Antenna Polarization

For any wireless transmission, whether it be mobile, satellite, radio antenna is a must. Choosing desired antenna for particular application needs some study. An antenna can be described as a transducer which converts electrical energy into electromagnetic energy and radiates in the air and viceversa. Electromagnetic signal requires no introduction which is described as combination of electric and magnetic fields oscillating perpendicular to each other and orthogonal to the propagation of wave. It is the electric field component component that is important for us in deciding the antenna type. One way of classifying the antennas is by knowing the type of orientation of E-plane. Types are,

• Linear polarization
• Vertical Polarization
• Horizontal Polarization
• Circular Polarization
• Right hand circularly polarized
• Left hand circularly polarized
• Elliptical polarization 

Linear Polarization: The direction of radiation is in the direction of propagation. In other terms, it is in plane with the direction of propagation.

Circular Polarization: The plane of polarization rotates in a circle. For a time period of one wave, the plane rotates one cycle.

Vertical Polarization:

• The electric filed is perpendicular to the surface of earth
• In case of broadcasting requirements, we use this type of polarized antennas.
• Mobile antennas use vertical polarization
• Other applications include AM broadcast, Whip antennas on all automobiles
• Man made RF noise also is vertically polarized which may cause interference to actual signal of transmission

Horizontal Polarization:

• The electric field is in plane (parallel) with earth's surface.
• Local TV transmissions use this type of polarization
• Generally for less distance, when user feels other noise may interfere with their transmission, they use horizontal polarization

Circular Polarization:

• If the rotation of field is clockwise when looking at the direction of propagation, then it is Right hand circular propagation
• If the rotation of field is counter clock wise when looking at the direction of propagation, then it is called left hand circular polarization.
• This type of antennas are preferred when change in environment conditions should not effect the polarization, satellite communications is one example.
• This polarization is little difficult to generate and is costly.
• Helical antennas, Yagi antennas are also used.
• Patch antennas used for satellite communications generate circular polarization

Elliptical polarization:

• When electric field rotates through all angles in circular polarization, the distance between maximum and minimum peaks is represented by a term called Axial ratio or Ellipticity.
• If Axial ratio is 0 dB, then it is circular polarization
• If axial ratio is >0 dB, then it is elliptical polarization

Other considerations:

• The transmission and reception must have antennas with same type of polarization to get maximum signal reception. Any variations from this may cause signal drop in reception in dB.
• When more than one antenna need to be placed on top of buildings, care to be taken that they should be placed at a distance from each other, so that both doesn't interfere.
• The surroundings also have a effect on transmission. whether it be buildings, hilly areas, the signal strength gets reduced by some amount described by terminology fading. Fading can be short term or long term. For a receiver at a short distance, short term fading is due to multiple reflections from buildings and other obstacles. for a receiver at longer distances, long term fading is due to hilly terains, buildings.
• To enhance reception, engineers employ more than one antenna, which is described by the term diversity reception. combining two outputs and determining a strongest signal is always cumbersome. Even the placement of antennas at a distance causes maintenance costs to increase.