antenna chapter 1

UNIT-IANTENNA FUNDAMENTALS

PREPARED BY: BEWNET GETACHEW

Introduction The electric charges are the sources of the electromagnetic

(EM) fields. When these sources are time varying, the EM waves propagates away from the source and the radiation takes place.

In general, the radiation can be considered as a process of transmitting energy.

The radiation of the electromagnetic wave into the space is effectively achieved by using a conducting or dielectric structure called antennas or radiators.

A metallic device used for radiating or receiving radio waves is called antenna.

According to IEEE, antenna is defined as a means for radiating or receiving radio waves. Thus antenna is regarded as a transition between the free space and transmission line.

The antenna is a matching device between free space and the transmission line.

Impedance matching: matches impedance of transmission line to the intrinsic impedance of free space to prevent wanted reflection back to source.

Two main purposes of Antenna

Antenna must be designed to direct the radiation in the desired direction.

Antenna - How it Works

The antenna converts radio frequency electrical energy fed to it (via the transmission line) to an electromagnetic wave propagated into space. Antenna is a transducer which converts electrical energy into EM wave and vice versa.

Antenna Fundamentals Antenna can be used as transmitting antenna or

receiving antenna. It has directional properties. It is the important component of a wireless communication system.

Different antennas are used in different systems. But all the antennas possess basic fundamental properties which are same for all.

-radiation pattern -radiation intensity - gain -directivity -power gain -antenna efficiency -effective aperture - radiation resistance, - beamwidth - bandwidth, etc. - Polarization

The type of system you are installing will help determine the type of antenna used. Generally speaking, there are two ‘types’ of antennae:1. Directional

- this type of antenna has a narrow beamwidth; with the power being more directional, greater distances are usually achieved but area coverage is sacrificed

- Yagi, Panel, Sector and Parabolic antennas2. Omni-Directional - this type of antenna has a wide

beamwidth and radiates 3600; with the power being more spread out, shorter distances are achieved but greater coverage attained

- Isotropic antenna

dBd and dBi

isotropic radiator

half-wave dipole

2.15dB

eg: 0dBd = 2.15dBi

Yagi Uda

Parabolic

Sectoral

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• Log periodic dipole array (LPDA) DipolesTransmission

line

- BW is smaller than LPDA- typical gain 12 – 14 dBi

Reflector Driven element (dipole)Directors

• Yagi antenna

Directional RadiationPattern

main lobe

main lobeside lobeback lobe

- very wide BW, with constant SWR- typical gain 10 dBi

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Antenna Radiation pattern

Directional Antenna Radiation Pattern

Horizontal plane Vertical plane

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Typical Radiation Pattern for a Yagi

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0

90

180

270 0 -3 -6 -10

-15

-20

-30dB

0

90

180

270 0 -3 -6 -10

-15

-20

-30dB

Typical Radiation Pattern for a Sector

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Pattern

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Upper Side Lobe Suppression (dB)

Side lobes

120° (eg) Peak

Peak - 10dB

Peak - 10dB

10dB Beamwidth

60° (eg) Peak

Peak - 3dB

Peak - 3dB

3dB Beamwidth

Beamwidth

An antennas polarization is relative to the E-field of antenna.

– If the E-field is horizontal, than the antenna is Horizontally Polarized.

– If the E-field is vertical, than the antenna is Vertically Polarized.

Polarization

No matter what polarity you choose, all antennas in the same RF network must be polarized identically regardless of the antenna type.

Polarization

Vertical Horizontal

Vertical Polarization:

The electric field is vertical to the ground (In the maximum gain

direction)

Horizontal Polarization:The electric field is parallel to the ground (In the maximum gain

direction)

Polarization Requirements for various frequencies

Polarization may deliberately be used to:– Increase isolation from unwanted signal sources (Cross

Polarization Discrimination (x-pol) typically 25 dB)– Reduce interference– Help define a specific coverage area

Horizontal

Vertical