Isotropic radiator

Animated diagram of waves from an isotropic source, represented by the red dot

An isotropic radiator ( English isotropic antenna ), also spherical radiator or isotropic antenna called, is a model or the hypothetical idealization of a point radiator, the isotropic (i. E. Equally in all directions in space) or receives and sends lossless. Because of its simplicity in the model in antenna technology, it is used as a reference in the form of an “imaginary reference antenna ”. With the exception of the lack of loss and the even power distribution, it is assumed that all other properties of the isotropic radiator are identical to the real antenna that is to be described.

In the case of isotropic radiators, it is assumed that the entire transmission power is evenly distributed over the area of a sphere. The power density in the distance is then: ${\ displaystyle P_ {TX}}$ ${\ displaystyle A}$ ${\ displaystyle S_ {O}}$ ${\ displaystyle {r}}$

{\ displaystyle S_ {O} = {\ frac {P_ {TX}} {A _ {\ text {sphere}}}} = {\ frac {P_ {TX}} {4 \ pi r ^ {2}}}}

As a transmitting antenna

In contrast to sources of longitudinally polarized waves such as sound sources, an antenna that emits transverse waves coherently and with a completely isotropic power distribution is theoretically not feasible. All antennas have a more or less pronounced directional characteristic . To characterize the directivity of a given antenna, its antenna diagram is compared with that of the fictitious isotropic radiator (isotropic radiator as reference antenna ). However, only a small angular range in the main beam direction of the antenna to be compared is compared. The difference then is the antenna gain .

Due to the sometimes very large numerical values of the comparison, this is almost exclusively given in a logarithmic measure, the decibel (dB). The dBi value indicates the antenna gain of an antenna in relation to the isotropic radiator as a reference antenna. For example, it is 2.15 dBi for a λ / 2 dipole antenna in the direction perpendicular to the antenna axis, and 1.8 dBi for a Hertzian dipole .

As a receiving antenna

Mathematically, an isotropic radiator can also be used as a receiving antenna. In order to be able to extract power from a field with a given power density (power per unit area), a receiving antenna needs an effective antenna area ( aperture ) A _W , which depends on the wavelength to be received : ${\ displaystyle \ lambda}$

{\ displaystyle A_ {W} = {\ frac {\ lambda ^ {2}} {4 \ pi}}}

A punctiform receiving antenna, on the other hand, has no surface and would therefore not be able to draw any power and would not work.

Approximation

In certain measuring methods for antennas, such as sunstrobe recording , the broad spectrum of solar radiation can be used as an approximation of a point source. The sun not only emits visible light , but also transmits in all frequency ranges with powers that are quite stable for the period of measurement. The approach of point sources is permissible despite the huge size of the sun's surface , as the sun is very far away from the measuring antenna.

literature

Jürgen Detlefsen, Uwe Siart: Basics of high frequency technology. 2nd, expanded edition. Oldenbourg, Munich / Vienna 2006, ISBN 3-486-57866-9 .
Communication technology. Volume 1: Fundamentals of high frequency technology. Institute for the Development of Modern Teaching Media e. V., Bremen, 1980.
Edgar Voges : high frequency technology. Volume 2: Power tubes, antennas and radio transmission, radio and radar technology. Hüthig, Heidelberg 1987, ISBN 3-7785-1270-6 , p. 134 ff .: Chapter 17.2: Directional factor and antenna gain.
Description of an isotropic receiving antenna for measurement purposes ( online )