Radio frequency line of sight is defined by Fresnel Zones which are ellipse shaped areas between any two radio antennas.
The distance between the two radio antennas and the frequency of operation are required to compute the radius of the Fresnel Zone.
A Fresnel zone is a cylindrical ellipse drawn between transmitter and receiver. The size of the ellipse is determined by the frequency of operation and the distance between the two sites.
Huygens formulates electro-magnetic wave character theory, based on Huygens’ theory, Fresnel formulate the concept of Fresnel zone which further explains the reflection and diffraction of electric waves and is proved in practice. The basic idea of Huygens principle is: light and electric-magnetic wave are both a kind of oscillation, the medium around them are flexible, and therefore, the oscillation of a point can be transmitted to the adjacent particle and spread around, and then it becomes the wave that is transmitted in the medium. Therefore, it can be considered that the oscillation of a point source is transmitted to the adjacent particles and forms secondary wave source and then tertiary wave source and so on. If the wave transmitted by the point source is spherical wave, then the wave before the secondary wave formed by the point source is spherical wave and that before the tertiary wave is also spherical wave, and the other is by analogy. In microwave communication, when the size of transmitting antenna is less than the microwave relay distance, the transmitting antenna can be considered to be a point source.
Supposed, for a microwave relay section, the transmitting point is T and the receiving point is R and the distance between stations is d, if on a flat surface, the distance from a moving point P to two fixed points (T and R) is a constant, the track of this point is an ellipse. And in the space, the track of this point is a rotary ellipsoid. In electric wave propagation, when the constant is d+λ/2, the ellipsoid obtained is called first Fresnel ellipsoid, and when the constant is d+2λ/2, the ellipsoid obtained is called second Fresnel ellipsoid..., when the constant is d+Nλ/2, the ellipsoid obtained is called Number N Fresnel ellipsoid.
If the Fresnel ellipsoid intersect with the waves transmitted from T or R, on the intersect interface, a series of circles and rings can be obtained and the center is a circle which is called the first Fresnel zone, the annulus (external circle minus internal circle) next to the first Fresnel zone is called second Fresnel zone and others by analogy. These rings and circles can be approximately considered as plane area graphics that are vertical to the ground and the ray between T and R. In practice, the influence of Fresnel zone on the project can be ignored.
The distance from any point on the Fresnel zone to the link between R and T is called Fresnel diameter and is represented by F. when the point is on the first Fresnel zone, the diameter is called the first Fresnel zone diameter.
Based on definitions of Fresnel ellipsoid and Fresnel zone, the first, second… and number N Fresnel zone diameter can be approximately expressed by:
Based on analysis, phases of field strength produced by adjacent Fresnel zones in the receiving point R are opposite. That is, the field strength produced by the second Fresnel zone is opposite to that produced by the first Fresnel, and the field strength produced by the third Fresnel zone is opposite to that produced by the second Fresnel.
Take the first Fresnel zone as a reference, the field strength produced by the zone with odd number makes the field strength of the receiving point strengthened, and that produced in by the zone with even number makes the field strength weakened. Field strength of the receiving point is sum of the vectors of the field strength of each Fresnel zone at the receiving point. In practice, for the slant of each Fresnel zone to the receiving point is different, thus Fresnel zones interfere with each other. The result of the vectors overlapped together is: the field strength of the receiving point which is obtained from all the Fresnel zones in the free space is approximately equal to the field strength produced by the first Fresnel zone at this point in the space.
What is a Fresnel zone and why is it important?
A Fresnel zone is a cylindrical ellipse drawn between transmitter and receiver. The size of the ellipse is determined by the frequency of operation and the distance between the two sites.
1. Huygens-Fresnel Principle
Huygens formulates electro-magnetic wave character theory, based on Huygens’ theory, Fresnel formulate the concept of Fresnel zone which further explains the reflection and diffraction of electric waves and is proved in practice. The basic idea of Huygens principle is: light and electric-magnetic wave are both a kind of oscillation, the medium around them are flexible, and therefore, the oscillation of a point can be transmitted to the adjacent particle and spread around, and then it becomes the wave that is transmitted in the medium. Therefore, it can be considered that the oscillation of a point source is transmitted to the adjacent particles and forms secondary wave source and then tertiary wave source and so on. If the wave transmitted by the point source is spherical wave, then the wave before the secondary wave formed by the point source is spherical wave and that before the tertiary wave is also spherical wave, and the other is by analogy. In microwave communication, when the size of transmitting antenna is less than the microwave relay distance, the transmitting antenna can be considered to be a point source.
2. Fresnel Ellipsoid
Supposed, for a microwave relay section, the transmitting point is T and the receiving point is R and the distance between stations is d, if on a flat surface, the distance from a moving point P to two fixed points (T and R) is a constant, the track of this point is an ellipse. And in the space, the track of this point is a rotary ellipsoid. In electric wave propagation, when the constant is d+λ/2, the ellipsoid obtained is called first Fresnel ellipsoid, and when the constant is d+2λ/2, the ellipsoid obtained is called second Fresnel ellipsoid..., when the constant is d+Nλ/2, the ellipsoid obtained is called Number N Fresnel ellipsoid.
3. Fresnel Zone
If the Fresnel ellipsoid intersect with the waves transmitted from T or R, on the intersect interface, a series of circles and rings can be obtained and the center is a circle which is called the first Fresnel zone, the annulus (external circle minus internal circle) next to the first Fresnel zone is called second Fresnel zone and others by analogy. These rings and circles can be approximately considered as plane area graphics that are vertical to the ground and the ray between T and R. In practice, the influence of Fresnel zone on the project can be ignored.
Also read How to calculate Fresnel Zone in Microwave System
4. Fresnel Zone Diameter
The distance from any point on the Fresnel zone to the link between R and T is called Fresnel diameter and is represented by F. when the point is on the first Fresnel zone, the diameter is called the first Fresnel zone diameter.
 |
| First Fresnel zone diameter |
Based on definitions of Fresnel ellipsoid and Fresnel zone, the first, second… and number N Fresnel zone diameter can be approximately expressed by:
5. Relationship between Field strength of Receiving Point and Energy of Each Fresnel Zone
Based on analysis, phases of field strength produced by adjacent Fresnel zones in the receiving point R are opposite. That is, the field strength produced by the second Fresnel zone is opposite to that produced by the first Fresnel, and the field strength produced by the third Fresnel zone is opposite to that produced by the second Fresnel.
Take the first Fresnel zone as a reference, the field strength produced by the zone with odd number makes the field strength of the receiving point strengthened, and that produced in by the zone with even number makes the field strength weakened. Field strength of the receiving point is sum of the vectors of the field strength of each Fresnel zone at the receiving point. In practice, for the slant of each Fresnel zone to the receiving point is different, thus Fresnel zones interfere with each other. The result of the vectors overlapped together is: the field strength of the receiving point which is obtained from all the Fresnel zones in the free space is approximately equal to the field strength produced by the first Fresnel zone at this point in the space.
Post a Comment