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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_ratio7/26/2019 Our Broad-Band Antenna
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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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Any Questions.?