Fresnel Zone
Figure 1: Fresnel zone.
d is the distance between the transmitter and the receiver,
b is the radius of the Fresnel zone.
Figure 1: Fresnel zone.
d is the distance between the transmitter and the receiver,
b is the radius of the Fresnel zone.
Fresnel Zone
In radio communications, a Fresnel zone is one of a theoretically infinite number of concentric ellipsoids of revolution which define volumes in the radiation pattern of a usually circular aperture. Fresnel zones result from diffraction by the circular aperture. Radio waves will travel in a straight line from the transmitter to the receiver normally. But if there are obstacles near the path, the radio waves reflected off by those objects may arrive out of phase with the signals that travel directly and reduce the power of the received signal (Interferences). This effect is a reason for the “Fading” of radio communication. On the other hand, the reflection can enhance the power of the received signal if the reflection and the direct signals arrive in phase.
The size of the first Fresnel zone is determined by the distance d between the emission and reception points and by the wavelength λ. The size of the semi-axis b of the ellipsoid can be determined with the help of the formula:
(1)
Based on the energy sense, for the unimpeded propagation of an electromagnetic wave between two points in the first Fresnel zone, there must be no obstacles, at least, whose dimensions are commensurate with the wavelength. The presence of obstacles in this area will lead to the emergence of reflected waves, which, interacting with the direct wave at the observation point, can attenuate the wave.
Figure 2: Fresnel zone for radar
Fresnel zone in radar
For radar systems, when constructing the Fresnel zone, in addition to the values listed above, the rotation of the antenna has an additional effect. If the parameters of the Fresnel zone are such that it touches the ground surface, an annular area or stripe appears on this surface (Figure 2). The transverse size of the stripe is determined by the condition that the wave phases at the beginning and the end of the stripe differ by 180°. Such stripes are sometimes also called Fresnel zones. The presence of surface irregularities within the Fresnel zone affects the formation of the radar antenna radiation pattern, the intensity of the wave at the target point, and, therefore, the power of the received echo signal. Approximately, the size of the Fresnel zone on the ground surface around the radar can be calculated by the formulas:
(2)
- h = height of antenna above ground
- λ = transmitters wavelength