Microstrip antennas became popular with the advent of cellular communication technology due to their compact size, light weight, ease of manufacture, reliability, and thin profile that allows for conformal mounting. If a single antenna can be used for multi-frequency operation, the complexity of the mobile communication antenna system could be reduced. The goals of the project are to become familiar with the concepts of microstrip antenna and to analyze the multiband behavior of microstrip antenna in terms of multiband characteristics, frequency separation of bands, dependence of resonant frequency on the dimensions and positions of the slots and the influence of the feed location. on the behavior of the antenna.
The project work would analyze typical rectangular microstrip antenna structures by modeling and simulating the antenna structure using HP High Frequency Structure Simulator (HP HFSS) software and draw conclusions about the findings. The project "Investigation of multiband behavior of microstrip antennas" requires an analysis of the electromagnetic behavior and characteristics of microstrip antennas. Microstrip antennas have become quite useful in recent years due to their compact size, light weight, thin profile making their mounting compliant, ease of manufacture, reliability and multi-band behavior.
Problem Statement
Various aspects will be studied to investigate the multiband characteristics of this antenna, which will be discussed further in this report. The increase in the number of mobile communication devices and satellite communications has meant an increase in the need for antennas that are integrated into ever smaller devices.
Scope of Study
From the literature review, the main interest would be to understand the basics of the microstrip antenna, which serves as an introduction to the microstrip antenna. Microstrip antenna configuration, characteristics, theory and concepts are prerequisite knowledge to follow this project. Related information such as boundary condition, fractals and transmission line characteristics are important to gain a clear picture of the project.
Micro strip Antenna Configuration and Characteristics
The thickness of the substrate, d, is usually very small compared to the wavelength (d<<λ).
Feeding Methods
Microstrip Line
Coaxial-Line
Aperture Coupling
Proximity Coupling
Boundaries Overview
- Port
- Perfect H
- Perfect E
- Symmetry Planes
- Ground Plane
- Conductor
- Resistor
- Radiation Boundary
- Restore
If radiation barriers are used in a structure, the ground plane acts as a shield for the far-field energy, preventing the waves from propagating beyond the ground plane. To model a surface as lossy, the parameters Siemens/meter and permittivity enable a surface to be specified that is an imperfect conductor. A surface or boundary can be modeled as an impedance, and the real and imaginary parts of an impedance can be described in ohms/square.
Radiative boundaries, also referred to as absorbing boundaries, enable a surface to be modeled as open: waves radiate outward, infinitely far into space. Radiation boundaries can also be placed relatively close to a structure and can be arbitrarily shaped. For example, it can be used to model a hom antenna aperture within a radiation boundary.
Fractals and Multi band Behavior of Antenna
Transmission Line Characteristic
Relationship ofMicrostrip Antenna Parameters
The table below shows the antenna dimension, resonant frequency and wavelength calculated from the equations above.
Scattering Parameters
Both ports are transmission lines with characteristic impedance Z0 • For transmission lines, voltage waves can propagate in two directions. From the above equation, each S parameter can be expressed in terms of the ratio of output and input voltage phasor. Each S parameter is the ratio of an outgoing wave to an incoming wave under the constraint that one of the ports is terminated with a non-reflective or matched load.
From the equations above, S11 is simply the reflection coefficient seen at port I when port 2 is terminated by a matched load. At the resonant frequency, the reflection is minimum (maximum power is radiated to the room), which gives high S-amplitude in negative dB scale as shown in the figure below.
Tool .............................................................................................................. l9
- Calculating Dimensions and Drawing the Geometric Model
- Defining Ports and Boundaries
- Solving for the S-Parameters
It is a general-purpose tool that can be used for a variety of electromagnetic (EM) modeling applications, including antenna design and analysis, machined component design and analysis, circuit design and analysis, and high-speed digital circuit design and analysis. The HP HFSS drawing interface (based on the industry standard AutoCAD drawing tool) can be used to draw the geometry of any structure of interest in modeling. The gates, materials, and boundaries of the structure must be identified before solving for the electromagnetic fields at the specified frequencies and desired accuracy.
This section describes the steps for modeling the structure and analyzing its electromagnetic behavior using HP HFSS. First, some dimensions of the structure (microstrip antenna) must be calculated using the relation formulas (see 2.6) - the dielectric constant, operating frequency, ground plane dimensions, patch dimensions and characteristic impedance can be preset values. This final geometric drawing can be shaded and displayed from any angle and in different colors for better visualization of the structure.
If the structure contains frequently used parts, it can be quickly drawn by selecting objects from the standard part library provided with the software. The materials from which the structure is made can be determined during the drawing of the structure or after the structure is completed. The next step is to determine the boundaries of the structure and determine and calibrate the gate.
If the S parameters are to be included in a circuit simulation, they can be renormalized to 50 ohms with a simple menu command. A gate-only solution must be specified for rapid 2D analysis of the gates of the structure. Animated shaded field plots shift the phase in which the plot is displayed, simulating the fields as they propagate through the structure in real time.
These shaded plots can be rotated to obtain different perspectives and an optimal viewing angle for the structure.
Effect of Changing the Patch Dimensions and Feed Locations
As can be observed from the results shown in Figure 4-1 to Figure 4-8, the resonance frequencies change when the dimension of b changes. When the feed line is placed in position 1, the graph in Figure 4-1 and Figure 4-3 shows an acceptable result, but Figure 4-5 and Figure 4-7 do not show the desired result. When the feed line is placed in position 2, the graph in Figure 4-6 and Figure 4-8 shows an acceptable result, while Figure 4-2 and Figure 4-4 do not show the desired result.
These results only show that the resonant frequencies at which antennas radiate most efficiently can be changed by arbitrarily changing the dimensions of the patch (with the feedline located at a specific location).
Varying Feedline
Effect of Slot within the Patch and Varying the Slot Position
When slots are tuned, the resonant frequencies for some slot locations change at certain values of S compared to those without the slot. Theoretically, with a slot in the patch, current will take a longer path through the patch, so the antenna will effectively have a larger dimension (a and b). Note that the location of the slot is not important now, as it is simply placed to see if the resonant frequencies will be shifted compared to the no slot.
For the rest of the simulations, S is chosen to be 3.6 em so that the computational time required to run the simulation can be reduced. A total of 15 window positions were chosen, as shown in Figure 4-11 below, to investigate the effect of the slot position on the resonant frequency of the antenna. Simulations with the window positioned along the line a/2 parallel to side b (as shown in Figure 4-13) were done to see if f1 would be shifted more.
Using the center of the patch as a reference, the center of the window at position 3 is 0.225 em away. Similarly, the center of the window at position 2 and position 3 is 0.45 em and 0.675 em away, respectively. From the three S-magnitude plots, the resonance frequencies only changed when the gap is placed in position 1.
This means that there is no significant shift of f1 when the window has been moved along sides a and b.
New Window Shape
Window Size
When the window size is changed from 0.1 to 0.9 times the patch size, the third resonance frequency component appears at window sizes of 0.7 and 0.8 times the patch size. If at least one of the dimensions of the window (a* and/or b*) has the length between 1.8 to 3 em, the corresponding resonant frequency will be somewhere between 3.45 and 5.75 GHz. The following simulation result shows the S-size plot when using the same window in Figure 3-7 with the size doubled.
Window Rotation
The simulation results showed an obvious relationship between the slicing dimensions and the feed location with the S-size plot or the multi-band characteristics of the microstrip antenna. Also, the resonant frequencies of the microstrip antenna can be shifted by inserting the slot or window inside the patch. This provides a means to control the value of the resonant frequency at constant patch dimension by manipulating the position, orientation and size of the slot.
Therefore, the dependence of the resonant frequencies of the microstrip antenna on the patch dimension can be reduced by the presence of the slot. However, the obtained results can be applied to rectangular microstrip patch antennas with sizes in terms of their ratio a to b (refer to figure 3-4) equal to the investigated cases. With more investigations on the slot effect, microstrip antennas can be designed with frequency splitting as a specification.
Further improvement of the project in the future may include the integration of these aspects - patch dimensions (a and b), feed locations and slot variations - to ensure the correct orientation of the microstrip antenna design. The scope of the project could be expanded to include investigations of other patch shapes and slot manipulations that could produce more than two effective resonant frequencies. Best On the Significance of Self-Similar Fractal Geometry in Determining the Multiband Behavior of the Sierpinski Gasket Antenna,” IEEE Antennas and Wireless Propagation Letters, Volume 1.
Werner og Suman Ganguly, februar An Overview of Fractal Antenna Engineering Research", IEEE Antennas and Propagation Magazine, bind 45, nr. 7] Carles Puente-Baliarda, Rafael Pous og Angel Cardama, april On the Behavior of the Sierpinski Multiband Fractal Antenna" , IEEE Transaction on Antennas and Propagation, bind 46, nr. 4, s. 517-524.
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