Impact of the Ground Structure on the Performance of ...

International Journal of Electronics Communication and Computer Engineering Volume 4, Issue 4, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209

Impact of the Ground Structure on the Performance of a Horizontal Dipole Antenna Aras Saeed Mahmood Department of Physics, University of Sulaimani, Kurdistan Region, Iraq E-mail: [email protected] Abstract – Among the radio frequency antennas, the dipole antenna or dipole aerial is one of the most important and commonly used types. The characteristics and the performance of these antennas are affected by many factors one of which is the ground structures. In this paper an attempt has been made using a new version of the Numerical Electromagnetic Code (4NEC2) simulation software to investigate the effect of seven different ground structures on the gain, radiation pattern, and the variation of the direction of maximum radiation of a horizontal dipole antenna operating at 300 MHz erected at different elevations. The gain varies with the elevation and directly proportional to the conductivity of the ground. The 20 m elevation was the best height for achieving a high gain above all the different ground structures. The deep of the nulls of the radiation patterns is directly proportional to the conductivity of the ground and the height of the antenna decides the direction of the maximum radiation.

transmitter’s power output and frequency, the Earth’s shape and conductivity along the transmission path, and the local weather conditions [3].

Keywords — Gain, Half Wave Dipole Antenna, Radiation Pattern, 4NEC2 Software.

I. INTRODUCTION Dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, in line with each other, with a small space between them. The radio frequency voltage is applied to the antenna at the center, between the two conductors. The geometry of this antenna, which is a cylindrical structure, is fully described by five parameters, including length, radius, feeding gap, frequency and wavelength. For a half wave dipole the length of the dipole should be half of the wavelength but practically it is 0.45 times of the wavelength. The dipole antenna consists of two terminals or "poles" into which radio frequency current flows. This current and the associated voltage causes and electromagnetic or radio signal to be radiated. Typically a dipole antenna is formed by two quarter wavelength conductors or elements placed back to back for a total length of λ/2. A standing wave on an element of a length λ/4 yields the greatest voltage differential, as one end of the element is at a node while the other is at an antinode of the wave. The larger the differential voltage, the greater the current between the elements [1]. A half-wavelength dipole is shown in Fig.1. Below the antenna, an approximation of the voltage distribution along the antenna is shown as a function of time. In addition, the corresponding approximation of the current distribution is also shown [2]. All Ultra High Frequency (UHF) and upper Very High Frequency (VHF) transmissions are by ground waves whose propagation is affected by the Earth’s electrical characteristics and by the amount of diffraction (bending) of the waves along the Earth’s curvature. The ground wave’s strength at the receiver depends on the

Fig.1. Dipole antenna with corresponding voltage and current distributions, showing one half of a wave cycle in total [2].

II. DESIGN AND SETUP OF THE DIPOLE ANTENNA The 300 MHz antenna used in this work has an approximate total length of 480 mm using the formula below [4] with a radius of 1 mm. 142.6 (1) Lengthof a  antenna (in meters)  2

frequency(in MHz)

The antenna erected above seven different ground structures having the dielectric constants and conductivity as in the table 1 below [5]. Table 1: Ground Types Land Type Dielectric Conductivity Constant (mhos/m) flat desert, cities 2.5 0.00022 mountains, steep rocky hills 7.0 0.0012 average ground 10.0 0.003 pastoral land, medium hills 13.0 0.011 rich farm land 22.5 0.065 rice paddy 34.0 0.15 sea water 81 5 The 4NEC2 simulation software has been used to investigate the radiation pattern, gain and variation of the direction of maximum radiation of the antenna for different ground structures. Copyright © 2013 IJECCE, All right reserved 1171


III. SIMULATION RESULTS 1. Gain The simulation software (4NEC2) was used to investigate of the variation of the gain with the height of the antenna above the ground. Fig. 2 below shows that, whatever the type of the ground, the proper height of the proposed antenna is 20m above the ground. The figure also shows that for the same height, the gain depends on the type of the ground. The higher the conductivity and the dielectric constant of the ground the higher the gain.

Fig.2. variation of the grain (dB) with the height of the antenna (m) above different ground structure

Fig.3. Radiation patterns for a dipole antenna operating at 300 MHz, 20 meters above different ground structures Copyright © 2013 IJECCE, All right reserved 1172

International Journal of Electronics Communication and Computer Engineering Volume 4, Issue 4, ISSN (Online): 2249–071X, ISSN (Print): 2278–4209 This is because of the fact that the dielectric constant or Earth’s surface conductivity determines how much of the surface wave signal energy will be absorbed or lost, i.e. the Conductivity of the local ground can greatly alter the strength of the transmitted or received signal [3]. The effect of imperfect ground, which has low conductivity compared to perfect ground, is more ohmic losses. The electric fields penetrate into the earth and excite currents. These currents give rise to ohmic losses which appear as an increase in the input ohmic resistance. Therefore the radiation efficiency of the antenna decreases as Rrl  (2) Rrl  ROHMIC Where  is the radiation efficiency, Rrl , is the radiation resistance, ROHMIC is ohmic resistance [5]. From [6] the radiation efficiency is given as P   rad (3) Pin And the gain 4 U ( ,  ) (4) (G )  Pin Where

Prad

is the radiated power,

Pin is the input

power and U ( ,  ) is the radiation intensity in ( ,  ) direction, so as the conductivity of the ground decreases the antenna gain decreases as it is shown in the fig. 2. Also in the VHF and UHF ranges, the presence of objects (e.g., buildings or towers) may produce strong reflections that arrive at the receiving antenna [3].

2. Radiation Pattern The IEEE Standard Definitions of Terms for Antennas defines the radiation pattern as [7]: “A mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. In most cases, the radiation pattern is determined in the far-field region and is represented as a function of the directional coordinates. Fig. 3 shows very explicitly how t he ground above which the antenna is erected affects the radiation pattern. The shapes of the radiation patterns of the proposed antenna erected at the same height (20m) above different ground structers have almost the same shape but the deep of the nulls depends differs. The ground with a higher conductivity causes the radiation pattern to have a deeper nulls or more directive pattern as shown in the same Fig.3.

3. Direction of maximum radiation From the previous Fig.3. The direction of the maxmium radiation from the antenna is   85o where  , is the angle from the z-axis and it is the same for all the ground types i.e. the conductivity or the dielectric constant of the ground has not any effect on the direction at which there is a maximum radiation. This direction is related to the altitude of the antenna above the ground not the structure of the ground type. Fig. 4 shows the variation of this direction with the height of the antenna above different ground structers tabulated in table 1.

The first four ground structures having a very low conductivity possess the same variation of the direction of maximun radiation with altitude of the antenna i.e.  varies from 65o to 85o for all the four ground types. For the higher ground conductivity property (the other three grounds) there are some fluctuations in the direction of the maximun radiation but the upper limit of this angle is the same up on any ground and it is depends on the kind and characteristics of the antenna.

Fig.4. variation of the angle of maximum radiation () with the antenna height above different ground structure

IV. CONCLUSION The 4NEC2 simulation software were used to investigate some properties of the horizontal dipole antenna operating at 300MHz erected at different heights above differene grounds. The following conclusions were odtained: 1. The gain of the antenna depends on the height and for the same height, the gain is proportional to the conductivity or the dielectric constant of the ground structers. 2. The shape of the radiation pattern of the same diople antenna is almost the same for different ground types. While the deep of the nulls depends on the conductivity of the ground. 3. The direction of maximum radiation of the antenna vaies with the height of the antenna and for the ground types with low conductivity property this variation is almost the same.

REFERENCES [1]

[2]

[3] [4] [5]

[6]

[7]

P. Singh, A. Sharma, N. Uniyal, and R. Kala, ‘‘Half-Wave Dipole Antenna for GSM Applications,’’ International Journal of Advanced Computer Research Vol. 2, No. 4, Issue.6, December 2012, PP. 354-357. J.A. Bean, ‘‘Thermal Infrared Detection Using Antenna-Coupled Metal-Oxide-Metal Diode, ’’PhD dissertation, University of Notre Dame- Indiana, 2008, Pages 24, and 25. J. E. Rhodes, ‘‘Antenna Handbook,’’ "MCRP 3-40. 3C", Department of the Navy, U.S. Marine Corps, June 1999, ch. 1. United States Army Signal Center and Fort Gordon, Georgia 30905-5000, ‘‘Antenna Theory, ’’February 2005, PP. 1-9. G. Turkes, ‘‘Tactical HF Field Expedient Antenna Performance,’’ M.Sc. Thesis, Naval Postgraduate School, Volume 1, March 1990, Page 8. G. W. M. Whyte, ‘‘Antennas for Wireless Sensor Network Applications,’’ Ph.D. thesis, University of Glasgow, 2008, PP. 73-75. C. A. Balanis, “Antenna Theory, Analysis and Design, 3rd Edition,’’ John Wiley and Sons, 2005, ch. 2.

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AUTHOR’S PROFILE Aras Saeed Mahmud received his B.Sc. degree in physics from the University of Sulaimani, Iraq in 1980. He was awarded his M.Sc. from University of Mosul/Iraq in physics (1985). From 1986 to 1994 he was the head of Quality Control Department for Distribution Transformer Factory in the Diala State Company for Electrical Industry. From 1994 to 1999 he was the head of the Physics Unit in the University of Sulaimani. He was the head of Physics Department from 1999 to 2002 in the same university. From 2002 to 2005 he studied towards his Ph.D. degree in the Electromagnetic Field Theory in the same university. From 2006 to 2010 he was the head of the Physics Department in the same university. Currently he is the head of the school of science education in the faculty of science and science education.

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