broadbanding technique for microstrip patch antenna
BROADBANDING TECHNIQUES FOR MICROSTRIP PATCH ANTENNAS NAME-ASHIT TOMAR BRANCH-ECE SEC-A SEM-6th ROLL NO.-0922231022
BRIEF INTRODUCTION OF PATCH ANTENNA•Microstrip antennas are planar resonant cavities that leak fromtheir edges and radiate•Microstrip consists of a metal strip on a dielectric substratecovered by a ground plane on the other side.•It is fabricated by etching the antenna element pattern in metaltrace bonded to an insulating dielectric substrate, such asa printed circuit board , with a continuous metal layer bonded tothe opposite side of the substrate which forms a ground plane.
NEED OF BROADBANDING OF PATCH ANTENNA•The main limitation of printed patches is theNarrow Bandwidth availability.WHY BW OF PATCH ANTENNA IS LIMITED?• The operating bandwidth of a single linearlypolarized patch antenna is limited by its inputVSWR(Standing Wave Ratio) and it is inverselyproportional to the Q factor of the patchresonator.
What Does Broadbanding Means Broadbanding concern with the increment of the operating Bandwidth *of the patch antenna. Bandwidth of patch antenna may be define as –The frequency range over which the radiated power is within 3dB of the incident power and the radiation pattern is essentially the same.
Techniques For Broadbanding1-Decreasing the Q factor of patch by increasing substrate height & decreasing the dielectric constant.2-Use the multiple resonator located in one plane.3- Electromagnetically coupled patch Antenna.4-Use of multilayer configuration with multilayer resonators stacked vertically.
Decreasing the Q factor of the patches by increasing the substrate height & lowering the dielectric constantBandwidth of patches can be given by the expression- BW= s-1/Q*s^(12)For s=2, & substituting for Q in terms of energy stored & power radiated we can write BW=√2hGe/πc √ εre εo be……………….(a)Where Ge is the edge conductancebe is the effective width ,h is the substrate thicknessεr is the effective dielectric constant.
Performance & Limitations From equation (a) we can conclude that the BW increases linearly with increase of “h”. Also BW increases when εr is reduced.BUT IT HAVE CERTAIN LIMITATIONS-1-For probe -fed patch antenna an increase in the thickness of substrate causes an increase in probe inductance which in turn creates input matching problems.2-For microstrip fed patches,increased substrate thickness causes an increase junction reactance,which creates spurious radiation as well as input match problem.
CONTINUED ……3-Thick substrates make it mechanically difficult to have antenna arrays conformal to curved surfaces(of aircraft,space craft, missile etc.).4-Many of the analysis and design techniques used (cavity model etc.) become inaccurate for thick substrate.
Use the multiple resonator located in one plane.•This configuration consist of 4triangular patches as shown inthe figure.•The central patch “A” is probe fed, the lower patch “B” is gapcoupled & other two patches are Cand D are coupled by shortsections of microstrip lines.
Performance & Limitations- A bandwidth which is 5.4 times the bandwidth of single rectangular patch antenna has been reported.But there are two problems associated with this configuration-1-Larger area requirement & consequent difficulty in using these configurations as array element.2-Variations of the radiation pattern over the impedance bandwidth of the configuration.
Multiple-Resonator Configuration with Patches Stacked Vertically In this approach two or more then two layers of dielectricsubstrate are used .Resonant patches are located on the top of each of the substratelayers & are stacked vertically.Two-layer configurations are mostcommon but three dielectric layers have also been used.It may be work in two ways1-When smaller patch is on the top,edges of both of the smaller &larger patches becomes as the source of radiation with the effectiveaperture shifting from the bigger patch to the smaller patch as thefrequency of operation is incresed.
Continued…..2-When the larger patch is on the top the upper patch constitutes the radiating aperture.The lower patch helps in the broadband excitation of the upper patch and is termed as the Feeder patch. If two patches have identical dimension in that condition the distinction between the Feeder & Radiator patches disappear s and the two functions merged. In most of these two patch configuration,the lower patch is fed via a probe or a microstrip line. For upper patch excitation capacitive coupling can be done.
Electromagnetically coupled patch Antenna(ECPA)As shown in the figure thisconfiguration consist of the threelayers namely cover, patch & groundplanePatch is sandwiched in betweenthe other two layersThe antenna is fed by the groundplane which is an advantage of theECPA.
ADVANTAGES OF ECPA The whole feed structure for the single antenna or array is located more closely to the ground so the radiation pattern that we get is less disturbed. Spacing to the patch and the ground plane is increased which causes in return of the enhanced bandwidth. The cover layer of ECPA is very useful in case of the Environmental protection which is further an advantage of the patch antenna.
CONCLUDING REMARKThe bandwidth of patch antenna can be increased by the height of the substrate or by decreasing the value of the dielectric constant of substrate.The problems that are faced in above method is over come by the introduction of the Electromagnetically coupled patch antenna.Between the two different method of constructing the multiresonator coupled patch configuration the vertically stacked patches require small area and does not suffer from pattern degradation with frequency.
Continued….The search for “ ideal” wideband printed microstrip antenna is still on.Perhaps a combination of various approaches discussed in this paper would lead to an optimum broadband configuration.We can look forward to continued research in this area.