Radio waves travel in a straight line, unless something refracts or reflects them. But the energy of radio waves is not “pencil thin.” They spread out the farther they get from the radiating source — like ripples from a rock thrown into a pond.
The area that the signal spreads out into is called the Fresnel zone (pronounced fra-nell). If there is an obstacle in the Fresnel zone, part of the radio signal will be diffracted or bent away from the straight-line path. The practical effect is that on a point-to-point radio link, this refraction will reduce the amount of RF energy reaching the receive antenna.
The thickness or radius of the Fresnel zone depends on the frequency of the signal — the higher the frequency,the smaller the Fresnel zone.
Line of sight between transmitter and receiver is a straight line and ray bending due to K-value variation is added to the terrain heights.
The modification of terrain heights is given by
(d1 x d2) / (12.74 x k)
K=1.33 for path lengths between 5 to 15 Kms
K=0.66 for longer path lengths of 15 to 30 Kms
There must be a clearance for the first Fresnel zone to avoid diffraction loss in addition to free space loss.
Clearance of 100% or greater at k=4/3
Clearance of 60% or greater at lower k values
In case of space diversity, the antenna can have a 60% clearance at k=4/3 plus allowance for tree growth, buildings (usually 3 meter)
In free space, radio signals propagate like light ie. they follow a straight line. Such a line between the transmitter and receiver, is referred to as Line-of-Sight.Microwave radio communication requires a clear line-of-sight (LOS) condition.Under normal atmospheric conditions, the radio horizon is around 30 percent beyond the optical horizon.Radio LOS takes into account the concept of Fresnel ellipsoids and their clearance criteria.
Free Space Loss
As signals spread out from a radiating source, the energy is spread out over a larger surface area. As this
occurs, the strength of that signal gets weaker. Free space loss (FSL), measured in dB, specifies how much the signal has weakened over a given distance. Figure 1 shows the formula to calculate FSL and what the theoretical loss would be at sample distances. The type of antenna used has no effect on FSL, since at any appreciable distance all antennas look like a pointsource radiator. Note that the difference in FSL between a 2.4 GHz link and a 5.8 GHz link is always about 8 dB,regardless of the distance. This is one of the reasons why 802.11a wireless local area network (WLAN) devices will have less than half the range of a 2.4 GHz WLAN device (e.g., 802.11b).
- Microwave Radio Links Requires Recurring Licence Cost.
- Bandwidth is Limited to STM1(155Mb/s).
- Medium is Exposed to Many Atomspheric uncertainites.
Advantages of Microwave Radio Link Communication.
- Less prone to accidental damage.
- Increased Bandwidth Availability
- Very Much Reliable and Less fade due to LOS
- Power Requirement are pretty low .
- Links across mountains and rivers are more economically feasible.
- They are easy to deploy and maintain.
- The medium is free.
Microwave Networks communicaion can be deifned as several Digital Microwave links Connected in a tandem providing end to end connectivity between two points.
Fast Facts on Microwave Network Communications:
- Microwave frequencies range from 2 to 60 GHz.
- Frequencies between 30 GHz and 300 GHz are referred to by IEEE as Millimeter waves (MMW) instead of Microwaves as their wavelengths are between 1 and 10mm.
- Frequencies < 3 GHz are normally used by Low capacity systems.
- Frequencies between 3 and 15 GHz are used by Medium and Large capacity systems.
- Frequencies > 15 GHz are normally used Large capacity systems for short haul links.
