Our Broad-Band Antenna

7/26/2019 Our Broad-Band Antenna

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Broad-Band Dipole Antenna

Ahmad Jan (E10007 - A)

Urooj Hayat


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Broad-Band Dipole Antenna

An antenna is said to be broadband when it

has high bandwidth, which provides excellent

performance for short to medium range

circuits.

An antenna can be made broadband by

increasing the volume it occupies. Hence, a

dipole antenna can be made more broadbandby increasing the radius of the dipole.


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This Broadband Dipole Antenna is an idealOmni-directional high frequency dipole with

frequency range 160 MHz to 500 MHz


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History / Background


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There was need of an antenna which is

suitable for all kinds of environment.This

antenna provides reliable, repeatable

measurements,Whether testing outdoors, or

in a shielded enclosure, the rugged(rough)

construction of the Broadband Dipole makes it

suitable for all conditions.


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Basic Principle of Radiation


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When the generator applied the AC voltage to

the antenna a current will flow through that

antenna which results produce an electric and

magnetic field that will radiate in air.


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Antenna Parameters


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Polarization

The polarization of an antenna is the

polarization of the wave radiated by the

antenna. It describes the orientation of the

electric field.

The polarization of a broadband antenna is of

horizontal type.


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Gain

As we know that Antenna gain is the ratio of

surface power radiated by the antenna to the

surface power radiated by a hypothetical

isotropic antenna.

The gain of broadband dipole antenna is

approximately around 5-8 dBi. (dbi means gain

in decibels compared to isotropic antenna).


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Directivity

Directivity (D) is defined as the ratio of the

radiation intensity P in a given direction from

the antenna to the radiation intensity

averaged over all directions.

Average radiation intensity is equal to the

total power radiated by the antenna divided

by 4.


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Axial Ratio

The axial ratio is the ratio of the magnitudes

of the major and minor axis of the dipole of

antenna defined by the electric field vector.


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Power Density

Let suppose we have a broadband dipole antenna havingbandwidth equal to that of dipole ,then the Averageradiatedpower density

(Pd) = (15 Im2

/ r2

) [ ( Cos (L Cos(theta)) - Cos(L) ) / Sin] 2

Where, = 2 / = Wave Length

Im= Magnetic CurrentL = Half Antenna Lengthr = radius = Angle


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Bandwidth

The bandwidth of an antenna refers to the

range of frequencies over which the antenna

can operate correctly. The antenna's

bandwidth is the number of Hz for which theantenna will exhibit an SWR less than 2 to1.

By this definition the bandwidth of broadband

dipole antenna is approximately

500-160=340 hz


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Beamwidth

An antenna's beam width is the angular

distance between the half power points.

Half the power expressed in decibels is

3dB, so the half power beamwidth is

sometimes referred to as the 3dB beamwidth.

The directive gain is inversely proportional to

the beamwidth,as the beamwidth decreases,

the directive gain increases.


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Front-to-Back Ratio

It is the ratio of signal strength transmitted in

a forward direction to that transmitted in a

backward direction. For receiving antennas,

the ratio of received-signal strength when theantenna is rotated 180.

FM BROADBAND DIPOLE ANTENNA has front

to back ratio 7.5 db.

(87.5 108 MHz)
http://en.wikipedia.org/wiki/Front-to-back_ratiohttp://en.wikipedia.org/wiki/Front-to-back_ratio


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High Efficiency

Broadbanding is achieved without the use of

resistors or tuning units for full antenna effi

ciency, with valuable power radiated rather

than lost in tuning devices.It is reliable forshort and medium range communications.


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Input Impedance

The input impedance of an antenna is defined

as the ratio of the voltage to current at the

antennas terminals.

Input impedance of this antenna is

theoretically calculated and has a standard

value of 50 ohms.


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Radiation Patterns


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Radiation pattern means a graphicalrepresentation of the radiation properties ofthe antenna as a function of space

coordinates. Radiation properties include radiation

intensity, field strength, direction and phase.

The radiation pattern of dipole antenna issame as that of the loop antenna which islinearly polarized.


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Radiation Pattern from 1 to 10 MHz


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Radiation Pattern from 11 to 20 MHz


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Fresnel Zones


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Radio waves will travel in a straight line fromthe transmitter to the receiver, if they areunobstructed. But if there are obstacles near

the path, the radio waves will reflecting offthose objects and may arrive out of phaseand reduce the power of the received signal.

Fresnel provided a means to calculate how outof phase the deflections between thetransmitter and the receptor.


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Fresnel Zone Calculation

For establishing Fresnel zones, first determine

the RF Line of Sight (RF L0S), which is a

straight line between the transmitting and

receiving antennas. Now the zone surroundingthe RF L0S is said to be the Fresnel zone.

The general equation for calculating the

Fresnel zone radius is:


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Fresnel Zone Clearance

The rule of thumb is that 60% of the 1st

Fresnel zone must be clear of obstacles.

This statement can be understand by the

following figure shown in next slide:


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EXAMPLES OF FRESNAL ZONES

DISRUPTED


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Applications of the Antenna


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For Digital T.V

A new digital TV receiving antenna, which can

generate a broad bandwidth covering the

complete spectrum of digital TV channels

(470-862 MHz) and has a simple structure.

The broadband dipole antenna is capable of

generating a broad bandwidth so as to cover

the complete spectrum of digital TV channels(470-862 MHz).


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For Wireless Communication

A printed dipole antenna with broad band is

proposed for modern wireless communication

systems.


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Other Applications

This broadband antenna are widely used by

military, commercial and broadcasting

services.

In most applications of UHF RFID systems are

preferred, which largely depends on the use of

broadband dipole antennas.


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Physical Structure / Material


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Physical Structure

The antenna comprises of two horizontalconical sections, the elements of which areconnected in the centre to a common feed

line. The antenna is mounted between twoguyed support masts.

Physical Dimensions

Height: 21 in.

Width: 24 in.

Weight: 1.13 kg.


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Configuration of Antenna

Horizontal configuration

Inverted V configuration

Antenna performance is very similar in both

horizontal and inverted "V" configurations.

The inverted "V" configuration requires a

single mast for installation.

For long range operation, the horizontal

configuration is recommended.


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Figure


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Material

The antenna is made from lightweight,weatherproof materials designed to withstandextreme environmental conditions .

The wire elements are constructed of high qualitystainless steel, and insulators are made from UVresistant, durable plastic.

The material utilized for the pole sections is a

composite known as FRP - fiberglass reinforcedplastic. It is extremely light weight, strong, andrigid.


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Accessories

Coaxial cables

Static protection kit

Masts(poles)

Supports

Insulators

Antenna wire


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Main Vendors


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Barker & Williamson CO Ltd

MICRO-ANT, INC.

Renair Antenna UK Ltd

Bushcomm antennas

Dannex Co Ltd

Parco electronics, co


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Refrences

Wikipedia Radio Shack 1974-75 unabridged Dictionary of

Electronics edited by Rudolf F. Graf, cat. no. 68-1030.

R. C. Johnson and H. Jasik (eds.),AntennaEngineering Handbook,

New York: McGraw-Hill, 1984.

W. L. Stutzman and G. A. Thiele,Antenna Theoryand Design, 2nded., New York: Wiley, 1998, pp.172, 173.


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