IJRRAS 15 (1) ● April 2013 www.arpapress.com/Volumes/Vol15Issue1/IJRRAS_15_1_15.pdf 107 DESIGN AND NUMERICAL ANALYSIS OF A SINGLE HALF-WAVE DIPOLE ANTENNA TRANSMITTING AT 235MHz USING METHOD OF MOMENT Adewumi Adebayo Segun 1 , Alade Michael Olasope 2 , Rotimi Cornelius Okeowo 3 & Akinleye Abiodun Ismail 4 1,2,4 Department of Pure and Applied Physics, Ladoke Akintola University of Technology, P.M.B 4000, Ogbomoso, Oyo State, Nigeria 3 Department of Physics, College of Education, P.M.B 750, Ikere-Ekiti, Ekiti State, Nigeria ABSTRACT This paper deals with design and numerical analysis of a single Half- wave dipole antenna suitable for transmitting UHF television signals at a frequency of 235MHz using Method of Moment. Two equations, namely the radiating field and the electric field strength equations were used to determine the variations in the electric field strength and free space loss with distance in kilometers. Other parameters such as the power radiated, gain and voltage standing wave ratio of the antenna at this frequency were also evaluated. The radiation patterns obtained shows that the antenna is a good radiator while the variations in electric field strength and free space loss with distance actually show the distance covered and the rate of loss of the signal transmitted at this particular frequency using half –wave dipole antenna. Keywords: Design, Numerical Analysis, Half-wave dipole Antenna, Method of moment, far field pattern 1. INTRODUCTION This work was motivated by a desire to improve on the performances of some antennas used for transmitting Television signals in some parts of south west region of Nigeria so as to enhance uniform distribution and higher efficient transmission of Television signals from base stations. The resulting analysis will enhance industry’s ability to design antennas that will meet performance specifications and enable Television broadcasting stations to know the importance of design and analysis of antennas before they are employed in transmissions. In today’s transmission systems, efficient antennas are required for transmission so as to produce high energy radiating signals that can be transmitted over a long distance for wide coverage before the signal is completely attenuated. An efficient radiating antenna coupled with high power transmitter will reduce the number of repeaters stations and eventually save cost of installations. Gautama A.K observed and reported that in order to ensure higher power radiation an antenna must have the ability to match the transmission line with the load impedance; it must also have the ability to transfer energy from electrostatic to electromagnetic energy or vice versa. In this study, the design and numerical analysis of a single short half-wave dipole Antenna suitable for transmitting at the same frequency as that of Nigeria television station NTA Ogbomosho (235MHz) is presented. Half – Wave dipole is chosen because of it wide acceptance in practice and its uses as a reference antenna. The characteristics of a Half-Wave dipole antenna in literature show that it has distributed capacitance and inductance which make it behave like a resonant circuit with voltage and current out of phase. Half – Wavelength dipole is one of the most commonly used antennas because its radiation resistance is 73 ohms, which is very near the 75-ohm characteristic impedance of some transmission lines, it matching to the line is simplified especially at resonance compare to other antennas. (Kennedy and Davis, 2005; Balanis, 2005). The diagram figure1 shows the transmitting antenna used for transmitting signal at a frequency of 235MHz in Nigeria Television Authority (NTA) station Ogbomosho, Oyo State, Nigeria. Ogbomosho is located on the latitude 8 0 08' 01" N and longitude 4 0 14' 48" E. This station transmits at frequency of 235MHz with a transmitters’ power of 5kw but from information gathered from other cities and towns of about 40km away, it has been observed that the signals hardy reach 35km away from the base station before it is completely attenuated. I think this is the main reason why government has to install another Nigerian Television transmitting station in Oyo town which is just 47km away from Ogbomosho which could have been covered by the same base station if efficient antenna properly match with transmission line cable and powered by efficient transmitter is used for transmission. Though the role of free space path loss and other attenuation factors cannot be overlooked yet the importance of a well designed antenna in transmitting systems both at the transmitting and receiving ends cannot be underrated.
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IJRRAS 15 (1) April 2013 www.arpapress.com/Volumes/Vol15Issue1/IJRRAS_15_1_15.pdf
107
DESIGN AND NUMERICAL ANALYSIS OF A SINGLE HALF-WAVE
DIPOLE ANTENNA TRANSMITTING AT 235MHz USING METHOD OF
MOMENT
Adewumi Adebayo Segun1, Alade Michael Olasope
2, Rotimi Cornelius Okeowo
3 & Akinleye Abiodun Ismail
4
1,2,4
Department of Pure and Applied Physics, Ladoke Akintola University of Technology,
P.M.B 4000, Ogbomoso, Oyo State, Nigeria 3Department of Physics, College of Education, P.M.B 750, Ikere-Ekiti, Ekiti State, Nigeria
ABSTRACT
This paper deals with design and numerical analysis of a single Half- wave dipole antenna suitable for transmitting
UHF television signals at a frequency of 235MHz using Method of Moment. Two equations, namely the radiating
field and the electric field strength equations were used to determine the variations in the electric field strength and
free space loss with distance in kilometers. Other parameters such as the power radiated, gain and voltage standing
wave ratio of the antenna at this frequency were also evaluated. The radiation patterns obtained shows that the
antenna is a good radiator while the variations in electric field strength and free space loss with distance actually
show the distance covered and the rate of loss of the signal transmitted at this particular frequency using half –wave
dipole antenna.
Keywords: Design, Numerical Analysis, Half-wave dipole Antenna, Method of moment, far field pattern
1. INTRODUCTION
This work was motivated by a desire to improve on the performances of some antennas used for transmitting
Television signals in some parts of south west region of Nigeria so as to enhance uniform distribution and higher
efficient transmission of Television signals from base stations. The resulting analysis will enhance industry’s ability
to design antennas that will meet performance specifications and enable Television broadcasting stations to know
the importance of design and analysis of antennas before they are employed in transmissions. In today’s
transmission systems, efficient antennas are required for transmission so as to produce high energy radiating signals
that can be transmitted over a long distance for wide coverage before the signal is completely attenuated. An
efficient radiating antenna coupled with high power transmitter will reduce the number of repeaters stations and
eventually save cost of installations.
Gautama A.K observed and reported that in order to ensure higher power radiation an antenna must have the ability
to match the transmission line with the load impedance; it must also have the ability to transfer energy from
electrostatic to electromagnetic energy or vice versa.
In this study, the design and numerical analysis of a single short half-wave dipole Antenna suitable for transmitting
at the same frequency as that of Nigeria television station NTA Ogbomosho (235MHz) is presented. Half – Wave
dipole is chosen because of it wide acceptance in practice and its uses as a reference antenna.
The characteristics of a Half-Wave dipole antenna in literature show that it has distributed capacitance and
inductance which make it behave like a resonant circuit with voltage and current out of phase. Half – Wavelength
dipole is one of the most commonly used antennas because its radiation resistance is 73 ohms, which is very near the
75-ohm characteristic impedance of some transmission lines, it matching to the line is simplified especially at
resonance compare to other antennas. (Kennedy and Davis, 2005; Balanis, 2005).
The diagram figure1 shows the transmitting antenna used for transmitting signal at a frequency of 235MHz in
Nigeria Television Authority (NTA) station Ogbomosho, Oyo State, Nigeria. Ogbomosho is located on the latitude
80 08' 01" N and longitude 4
0 14' 48" E. This station transmits at frequency of 235MHz with a transmitters’ power
of 5kw but from information gathered from other cities and towns of about 40km away, it has been observed that the
signals hardy reach 35km away from the base station before it is completely attenuated. I think this is the main
reason why government has to install another Nigerian Television transmitting station in Oyo town which is just
47km away from Ogbomosho which could have been covered by the same base station if efficient antenna properly
match with transmission line cable and powered by efficient transmitter is used for transmission. Though the role of
free space path loss and other attenuation factors cannot be overlooked yet the importance of a well designed
antenna in transmitting systems both at the transmitting and receiving ends cannot be underrated.
IJRRAS 15 (1) April 2013 Segun & al. Analysis of a Single Half-Wave Dipole Antenna
108
Figure.1 Nigeria Television Authority (NTA) Transmitting Antenna Ogbomosho.
However, in this paper a single Half -wave dipole antenna suitable for transmitting at the same frequency (235MHz)
as that of NTA Ogbomosho is designed and analyzed numerically to observe its performances and effectiveness
over the designed frequency using method of moment.
2. DESIGN ANALYSIS
Step1: The initial condition for the design of the half-wave dipole antenna expected to operate at the design
frequency of 235MHz is that the half-wave dipole antenna length dl shown in figure 2 is assumed to be 0.5𝜆, that is
dl is λ/2 long such that λ = c/f.
Figure2. Diagram showing the Half- wave dipole antenna located in space.
Since the transmitting half-wave dipole antenna is usually located in free space carrying an oscillating current, it will
give rise to two fields 𝐻∅ (Far field radiation) and 𝐸𝜃 (Electric field strength) which can be obtained using the
equation (Gautam, 2009).
𝐻∅ =𝐼𝑜𝑑𝑙𝑠𝑖𝑛𝜃
4𝜋[𝜔
𝑟𝑐𝑐𝑜𝑠𝜔 𝑡 −
𝑟
𝑐 ] 1
and
𝐸𝜃 =𝐼𝑜𝑑𝑙𝑠𝑖𝑛𝜃
4𝜋𝜖 𝑐2𝑟[𝑐𝑜𝑠𝜔(𝑡 −
𝑟
𝑐] 2
where dl is the dipole antenna length (0.5λ), r is the distance of the far field region from the dipole element usually
θ
P Z
X
Y
ϕ
dl
IJRRAS 15 (1) April 2013 Segun & al. Analysis of a Single Half-Wave Dipole Antenna
109
greater than 2(𝑑𝑙)2 𝜆 chosen to be 1.5m.
λ is the wavelength (1.276595745m).
C is the speed of light (3 × 108𝑚/𝑠).
t is the period (1/f)
𝜔 = 2𝜋𝑓, 𝑓 is the frequency (235MHz).
θ is the angle of radiation in degree (0-3600)
𝜖 is the permittivity of free space ( 8.85 × 10−12𝐹/𝑚 )
𝑖𝑜 is the peak value of current.
dl is the length of the dipole antenna.
2.1 Determination of the peak value of the dipole antenna current using method of moment
step2: To determine the peak value of the dipole current 𝑖𝑜 using method of moment (circuit equation) technique,
the step is as follow: the Half-wave dipole antenna is divided into 4 segments Δz’= 0.125λ long and each assumed to
have a uniform current over each segment given by 𝑖1 𝑖2 𝑖3 𝑎𝑛𝑑 𝑖4 as shown in figure 3.
a
Figure3. Single Half-wave dipole antenna 0.5λ long divided into 4 segments Δz1=Δz2=Δz3=Δz4=0.125λ long
Where the radius a = r11 = r22 = r33 = r44 = 0.001.
The Half-wave dipole is assumed to be symmetrical so that
r12 = r21 = r23 = r32= r34 = r43 3
r13 = r31 = r24 = r42 and r14 = r41 4
Also
G11=G22=G33=G44, 5
ΔZ4
ΔZ3
ΔZ2
ΔZ1
IJRRAS 15 (1) April 2013 Segun & al. Analysis of a Single Half-Wave Dipole Antenna
110
G12=G21=G23=G32=G34=G43, 6
G13=G31=G24=G42and G14=G41. 7
The peak value of the current was determined as follow Using equation 8, 9, 10and 11 (Kraus et al., 2002; John