Maximum usable frequency

In radio transmission maximum usable frequency (MUF) is the highest radio frequency that can be used for transmission between two points via reflection from the ionosphere (skywave or "skip" propagation) at a specified time, independent of transmitter power. This index is especially useful in regard to shortwave transmissions.

In shortwave radio communication, a major mode of long distance propagation is for the radio waves to reflect off the ionized layers of the atmosphere and return diagonally back to Earth. In this way radio waves can travel beyond the horizon, around the curve of the Earth. However the refractive index of the ionosphere decreases with increasing frequency, so there is an upper limit to the frequency which can be used. Above this frequency the radio waves are not reflected by the ionosphere but are transmitted through it into space.

The ionization of the atmosphere varies with time of day and season as well as with solar conditions, so the upper frequency limit for skywave communication varies on an hourly basis. MUF is a median frequency, defined as the highest frequency at which skywave communication is possible 50% of the days in a month, as opposed to the lowest usable high frequency (LUF) which is the frequency at which communication is possible 90% of the days, and the Frequency of optimum transmission (FOT).

Typically the MUF is a predicted number. Given the maximum observed frequency (MOF) for a mode on each day of the month at a given hour, the MUF is the highest frequency for which an ionospheric communications path is predicted on 50% of the days of the month.

On a given day, communications may or may not succeed at the MUF. Commonly, the optimal operating frequency for a given path is estimated at 80 to 90% of the MUF. As a rule of thumb the MUF is approximately 3 times the critical frequency.[1]

[2]

where the critical frequency is the highest frequency reflected for a signal propagating directly upward and Θ is the angle of incidence.[3]

Optimum Working Frequency

Another important parameter used in skywave propagation is the optimum working frequency (OWF), which estimates the maximum frequency that must be used for a given critical frequency and incident angle. It is the frequency chosen to avoid the irregularities of the atmosphere.

gollark: Why is my documents folder 25GB?
gollark: Apparently my `.config` folder is 1GB.
gollark: ```Filesystem 1K-blocks Used Available Use% Mounted ondev 4049896 0 4049896 0% /devrun 4093740 1156 4092584 1% /run/dev/mapper/arch-root 62914560 36713744 25937136 59% /tmpfs 4093740 32672 4061068 1% /dev/shmtmpfs 4093740 0 4093740 0% /sys/fs/cgrouptmpfs 4093740 8236 4085504 1% /tmp/dev/sda1 1046512 72504 974008 7% /boot/dev/mapper/arch-home 163371072 142836388 20534684 88% /hometmpfs 818748 32 818716 1% /run/user/1000```
gollark: Well, my server holds other stuff, and it has 1TB.
gollark: I had to clean out my home folder a while ago.

See also

Sources

 This article incorporates public domain material from the General Services Administration document: "Federal Standard 1037C". (in support of MIL-STD-188)

  1. Practical antenna handbook, 5th edition. ISBN 978-0-07-163958-3
  2. http://www.tutorvista.com/content/physics/physics-iv/communication-systems/sky-wave-propagation.php
  3. Poole, Ian. "Lowest & Maximum Usable Frequency, Critical Frequency". Electronics Notes. Retrieved 23 April 2014.
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