Bandwidth enhancement of Microstrip Antennas

Microstrip antennas have become the favorite choice of antenna designers because they offer the attractive advantages of low profile, light weight, easy fabrication and easy integration with circuits. The disadvantage of microstrip antenna is its narrow bandwidth. During the recent years, the capacity issue for wireless communication applications is not easily solved, with the expansion in mobile communication systems and the number of people using their services, much effort has been devoted to the bandwidth enhancement of microstrip antennas, and many techniques have been proposed. In addition, applications in present-day mobile communication systems usually require smaller antenna size in order to meet the miniaturization requirements of mobile units. The techniques used in miniaturization further spoil the bandwidth of the antenna. A comprehensive enumeration of all the techniques used in this regard follows:

• Use of impedance matching network
• Multiple resonators arranged in stacked or co-planar structure
• Using lossy materials
• Reactive loading using U-shaped slot
• Capacitively probe-fed structure
• L-probe feeding
• Using thick foam substrate
• Using proximity-coupling
• Using a gap-coupled probe feed or a capacitively coupled feed
• Meandered ground plane
• Slot-loading
• Stacked shorted patch
• Feed modification
• Chip resistor loading
• Teardrop dipole in an open sleeve structure
• Using air substrates
• Using shorting posts between the patch and the ground plane
• Three dimensional patches like V-shaped patch or wedge -shaped patch

Electromagnetic Simulation of Antennas

Electromagnetic simulation is a new technology to yield high accuracy analysis and design of complicated microwave and RF printed circuit, antennas, high speed digital circuits and other electronic components.

The dimensions and other calculated parameters(using MATLAB) are not sufficient to directly fabricate an antenna unless they are further simulated and tested. This is done to increase the efficiency of the design process as it reduces the number of costly manufacture-and-test cycles. The deviation of the actual results from the calculated values is due to many reasons

• Low accuracy of the model used to calculate the parameters.
• Assumptions taken for modeling the antenna on a certain model may not be valid under practical conditions.
• Neglecting certain parameters in he theoretical calculations for expediency may affect the characteristics.
• And finally to account for certain unforeseen conditions that might occur practically for which the theoretical model may not be sufficient, but rigorous simulators take this into account.

I am including a list of the softwares one could use for this purpose. Most of them let you download a evaluation version on making a request. I have personally used IE3D and Ansoft HFSS, and I found both of them quiet effective. By the way the list of softwares is by no means exhaustive.


For planar antennas

1. IE3D is a full-wave, method-of-moments based electromagnetic simulator solving the current distribution on 3D and multilayer structures of general shape. It has been widely used in the design of MMICs, RFICs, LTCC circuits, microwave/millimeter-wave circuits, IC interconnects and packages, HTS circuits, patch antennas, wire antennas, and other RF/wireless antennas.

You can also request a 45 days evaluation version on the Zealand site.

2. Momentum is a 3-D planar electromagnetic (EM) simulator used for passive circuit analysis. It accepts arbitrary design geometries (including multi-layer structures) and accurately simulates complex EM effects including coupling and parasitics. Momentum works together with Agilent's Advanced Design System (ADS) to compute S-, Y-, and Z-parameters of general planar circuits. Microstrip, stripline, slotline, coplanar waveguide, and other circuit topologies can be analyzed quickly and accurately with Momentum. Vias that connect one layer to another can also be simulated; enabling design engineers to more fully and accurately simulate multilayer RF/MMIC's, printed circuit boards, hybrids, and Multi-Chip Modules (MCMs).

The simulator is based on the Method of Moments (MoM) technology that is particularly efficient for analyzing planar conductor and resistor geometries. It is designed to evaluate multi-layer planar geometries and generate EM accurate models that can then be used directly in ADS circuit simulators including Harmonic Balance, Convolution, and Circuit Envelope.

Circuit simulators must explicitly account for signal coupling and are also limited to designs that can be constructed only from available circuit models. The Momentum EM simulator overcomes the limitations of available models by implicitly accounting for signal coupling.

For 3D structures

1. Ansoft HFSS (High Frequency Structure Simulator) is an integrated full-wave electromagnetic simulation and optimization package for analysis and design of 3-dimensional microstrip
antennas and high frequency printed circuits and digital circuits such as microwave and millimeter-wave integrated circuits (MMICs) and high speed printed circuit boards (PCBs). HFSS™ utilizes a 3D full-wave Finite Element Method (FEM) to compute the electrical behavior of high-frequency and high-speed components. With HFSS, engineers can extract parasitic parameters (S, Y, Z), visualize 3D electromagnetic fields (near- and far-field), and generate Full-Wave SPICE™ models to effectively evaluate signal quality, including transmission path losses, reflection loss due to impedance mismatches, parasitic coupling, and radiation.


2. CS Microwave Studio is a tool for the 3D EM simulation of high frequency components. Applications include typical MW&RF like in mobile communication, wireless design, but also increasingly signal integrity, and EMC/EMI.

CST promotes Complete Technology for 3D EM. Users of the software are given flexibility in tackling a wide application range through a variety of available solver technologies. Beside the flagship module, the broadly applicable Time Domain solver and the Frequency Domain solver which simulates on hexahedral as well as on tetrahedral grids, CST MWS offers further solver modules for specific applications. Filters for the import of specific CAD files and the extraction of SPICE parameters enhance design possibilities and save time. In addition, CST MWS is embedded in various industry standard workflows through the CST DESIGN ENVIRONMENT™.

3. Empire is a powerful and fast 3D electromagnetic field software based on 3D FINITE DIFFERENCE TIME DOMAIN METHOD which includes specially modified algorithms and methods for efficient utilization of multiple floating points and caches to achieve ultra fast solution times and excellent accuracy. Empire is 15 to 20 times faster than any other 3Dsimulation software on the market. The recently launched new GUI (GANYMEDE) offers new tools for easy construction and modification of the objects analyzed. The polygon editor, priority modelling modes and built in script allow very fast set up of solutions and modifications on the go without the necessity of restarting problems from scratch analysis of devices such as Helix antennas for example are analysed and optimized in minutes. Empire claims to be effectively faster than other relevant simulation software available in the market.

Matlab Patch Calculator GUI

This is a snapshot of the GUI I created for calculating rectangular patch antennas. It uses the Transmission Model to calculate the dimensions and the Cavity model to calculate radiation patterns.



In order to download this file please visit my project page at the Matlab Central File Exchange. And dont forget to leave your reviews and comments.

Cavity Model

Another model available at our disposal in order to analyze microstrip antennas is the cavity model. This model is different from transmission line model as in it is provides a better way to model the radiation patterns and is closer in the physical interpretation of the antenna characteristics. The normalized fields within the dielectric can be found more accurately by treating the region as a cavity bounded by electric conductors (above and below) and by magnetic walls along the perimeter of the patch. An attempt is made to present a physical interpretation into the formation of the fields within the cavity and radiation through its side walls.

Physical Insight
When the microstrip patch is energized, a charge distribution is established on the upper and lower surfaces of the patch, as well as on the surface of the ground plane. The charge distribution is controlled by two mechanisms; an attractive and a repulsive mechanism. The attractive mechanism is between the corresponding opposite charges on the bottom side of the patch and the ground plane, which tends to maintain the charge concentration on the bottom of the patch. The repulsive mechanism is between the like charges on the bottom surface of the patch, which tends to push the charges from the bottom around its edges to the top surface of the patch.

E-plane pattern

H-plane pattern

Slot Conductance
Single slot conductance

Where,

Mutual Conductance

Where, J0 is the Bessel function of the first kind and order zero.

Characteristic impedance of microstrip line feed
For W0/h ≤ 1

For W0/h > 1

Beamwidths
E-plane

H-plane

Transmission Line Model

Transmission line method is the easiest method as compared to the rest of the methods. This method represents the rectangular microstrip antenna as an array of two radiating slots, separated by a low impedance transmission line of certain length.

The following effects are taken into account for this model:

Fringing Effects: As the dimensions of the patch are finite along the length and the width, the fields at the edges of the patch undergo fringing i.e. the field exists outside the dielectric thus causing a change in the effective dielectric constant. It is a function of the dimensions of the patch and the height of the substrate.

The above diagram shows a patch antenna from the Transmission Line Model perspective. We can observe the fringing at the edges increasing the effective length.

Effective length and Width: Due to fringing effect, electrically the patch dimensions will be bigger than its physical dimensions. A formula to calculate the effective length Leff is shown below.

Where, an approximate relation for the normalized extension of length ΔL is given below:

For an efficient radiator, a practical width that leads to good radiation efficiencies is,

Conductance: Each radiating slot is represented by a parallel equivalent admittance Y(with conductance G and susceptance B). The slots are labeled as Θ1 and Θ2. The equivalent admittance of a slot is given by,

Y1 = G1 + B1

Where for a slot of finite width (W)

For,

Since both the slots are identical, its equivalent admittance is;

Y2= Y1, G2=G1, B2=B1

The conductance of each slot can be obtained by using field expression from cavity model. In general, the conductance is defined as

The above list of equations obtained from the transmission line model can be used to calculate parameters for analysis and synthesis of antenna. For these purposes, number of models may be used in conjunction with each other.

Design by Modelling

Analytical models are devised to provide an understanding of the operating principles that could be useful for a new design, for modifications to an existing design, and for the development of new antenna configurations. The objectives of antenna analysis are to predict the radiation characteristics such as radiation patterns, gain, polarization as well as near field characteristics such as input impedance, impedance bandwidth and antenna efficiency. The analysis of microstrip antennas is complicated by the presence of dielectric inhomogeneity, inhomogeneous boundary conditions, narrow frequency band characteristics, and a wide variety of feed, patch shapes and substrate configurations. Thus a trade-off is reached between the complexity and accuracy of the solution by compromising on one or more of the features.
The model chosen for analysis is a good one if it has the following characteristics.

• It can be used to calculate all impedance and radiation characteristics of the antenna under discussion.
• Its results are accurate enough for its intended purpose.
• It is as simple as possible, while providing the required accuracy for impedance and radiation properties.
• It lends itself to interpretation in terms of known physical phenomena.

Feeding Methods

A feedline is used to excite to radiate by direct or indirect contact. There are many different methods of feeding and four most popular methods are microstrip line feed, coaxial probe, aperture coupling and proximity coupling.

 

Microstrip Line Feeding

Microstrip line feed is one of the easier methods to fabricate as it is a just conducting strip connecting to the patch and therefore can be consider as extension of patch. It is simple to model and easy to match by controlling the inset position. However the disadvantage of this method is that as substrate thickness increases, surface wave and spurious feed radiation increases which limit the bandwidth.

Coaxial Feeding

 Coaxial feeding is feeding method in which that the inner conductor of the coaxial is attached to the radiation patch of the antenna while the outer conductor is connected to the ground plane.

Advantages

•         Easy of fabrication
  
•         Easy to match
  
•         Low spurious radiation
  

Disadvantages

• Narrow bandwidth
• Difficult to model specially for thick substrate
• Possess inherent asymmetries which generate higher order modes which produce cross-polarization radiation.

Aperture Coupling

 Aperture coupling consist of two different substrate separated by a ground plane. On the bottom side of lower substrate there is a microstrip feed line whose energy is coupled to the patch through a slot on the ground plane separating two substrates. This arrangement allows independent optimization of the feed mechanism and the radiating element. Normally top substrate uses a thick low dielectric constant substrate while for the bottom substrate; it is the high dielectric substrate. The ground plane, which is in the middle, isolates the feed from radiation element and minimizes interference of spurious radiation for pattern formation and polarization purity.

 Advantages

• Allows independent optimization of feed mechanism element.

Proximity Coupling

Proximity coupling has the largest bandwidth, has low spurious radiation. However fabrication is difficult. Length of feeding stub and width-to-length ratio of patch is used to control the match.

Microstrip Antenna Configurations

Microstrip antennas are characterized by a large number of physical parameters as compared to conventional microwave antennas. They can be design to have varied geometrical shapes and sizes. They can be broadly divided into four main categories.

 

Microstrip Patch Antenna

 A microstrip patch antenna is essentially a conducting patch of planar or non planar geometry on one side of dielectric substrate with a ground plane on other side. The radiation characteristics of different shapes of patch are similar despite the difference in geometrical shape because fundamentally all of them behave like a dipole. The figure below shows a rectangular patch having microstrip line feeding and a circular patch with coaxial feeding.

Microstrip or Printed Dipole Antenna

 Microstrip or printed dipoles differ from rectangular patch antenna in their length-to-width ratio. The width of dipole is typically less than 0.05 of wavelength. The radiation pattern of a dipole and patch are similar due to similar longitudinal current distributions, but on the contrary, radiation resistance, bandwidth and cross polar radiation between the two types differ. Microstrip dipoles are attractive elements owing to their desirable properties such as small size and linear polarization. They are well suited for higher frequencies for which the substrate can be electrically thick, and can therefore attend significant bandwidth.

 

Printed Slot Antenna

 Printed slot antennas comprise a slot in the ground plane of a grounded substrate. The slot can have any shape. Theoretically most of the microstrip patch can be realized in the form of a printed slot. Like patch antennas, slot antennas can either be fed microstrip line or coplanar waveguide. They are generally bidirectional radiators: unidirectional radiation is obtained by using a reflector plate on one side of the slot.

Microstrip Traveling Wave Antenna

 Microstrip traveling wave antennas consist of chain shaped periodic conductors or a long microstrip line of sufficient width to support a TE mode. The other end of traveling wave antenna is terminated in a matched resistive load to avoid the standing waves on the antenna. Traveling wave microstrip antenna can be designed so that the main beam lies in any direction from broadside to end fire.

 

Basic Concepts and Terminology

In this post I enlist some basic concepts and terminology involved in the domain of microstrip antennas. A proper comprehension of these concepts is imperative before any further deliberation on microstrip antennas.

Stripline and Microstrip

The terms stripline and microstrip are often encountered in the literature, in connection with both transmission lines and antennas. A stripline or triplet device is a combination of three parallel conducting layers separated by two thin dielectric substrates, the center conductor of which is analogous to the center conductor of coaxial transmission line. If the center conductor couples to a resonant slot cut orthogonally in upper conductor, the device is said to be a stripline radiator.

On the other hand, microstrip consists of two parallel conducting layers separated by a single thin dielectric substrate. The lower conductor functions as a ground plane and the upper conductor may be a simple resonant circular or rectangular patch, or a resonant dipole, or a monolithically printed array of patches or dipoles and the associated feed network.

Single Microstrip Line

A basic microstrip transmission lines consists of a conductive strip and a ground lane separated by a dielectric as shown in the figure below.

Fig: Single microstrip line

The figure above shows the general structure of microstrip line. The conduction strip, which is the microstrip line, has a width W, a thickness t. It is placed on the top of dielectric substrate, which has a relative dielectric constant €r, with a thickness of h, and a ground plane below on the substrste.

The single transmission line forms the basics of microstrip and therefore a basic knowledge of its working imperative. As mention earlier, two parameters, the effective dielectric constant €r and the characteristic impedance describe transmission characteristics.

Microstrip Antenna

A microstrip antenna in its basic form consists of a metallic radiating patch on one side of dielectric substrate, which has a ground plane on other side. The radiating patch can be considered as extension of microstrip line.

Fig: Rectangular Microstrip patch antenna

Assuming a voltage input at feedline, when operating in transmission mode, current is excited on feedline to the patch of a vertical electric field between the patch and the ground plane. So therefore the patch element resonates at certain wavelength and this result in radiation.

An Introduction to Microstrip Antennas

Microstrip Antennas started out as an extension of microstrip circuit elements, a modification of microstrip structures into radiating elements. But over the years due to the compact and thin profile structure of the microstrip antennas which are amenable to conformal configurations, and numerous other advantages as well, microstrip antennas have been a subject of extensive research and  extensive commercial use.

A Little bit of history:

Deschamps was the first to give the concept of microstrip radiators way back in 1953. But as is the case with most scientific breakthroughs of that time, the first patented documentation of microstrip antennas is in the name of Gutton and Baissinot of France in 1955. The first ones to make practical antennas were Howell and Munson.

Robert (Bob) Eugene Munson is regarded as the father of practical microwave patch antennas. Although the patch antenna was first theorized by G. A. Deschamps in 1953, but it was not put to use for many years (by Munson), first on a datalink for Sidewinder missile, then on Sprintmissile's semi-active seeker.

So was it really Munson's genius or was he just in the right place at the right time??

The 1960s and 1970s witnessed 

• the move to higher frequencies and solid state devices
  
• the move away from chassis and terminal wiring to printed circuits
  
• a demanding need for conformal arrays (the Sprint missile array was on the curved surface of the nose cone).
  

At this point of time. the height of the Cold War, Munson was a defense worker at Ball Aerospace. So Munson had just the right needs and facilities at his disposal for development of microstrip antennas, which the likes of Deschamps, Gutton and Baissinot were not able to take advantage of. These situational advantages apart it must have taken all of Munson's genius to make such outstanding progress in this field. Munson's name appears on many U.S patents, 29 at last count just on antennas. Today he's retired and living on a 160 acre vegetable farm outside Boulder. Financial benifits apart Munson's reputation in the field of microwave engineering exceeds all his contemporaries. The above information on Munson was obtained from the Microwave hall of fame, whereas a simple Google search on Deschamps hardly yielded any results.