Review of Various Broadband Techniques for MSAs

Definition of Bandwidth

Modified Shape Patches

The regularMSA configurations, such as rectangular and circular patches havebeen modified to rectangular ring [38] and circular ring [39], respectively, toenhance the BW. The larger BW is because of a reduction in the qualityfactor Q of the patch resonator, which is due to less energy stored beneaththe patch and higher radiation. When a U-shaped slot is cut inside therectangular patch, it gives a BW of approximately 40% for VSWR £ 2 [40].Similar results are obtained when a U-slot is cut inside a circular or atriangular MSA [41, 42]. These configurations are discussed in detail inChapter 6.

Planar Multiresonator ConfigurationsThe planar stagger–tuned coupled multiple resonators yield wide BW in thesame way as in the case of multistage tuned circuits. Several configurationsare available yielding BW of 5–25% [43–49]. Various parasitic patches likenarrow strips, shorted quarter-wavelength rectangular patches, and rectangularresonator patches have been gap-coupled to the central-fed rectangularpatch. Three combinations of gap-coupled rectangular patches are shown inFigure 1.5. To reduce the criticality of the gap coupling, direct coupling asdepicted in Figure 1.6 has been used to obtain broad BW. Both gap anddirect (hybrid) coupling have been used with circular MSAs (CMSAs) andequilateral triangular MSAs (ETMSAs) to yield broad BW.

Multilayer Configurations

In the multilayer configuration, two or more patches on different layers ofthe dielectric substrate are stacked on each other. Based on the couplingmechanism, these configurations are categorized as electromagnetically coupledor aperture-coupled MSAs.

The concept of log-periodic antenna has been applied to MSA to obtain amulti-octave BW. In this configuration, the patch dimensions are increased logarithmically and the subsequent patches are fed at 180° out of phasewith respect to the previous patch [67–70]. The main disadvantage of thisconfiguration is that the radiation pattern varies significantly over the impedanceBW as described in Chapter 5.

Ferrite Substrate-Based Broadband MSAsThe multiresonant behavior of a patch on a ferrite substrate yields a broadBW of about three octaves by changing the magnetic field. Also, the dimensions of the patch are reduced because of the high dielectric constant of the ferrite substrate. However, the efficiency of these antennas is poor because of lossy substrate and requires external magnetic fields, which makes it bulky [71–73]. The methods for increasing the BW of MSA are continuously getting upgraded. The search for an ideal broadband MSA is still continuing. Perhaps a combination of various approaches would lead to an optimum broadband configuration.

Broadband Compact MSAsThe size of a half-wavelength (l /2) RMSA is too large in the ultra-high frequency (UHF) band. There is a need for a compact MSA for personal mobile communication and other applications. A shorted l /4 RMSA has the same resonance frequency as that of a l /2 RMSA, with half the area [74]. The resonance frequency reduces further as the width of the shorting plate decreases [74, 75]. Similarly, compact MSA in circular and triangular configurations is realized by placing shorting posts at the zero potential lines [76, 77]. A single shorting post yields a maximum reduction in the resonance frequency of the rectangular, circular, and triangular MSAs [78–80]. The compact antennas have also been realized by cutting slots in regularly shaped antennas. The requirements of these compact broadband MSAs will increase in the future due to the ever-growing miniaturization of communication systems. The BW of the compact MSA has been increased in both planar as well as multilayer configurations

Tunable and Dual-Band MSAs

Tunable MSAs are of interest in many systems as they can be tuned over a large frequency range. These tunable antennas provide an alternative to large- BW antennas, especially when a large BW is required for encompassing several narrowband channels. The tunable MSA is realized by changing the length of the small stub attached to the regularly shaped MSA [84, 85], or by changing the number of shorting posts used to make a compact configuration [74, 76, 77]. Tunability is also achieved by integrating active devices such as varactor or PIN diodes along with the MSA [86]. When an antenna must operate at two frequencies that are far apart, a dual-frequency antenna can be used to avoid the use of two separate antennas. When two or more resonance frequencies of a MSA are close to each other, a broad BW is obtained. When these are separated, dual-band operation is obtained. In general, all the methods described earlier for increasing the BW of MSAs can be utilized to obtain dual-band characteristics. In the single-layer MSA, dual-band operation is achieved by using either slot or shorting pins or varactor or optically tuned diodes or by selecting the proper length of a stub [9, 85–89]. In multilayer configurations, either electromagnetic or aperture coupling could be used for dual-frequency operation [90–92]. The separation between the two frequencies is obtained by adjusting the air gap between the two layers or by changing the dimensionsof the patches.

Broadband Circularly Polarized MSAsCP is particularly useful for a number of radar, communication, and navigation systems because the rotational orientations of the transmitter and the receiver antennas are unimportant in relation to the received signal strength. With linearly polarized signals, on the other hand, there will be only very weak reception if the transmitter and receiver antennas are nearly orthogonal. Also, the circularly polarized wave reverses its sense of polarization from right-hand to left-hand CP and vice versa after reflection from regular objects. The system will then tend to discriminate the reception of such reflected signals from other signals arising from direct paths on reflections from irregular shapes. CP is generated when two orthogonal modes are excited in a phase quadrature with equal magnitude. A rectangular or circular MSA generates CP when fed at two orthogonal points with equal amplitude and 90° phase difference. CP can also be generated by using a single-feed MSA. Singlefeed MSA configurations include diagonal fed nearly square, corner chopped square, and square with diagonal slot. Similar variations are possible forcircular and triangular MSAs. Broadband CP is obtained by using dual-feed multiple planar or stacked patches, or single-feed MSA in a sequential rotation array configuration.

Broadband Planar Monopole Antennas

MSA in its regular shape cannot yield multi-octaveBWbecause of its resonantnature. Some modification of the MSA configuration is required to obtainan octave BW. If a rectangular patch without the substrate and ground planeis fed at the edge by a coaxial feed with a perpendicular ground plane, thenthe patch will have an effective dielectric constant equal to 1 with large h.Both of these factors yield broad BW. This modified configuration can bethought of as a planar rectangular monopole antenna [99]. Other configurationssuch as triangular, hexagonal, circular, and elliptical monopoles alsoyield broad BW. An elliptical monopole with an ellipticity of 1.1 yields BWof 1:11 for VSWR > 2.

L-shaped probe:

Capacitive “top hat” on probe:

Top view

13

Improving BandwidthProbe Compensation

As the substrate thickness increases the probe inductance limits the bandwidth – so we

compensate for it.

SSFIP: Strip Slot Foam Inverted Patch (a version of the ACP).

Microstrip substrate

Patch

Microstrip line Slot

Foam

Patch substrate

Bandwidths greater than 25% have been achieved. Increased bandwidth is due to the thick foam substrate and

also a dual-tuned resonance (patch+slot).

14

Improving Bandwidth

Note: There is no probe inductance to worry about here.

J.-F. Zürcher and F. E. Gardiol, Broadband Patch Antennas, Artech House, Norwood, MA, 1995.

Bandwidth increase is due to thick low-permittivity antenna substrates and a dual or triple-tuned resonance.

Bandwidths of 25% have been achieved using a probe feed. Bandwidths of 100% have been achieved using an ACP feed.

Microstrip substrate

Driven patch

Microstrip lineSlot

Patch substrates

Parasitic patch

15

Improving BandwidthStacked Patches

Bandwidth (S11 = -10 dB) is about 100%

Stacked patch with ACP feed3 4 5 6 7 8 9 10 11 12

Frequency (GHz)

-40

-35

-30

-25

-20

-15

-10

-5

0

Ret

urn

Loss

(dB

)

MeasuredComputed

16

Improving BandwidthStacked Patches

(Photo courtesy of Dr. Rodney B. Waterhouse)

Stacked patch with ACP feed

0

0.2

0.5 1 2 5 1018

017

016

015

014

0

130

120110 100 90 80 70

6050

4030

2010

0-10

-20-30

-40

-50-60

-70-80-90-100-110-120-130

-140

-150

-160

-170

4 GHz

13 GHz

Two extra loops are observed on the Smith chart.

17

Stacked Patches

Improving Bandwidth

(Photo courtesy of Dr. Rodney B. Waterhouse)

Radiating Edges Gap Coupled Microstrip Antennas

(REGCOMA).

Non-Radiating Edges Gap Coupled Microstrip Antennas

(NEGCOMA)

Four-Edges Gap Coupled Microstrip Antennas

(FEGCOMA)

Bandwidth improvement factor:REGCOMA: 3.0, NEGCOMA: 3.0, FEGCOMA: 5.0?

Mush of this work was pioneered by

K. C. Gupta.

18

Improving BandwidthParasitic Patches

Radiating Edges Direct Coupled Microstrip Antennas

(REDCOMA).

Non-Radiating Edges Direct Coupled Microstrip Antennas

(NEDCOMA)

Four-Edges Direct Coupled Microstrip Antennas

(FEDCOMA)

Bandwidth improvement factor:REDCOMA: 5.0, NEDCOMA: 5.0, FEDCOMA: 7.0

19

Improving BandwidthDirect-Coupled Patches

The introduction of a U-shaped slot can give a significant bandwidth (10%-40%).

(This is due to a double resonance effect, with two different modes.)

“Single Layer Single Patch Wideband Microstrip Antenna,” T. Huynh and K. F. Lee, Electronics Letters, Vol. 31, No. 16, pp. 1310-1312, 1986.

20

Improving BandwidthU-Shaped Slot

A 44% bandwidth was achieved.

Y. X. Guo, K. M. Luk, and Y. L. Chow, “Double U-Slot Rectangular Patch Antenna,” Electronics Letters, Vol. 34, No. 19, pp. 1805-1806, 1998.

21

Improving BandwidthDouble U-Slot

A modification of the U-slot patch.

A bandwidth of 34% was achieved (40% using a capacitive “washer” to compensate for the probe inductance).

B. L. Ooi and Q. Shen, “A Novel E-shaped Broadband Microstrip Patch Antenna,” Microwave and Optical Technology Letters, vol. 27, No. 5, pp. 348-352, 2000.

22

Improving BandwidthE Patch

Multi-Band Antennas

General Principle:

Introduce multiple resonance paths into the antenna.

A multi-band antenna is sometimes more desirable than a broadband antenna, if multiple narrow-band channels are to be covered.

23

Dual-band E patch

High-band

Low-band

Low-band

Feed

Dual-band patch with parasitic strip

Low-band

High-band

Feed

24

Multi-Band Antennas

1.4 Methods of AnalysisThe MSA generally has a two-dimensional radiating patch on a thin dielectricsubstrate and therefore may be categorized as a two-dimensional planarcomponent for analysis purposes. The analysis methods for MSAs can bebroadly divided into two groups.In the first group, the methods are based on equivalent magnetic currentdistribution around the patch edges (similar to slot antennas). There arethree popular analytical techniques:· The transmission line model;· The cavity model;· The MNM.In the second group, the methods are based on the electric currentdistribution on the patch conductor and the ground plane (similar to dipoleantennas, used in conjunction with full-wave simulation/numerical analysismethods). Some of the numerical methods for analyzing MSAs are listed asfollows:

· The method of moments (MoM);· The finite-element method (FEM);· The spectral domain technique (SDT);· The finite-difference time domain (FDTD) method.This section briefly describes these methods.

1.4.1 Transmission Line ModelThe transmission line model is very simple and helpful in understandingthe basic performance of a MSA. The microstrip radiator element is viewedas a transmission line resonator with no transverse field variations (the fieldonly varies along the length), and the radiation occurs mainly from thefringing fields at the open circuited ends. The patch is represented by twoslots that are spaced by the length of the resonator. This model was originallydeveloped for rectangular patches but has been extended for generalizedpatch shapes. Many variations of this method have been used to analyze theMSA [9, 20–22].Although the transmission line model is easy to use, all types of configurationscan not be analyzed using this model since it does not take care ofvariation of field in the orthogonal direction to the direction of propagation.

1.4.2 Cavity ModelIn the cavity model, the region between the patch and the ground plane istreated as a cavity that is surrounded by magnetic walls around the peripheryand by electric walls from the top and bottom sides. Since thin substratesare used, the field inside the cavity is uniform along the thickness of thesubstrate [23–25]. The fields underneath the patch for regular shapes suchas rectangular, circular, triangular, and sectoral shapes can be expressed asa summation of the various resonant modes of the two-dimensional resonator.The fringing fields around the periphery are taken care of by extendingthe patch boundary outward so that the effective dimensions are larger thanthe physical dimensions of the patch. The effect of the radiation from theantenna and the conductor loss are accounted for by adding these losses tothe loss tangent of the dielectric substrate. The far field and radiated powerare computed from the equivalent magnetic current around the periphery.An alternate way of incorporating the radiation effect in the cavitymodel is by introducing an impedance boundary condition at the walls ofthe cavity. The fringing fields and the radiated power are not included inside the cavity but are localized at the edges of the cavity. However, the solutionfor the far field, with admittance walls is difficult to evaluate

1.4.3 MNMThe MNM for analyzing the MSA is an extension of the cavity model [9,26, 27]. In this method, the electromagnetic fields underneath the patchand outside the patch are modeled separately. The patch is analyzed as atwo-dimensional planar network, with a multiple number of ports locatedaround the periphery. The multiport impedance matrix of the patch isobtained from its two-dimensional Green’s function. The fringing fieldsalong the periphery and the radiated fields are incorporated by adding anequivalent edge admittance network. The segmentation method is then usedto find the overall impedance matrix. The radiated fields are obtained from thevoltage distribution around the periphery. Appendix C details this method.The above three analytical methods offer both simplicity and physicalinsight. In the latter two methods, the radiation from the MSA is calculatedfrom the equivalent magnetic current distribution around the peripheryof the radiating patch, which is obtained from the corresponding voltagedistribution. Thus, the MSA analysis problem reduces to that of finding theedge voltage distribution for a given excitation and for a specified mode.These methods are accurate for regular patch geometries, but—except forMNMwith contour integration techniques—they are not suited for arbitraryshaped patch configurations. For complex geometries, the numerical techniquesdescribed below are employed [9].

1.4.4 MoMIn the MoM, the surface currents are used to model the microstrip patch,and volume polarization currents in the dielectric slab are used to modelthe fields in the dielectric slab. An integral equation is formulated for theunknown currents on the microstrip patches and the feed lines and theirimages in the ground plane [28]. The integral equations are transformedinto algebraic equations that can be easily solved using a computer. Thismethod takes into account the fringing fields outside the physical boundaryof the two-dimensional patch, thus providing a more exact solution. Thisbook makes extensive use of a commercially available software IE3D [29]based on MoM to analyze various MSA configurations.

1.4.5 FEMThe FEM, unlike the MoM, is suitable for volumetric configurations. Inthis method, the region of interest is divided into any number of finite surfacesor volume elements depending upon the planar or volumetric structures tobe analyzed [30]. These discretized units, generally referred to as finiteelements, can be any well-defined geometrical shapes such as triangularelements for planar configurations and tetrahedral and prismatic elementsfor three-dimensional configurations, which are suitable even for curvedgeometry. It involves the integration of certain basis functions over the entireconducting patch, which is divided into a number of subsections. Theproblem of solving wave equations with inhomogeneous boundary conditionsis tackled by decomposing it into two boundary value problems, one withLaplace’s equation with an inhomogeneous boundary and the other correspondingto an inhomogeneous wave equation with a homogeneous boundarycondition

1.4.6 SDTIn the SDT, a two-dimensional Fourier transform along the two orthogonaldirections of the patch in the plane of substrate is employed. Boundaryconditions are applied in Fourier transform plane. The current distributionon the conducting patch is expanded in terms of chosen basis functions,and the resulting matrix equation is solved to evaluate the electric currentdistribution on the conducting patch and the equivalent magnetic currentdistribution on the surrounding substrate surface. The various parametersof the antennas are then evaluated 1.4.7 FDTD MethodThe FDTD method is well-suited for MSAs, as it can conveniently modelnumerous structural inhomogenities encountered in these configurations[10]. It can also predict the response of the MSA over the wide BW witha single simulation. In this technique, spatial as well as time grid for theelectric and magnetic fields are generated over which the solution is required.The spatial discretizations along three Cartesian coordinates are taken to besame. The E cell edges are aligned with the boundary of the configurationand H-fields are assumed to be located at the center of each E cell. Eachcell contains information about material characteristics. The cells containingthe sources are excited with a suitable excitation function, which propagatesalong the structure. The discretized time variations of the fields are deter

mined at desired locations. Using a line integral of the electric field, thevoltage across the two locations can be obtained. The current is computedby a loop integral of the magnetic field surrounding the conductor, wherethe Fourier transform yields a frequency response.The above numerical techniques, which are based on the electric currentdistribution on the patch conductor and the ground plane, give resultsfor any arbitrarily shaped antenna with good accuracy, but they are timeconsuming.These methods can be used to plot current distributions onpatches but otherwise provide little of the physical insight required forantenna design.

Feeding Techniques

Feeding Techniques: Coaxial feed Microstrip feed Proximity coupled microstrip feed Aperture coupled microstrip feed Coplanar wave guide Line Feed

1-Microstrip Line Feed :In this type of feed technique, a conducting strip is connected directly to the edge of themicrostrip patch.

This kind of feed arrangement has the advantage that the feed can be etched on thesame substrate to provide a planar structure.

2-Coaxial Feed :-The Coaxial feed or probe feed is a very common technique used for feeding Microstrippatch antennas.

Probe fed Rectangular Microstrip Patch Antenna from top

Probe fed Rectangular Microstrip Patch Antenna from side view

The main advantage of this type of feeding scheme is that the feed can be placed at anydesired location inside the patch in order to match with its input impedance.

This feed method is easy to fabricate and has low spurious radiation.

However, its major disadvantage is that itCoaxial Ground Plane Connector SubstratePatch provides narrow bandwidth and is difficult to model since a hole has to be drilled in the substrate . and the connector protrudes outside the ground plane, thus not making it completely planar for thick substrates .

3-Aperture Coupled Feed

In this type of feed technique, the radiating patch and the microstrip feed line are separated by the ground plane .

Coupling between the patch and the feedline is made through a slot or an aperture in the ground plane.

Aperture-coupled feedThe coupling aperture is usually centered under the patch, leading to lower cross polarization due to symmetry of the configuration.

The amount of coupling from the feed line to the patch is determined by the shape, size and location of the aperture.

Comparing the different feed techniques :-