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Posted By: technopediasite

There is no straight forward answer for this question. Definitely one can point out Vertical Polarization as the best in first view because it is more prone to rain fading. Rain droplets are generally flattened with increases in size. I will try to show you in figure. We can understand very easily with the help of below figure, and thus Vertical Polarization is more prone and less affected.
However, horizontal polarization is very much used to avoid interface, in case nearby areas are using vertical polarization.

So vertical polarization is generally used for high frequency links, because high frequencies are more prone to rain fading and horizontal polarization is generally used to avoid interference. However, this cannot be treated as rule. Each operator is free to decide.
Here I would like to discuss about the Polarization, may be someone can think what is the polarization & how many types of polarization can be use for MW Links.

Polarization: Polarization defines the way of movement of MW waves in air.  It can be either Linear or Circular.

Type of Polarization
1.  Linear - can be sub-divided into Vertical and Horizontal.

2.  Circular

VERTICAL POLARIZATION: An electromagnetic wave is said to be following Vertical Polarization if its electrical component is perpendicular to the horizon of earth.

HORIZONTAL POLARIZATION: An electromagnetic wave is said to be following Horizontal Polarization if its electrical component is parallel to the horizon of earth.

CIRCULAR POLARIZATION: An electromagnetic wave is said to be following Circular Polarization if it radiates electric and magnetic field in all directions i.e. they keep on rotating.  Phase is the deciding factor here.  Don't worry about this... We generally do not use this in MW links.
Some of the major factors affect the MW link, I would like to also explain this factors.
Reflection is one of the major factors that affect MW link, below figure explain this phenomena.

Water is good reflector.  Reflected Wave can have different phase and amplitude as compared to LOS wave.  Thus, this causes Fading of signal at receiver and this fading is called Multi Path Fading.To overcome this problem, we either adjust antenna heights at two ends to avoid major source of reflection or to reduce its intensity.  Another solution is to use Space Diversity

Diffraction of wave occurs when bending takes place at sharp irregular edges.  This diffracted wave can interfere very much with desired signal.

Scattering of ray of light occurs when object it strikes is of smaller size that its own wavelength.

Above 10 GHz, absorption in atmosphere becomes dominant.  Rain droplets become comparable to wavelength.
This absorption can be 2 dB/Km or can be as high as 3 dB/Km in case of rain.

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