design of circularly polarized microstrip antenna using truncated corner method

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Design of single feed circularly polarized microstrip antenna using truncated corner method Mohd Aly Rajaie bin Halim and Puan Elfarizanis bt Baharudin Department of Electronic communication Engineering Faculty of Electrical and Electronic Engineering Kolej Universiti Teknologi Tun Hussein Onn Beg Berkunci 101, 86400 Parit Raja , Batu Pahat, Johor E-mail: [email protected] ABSTRACT Circularly polarized (CP) antennas are often used in deep space or satellite communications. In general, an antenna will radiate an elliptical polarization, which is defined by axial ratio, tilt angle, and sense of rotation. This project was developed to implement the concept of circularly polarized microstrip antenna. A simple design for circularly polarized single feed microstrip square patch antenna is presented. The design method consists of pair of truncated corners with suitable cutting side length is introduced at the patch corner as perturbation elements or perturbation segments. The main objective of this project is to design and investigate the characteristic of the circularly polarized microstrip antenna that operate at 2.4 GHz frequency operation. Keywords-: circular polarization, return loss, axial ratio, bandwidth, voltage standing wave ratio, left hand circular polarization, right hand polarization. 1. INTRODUCTION Microstrip antenna (MA) consists of a patch of metallization on a grounded substrate. These are low profile, lightweight antennas, most suitable for aerospace and mobile application. Because of their low-power handling capability, these antennas can be used in low-power transmitting and receiving application. Microstrip antenna offers over than other type of antennas due to their advantages and increasingly used in a variety applications such as military, industry, and wireless communication. Increasing demands for circular polarization microstrip antenna for communication systems has directed researchers to improve the performance of circularly polarized microstrip antenna. This project is a hardware and software design. It will involve the planning, designing, 1

Transcript of design of circularly polarized microstrip antenna using truncated corner method

Page 1: design of circularly polarized microstrip antenna using truncated corner method

Design of single feed circularly polarized microstrip antenna using truncated corner method

Mohd Aly Rajaie bin Halim and Puan Elfarizanis bt Baharudin

Department of Electronic communication EngineeringFaculty of Electrical and Electronic Engineering

Kolej Universiti Teknologi Tun Hussein OnnBeg Berkunci 101, 86400 Parit Raja , Batu Pahat, Johor

E-mail: [email protected]

ABSTRACTCircularly polarized (CP) antennas are often used in deep space or satellite communications. In general, an antenna will radiate an elliptical polarization, which is defined by axial ratio, tilt angle, and sense of rotation. This project was developed to implement the concept of circularly polarized microstrip antenna. A simple design for circularly polarized single feed microstrip square patch antenna is presented. The design method consists of pair of truncated corners with suitable cutting side length is introduced at the patch corner as perturbation elements or perturbation segments. The main objective of this project is to design and investigate the characteristic of the circularly polarized microstrip antenna that operate at 2.4 GHz frequency operation. Keywords-: circular polarization, return loss, axial ratio, bandwidth, voltage standing wave ratio, left hand circular polarization, right hand polarization.

1. INTRODUCTION

Microstrip antenna (MA) consists of a patch of metallization on a grounded substrate. These are low profile, lightweight antennas, most suitable for aerospace and mobile application. Because of their low-power handling capability, these antennas can be used in low-power transmitting and receiving application. Microstrip antenna offers over than other type of antennas due to their advantages and increasingly used in a variety applications such as military, industry, and wireless communication. Increasing demands for circular polarization microstrip

antenna for communication systems has directed researchers to improve the performance of circularly polarized microstrip antenna. This project is a hardware and software design. It will involve the planning, designing, simulating, fabricating, and testing process. The simulation process will be done using software while the fabrication process will be done using milling machine. The objectives of this project are to design and fabricate the circularly polarized microstrip antenna using truncated square patch which can operated at 2.4 GHz frequency resonant. The other objectives are to investigate the characteristics of this antenna and analyze the overall results that get from simulation and measurement.

2. POLARIZATION

Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two special cases of elliptical polarization are linear polarization and circular polarization. The initial polarization of a radio wave is determined by the antenna. In circular polarization, the electric field vector appears to be rotating with circular motion about the direction of propagation, making one full turn for each RF cycle. This rotation may be right-hand or left-hand. Choice of polarization is one of the design choices available to the RF system designer. There are several advantages of circular polarization compare with other polarization such as reflectivity, absorption, phasing issues, multi-path, inclement weather, and line of sight.

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3. INPUT RETURN LOSS (S11)

Input return loss or S parameter is known as scattering parameters. S parameter is a set of parameters describing the scattering and reflection of traveling waves when a network is inserted into transmission line. S parameters are normally used to characterize high frequency network, where simple models valid at lower frequencies cannot be applied.

4. BANDWIDTH

The bandwidth can be the range of frequencies on either side of the center frequency where the antenna characteristics like input impedance, radiation pattern, beam width, polarization, side lobe level or gain, are close to those values which have been obtained at the center frequency. The bandwidth of a broadband antenna can be defined as the ratio of the upper to lower frequencies of acceptable operation

5. SINGLE FEED CP ANTENNA

Various type of circularly polarized antennas have been reported so far with several feeding techniques [3]. Two commonly used CP antennas are a single feed corner truncated square patch antenna and a dual feed square patch with 90 degrees phase shift between two feeds [4].

In this paper truncated corner square patch antenna is used. In this configuration, the orthogonal field components in phase quadrature are excited by feeding it using coaxial feed. The signal injected by the feed fields to propagate in one direction guided transmission line formed by the patch. The perturbation section scatters the feed fields into a mode that is partially orthogonal to the previous mode. In actually, perturbations create two new modes modifying the original modes. For circular polarization these two modes must have equal amplitudes and differ in phase by 90 degrees which can controlled by proper setting of feed point and perturbation section [5]. The condition for circular polarization and expressions for the size of perturbation and orthogonal modes are related to antenna unloaded Q. the relations are expressed as follows [7]:

(1)

Where, Δs is the areas of the perturbations with x length and Q is the quality factor for a square patch with dimension a. Figure 1 shows the geometry of CP antenna.

Figure 1: Circularly polarized single fed patch antenna

6. METHODOLOGY

There are a few things need to be considered in this project. Some important steps must be followed to ensure the aim of the project

achieved. They are started by planning, designing, then simulating, fabricating, and testing process.

Figure 2: The methodology of the project.

A. Planning

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There are many materials can be used in order to design a microstrip antenna, for example, RT/DUROID 5880, R3003, R04003, and FR4. However, FR-4 is decided to be used since the cost is cheaper rather than others. The FR-4 has the dielectric constant 4.5 and thickness 1.6 mm.

B. Design parameters

The antenna was aimed to operate at 2.4 GHz with input impedance 50Ω. Measured resonant frequency is usually lower than the resonant frequency obtained by software due to manufacture tolerance, measurement tolerance and accuracy of software. To calculate the perturbation segments, it is necessary to know the unloaded Q value of the antenna as a function of substrate thickness and dielectric constant. Standard equations are given to calculate the Q in [3] and [5]. After determining length the square patch, quality factor related to the substrate thickness is calculated. Then amount of perturbation is calculated by (1). Consequently, the dimension of the antenna is:a=29.118mm, x=5.34mm,

Figure 3: The truncated corner microstrip antenna

C. Simulation

The simulation process of the antenna is using CST microwave studio. Using software, the truncated segments as well as the patch dimension is adjusted to yield a peak axial ratio at the desired frequency.

D. Fabrication

In this paper single fed CP antenna are designed with using FR-4 as substrate layer with εr=4.5, dielectric thickness 1.6 mm. The antenna is fabricated at UTHM Printed Circuit Board (PCB) Fabrication Laboratory. The SMA connector is soldered near the edge of the patch and a radiation characteristic for this element is measured. Figure 4 shows the CP antenna after fabricated process.

Figure 4: CP truncated corner microstrip antenna

E.Testing

There are two types of testing or measurements involve in this project. One is the return loss, input impedance, vswr, test using network analyzer. The other part is the antenna radiation pattern test which involves the use of equipments like rotating antenna platform, transmitter, and using CASSY Lab software. Figure 5 shows the measurement of CP microstrip antenna.

Figure 5: Radiation pattern setup

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7. RESULT AND ANALYSIS

In figures 6 and 7 simulated and measured results of return loss of antenna have been represented. In simulation process the antenna has been considered lossless and therefore minimum of measured return loss is different from simulated result. Based on figure 6, the input return loss of the antenna at 2.4 GHz operating frequency is -26.81 dB, this value is larger than -10 db and it’s accepted in order to fabricate an antenna. Besides that, this value also shows the feeder point is the optimum feeder point. In figure 7, the input return loss for this antenna is -23.785 db at 2.4 GHz. Figure 8 show the Comparison input return loss (dB) from simulation and measurement. it is clearly seen that there is a frequency shift between the measured and simulation of the microstrip antenna. The exact value is 2.415 GHz which is the value of return loss is -27.3 dB. This means that the measured resonant frequency is shifted 15 MHz over the operating frequency. It is mainly due to the size of the patch especially the length (L) and the perturbation segments for the microstrip antenna. Due to inaccuracy in fabrication process, the size of patch will either increase or decrease. This will definitely bring to the shifting in the resonant frequency which the increase in the length of the patch will decrease the resonant frequency or vice versa. The difference in return loss reading between the measured and simulated result is mainly caused by the feed-point location. The effect of the feed-point location will bring to the condition where the antenna is either matched or mismatched with the feed line. If the antenna is mismatch, not all the available power from the source is delivered to antenna. This loss is called return loss (RL). the variation of return loss value indicates the variation in the reflection coefficient value. The decrease in return loss (<negative) also indicates increase in the mismatch between the antenna and the transmission line.

Figure 6: Simulated input return loss

Figure 7: Measured input return loss

Figure 8: Comparison input return loss.

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Another important measurement for microstrip antenna is standing wave ratio (SWR). it means that the indication of the match between input impedance of antenna and impedance of the signal source. Figure 9 show the value of VSWR on this antenna. The VSWR for this antenna is 1.096 and it’s below 2, so, this antenna can operate at 2.4 GHz frequency operation. Figure 10 show the measurement result of this antenna, the value of VSWR for this antenna is 1.1371 and its means that this value lowers than 2. So, this antenna is well match and can operate at 2.4 GHz operating frequency. Since the VSWR, return loss and reflection coefficient are related to each other which are use to determine the matching between antenna and transmission line. Therefore, the reason that causes the difference in VSWR reading due the improper placement of feed-point location.

Figure 9: voltage standing wave ratio (simulated)

Figure 10: voltage standing wave ratio (measured)

Figure 11: Comparison value of VSWR

Figure 11 illustrates the impedance locus from 2.0 GHz to 3 GHz. The simulated input impedance at 2.4 GHZ operating frequency is 54.41 – j 1.807 Ω. This means that the antenna is not well match to the feed line of 50 Ω, and the figure 12 show the measurement result of input impedance, the input impedance at 2.4 GHz for this antenna is 54.053 + j 5.2129 Ω. This measured value also not well match to the feed line of 50Ω.

Figure 11: Input impedance (simulated)

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Figure 12: Input impedance (measurement)

Figure 13 and 14 shows the polarization of this antenna. This antenna has two type polarizations, which is left polarization and right polarization. From figure 4.12, at frequency 2.4 GHz, this antenna has angular width (3db) about 81.3 degrees and for right polarization, the angular width (3db) about 92.3 deg.

Figure 13: Left polarization (LHCP)

Figure 14: Right polarization (RHCP)The antenna gain of this microstrip antenna is defined in far-field region. From simulation at figure 15, the gain of this antenna is 6.293 dB at operating frequency of 2.4 GHz.

Figure 15: Simulated gain of microstrip antenna.

The radiation pattern in the ABS, THETA, and PHI component in 2D for the microstrip antenna are shown in Figure 16, 17, 18. THETA component is also can be termed as elevation, which means the angle varies from -90 degrees (straight down) to +90 degrees (overhead). Besides, PHI component is also can be termed as azimuth angle, which mean the angle varies from 0 to 360 degrees. From Figure 16, it can seen that at 2.4 GHz frequency, the main lobe magnitude for microstrip antenna is -6.3 dBi, and the value of main lobe direction and angular width (3dB) is 5.0 and 158.2 degrees. Besides, Figure 18 shows the radiation patterns in PHI component or azimuth angle for microstrip antenna. At 2.4 GHz frequency, the main lobe magnitude for microstrip antenna is 6.1 dBi., and the value of angular width (3dB) is 87.1 degrees.

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Figure 16: Radiation pattern for theta component

Figure 17: Radiation pattern for ABS component

Figure 18: Radiation pattern for PHI component.

8. CONCLUSION

The circularly polarized truncated square microstrip antenna at 2.4 GHz has been designed, fabricated and tested. Measured results of resonant frequency, return loss, radiation pattern, bandwidth were presented. The objective of this project not fulfilled reached because the resonant frequency is shifted 15 MHz over the 2.4 GHz. Since bandwidth is related to quality factor of the square patch. By comparing the simulated values obtained from the CST microwave studio with the measured values of the fabricated microstrip antenna, it ca be seen that the real antenna actually have better performance than predicted by the simulation software. Through this project, much experience and knowledge about microstrip antenna have gained. From the design process, the knowledge of using CST microwave studio to design the microstrip antenna would be an invaluable tool for my future career as an engineer. During the testing process, the experience of using network analyzer, CASSY Lab, will enhance my hand on skills.

ACKNOWLEDGMENT

The author would like to thank to his supervisor Puan Elfarizanis Bt Baharudin who throughout the year has been advising, guiding, supporting and keep motivating him till the completion of the project and for all companions and friends for supporting the author till the completion of the project.

REFERENCES

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[5] Johan Laqerqvist (2002) . “Design and Analysis of Electrically Steerable Microwave Antenna for Ground to Air Use.” Lulea University of Technology : Master’s Thesis.[6] D.M Pozar, “ Microstrip Antenna”, Proc,

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[8] Kumar,girish. and Ray, K.P. (2003). “Broadband Microstrip Antennas.” Artech House.

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[11] M. Niroojazi, M.N. Azarmanesh. “ Practical Design of Single Feed Truncated Corner Microstrip Antenna”. IEEE, (CNSR’04).

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