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The principal attributes of microstrip antennas are summarized in Table 2. TABLE 2: MICROSTRIP ANTENNA ATTRIBUTES 3 , 4 POSITIVE ATTRIBUTE NEGATIVE ATTRIBUTE Low cost fabrication – printed circuit manufacturing methods Narrow bandwidth – bandwidth increase generally requires increasing volume Surface conformable – facilitated by flexible substrate materials Sensitive to temperature and humidity – low loss substrates utilize PTFE in composite Mechanically stable – dielectric substrates may use composite ceramic filled construction Limitation on maximum gain Polarization diversity – readily achieved using alternate feed methods Poor cross polarization – limited element and feed isolation Flexible gain and pattern options – readily achieved using alternate feed methods and array techniques Spurious radiation – surface and other propagation modes Ease of integration with other passive and active functions – achieved via compatibility with passive and active components Low efficiency due to dielectric and conductor losses Low profile – low profile planar construction Modest power handling In a properly designed microstrip antenna, the radiation intensity is in a direction normal to the radiating element, i.e. broadside. For the rectangular microstrip antenna, the length, L, is typically one-third to one-half wavelength long depending upon the substrate relative dielectric constant, which is commonly 2.0 to 10.0; the lower values of dielectric constant yielding higher efficiency. The substrate height, h, is also a critical parameter with respect to efficiency and bandwidth, as well as reducing undesired propagation modes at the conductor edges and within the substrate. There are several techniques available for the introduction of RF energy to the radiating microstrip via the feed line structure. In some cases, alterations in the feed line structure have the potential for attendant changes to the efficiency, gain and bandwidth of the microstrip antenna. The most common feed structure for the rectangular microstrip antenna is direct attachment at the radiating edge as illustrated. The rectangular microstrip antenna geometry is most popular, however, alternate shapes, e.g. circular and triangular, provide utility in certain applications. Thin strips for the implementation of half-wavelength dipoles are attractive for increasing the operational bandwidth. To maintain brevity, the emphasis within this tutorial is restricted to microstrip antennas of rectangular geometry. 3 Bancroft, R., Microstrip and Printed Antenna Design, 2nd Ed., Scitech Publishing, Raleigh, NC, 2009, pp. 5-6. 4 Bahl, I. J. and Bhartia, P., Microstrip Antennas, Artech House, Dedham, MA, 1980, pp. 2-4. Microstrip Antenna Design 4 www.cadence.com/go/awr

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