Path loss

In physics, the path loss L describes the loss of electromagnetic power P between a transmitter and a receiver. A low path loss usually indicates a good reception situation.

The path loss includes all losses on the way from the transmitter to the receiver, such as free space attenuation , absorption losses when penetrating media ( atmosphere ), losses due to diffraction and shadowing as well as losses due to different propagation paths due to diffraction at obstacles within the Fresnel zone .

In the case of radiation coupling, a sufficiently large path loss on the coupling path between the interference source (transmitter) and the interference sink (receiver) is a characteristic of electromagnetic compatibility with respect to undesired signals . Increasing the path loss, for example by increasing the spatial distance between the sink and the source, is then an interference suppression measure.

Propagation model calculations

There are basically two different ways of determining the path loss for a radio transmission channel. The first is the deterministic method , in which the propagation conditions are physically analyzed and then their influence is calculated. However, this is only possible to a limited extent and will only lead to the goal if the propagation is as trouble-free as possible. In practice, this is usually only useful for connections between the earth station and satellites as well as for radio connections on the earth's surface with an extremely short distance (for example for a radio mouse ).

Another method uses stochastic and statistical means. These calculations are carried out with computer programs that are based, for example, on a terrain profile and special conditions in the area. Most of these methods are based on extensive investigations into the propagation conditions. Several wave propagation models are known:

the wave propagation model by Lee,
the Okumura Hata model,
the COST-Walfisch-Ikegami model,
the COST-231-HATA model.

As a result, proportionalities between the path loss and the distance are output. These can be between to . Some models add a so-called environmental correction factor between −2 and −20 dB. ${\ displaystyle F \ propto r ^ {1 {,} 6}}$ ${\ displaystyle F \ propto r ^ {5}}$

Components of path loss

There are many causes for the attenuations for electromagnetic waves combined in a path loss:

Free space attenuation: As the energy spreads over an ever larger area as it propagates, the power density on a given area is reduced. The free space attenuation is determined by using isotropic antennas. The size of an isotropic antenna is not the point, but an area in the order of magnitude of the wavelength used. As a result, the free space attenuation becomes frequency-dependent.

Absorption losses: Absorption losses occur when the electromagnetic waves pass through a medium that is not completely transparent to electromagnetic waves. In the earth's atmosphere, these absorption losses occur, for example, on raindrops, fog or clouds, as well as on individual molecules such as oxygen.

Diffraction losses: If there is an obstacle in the propagation path, the signal will bend around this obstacle. Even after a short distance, there is no longer a shadow behind this obstacle. However, additional losses occur in diffraction.

Losses due to multipath propagation: In a real environment, the electromagnetic waves are reflected on various objects. You can reach the recipient on different distances. This results in phase differences that weaken the signal through interference ( multipath effect ).

Influence of the earth's surface: It is not just shadows from elevations that prevent it from spreading. The conductivity of the earth's surface also has a significant influence. The electromagnetic waves spread best over the surface of water or wetlands. Dry sandy soil causes greater damping.

Obstacles such as buildings or vegetation: The electromagnetic waves are not only reflected by these obstacles, but also absorbed by them. Damp foliage is particularly effective at dampening the spread. Multiple reflections can occur within walls, which then also leads to interference.

the atmosphere: Reflections on layers of the ionosphere can also have a positive effect on the range of the propagation of electromagnetic waves. Interference often occurs here too, which can lead to the signal being extinguished ( fading ).

Free space expansion

The path loss in decibels for propagation in free space without interference is:

{\ displaystyle {\ frac {L _ {\ mathrm {F}}} {\ mathrm {dB}}} = 20 \ log _ {10} (f \, / \, \ mathrm {Hz}) +20 \ log _ {10} (d \, / \, \ mathrm {m}) +20 \ log _ {10} {\ frac {4 \ pi} {c}}}

f : frequency
d : distance between transmitter and receiver
c : speed of light

The propagation of free space occurs, for example, in the radio connection between two satellites that have a clear visual view of one another. Only in this special case is the path loss exclusively characterized by the free space attenuation.

Two way propagation

Transmitter and reflection on a flat surface

The above Equation applies to free space. If the wave propagates over an electrically conductive plane, it is usually received by the receiver both directly and indirectly, mirrored by the plane. One speaks of two-way propagation.

{\ displaystyle L _ {\ mathrm {P}} = - g _ {\ mathrm {S}} -g _ {\ mathrm {E}} -20 \ log h _ {\ mathrm {S}} -20 \ log h _ {\ mathrm {E}} -40 \ log d}

Approximation for

{\ displaystyle h _ {\ mathrm {S}} + h _ {\ mathrm {E}} \ ll d}

h _S and h _E indicate the heights of the transmitting and receiving antenna above the plane. When spreading over the plane, the path loss increases much faster (with the power of 4) than in free space (with the power of 2).

literature

Jürgen Detlefsen, Uwe Siart: Basics of high frequency technology. 2nd edition, Oldenbourg, Munich Vienna 2006, ISBN 3-486-57866-9
Wolfgang Frohberg, Horst Kolloschie, Helmut Löffler : Paperback of communications engineering. Hanser, 2008.
Herbert Zwaraber: Practical setup and testing of antenna systems. 9th edition, Hüthig, Heidelberg 1989, ISBN 3-7785-1807-0

Individual evidence

↑ Tutorial Radio Signal Path Loss from radio-electronics.com (accessed January 17, 2014)

[1] Tutorial Radio Signal Path Loss from radio-electronics.com (accessed January 17, 2014)