph showing the virtual reflection height as a function of frequency. From this, fundamental parameters like foF2, hmF2, and MUF are extracted.

There are about 100 active ionosondes worldwide, operated by organizations like GIRO, NOAA, and SWS Australia.

The foF2 (F2 layer critical frequency) is the most important parameter. It represents the maximum frequency that is reflected when the signal hits the ionosphere vertically.

• If you transmit at frequency < foF2: the signal is reflected back to Earth
• If you transmit at frequency > foF2: the signal passes through the ionosphere and is lost in space

For oblique (DX) contacts, the MUF is about 3 times the foF2. So if foF2 = 7 MHz, you can use frequencies up to ~21 MHz for long-distance contacts.

Besides foF2, ionosondes measure:

• hmF2: height of the F2 layer maximum (typically 250-400 km). Affects skip distance.
• MUF(3000): Maximum Usable Frequency for a 3000 km path, calculated from foF2 and hmF2.
• foE: E layer critical frequency (lower). Useful for NVIS and regional propagation.
• foF1: F1 layer critical frequency, present only during daytime.

How to use ionosonde data for your contacts:

• Check nearby ionosondes: data from the nearest ionosonde indicates local conditions
• Estimate the MUF: multiply foF2 x 3 to get an idea of the maximum frequency for DX
• Monitor trends: if foF2 is rising, higher bands are opening
• Compare multiple stations: to understand conditions along your contact path
• Consider the time: foF2 is highest around local noon and lowest at night

Remember: this is real measured data, not theoretical models!