In optics and radio communications, a Fresnel zone, named for physicist Augustin-Jean Fresnel,
is one of a number of concentric ellipsoids which define volumes in the radiation pattern
of a circular aperture. Fresnel zones result from diffraction by the circular aperture.
The cross section of the first Fresnel zone is circular. Subsequent Fresnel zones are
annular in cross section, and concentric with the first.
Importance of Fresnel zones If unobstructed, radio waves will travel in
a straight line from the transmitter to the receiver. But if there are reflective surfaces
along the path, such as bodies of water or smooth terrain, the radio waves reflecting
off those surfaces may arrive either out of phase or in phase with the signals that travel
directly to the receiver. Waves that reflect off of surfaces within an even Fresnel zone
are out of phase with the direct-path wave and reduce the power of the received signal.
Waves that reflect off of surfaces within an odd Fresnel zone are in phase with the
direct-path wave and can enhance the power of the received signal. 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--where a given obstacle will cause mostly in phase or mostly out of phase
reflections between the transmitter and the receiver. Obstacles in the first Fresnel zone
will create signals with a path-length phase shift of 0 to 180 degrees, in the second zone
they will be 180 to 360 degrees out of phase, and so on. Even numbered zones have the maximum
phase cancelling effect and odd numbered zones may actually add to the signal power.
To maximize receiver strength, one needs to minimize the effect of obstruction loss by
removing obstacles from the radio frequency line of sight. The strongest signals are on
the direct line between transmitter and receiver and always lie in the first Fresnel zone.
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, 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:
where, Fn = The nth Fresnel Zone radius in metres
d1 = The distance of P from one end in metres d2 = The distance of P from the other end
in metres = The wavelength of the transmitted signal
in metres The cross sectional radius of each Fresnel
zone is the longest at the midpoint of the RF LoS, shrinking to a point at the antenna
on each end. For practical applications, it is often useful to know the maximum radius
of the first Fresnel zone. From the above formula, the following formulas can be derived,
using , , and . Now we have 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. In SI:
where r = radius in metres
D = total distance in kilometres f = frequency transmitted in gigahertz.
Or in imperial units:
where r = radius in feet
D = total distance in miles f = frequency transmitted in Gigahertz.
See also
References
This article incorporates public domain material from the General Services Administration
document "Federal Standard 1037C". External links
Fresnel Zone Calculator and Elevation Chart Fresnel Zone Calculator
FEN Fresnel Zone Calculator More Fresnel Zone Details
R.E. Sherriff, Understanding the Fresnel zone VHFMicrowave Radio Propagation: A Primer for
Digital Experimenters