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- 1. Micro-Strip AntennasOutlinesIntroduction .Micro-Strip Antennas Types .Micro-Strip Antennas Shapes .Types of Substrates (Dielectric Media) .Comparison of various types of flat profile printed antennas .Advantages & DisAdvantages of MSAs .Applications of MSAs .Radiation patterns of MSAs .How to Optimizing the Substrate Properties for IncreasedBandwidth ?Comparing the different feed techniques .Presented by:-Tarek Nader Esmat 04 -12-2012
2. IntroductionIn its most basic form, a Microstrip patch antenna consists of a radiating patch on oneside of a dielectric substrate which has a ground plane on the other sideFor good antenna performance, a thick dielectric substrate having a low dielectric constant isdesirable since this provides better efficiency, larger bandwidth and better radiation . In genaral Micro strip antennas are alsoknown as PRINTED ANTENNAS . These are mostly used at microwave frequencies. Because the size of the antenna is directlytied the wavelength at the resonantfrequency. Micro strip patch antenna or patch antennais a narrowband wide-beam antenna. Structure of a Microstrip Patch Antenna 3. The patch usually fed along the centerline to symmetry and thusminimize excitation of undesirable modes. Micro strip antennas are easy to fabricate and comfortable oncurved surface . The directivity is fairly insensitive to the substrate thickness. Micro strip patch antennas patches are in variety of shapes ,such as rectangular , square , triangular and circulator etc. 4. Patch Shapes Are:(a) Single radiating patches(b) Single slot radiator 5. (c) Microstrip traveling wave antennas Comb MTWA Meander Line Type MTWA Rectangular Loop Type MTWAFranklin Type MTWA The open end of the long TEM line is terminated in a matched resistive load. 6. (d) Microstrip antenna arrays 7. Substrates are: The most commonly used substrates are,1) Honey comb(dielectric constant=1.07)2)Duroid(dielectric constant=2.32)3)Quartz(dielectric constant=3.8)4)Alumina(dielectric constant=10) A thicker substrate will increase the radiation power, reduce conductor loss and improve Band width. 8. Comparison of various types of flat profile printed antennas:Charaterstics Microstrip patch Microstrip slot Printed diploeantennaantenna antennaProfile Thin ThinThinFabrication Very easyEasyEasyPolarizationBoth linear& Both linear&Linearcircular circularDual freq operation Possible PossiblePossibleShape Any shapeRec &circle Rec &tiangularSpurious radiationExists Existsexists 9. Advantages: Low fabrication cost, hence can be manufactured in largequantities. Easily integrated with microwave integrated circuits (MICs). Capable of dual and triple frequency operations. Supports both, linear as well as circular polarization. Low cost , Less size , Low Mass . Mechanically robust when mounted on rigid surfaces. High Performance Light weight and low volume. 10. Disadvantages: Narrow bandwidth associated with tolerence problem Lower Gain(Nearly 6db) . Large ohmic losses in feed structure of arrays. Excitation of surface waves . Most microstrip antennas radiate into half-space . Relatively low efficiency (due to dielectric and conductorlosses) . relatively high level of cross polarization radiationSpurious feed radiation (surface waves, strips, etc.) Inherently low impedance bandwidth.Low efficiency .Extraneous radiation from feeds and junctions .Low power handling capacity. 11. Remedies:Low power and low gain can overcome by arrays configuration.Surface wave associated limitations such as poor efficiency,increased mutual coupling , reduced gain and radiation pattern can overcome.The band width can increase up to 60% by using some special techniques. 12. Applications:Used in mobile satellite communication system.Direct broad cast telivision(DBS).Wire less LANS.Feed elements in coaxial systemGPS system.Missiles and telementryUHF Patch Antennas for Space 13. Why we use Microstrip Patch Antennas ? Used for some reasons: Flat surface makes them ideal for mounting on airplane Impedance matching fairly simple Microstrip patch antennas have a very high antenna quality factor(Q). Q represents the losses associated with the antenna and a large Q leads to narrow bandwidthand low efficiency. Q can be reduced by increasing the thickness of the dielectric substrate. But as the thicknessincreases, an increasing fraction of the total power delivered by the source goes into a surfacewave Calculating Patch Length:- o PL3.009"2r 14. UHF Patch Antennas for SpaceAntenna Development Corporation, Inc.(AntDevCo) employees have designed andmanufactured spacecraft microstrip patch antennas for many small spacecraft programs.These antennas are capable of supporting high data rates to at least 10 Watts of transmittedpower.Applications include GPS, USAF SGLS, NASA SN (Including TDRSS forward/return pairs), radartransponder, and the NASA DSN.The antennas can be supplied with LHCP, RHCP, or linear polarization .UHF Patch antenna 15. Radiation patterns of a rectangular microstrip patch antennaThe directivity of a microstrip antenna as a function of dielectric constant computed :- 16. Radiation efficiency, h, and unloaded radiation Q, Q o, as a functionof substrate thickness .Figure shows that :A microstrip patch that uses a thicker substrate is more efficient.In addition, as the substrate thickness increases, the radiation Q of the antenna decreases.Thus, impedance bandwidth increases with increasing substrate thickness. 17. Optimizing the Substrate Properties for Increased BandwidthThe easiest way to increase the bandwidth of an MSA is to :1) Print the antenna on a thicker substrate. However, thick substrates tend to trap surface wave modes , especially if the dielectricconstant of the substrate is high . Finally if the substrate is very thick, radiating modes higher than the fundamental will beexcited.2) Decrease the dielectric constant of the substrate. However, this has detrimental effects on antenna size reduction since the resonant length ofan MSA is shorter for higher substrate dielectric constant.. In addition, the directivity of the MSA depends on the dielectric constant of the substrate.3) Stack two patches on top of each other separated by a dielectric substrate or spacers.The application involved two identical circular patches stacked on top of each other. The lowerpatch was fed using a coaxial probe feed, and the top patch was electromagnetically coupled tothe lower one . A stacked circular patch EMC-MSA fed using a coaxial probe. 18. Comparing the different feed techniques :- 19. Feeding Techniques: Coaxial feed Microstrip feed Proximity coupled microstrip feed Aperture coupled microstrip feed Coplanar wave guide Line Feed1-Microstrip Line Feed :In this type of feed technique, a conductingstrip is connected directly to the edge of themicrostrip patch.This kind of feed arrangement has theadvantage that the feed can be etched on thesame substrate to provide a planar structure. 20. 2-Coaxial Feed :-The Coaxial feed or probe feed is a very commontechnique used for feeding Microstrippatch antennas.The main advantage of this type of feedingscheme is that the feed can be placed at anydesired location inside the patch in order tomatch with its input impedance.This feed method is easy to fabricate and hasProbe fed Rectangular Microstrip Patchlow spurious radiation.Antenna from topHowever, its major disadvantage is that itCoaxial Ground Plane Connector SubstratePatch provides narrow bandwidth and isdifficult to model since a hole has to be drilledin the substrate . and the connector protrudesoutside the ground plane, thus not making itcompletely planar for thick substrates .Probe fed Rectangular Microstrip Patch Antenna fromside view 21. 3-Aperture Coupled FeedIn this type of feed technique, the radiating patchand the microstrip feed line are separated by theground plane .Coupling between the patch and the feedline is made through a slot or an aperture inthe ground plane.The coupling aperture is usually centered under the Aperture-coupled feedpatch, leading to lower cross polarization due to symmetryof the configuration.The amount of coupling from the feed line to the patch is determined by the shape, size andlocation of the aperture. 22. 4-Proximity Coupled FeedThis type of feed technique is also called as theelectromagnetic coupling scheme .Proximity-coupled FeedTwo dielectric substrates are used such that the feed line is between the two substrates andthe radiating patch is on top of the upper substrate.The main advantage of this feed technique is that it eliminates spurious feed radiation andprovides very high bandwidth due to overall increase in the thickness of the microstrip patchantenna.This scheme also provides choices between two different dielectric media, one for the patchand one for the feed line to optimize the individual performances.Matching can be achieved by controlling the length of the feed line and the width-to-line ratioof the patch.The major disadvantage of this feed scheme is that it is difficult to fabricate because of thetwo dielectric layers which need proper alignment.Also, there is an increase in the overall thickness of the antenna.