CF vs MUF: Understanding Critical and Maximum Usable Frequencies

This article explains the concepts of Critical Frequency (CF) and Maximum Usable Frequency (MUF), highlighting the difference between them. We’ll also cover the formulas used to calculate each.

The Ionosphere and Radio Wave Propagation

Radio waves travel between transmitters and receivers using antennas. Crucially, the Earth is surrounded by layers, including the troposphere, stratosphere, and ionosphere. The ionosphere, located approximately 90 to 250 km above the Earth, plays a vital role in radio wave propagation. Both CF and MUF are directly related to the properties of this layer.

CF | Critical Frequency Explained

Critical Frequency (CF) is a key parameter when considering radio wave transmission. It represents the highest frequency at which a radio wave, transmitted vertically, will be reflected back to Earth from the ionosphere. Frequencies higher than the critical frequency will penetrate the ionospheric layer instead of being reflected.

Critical Frequency Formula

The critical frequency is determined by the ionization density of the ionosphere. The formula is given as:

Equation-1: CF Formula

The critical frequency depends on atmospheric conditions and the angle of transmission from the antenna. This means the CF value can change throughout the day as the ionosphere’s properties vary. In the equation, N_max represents the maximum electron density (or ionization density) within the ionosphere.

MUF | Maximum Usable Frequency Explained

While critical frequency represents the highest frequency reflected at vertical incidence, it’s not the highest frequency that can be reflected. The highest reflectable frequency depends on both the angle of incidence and the distance between the transmitting and receiving antennas.

Maximum Usable Frequency Formula

Maximum Usable Frequency (MUF) is the maximum frequency that can be used for reliable communication between two points via ionospheric reflection, for a specific distance. As shown in the MUF formula, it depends on both the critical frequency and the angle of incidence. MUF is typically 3 to 4 times higher than the critical frequency.

Key Difference between CF and MUF

The primary difference is that CF refers to the vertical incidence reflection limit, while MUF considers the angle of incidence and distance between the transmitter and receiver. MUF is the highest frequency that will work for a given path, taking into account the curvature of the Earth and the angle at which the radio wave strikes the ionosphere.