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ANTENNAS AND WAVE PROPAGATION

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Propagation Mechanism

Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal
Refraction - When there is a transition from one medium to another.
Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave.
Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

Perfect conductor & Dielectric

Perfect conductors reflect with no attenuation
Dielectrics reflect a fraction of incident energy
•Reflection induces 180 phase shift

Refraction properties

When wave enters a region with a higher dielectric constant ( a lower propagation velocity) it bends toward the normal.
Critical angle. (large incident angle + wave travels to area of low dielectric constant. Extreme case is total internal reflection)

Fresnel Zones

Alternate zones differ in phase by 180
Line of sight (LOS) corresponds to 1st zone
If LOS is partially blocked, 2nd zone can destructively interfere (diffraction loss)

ANTENNA AND WAVE PROPAGATION

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Introduction

•Antenna is a Essential element in wireless system.
•Used to transmit and receiveElectromagnetic waves from free space.
•Transmission media cost is zero.

EM waves:

•Velocity :3x10^8 m/s.
•Direction of Electric and magnetic field is perpendicular to each other.
•Also Direction of Electric and magnetic field is perpendicular to direction of propagation.
•Action is similar as water waves but it is invisible.
•Standing waves.

Antennas

Receiving Antenna:Any structure designed to efficiently receive electromagnetic radiation is called a transmitting antenna
We also know that an electromagnetic field will induce currentin a wire. The shape and size of the structure determines how efficiently the field is converted into current, or put another way, determines how well the radiation is captured. The shape and size also determines from which direction the radiation is preferentially captured.

Antenna parameters

•Antenna Impedance : Technically, antenna Impedance is the ratio at any given point in the antenna of voltageto currentat that point.
•Effective Antenna aperture is the ratioof the available power at the terminal of the antenna to the power flux density .
•The gainis also called as directive gain when the antenna radiate power in a particular direction relative to the average power radiated by the Antenna.
•Directivityof an antenna is the ratio of the radiation intensity in a particulardirection and the radiation intensity averaged over all directions.
•Power Beam width (HPBW) is the anglebetween two vectors, originatingat the pattern’s origin and passing through these points of the major lobewhere the radiation intensity is half its maximum.