As a Fresnel zone [ fʀɛnɛl - ], named after Augustin -Jean Fresnel, is called in a radio transmission specific spatial regions between the transmitting and receiving antenna. The significance of the Fresnel zones is that due to the wave nature of the propagation of the electromagnetic radiation can be disturbed by obstacles, even if line of sight between the transmitting and receiving antenna consists. The amount of additional attenuation caused by obstacles can be calculated or estimated using the Fresnelzonenbetrachtung.
The first Fresnel zone is an imaginary ellipsoid of rotation between the antennas that are located at the foci of the ellipsoid. In the adjacent figure, the first Fresnel zone is shown on a hilly surface. At the edge of the ellipsoid of the detour for the diffracted signal at obstacles is half a wavelength. Within a Fresnel Zone is the path difference, ie the difference between two propagation paths less than half a wavelength.
In the region of the first Fresnel zone of the main part of the energy is transmitted. This zone should be free of obstructions (such as buildings, trees, mountains ). This is not the case, the transmission is attenuated. If the first Fresnel zone covered in half, so the excess loss is 6 dB at the receiving antenna. Under certain circumstances, the reception is then disturbed or completely interrupted. In the figure above, the first Fresnel zone is free of obstacles, so that a quasi undamped radio transmission is possible.
The second and higher Fresnel zones - thicker and a little longer - with a path difference of have in practice only a minor role and are usually neglected in simple calculations.
The maximum radius (half the thickness) of the Fresnel zone is frequency-dependent: at high frequencies with short wavelengths decreases. Due to the curvature of the earth and at a great distance of the antennas can therefore already be a noticeable attenuation at low transmission frequencies to each other, although there is still a direct optical sight between the transmitting and receiving antenna.
The location-dependent radius of the nth Fresnel zone can be approximated by:
Where n is the number of the Fresnel zone, the wavelength of the signal, the radio field length of the microwave field, defined as the distance between the antennas, and respectively the distance between the plane in question and the transmitter or receiver. However, this approach does not apply to radii near the transmitter or receiver.
In the middle between the transmitter and receiver is the maximum radius of the first Fresnel zone given by:
This maximum radius increases thus proportional to the square of antenna spacing and wavelength. Their geometric mean results in just the maximum diameter.
Obstacles outside the first Fresnel zone, it may be because of diffraction and interference even be a slight amplification of the signal. Slightly because most of the energy in the first Fresnel zone is transferred.