IJCST Vol. 6, ISSue 4, oCT - DeC 2015

w w w . i j c s t . c o m InternatIonal Journal of Computer SCIenCe and teChnology 253

ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)

Rectangular Microstrip Patch Antenna1Madhuri Garg, 2Nisha Singh

1Dept. of Physics, Sri Sai University, Palampur, Himachal, Pradesh, India2Dept. of Computer Application, TES College of Arts and Technology, Amritsar, Punjab, India

AbstractA rectangular microstrip patch antenna is a form of antenna which consists of a rectangular patch. This patch is of any planar or non-planar geometry on one side of dielectric substrate and a ground plane on the other side. Micro strip patch antenna have low profile configuration, narrow bandwidth and is capable of dual and triple frequency operations. Patch used is made of conducting material such as gold tin and nickel. The rectangular patch can be easily analysed using transmission line model and cavity model. Transmission line model yields less accurate results and lacks versatility. In cavity model the interior region of dielectric substrate is modeled as cavity bounded by electric walls on top and bottom.

KeywordsMicrostrip, Dielectric Substrate, Low Planar Configuration, Dual Frequency, Tripple Frequency.

I. IntroductionIn the micro strip antenna the upper surface of the dielectric substrate supports the printed conducting strip which is suitably contoured while the lower surface of the substrate is backed by a conducting ground plane. Such antenna sometimes called a printed antenna because the fabrication procedure is similar to that of a printed circuit board. Many types of micro strip antennas have been evolved which are variations of the basic structure. Micro strip antennas can be designed as very thin planar printed antennas and they are very useful elements for communication applications.

A. Origin Microstrip geometries that radiate electromagnetic waves were originally being used from early fifties. The realization of a microstrip like antenna integrated with microstrip transmission line was developed in 1953 by Deschamps. Gutton and Bassinot patented a microstrip design in 1955. However first practical antennae was experimented after 20 years. Howell and muson introduced the first practical antenna.There are different advantages and disadvantages of microstrip patch antenna which are as described below

1. AdvantagesLight weight and low volume.• Low fabrication cost, hence can be manufactured in large • quantities.Supports both, linear as well as circular polarization.• Can be easily integrated with Microwave Integrated Circuits • (MICs).Capable of dual and triple frequency operations.•

2. DisadvantagesNarrow bandwidth• Low efficiency• Low Gain• Low power handling capacity.•

B. Microstrip Patch AntennaMicrostrip patch antenna is a type of microstrip antenna. It is the most common form of antennas .It consists of a conducting patch of any planar or non-planer geometery on one side of a dielectric substrate and a ground plane on other side. The metallic patch is normally made of conducting material such as copper, gold, tin and nickel and the metal must be corrosion resistant. Patch can be of any shape such as rectangular, circular ring etc. Microstrip patch antennas have low profile configuration and are capable of dual and triple frequency. Due to these advantages these antennas are most suitable for aerospace and mobile applications. However narrow bandwidth, lower gain, extraneous radiations from feed and junction are their main disadvantages. To overcome these limitations these antennas can be further loaded with stubs, shorting pins, diodes to obtain compactness dual frequency operations, frequency agility and polarization control. Thus these antennas are finding increasing applications in commercial sector of industry especially in GPS (Global Positioning System), SDARS(Satellite Digital Audio Radio Services) and WLAN (Wireless Local Area Network).

C. Rectangular PatchThe rectangular patch is by far the most widely used. It is very easy to analyze using both the transmission line model and cavity model which are most accurate for this substrates.

Fig. 1: Rectangular Microstrip Patch Antenna

1. Transmission Line ModelThe transmission line model is the easiest of all models but it yields less accurate result and it lakes the versatility. A rectangular microstrip antenna can be represented as an array of two radiating narrow apertures (slots) each of width w and height h, separated by a distance L. Basically the transmission line model represents the microstrip antenna by two slots, separated by a low-impedance Zc transmission line of length L.

It is used to determine the input performance of a rectangular patch antenna. This model is only used in rectangular patch antenna. This model is only used in rectangular patch antenna and not in any other patch form. This model represents the microstrip antenna by two slots of width w and height h separated by a transmission line of length L. The microstrip is essentially a nonhomogeneous line of two dielectrics, typically the substrate and air.

IJCST Vol. 6, ISSue 4, oCT - DeC 2015 ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)

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Fig. 2: Microstrip Line

Fig. 3: Electric Field Lines

Hence, as seen from Fig. 3, most of the electric field lines reside in the substrate and parts of some lines in air. As a result, this transmission line cannot support pure Transverse-Electric-Magnetic (TEM) mode of transmission, since the phase velocities would be different in the air and the substrate. Instead, the dominant mode of propagation would be the quasi-TEM mode. Hence, an effective dielectric constant (εreff ) must be obtained in order to account for the fringing and the wave propagation in the line. The value of εreff is slightly less than εr because the fringing fields around the periphery of the patch are not confined in the dielectric substrate but are also spread in the air as shown in Fig. 3 above. The expression for εreff is given as [13].

(1)

Where, εreff = Effective dielectric constant εr = Dielectric constant of substrate h = Height of dielectric substrate W = Width of the patch

Consider Fig. 4 below, which shows a rectangular microstrip patch antenna of length L, width W resting on a substrate of height h. The co-ordinate axis is selected such that the length is along the x direction, width is along the y direction and the height is along the z directio

Fig. 4: Microstrip Patch Antenna

In order to operate in the fundamental TM10 mode, the length of the patch must be slightly less than λ/ 2 where λ is the wavelength in the dielectric medium and is equal to λo / εreff εreff where λo is the free space wavelength. The TM10 mode implies that the field varies one λ/ 2 cycle along the length, and there is no variation along the width of the patch. In the Fig. 5 shown below, the microstrip patch antenna is represented by two slots, separated by a transmission line of length L and open circuited at both the ends. Along the width of the patch, the voltage is maximum and current is minimum due to the open ends. The fields at the edges can be resolved into normal and tangential components with respect to the ground plane.

Fig. 5: Top View of Antenna

Fig. 6: Side View of Antenna

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It is seen from Fig. 6 that the normal components of the electric field at the two edges along the width are in opposite directions and thus out of phase since the patch is λ/2 long and hence they cancel each other in the broadside direction. The tangential components (seen in Fig. 6), which are in phase, means that the resulting fields combine to give maximum radiated field normal to the surface of the structure. Hence the edges along the width can be represented as two radiating slots, which are λ/2 apart and excited in phase and radiating in the half space above the ground plane. The fringing fields along the width can be modeled as radiating slots and electrically the patch of the microstrip antenna looks greater than its physical dimensions. The dimensions of the patch along its length have now been extended on each end by a distance ∆L, which is given empirically as [14].

(2)

The effective length of the patch Leff now becomes:

Leff = L +2∆L (3)

For a given resonance frequency fo, the effective length is given by as:

(4)

For a rectangular Microstrip patch antenna, the resonance frequency for any TMmn mode is given as:

(5)

Where m and n are modes along L and W respectively.For efficient radiation, the width W is given by Bahl and Bhartia as:

(6)

II. Cavity ModelAlthough the transmission line model discussed in the previous section is easy to use, it has some inherent disadvantages. Specifically, it is useful for patches of rectangular design and it ignores field variations along the radiating edges. These disadvantages can be overcome by using the cavity model. A brief overview of this model is given below.

In this model, the interior region of the dielectric substrate is modeled as a cavity bounded by electric walls on the top and bottom. The basis for this assumption is the following observations for thin substrates ( h<<λ) .

Since the substrate is thin, the fields in the interior region do • not vary much in the z direction, i.e. normal to the patch. The electric field is • z directed only, and the magnetic field has only the transverse components Hx and Hy in the region bounded by the patch metallization and the ground plane. This observation provides for the electric walls at the top and the bottom

Fig. 7: Charge Distribution and Current Density Creation on the Microstrip Patch

Consider fig. 7 shown above. When the microstrip patch is provided power, a charge distribution is seen on the upper and lower surfaces of the patch and at the bottom of the ground plane. This charge distribution is controlled by two mechanisms-an attractive mechanism and a repulsive mechanism . The attractive mechanism is between the opposite charges on the bottom side of the patch and the ground plane, which helps in keeping the charge concentration intact at the bottom of the patch. The repulsive mechanism is between the like charges on the bottom surface of the patch, which causes pushing of some charges from the bottom, to the top of the patch. As a result of this charge movement, currents flow at the top and bottom surface of the patch. The cavity model assumes that the height to width ratio (i.e. height of substrate and width of the patch) is very small and as a result of this the attractive mechanism dominates and causes most of the charge concentration and the current to be below the patch surface. Much less current would flow on the top surface of the patch and as the height to width ratio further decreases, the current on the top surface of the patch would be almost equal to zero, which would not allow the creation of any tangential magnetic field components to the patch edges. Hence, the four sidewalls could be modeled as perfectly magnetic conducting surfaces. This implies that the magnetic fields and the electric field distribution beneath the patch would not be disturbed. However, in practice, a finite width to height ratio would be there and this would not make the tangential magnetic fields to be completely zero, but they being very small, the side walls could be approximated to be perfectly magnetic conducting.

IJCST Vol. 6, ISSue 4, oCT - DeC 2015 ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)

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III. Radiation EquationsThe radiation equations are as follows:-

IV. Rectangular Patch Parameters

A. DirectivityDirectivity of an antennae is defined as” the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4π. If the direction is not specified the direction of maximum radiation intensity is implied.” Simply we can say that the directivity of a non isotropic source is equal to the ratio of its radiation intensity in a given direction over that of an isotropic source.Mathematically, directivity can be written as

(7)

If the direction is not specified, it implies the direction of maximum radiation intensity (max directivity) is expressed as

(8)

Where,D = directivity (dimensionless)

=D = maximum directivity (dimensionless)U = radiation intensity (w/unit solid angle)Umax = maximum radiation intensity (w/unit solid angle)U0 = radiation intensity of isotropic source (w/unit solid angle)Prad = total radiated power (w)

For an isotropic source, it is very obvious from above equations that the directivity is unity since U, Umax and U0 are equal to each other.SINGLE SLOTS (k0h<<1)

The directivity of single slot can be expressed as

(9)

Asymptotically the directivity of single slot can be expressed asD1= 3.3(dim ensionless) = 5.2dB W<< λ0

TWO SLOTS (K0h<<1)

The directivity of two slots can be expressed as

(10)

Asymptotically the directivity of two slots can be expressed asD1= {6.6(dim ensionless) = 8.2 dB W<< λ0

B. GainAnother useful measure describing the performance of an antenna is the gain. Although the gain of the antenna is closely related to directivity. It is a measure that takes into account the efficiency of the antenna as well as directional capabilities, whereas the directivity is a measure that describes only the directional properties of the antenna and is therefore controlled only by the pattern. Gain of an antenna is defined as “the ratio of the intensity in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically”.

Thus,

(11)

The gain of a rectangular microstrip patch antenna with air dielectric can be very roughly estimated as follows. Since the length of the patch is half a wavelength, which is same as the length of a resonant dipole, we get about 2dB of gain from the directivity relative to the vertical axis of patch.

C. DesignBased on the simplified formulations that has been described, a design procedure is outlined which leads to practical design of rectangular microstrip antenna. The procedure assumes that the specified information includes the dielectric constant of the substrate (εr), the resonant frequency (fr), and the height of the substrate (h). The procedure is as follows:-

Specify: εr, fr and hDetermine: W, L

Design procedure:1. For an efficient radiator, a practical width that leads to good radiation efficiencies is [16]

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w w w . i j c s t . c o m InternatIonal Journal of Computer SCIenCe and teChnology 257

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(12)

Where μ0 is the free space velocity of light.

2. Determine the effective dielectric constant of the microstrip antenna using (1)3. Once W is found, determine the extension of the length L using (2)4. The actual length of the patch can now be determined by solving for L, or

(13)

V. Result and Summary

A. ResultDesigning a rectangular microstrip antenna using a substrate (RT/duroid 5880) with dielectric constant of 2.2h = 0.1588cm (0.0625 inches) so as to resonate at 10GHz.

Solution:- Using (12), the width W of the patch is

(14)

The effective dielectric constant of the patch is founded using (1), or

(15)

The extended incremental length of the patch is using (2)

The actual length L of the patch is

An experimental rectangular patch based on this design was built and tested. It is probe fed from underneath by a coaxial line.

VI. ConclusionIn this desertation the techniques of transmission line model and cavity model are used for the analysis of rectangular microstrip patch antenna parameters. The parameters determined are gain and directivity. Since numerous work has been done on rectangular microstrip patch antenna since 1950’s so now new form of patch antennas are used such as ring and circular. The bandwidth of all these antennas are low so we can use loading techniques to enhance the bandwidth of antennas.

References[1] Deschamps, G.A.,“Microstrip microwave antennas,” 3rd

USAF Symposium on Antennas, 1953.[2] Gutton, H., G.Bassinot,“Flat Aerial for ultra high frequencies,”

French patent no. 70313, 1955.[3] Howell, J.Q.,“Microstripantennas,” IEEE AP-S Int.Symp.

Digest, pp. 177-180, 1972.[4] Munson, R.E.,“Conformal microstrip antennas and microstrip

phased arrays,” IEEETrans.Antannas, Vol. AP-22, pp. 74-78, 1974.

[5] Carver, K.R, J.Mink,“Microstrip antenna technology,” IEEE Trans. Antennas Propag., Vol. AP-29, pp. 2-24, 1981.

[6] James, J., R, P.S.Hall,“Handbook of microstrip antennas,” Peter, Prengrinus, London, UK, 1989.

[7] Richards, W.F., Lo Y.T., D.Harrison , (1981) “, An improved theory of microstrip antennas and applications,” IEEE Trans. Antennas Propag., Vol. AP-29, pp. 38-46

[8] IERana, N.G Alexopoulos,“ Current Distribution and Input Impedence of Printed dipoles,” IEEE Trans. Antennas Propagat. Vol. AP -29, No. 1, pp. 106-111, 1981.

[9] M.C Bailey, M.D Deshpande,“Integral Equation Formulation of Microstrip Antennas,” IEEE Trans.AntennasPropagat. Vol. AP-30, No. 4, pp. 651-656, 1982.

[10] J.R Mosig, F.E Gardiol,“General Integral Equation Formulation for MicrostripAntannas and Scatterers ,” Proc. Inst. Elect. Eng, Pt. H, Vol. 132, pp. 424-432, 1985.

[11] J.T Aberle, D.M Pozar,“ Analysis of Infinite Arrays of One and Two – Probe- Fed Circular Patches,” IEEE Trans. Antennas Propagat. Vol. AP -38, No. 4, pp. 421-432, 1990.

[12] DH. Schaubert, D.M. Pozar, A.Adrian,“Effect of Microstrip Antenna Substrate Thickness and Permittivity. Comparison of theories and Experiment, ” IEEE Trans. Antennas Propagat. Vol. AP-37, No. 6, pp. 677-682, 1989.

[13] C.A Balanis,“Advanced Engineering Electromagnetics, John Wiley and Sons,” New York, 1989.

[14] E.O Hammerstad,“Equations for Microstrip Circuit Design,” Proc . European Microwave Conf., pp. 268-272, 1975.

[15] W.F .Richards,“Microstrip Antennas,” Chapter 10 in Antenna Handbook. Theory, Applications and Design (Y.T.Lo and S.W.Lee, eds), Van Nostrand Reinhold Co., New York, 1988.

[16] I.J Bahl, P. Bhartia,“Microstrip Antennas, ArtechHouse, Dedham,” MA, 1980.