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Microstrip Antennas

Microstrip Antennas. Presented by: Dr. Rod Waterhouse School of Electrical & Computer Systems Eng . RMIT University Melbourne, Australia*. *Now located at Pharad, LLC, Glen Burnie, MD. email : r.waterhouse@ieee.org. Outline. Introduction to Microstrip Patch Antennas (MPAs)

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Microstrip Antennas

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  1. Microstrip Antennas Presented by: Dr. Rod Waterhouse School of Electrical & Computer Systems Eng. RMIT University Melbourne, Australia* *Now located at Pharad, LLC, Glen Burnie, MD email: r.waterhouse@ieee.org

  2. Outline • Introduction to Microstrip Patch Antennas (MPAs) • Characteristics of MPAs • Excitation methods (comparison) • Techniques to improve bandwidth • CP generation methods • Microstrip patch arrays • Omni-directional antennas

  3. Geometry of Microstrip Antenna • The basic antenna consists of a thin metallic patch separated from the ground plane by a dielectric layer • Usually used at microwave frequencies

  4. Applications of Microstrip Antennas • Aerospace vehicles including high-performance aircraft, spacecraft, satellites and missiles • Mobile radios, phones and pagers • Base stations for personal communications • Large ground-based phased array antennas

  5. Stand-alone Antenna Element A coaxially-fed aperture-coupled stacked patch antenna

  6. Array of Microstrip Antenna Elements An 8 element linear array of microstrip patches for conformal mounting on a cylindrical structure

  7. Microstrip Antenna Integrated into a System: HIC Antenna Base-station for 28-43 GHz MPA microstrip antenna filter DC supply Micro-D connector K-connector LNA PD fiber input with collimating lens diplexer

  8. Advantages of Microstrip Antennas • Low profile: h<<l0 • Conformal • Light weight • Inexpensive • Robust • MMIC compatible

  9. Disadvantages of Microstrip Antennas • Small bandwidth: BW h/l0 • Poor efficiency: losses due to surface waves in addition to conductor and dielectric losses • Trade-off between scan coverage and bandwidth in large phased arrays • Difficult to design - require full wave analysis

  10. Microstrip Antenna Shapes

  11. Rectangular Patch: Directivity

  12. Rectangular Patch: Bandwidth

  13. Rectangular Patch: Efficiency Note: Material losses are neglected here.

  14. Rectangular Patch: Radiation Pattern Neglects substrate • E-plane pattern • H-plane pattern

  15. Rectangular Patch: Impedance Level

  16. Rectangular Patch: Input Impedance

  17. Circular Patch: Geometry

  18. Circular Patch: Radiation Pattern Neglects substrate • E-plane pattern • H-plane pattern

  19. Circular Sector Patch

  20. Feeding Methods • Microstrip line feed • easy to fabricate • easy to match by • controlling inset position • “easy” to model (approximately) • spurious feed-line radiation • and surface-wave excitation

  21. Feeding Methods (Cont.) • Probe feed • easy to fabricate • easy to match by • controlling position • requires via hole • adds probe inductance (matching is difficult with thicker substrates)

  22. Feeding Methods (Cont.) • Aperture-coupled feed • can give broader bandwidth • more difficult to fabricate • allows independent optimisation of feed substrate • separates antenna and feed-circuit layers • some spurious radiation • due to slot

  23. Feeding Methods (Cont.) • Proximity-coupled feed • Can give broader bandwidth • low spurious radiation • more difficult to fabricate

  24. Equivalent Circuits for Typical Feeds

  25. Methods of Analysis • Transmission-line model • simplest • good physical insight • less accurate, valid only for rectangular patches • Cavity model • more complicated • good physical insight, more accurate, more versatile • Integral Equation/Moment Method • most complicated, less physical insight, very accurate& versatile • Differential Equation Techniques (FDTD, FEM) • complicated, computationally slow • most versatile and can take into consideration surrounding environment

  26. Printed Antenna Efficiency Antenna Feed Loss Measured Efficiency Probe-fed 0 dB 96 ± 2% MS-Line 0.02 dB 91 ± 2% Aperture Coupled 0.26 dB 85 ± 2 % Note: Direct-contact feeding techniques are more efficient.

  27. Surface Wave Effects • One of the main disadvantages of printed antennas excitation of surface waves hsw = Prad Prad + Psw • trends: er: hsw thickness: hsw

  28. Surface Wave Effects (cont.) Mutual coupling Diffraction at substrate edges (SLL, polarisation?) Interference Active devices

  29. z Rcrit d  r Principle of SW Reduction • Minimize the TM0 surface wave mode to improve microstrip antenna efficiency • D. R. Jackson, et. al., “Microstrip Patch Designs That Do Not Excite Surface Waves,” IEEE Trans. Antennas Propagat., vol. 41, No 8, August 1993, pp. 1026-1037. There is some critical radius which minimizes the excitation of the TM0 surface wave mode TM0 Rcrit= ´1n

  30. Improved Surface Wave Efficiency Other techniques: (1) Use ‘hi-lo’ structures to be discussed later. (2) Photonic Band Gap Structures • Several methods: (a) Eli Yablonovitch’s method (UCLA) (b) Tatsuo Itoh’s method (UCLA) (c) University of Michigan’s method

  31. Bandwidth Enhancement • For a single layer geometry • use low dielectric constant material such as hard foam (er= 1) • use thick substrate (h > 0.05l0) • Non-contact feed techniques • Proxy-coupled patches • Achieve a dual resonance: • with aperture coupled patches • with stacked patches • Use parasitic coupled patches (horizontally)

  32. Proximity Coupled Patch Disadvantage: more difficult to construct (alignment of layers important), surface wave losses higher, therefore less gain to active devices BW10dB = 10 %

  33. x z  y R2 R1 r2 d2 (xp, yp) r1 d1 Probe-fed Stacked Patch Advantages: robust, ease of construction, good isolation between feed network and antenna element Disadvantages: cross-polarisation levels, difficult to design Analysis: full-wave spectral domain with attachment mode

  34. Probe-fed Stacked Patch (cont.) Substrate Parameters r1= 2.2 d1 = 3.048 mm r2= 1.07 d2= 9.0 mm BW10dB = 23%

  35. Probe-fed Stacked Patch (cont.) Gain = 8.4 dBi

  36. Low dielectric constant material Patch conductor Patch conductor Edge feeding High dielectric constant grounded substrate Parasitic patch Low dielectric constant laminate OEIC layers Diplexer and patch Hi-lo Stacked Patch • Combination of high and low dielectric constant substrates • Arbitrary shaped edge-fed patch on high er (e.g. er >10) substrate • Parasitic patch stacked on low er (er =1.07) substrate

  37. Hi-lo Stacked Patch (cont.) Input Impedance of a hi-lo stacked patch antenna (BW10dB = 25%) Radiation pattern of a hi-lo printed antenna

  38. Aperture-coupled Patch (ACP) Advantages: high bandwidth, polarisation purity Disadvantages: F/B ratio, construction Analysis: full-wave spectral-domain integral equation

  39. Aperture-coupled Patch (cont.) Substrate Parameters ra = 1.07 da = 15.0mm BW10dB = 26 %

  40. Aperture-coupled Patch (cont.) Gain = 8.0 dBi

  41. patch feed network metallic cavity coupling slot Cavity-Backed Aperture Coupled Patch a b • Advantages: high bandwidth, polarisation purity, no backward slot radiation • Disadvantages: construction • Analysis: full-wave spectral domain integral equation

  42. Cavity-Backed ACP (cont.) Substrate Parameters ra = 1.07 da = 14.0 mm BW10dB = 21 %

  43. Cavity-Backed ACP (cont.) Gain = 9.0 dBi

  44. Aperture-coupled Stacked Patch (ACSP) Advantages: bandwidth, polarisation purity, better F/B Disadvantages: construction Analysis: full-wave spectral-domain integral equation

  45. Aperture-coupled Stacked Patch (cont.) • Bandwidths in excess of an octave!! • Good radiation performance across bandwidth.

  46. Aperture-coupled Stacked Patch (cont.)

  47. CPW-fed Stacked Patch BW10dB = 40% Gain = 7.0 dBi

  48. Parasitic Gap Coupled Patches

  49. Circular Polarization • CP can be achieved using various feed arrangements and/or slight modifications to the elements • CP is obtained when 2 orthogonal but otherwise identical modes are excited with a 90o phase difference between them. • Techniques include • single feed • simplest, narrowest bandwidth CP (less than impedance BW) • dual feed • requires additional phasing in feed network, broader bandwidth • synchronous subarray • most complicated geometry, best performance

  50. Single-Feed Arrangements for CP

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