Maximum Usable Frequency

The frequency of the blue signal is greater than the MUF and passes into space. The frequency of the green signal is less than that of the MUF, but greater than the MUF of the E-layer, and is in the range of the FOT. The frequency of the orange signal is less than the MUF of the E-layer and greater than that of the LUF. The frequency of the red signal is lower than the LUF and is absorbed in the D-layer.

The Maximum Usable Frequency (MUF) is the highest frequency that can be used for a shortwave radio link between two locations . Only the jump distance between transmitter and receiver determines a specific frequency. For example, the MUF for a 3000 km jump is referred to as MUF (3000).

description

The MUF is the highest frequency at which reflection from the ionosphere is possible. It enables a reliable connection in at least 50% of the reception time. However, this is not enough to operate a reliable radio link. A Frequency of Optimum Transmission (FOT) was therefore defined that is 15% lower than the MUF. The FOT guarantees a reliable radio connection 90% of the time.

The frequency usage window for radio waves is between the LUF and MUF. If the window closes, a so-called short wave fade (out) occurs.

The MUF changes depending on the time of day and time of year. Furthermore, solar activity and the sunspot cycle have a strong influence on the MUF.

The counterpart to the MUF is the LUF, the Lowest Usable Frequency , which represents the lower limit frequency for a reliable radio link. The frequency window that can be used for a radio link results from both. This can be closed if the LUF is higher than the MUF, for example after an X-ray outbreak from the sun (X-ray event). Then the increase in attenuation in the D-layer of the ionosphere resulting from the eruption causes the LUF to rise so strongly that all rays that are still able to penetrate the D-layer can no longer be reflected by the F-layer above , because their frequency is too high the MUF lies. This is known as a short-wave fade (out) .

The MUF can be determined approximately from the critical frequency of the ionosphere as follows:

${\displaystyle f_{\mathrm {muf} }\approx {\frac {f_{\mathrm {critical} }}{\sin \alpha }}={\frac {f_{\mathrm {critical} }}{\cos \phi }}=f_{\mathrm {critical} }\cdot \sec \phi =f_{\mathrm {critical} }\cdot {\sqrt {1+\left({\frac {d}{2\cdot h_{\mathrm {virt} }}}\right)^{2}}}}$

with = beam angle of the shaft relative to the horizon, = incidence angle relative to the perpendicular, = distance between the transmitting and the receiving, = virtual height of reflection. ${\displaystyle \alpha }$ ${\displaystyle \phi }$ ${\displaystyle d}$ ${\displaystyle h_{\mathrm {virt} }}$

Virtual and real level of reflection

The relationship between virtual and real reflection heights in the ionosphere is as follows:

Virtual height: ${\displaystyle h_{\mathrm {virt} }={\overline {PE}}}$ Real height: ${\displaystyle h_{\mathrm {real} }={\overline {PB}}=h+z_{0}}$ Jump distance: ${\displaystyle d={\overline {TR}}}$

Relationship between virtual and real reflection level

Framework values: ${\displaystyle \phi \leq \phi _{\mathrm {max} }\approx 74^{\circ }}$ ${\displaystyle h_{F_{2}}\approx 300\,\mathrm {km} }$ ${\displaystyle d\leq 4000\,\mathrm {km} }$

See also

• Lowest Usable Frequency (LUF)
• Reflection on the ionosphere
• Radio weather

literature

• J. Vastenhoud: Kurzwave Reception Practice . Hüthig, Heidelberg ISBN 3-7785-0816-4 .

Web links

• MUF, LUF, and FOT - The Basics of the Maximum Usable Frequency . HFRadio.org
• spaceweather.com

Individual evidence

1. ^ MUF, LUF, and FOT - The Basics of the Maximum Usable Frequency . HFRadio.org
2. ↑ Eckart Moltrecht: amateur radio course for the amateur radio license class E . ( Memento from June 21, 2008 in the Internet Archive )
3. ^ Tom Beer: The Aerospace Environment . P. 80