GOLLIS UNIVERSITY Department of Telecommunication Engineering

WIRELESS COMMUNICATION

CHAPTER-2ANTENNAS AND PROPAGATION

Lecturer Khadar A. Muse

Introduction₭ An antenna is an electrical conductor or system of conductors. used either for radiating electromagnetic energy or for collecting electromagnetic energy

₭ An antenna is a transducer that converts radio frequency to electromagnetic waves that are radiated into space◦ Transmission - radiates electromagnetic energy into space◦ Reception - collects electromagnetic energy from space.₭ In two-way communication, the same antenna can be used for

transmission and reception

Radiation Patterns•An antenna will radiate power in all directions. • Graphical representation of radiation properties of an antenna.• The radiation pattern provides a convenient means of determining

the beam width of an antenna, which is a common measure of the directivity of an antenna. • Depicted as two-dimensional cross section

Types of Antennas

₭ Dipole antennas

₭Parabolic Reflective Antenna

₭Isotropic antenna

Dipole Antenna Dipoles Two of the simplest and most basic antennas are the half-wave dipole, or Hertz, antenna (Figure 5.2a) and the quarter-wave vertical, or Marconi, antenna (Figure 5.2b). The half-wave dipole consists of two straight collinear conductors of equal length, separated by a small gap. The length of the antenna is one-half the wavelength of the signal that can be transmitted most efficiently. A vertical quarter wave antenna is the type commonly used for automobile radios and portable radios. A half-wave dipole has a uniform or omnidirectional radiation pattern.

Parabolic Reflective AntennaParabolic Reflective Antenna An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. The fixed point is called the focus and the fixed line is called the directory (Figure 5.4a).

Isotropic antenna The simplest pattern is produced by an idealized antenna known as the isotropic antenna. An isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center.

Antenna Gain Antenna gain

◦ Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

Effective area◦ Related to physical size and shape of antenna

•Relationship between antenna gain and effective area

• G = antenna gain• Ae = effective area• f = carrier frequency• c = speed of light (» 3 x 108 m/s)• = carrier wavelength

2

2

2

44c

AfAG ee

Propagation Modes

₭ Ground-wave propagation

₭ Sky-wave propagation

₭ Line-of-sight propagation

Ground Wave PropagationFollows contour of the earth.

Ground waves must have vertical polarization to be propagated from an antenna. Horizontally polarized waves are absorbed or shorted by the earth.

Can Propagate considerable distances. The signals can propagate for hundreds and sometimes thousands of miles at these low frequencies.

Ground wave propagation is strongest at the low- and medium-frequency ranges. That is, ground waves are the main signal path for radio signals in the 30-kHz to 3-MHz range.

The best-known example of ground wave communication is AM radio. AM broadcast signals are propagated primarily by ground waves during the day and by sky waves at night.The best propagation of ground waves occurs over salt water because the water is an excellent conductor.

Sky Wave PropagationSignal reflected from ionized layer of atmosphere back down to earth.Signal can travel a number of hops, back and forth between ionosphere and earth’s surface.With this propagation mode, a signal can be picked up thousands of kilometers from the transmitter. Reflection effect caused by refraction. This bending of the signal is caused by refraction in a region of the upper atmosphere known as the ionosphere.

Ultraviolet radiation from the sun causes the upper atmosphere to ionize, i.e., to become electrically charged. The atoms take on or lose electrons, becoming positive or negative ions.If a receiver lies in that area between the place where the ground wave is fully attenuated and the point of first reflection from the earth, no signal will be received. This area is called the skip zone.Examples• Amateur radio• CB radio

Line-of-Sight Propagation•The third method of radio signal propagation is by direct waves, or space waves.

•A direct wave travels in a straight line directly from the transmitting antenna to the receiving antenna. Direct wave radio signaling is often referred to as line-of-sight communication.

•Direct or space waves are not refracted, nor do they follow the curvature of the earth.

•Above 30 MHz, neither ground wave nor sky wave propagation modes operate, nor must communication be by line of sight.

Line-of-sight communication is characteristic of most radio signals with a frequency above approximately 30 MHz, particularly VHF, UHF, and microwave signals.Transmission distances at those frequencies are extremely limited, and it is obvious why very high transmitting antennas must be used for FM and TV broadcasts. The antennas for transmitters and receivers operating at the very high frequencies are typically located on top of tall buildings or on mountains, which greatly increases the range of transmission and reception.

Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction

the most significant impairments are

AttenuationThe strength of a signal falls off with distance over any transmission medium.

Attenuation introduces three factors for the transmission engineer.

1.A received signal must have sufficient strength so that the electronic circuitry in the receiver can detect and interpret the signal.

2.The signal must maintain a level sufficiently higher than noise to be received without error.

3.Attenuation is greater at higher frequencies, causing distortion.

Free Space Loss For any type of wireless communication the signal disperses with distance.For satellite communication this is the primary mode of signal loss.

Noise For any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception. Noise may be divided into four categories: Thermal noise Intermodulation noise Crosstalk Impulse noise

Atmospheric Absorption An additional loss between the transmitting and receiving antennas is atmospheric absorption. Water vapor and oxygen contribute most to attenuation. Rain and fog (suspended water droplets) cause scattering of radio waves that results in attenuation. This can be a major cause of signal loss

Multipath For wireless facilities where there is a relatively free choice of where antennas are to be located, they can be placed so that if there are no nearby interfering obstacles, there is a direct line-of-sight path from transmitter to receiver. The signal can be reflected by such obstacles so that multiple copies of the signal with varying delays can be received.

RefractionRadio waves are refracted (or bent) when they propagate through the atmosphere. The refraction is caused by changes in the speed of the signal with altitude or by other spatial changes in the atmospheric conditions. Normally, the speed of the signal increases with altitude, causing radio waves to bend downward.

Types of Fading Reading Assignment

•Fast fading

•Slow fading

•Flat fading

•Selective fading

•Rayleigh fading

•Rician fading

Forward Error Correction•Transmitter adds error-correcting code to data block• Code is a function of the data bits

•Receiver calculates error-correcting code from incoming data bits• If calculated code matches incoming code, no error occurred• If error-correcting codes don’t match, receiver attempts to

determine bits in error and correct

THE END