What is maximum usable frequency-MUF,in context of sky wave propogation?how is it related to critical freq.?

Question

I want to know the basic idea about the topic in this question.

Answer 1

There are different ways of propagation of radio waves in the atmosphere, they are, ground waves, which are used by the vlf, and lf frequencies, that follow the curvature of the earth, then they are sky wave, which follow straight lines, and bounce back after a certain distance from the atmosphere, expecially from the troposphere.
The maximum usable frequency in any method of propagation refers to, the maximum value of the radio frequency that can be used for a particular method of propagation, without suffering the effects of attenuation, fading, interference etc.
In the context of sky wave propagation, the MUF is governed by the fact that too high a frequency, ( equivalent to 500 mhz or higher) will not be reflected back to the earth by the atmosphere, but will only bend slightly, and continue outward.
Since the reflection does not take place from a point, but is actually a gradual deflection, like refraction of light when it moves from a rarer to a denser medium. similarly the atmosphere has different layers, of varying densities, which causes this phenomena to happen.

Answer 2

The ionosphere (charged part of the atmosphere) can refract (bend) radio waves back to earth so if you send a signal out toward the horizon and not straight up, it'll bend back to the earth and arrive over the horizon. It's the main way to send signals in the shortwave (high frequency) bands of 3 to 30 MHz. Since the conditions of the ionosphere vary with time of day, time of year and point in the sunspot cycle, the highest frequency of waves that will bend back varies. Above that frequency, the signals just go out in to space. The highest frequency is called the MUF ... Here's a really good tutorial thanks to out tax dollars and the US Navy.

http://www.ips.gov.au/Category/Educational/Other%20Topics/Radio%20Communication/Intro%20to%20HF%20Radio.pdf

Answer 3

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