The construction of a low-cost and reliable array antenna system is required to facilitate the practical use of the adaptive antenna in the mobile communication field. Patch antennas offer effective low profile design for a wide range of wireless application. They are inexpensive to fabricate, light in weight and can be made conformable with planar and non planar surfaces.
The demand of a compact smart antenna design with low cost, light weight and miniature size lead us to study the triangular patch array antenna. In this work, a very comprehensive review of the applications and investigations on triangular microstrip patch (TMP) has been presented.
The main purpose of this work is to investigate the smart nature of this specific shape antenna array for smart antenna applications. A narrow beam-width antenna would be required for smart antenna applications to reduce the effects of multipath fading. A thorough design study was conducted in order to determine the optimal triangular element dimensions to be used in the array.
The initial element design was done by optimizing a single array. It was then used to simulate finite arrays ranging from one to six patches. In this design, six triangular patch arrays are selected. They are equally distributed and centered in the middle and being arranged on a hexagonal substrate. Therefore, it will minimize the antenna size and predominantly compacted.
After successful simulation, the final six patch arrays were fabricated on hexagonal shape FR4. A good return loss has been achieved during simulation and after fabrication. Radiation pattern for this patch antenna has been simulated and measured which has a beamwidth of (62.20). The wide beamwidth of radiation pattern shows that it is not really applicable for smart antenna. The software used for simulation of this design is the CST Microwave Studio (CST MWS), a specialist analytical tool for fast and accurate 3D EM simulation of high frequency design.
Introduction
Microstrip Antennas have achieved a lot of attention in the previous few decades and many research studies have been published in this field. To improve the performance of microstrip antennas, an idea of utilizing triangular configurations is proposed. Before going into the details study of the proposed design and different researchers approach, the basic concept and parameters generally used in antenna design will be discussed for better understanding.
Fundamental Antenna Parameters
Among the most fundamental antenna parameters are impedance bandwidth, radiation pattern, directivity, efficiency and gain. Other characterizing parameters that will be discussed are half-power beamwidth, polarization and range. All of the aforementioned antenna parameters are necessary to fully characterize an antenna and determine whether an antenna is optimized for a certain application. For our design, the most important parameters which we will take into consideration are the Return loss, Directivity, Gain, and Radiation pattern.
To improve the performance of a microstrip patch antenna, we choose the triangular shape with some slots on the patch. We will see that this approach will miniaturize the size of antenna and also improve the performance of microstrip smart antenna system.
Method of Analysis
There are many methods of analysis for microstrip antennas. The most popular models for the analysis of Microstrip patch antennas are the transmission line model, cavity model, and full wave model (Balanis, 2001) (which include primarily integral equations/Moment Method).
Transmission Line Model
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.
(a) Microstrip Line (b) Electric Field Line
Figure 2.11 Microstrip line and its electric field lines
Hence, as seen from Figure 2.11, 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 ( ) must be obtained in order to account for the fringing and the wave propagation in the line. The value of is slightly less then 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 Figure 2.11 above. The expression for is given by (Balkanise, 2001) as:
(2.15)
Where,
= Effective dielectric constant
= Dielectric constant of substrate
h = Height of dielectric substrate
W = Width of the patch
Consider Figure 2.12 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 direction. The same setup will apply to triangular shape antenna.
Figure 2.12 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 where λ0 is the free space wavelength. The TM10 mode implies that the field varies one λ / 2 cycles along the length, and there is no variation along the width of the patch. In the Figure 2.13 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.
(a) Top view (b) Side view
Figure 2.13 (a) Top view, and (b) Side view of antenna
For the triangular shape microstrip patch antenna the effective side length can be calculated (Hassani, H.R et al, 1992) as:
(2.16)
Where, 'a' is the Side length and 'ae' is the effective side length of the patch.
For a triangular Microstrip patch antenna, the resonance frequency for any TMmn (kmn) mode is given by (Bahl & Bhartia, et al, 2001) as:
(2.17)
Where 'c' is the velocity of light, and m, and n shows the modes of operations.
is the dielectric constant, and 'a' is the side length of patch.
For m = 1, n = 0, equation 2.17 can be written as:
(2.18)
Patch Size Calculation
This process provides a reasonably accurate starting point although it does not provide the final patch dimensions. Antenna size depends heavily on the frequency band of operation. For example, a low-frequency band antenna can have a height of several meters (i.e., radio station transmit antennas), while another operating in a much higher frequency band can have several centimetres of length only (i.e., a cell phone antenna).
There are several other factors that contribute to the dimension of the antenna and its behaviour such as the substrate material used and its thickness. The equations have been obtained from (Bahl B. et al, 2001) and the values calculated refer to the dimensions illustrated in Figure 4.2.
Figure 4.2 Dimensions of a single layer element
Specified parameters:
Resonant Frequency: fr = 2.256 GHz
Substrate Permittivity: εr = 4.5
Substrate Thickness: h = 1.6 mm
The width (W), side length (a), and thickness (h) of the patch play a major role in determining its resonance frequency and characteristics. The material of the substrate and its thickness are also important. The thickness of the patch is usually given as a weight, i.e., 1 oz copper corresponds to about 1.2 mil in height (1 mil = 10−3 in).
The transmission line model described in chapter 2 will be used to design antenna.
To design the triangular microstrip patch array, we have to follow the following steps, for antenna parameters.
Step 1: Calculation of Resonance Frequency
Resonance frequency corresponding to various modes describes by kmn is given by (Bahl B. et al, 2001)
(4.1)
The other set of suggestions proposes replacing both 'a' and '' with their effective values. An expression for '' has been arrived at by curve fitting the experimental and theoretical results for the resonance frequency for TM10 mode. It is given by:
(4.2)
Step 2: Calculation of the Effective Side Length
The effective side length of equilateral triangle can be calculated by equation 2.16 given in chapter 2, as
(4.3)
Since, all sides of equilateral triangle are equal, therefore to calculate one side will be the same for all sides. By using the above formula we will get the effective side length of all the three side.
Bahl and Bhartia (Bahl I.J & Bhartia P, 1980) proposed that in addition to replacing 'a' in (4.1), should also be replaced by the effective value
(4.4)
Substituting , and , we will get
Step 3: Calculation of Effective Dielectric Constant
Substituting the values of, h, and 'a' in equation (4.4) gives the effective dielectric constant as:
Table 4.3 and 4.4 will simplify the parameters used in our design.
Table 4.3: Calculated parameters of triangular patch antenna
Dimension
Calculated (mm)
Implemented (mm)
Antenna length
41.791
42
Microstrip feed width w
3
3.0
Slot length s
2.5
2.5
Table 4.4: Implemented parameters of triangular patch antenna
Resonance frequency (fr)
2.256 GHz
Side length of patch (a)
42 mm
Substrate permittivity ()
4.5
Substrate height (h)
1.6 mm
Slot (S)
2.5 mm
After the calculations of all necessary parameters of our design, we will go one step further, to simulate our design.
CST Simulation and Optimization
The single element design was put into CST with the previous calculated dimensions and the following initial dimensions:
Width of 50 Ohm input transmission line: w = 3 mm
Length of 50 Ohm input transmission line: l = 10 mm
The transmission line was located at the edge of the triangular patch in the centre of the width side. (The set up for CST is described in appendix A).
The design was then simulated for range of frequencies (typically 1.0 to 3.0 GHz) and s-parameters were inspected. This scattering parameter is also known as return loss and it specifies the ratio of the reflected signal to the input signal. It is usually used to determine how well the feedline is matched relative to the antenna patch (in terms of impedances). From this graph the impedance bandwidth can also be measured. It is defined as being the range of frequencies for which the return loss response is below -10dB. Figure 4.3 shows the return loss that was obtained.
s11
Figure 4.3: Return loss for single patch
During simulation, it has observed that which parameters affect the antenna characteristics. The length L of the patch determines the resonant frequency whereas the patch width W and transmission line length determine the coupling between the transmission line and the patch element and therefore the return loss and bandwidth.
In order to discuss the other parameters for single patch its better to take the 6 patch triangular antenna and discuss all the parameters for it. So it will cover the necessary part of single as well as full antenna.
Six Patch Triangular Microstrip Antenna Design
All the calculation for 6 patch triangular microstrip antenna design is the same as for the single patch. The only difference is the hexagonal shape of the substrate, which will make our design prominent in case of shape and miniaturization. The proposed antenna geometry of our design is shown in Figure 4.4, by using CST software.
Figure 4.4 Triangular microstrip patch antenna using CST
After the successful simulation results of triangular microstrip patch antenna, the final fabrication of our design is needed. The final fabricated triangular microstrip patch array design is shown in Figure 4.5 below.
Figure 4.5: Triangular patch antenna through fabrication
To explain all the important antenna parameters in details, its better to include a new chapter specified only to simulation and test and measurement results and discussion. In Chapter 5, all the necessary parameters are explained in details.
4.8 Summary
This chapter has given a great emphasis on design aspects in implementing triangular patch antenna. Calculation steps for triangular patch antenna are meticulously elaborated. Similarly simulation steps are described in Appendix A. Design calculations for all side length, and width of antenna is also explained in detail. Furthermore, the details of dielectric constant of FR4 are also explained in details.
The 6 feeding microstrip triangular patch antenna operating on 2.256GHz and match with 50Ω impedance are designed. The fabricated triangular microstrip antenna parameters, which are the S-parameter, radiation pattern and the antenna gain, were measured. These parameters were then compared with the simulation results.
Triangular microstrip Antenna Simulation Results
The designed triangular microstrip antenna was simulated using CST Microwave Studio for its S-parameter performances, radiation patterns, and antenna gain. During the simulation, unit was set to mm, and GHz and the frequency range was set from 1.0 to 3.0 GHz. The maximum stop frequency sustained by the computer system in use was 3 GHz.
S-Parameter Simulation Results And Discussion
S- Parameter result for Single Feed
Figure 5.1 shows the S-parameter simulation results of S11, S22, S33, S44, S55 and S66 for port 1, port 2, port 3, port 4, port 5, and port 6 respectively. As we can see that the return loss for port 1, port 3, port 4, and port 6 have almost the same return loss at frequency 2.256 GHz, similarly port 2 and port 5 have the same return loss at 2.256 GHz. This is because the electric field for different position of the patch is different, and the dielectric constant of material is changing with the change of electric field to the position of the patch.
s11
s22
(a) S11
(b) S22
s33
s44
(c) S33
(d) S44
s55
s66
(e) S55
(f) S66
Figure 5.1: Return loss Result from the S-parameter CST simulation. (a) S11 (b) S22 (c)S33 (d) S44 (e) S55 (f) S66
All the single port excitation shows good return loss at value below -10dB. The best return loss we have obtained is -40.8 dB at port 4. The return loss is the best value of infinite or when the reflection coefficient, is equal 0. In practical, it is impossible to get the infinite log magnitude for the reflected signal since there will be losses whether due to the dielectric loss, conductor loss or surface loss. Hence, the task is to minimize the losses by matching load impedance to the characteristic impedance which is 50Ω. This can be done by adjusting the length of dimension of the antenna to 44mm and the width of the feeder to 2mm. From the results, the return loss (dB) for each ports is below -22dB. This means the reflected signals are low when operate in the resonant frequency.
Figure 5.2, shows the S-parameter simulation results by feeding two ports simultaneously. The adjacent port 1 and 2 has return loss of -46.17 dB at resonance frequency 2.222 GHz, which shows that the combined effect port 1 and 2 give the better return loss. The S- parameter simulation results for the port (1, 3) is -21.79 at resonant frequency 2.258 GHz. The return loss for port (1, 4) is -40.89 at the same resonance frequency 2.26 GHz.
From S-parameters result, it can be concluded that the return loss at different port and different position are changing. The return loss is still below -10 dB, which shows the matching of load impedance to the characteristic impedance.
port1,2
(a) Return loss result of port (1, 2)
port1,3
(b) Return loss result of port (1, 3)
port1,4
(c) Return loss result of port (1, 4)
Figure 5.2 S- parameters result for (a) port (1, 2) (b) port (1, 3) (c) port (1, 4)
portsemua
Figure 5.3 CST S- parameters result for all ports simultaneously
The summary of S- parameters results for excitation of single port and two adjacent ports at a time, alternate ports, and opposite ports are shown in Table 5.1 and Table 5.2
Table 5.1 Resonance frequency and return loss for single port excitation
Excitation
Cut off frequency
Return Loss
Port 1
2.256 GHz
-39.92
Port 2
2.256 GHz
-22.87
Port 3
2.256 GHz
-39.91
Port 4
2.256 GHz
-40.8
Port 5
2.256 GHz
-19.81
Port 6
2.256 GHz
-40.78
Table 5.2 Resonance frequency and Return loss for different port excitation
Excitation
Cut off frequency
Return Loss
Port1,2
2.222GHz
-46.17
Port1,3
2.258GHz
-21.79
Port1,4
2.26GHz
-40.89
All ports
2.176GHz
-19.77
From Table 5.1, it can be observed that resonant frequency for each port are the same and return loss for each port are different. From Table 5.2, the resonance frequency for excitation of port1, 2, port1, 3 and port1, 4 are changing slightly with different position, but still have good return loss, which is less than -10 dB. However, simultaneous excitation for all port indicates slightly differences from single port. From the Table 5.1, resonant frequency and return loss are 2.176GHz and -19.77dB respectively for port 1, 4. This difference is due to the array excitation of the six ports which produces fields that in turn interfere constructively and destructively.
