MUF maximum useable frequency:
In radio communication, the maximum usable frequency (MUF) is the highest frequency that can be effectively used for transmission between two points, via ionospheric reflection, at a specific time. In shortwave radio, long-distance communication is made possible by bouncing radio waves off the ionosphere’s layers. This reflection allows radio signals to travel beyond the horizon and follow the curvature of the Earth. However, the ionosphere’s ability to reflect radio waves diminishes as the frequency increases, imposing an upper limit on usable frequencies. Beyond this limit, radio waves are no longer reflected but instead pass through the ionosphere into space.
To ensure a good radio link between two locations, the operating frequency is normally chosen below the predicted MUF. For example, if the MUF is 12 MHz, the best working frequency will be somewhere between 6 and 12 MHz. In this case, you’d probably choose somewhere around 10 MHz.
Critical frequency:
The critical frequency serves as an indicator as to the state of the ionosphere. It is determined by transmitting a signal vertically towards the ionosphere and observing whether it is reflected back to the transmitting site. By measuring the time taken for the signal to return, the height of the ionospheric layer can be estimated. As the signal frequency increases, a threshold is reached where the signal no longer reflects, but instead penetrates the layer, passing through to a higher layer or through the entire ionosphere. This threshold is known as the critical frequency.
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