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A Fresnel zone (// fray-NEL), named for physicist Augustin-Jean Fresnel, is a series of concentric ellipsoidal regions of space between and around a transmitting antenna and a receiving antenna system. Each region is defined by the phase-shift (or lack thereof) which ultimately occurs when a transmitted sine-wave deflects off an object within that region and continues onward to the receiver. These regions can be important to take into account because a sine wave following one of these paths may arrive at the receiver out of sync with the line-of-sight sine wave by anywhere from 1 degree to 359 degrees (or more). If an object is within a particular Fresnel zone region, and a part of the original signal, which is not moving towards the receiving antenna and would otherwise have continued onward into space, is partially deflected by that object back to the receiving antenna, it can result in constructive amplification or destructive interference at the receiver depending on the path length and the resulting degree of shift of the arriving deflected wave.
Fresnel zones are seen in optics, radio communications, electrodynamics, seismology, acoustics, gravitational radiation, and other situations involving the radiation of waves and multipath propagation. Fresnel zone computations are used to anticipate obstacle clearances required when designing highly directive systems such as microwave parabolic antenna systems.
This is the cause of the picket-fencing effect when either the radio transmitter or receiver is moving and the high and low signal strength zones are above and below the receiver cut off threshold.
Importance of Fresnel zones
Fresnel zones are concentric ellipses (e.g. 1, 2, 3) centered around the direct transmission path (AB).
The 1st region is the solid ellipse in which the direct line-of-sight signal passes through. If a rogue part of the transmitted signal bounces off an object within this region and then moves on to the receiving antenna, the shift will be something less than a quarter-length wave, which is to say it will be something less than a 90 degree shift (ACB). In other words, its effect regarding phase-shift alone will be minimal. Therefore, this bounced signal can potentially result in having a positive impact on the receiver, as it is receiving a stronger signal than it would have without the deflection, and the additional signal will potentially be mostly in-phase. However, the positive attributes of this deflection also depends on the polarity of the signal relative to the object (see the section on polarity below).
The 2nd region surrounds the 1st region but doesn't include the first region. If a reflective object is located in the 2nd region, the rogue sine-wave which has bounced from this object and has been captured by the receiver will be shifted more than 90 degrees but less than 270 degrees because of the increased path length, and will potentially be received out-of-phase. Generally this is unfavorable. But again, this depends on polarity (explained below).
The 3rd region surrounds the 2nd region and deflected waves captured by the receiver will have the same effect as a wave in the 1st region. That is, the sine wave will have shifted more than 270 degrees but less than 450 degrees (ideally it would be a 360 degree shift) and will therefore arrive at the receiver with the same shift as a signal might arrive from the 1st region. A wave deflected from this region has the potential to be shifted precisely one wavelength so that it is exactly in sync with the line-of-sight wave when it arrives at the receiving antenna.
The 4th region surrounds the 3rd region and is similar to the 2nd region. And so on.
If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver. But if there are reflective surfaces that come in contact with a rogue transmitted wave, such as bodies of water, smooth terrain, roof tops, sides of buildings, etc., the radio waves deflecting off those surfaces may arrive either out-of-phase or in-phase with the signals that travel directly to the receiver. Sometimes this results in the counter-intuitive finding that reducing the height of an antenna increases the signal-to-noise ratio.
Fresnel provided a means to calculate where the zones are — that is, whether a given obstacle will cause mostly in-phase or mostly out-of-phase deflections between the transmitter and the receiver.
To maximize signal strength, one needs to minimize the effect of obstruction loss by removing obstacles from the radio frequency line of sight (RF LoS). The strongest signals are on the direct line between transmitter and receiver and always lie in the first Fresnel zone.
As explained above, the Fresnel Zone can be used to determine whether the bounced signal will be received in-phase or out-of-phase, but the polarity of a radio-frequency (RF) signal can greatly influence what actually happens at the receiving end of the transmission. Regarding polarity, an RF signal can be transmitted in different ways.
1) Linear Polarity - the sine wave moves on a plane.
a) Vertical polarity - the sine wave moves on a vertical plane.
b) Horizontal polarity - the sine wave moves on a horizontal plane.
2) Circular Polarity - the sine wave moves in 3-D, and moves in a tight circle as it leaves the transmitting antenna.
a) RHCP - The transmission moves clockwise as it leaves the transmitter, known as right-hand circular polarization.
b) LHCP - The transmission moves counter-clockwise, and is known as left-hand circular polarization.
Why does polarity matter?
If a signal is vertical polarity and it deflects off a horizontal object such as a flat roof, and then bounces up to a receiving antenna, and if the roof is within the 1st region of the Fresnel zone, the resulting signal will be upside down relative to the original signal! This means the high points of the sine wave are now low points, and vice versa. Hence, even though one would expect minimal change in phase in the first Fresnel region, the bounced signal will arrive out-of-phase. This would be unfavorable. So, the installer of the antenna system must take this into consideration and either move the transmitting antenna, receiving antenna, or both, to minimize or remove the interfering roof-deflected phase-shifted signal. Or, he can increase the height of either one or both the transmitting and receiving antenna so that the object (roof) that is deflecting the signal is in the 2nd region rather than the 1st (the upside down signal would behave as if it was right side up by the time it reached the receiver due to the half-wave phase shift in region 2). Or, he can simply change the polarity to horizontal!
If a signal is horizontal polarity and it deflects off a horizontal object such as a flat roof, and then bounces up to a receiving antenna, and if the roof is within the 1st region of the Fresnel zone, the resulting signal will be received favorably - as it will be in-phase. The left and right extremes of the sine wave will not be negatively impacted by the deflection of the roof. In fact, this will result in a stronger signal than if there was no deflection!
For an analogy to more easily understand the differences in deflected vertical and horizontal polarity signals, place a mirror on the floor in the middle of a room. Have somebody hold a flashlight on the other side of the room. The flashlight represents a signal and your eyes are the receiver. The mirror represents a flat roof within region 1 of the Fresnel Zone. Have the flashlight move up and down representing vertical polarity. Note that in the mirror, the flashlight moves in the opposite direction, that is, it moves down and up rather than up and down. This is out-of-phase. Now have the flashlight move to the left and right representing horizontal polarity. If you look in the mirror, the reflected image of the flashlight moves exactly in tandem with the actual flashlight. Left is left, right is right. This is in-phase.
If the signal is circular polarized, the Fresnel zone will have no effect, because a deflected circular polarized signal changes rotation upon deflection and the result is to become completely invisible to the receiver, regardless of whether is arrives in phase or out of phase. For example, a RHCP signal that hits a street, or a wall, or anything else, then becomes a LHCP signal, and is therefore invisible to the RHCP receiving antenna, regardless of whether it arrives at the receiver in-phase or out-of-phase.
Numerous examples can be made regarding the different regions of the Fresnel zone and whether the obstacles providing bounce are sides of buildings (vertical), or streets/flat roofs (horizontal). But the same logic regarding polarity and its effects applies to the 2nd region, 3rd region, and so forth.
Determining Fresnel zone clearance
The concept of Fresnel zone clearance may be used to analyze interference by obstacles near the path of a radio beam. The first zone must be kept largely free from obstructions to avoid interfering with the radio reception. However, some obstruction of the Fresnel zones can often be tolerated. As a rule of thumb the maximum obstruction allowable is 40%, but the recommended obstruction is 20% or less.
For establishing Fresnel zones, first determine the RF Line of Sight (RF LoS), which in simple terms is a straight line between the transmitting and receiving antennas. Now the zone surrounding the RF LoS is said to be the Fresnel zone.
The general equation for calculating the Fresnel zone radius at any point P in between the endpoints of the link is the following:
- is the th Fresnel Zone radius,
- is the distance of P from one end,
- is the distance of P from the other end,
- is the wavelength of the transmitted signal.
The cross sectional radius of each Fresnel zone is the longest at the midpoint of the RF LoS, dubious ] For practical applications, it is often useful to know the maximum radius of the first Fresnel zone. Using , , and in the above formula gives[
- is the distance between the two antennas,
- is the frequency of the transmitted signal,
- ≈ ×108 m/s is the 2.997speed of light in the air.
Substitution of the numeric value for followed by a unit conversion results in an easy way to calculate the radius of the first Fresnel zone , knowing the distance between the two antennas and the frequency of the transmitted signal :
- Coleman, Westcott, David, David (2012). Certified Wireless Network Administrator Official Study Guide. 111 River St. Hoboken, NJ 07030: John Wiley & Sons, Inc. p. 126. ISBN 978-1-118-26295-5.
- "Fresnel Zone Clearance". softwright.com. Retrieved 2008-02-21.
- Tomasi, Wayne. Electronic Communication Systems - Fundamentals Through Advanced. Pearson. p. 1023.
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).