AWR eBooks

Transmission Line Model The rectangular microstrip antenna is uniquely represented by the transmission line model, which provides a simple analysis method as well as an intuitive understanding of the radiation mechanics and other operational parameters. It is also instructive to examine the relative magnitude of the E- field below the conductor in order to gain additional physical insight. To that end, Figure 4 is offered. Note specifically the E-field fringing at each end of the microstrip line as well as the dual dielectric occupancy mentioned earlier. The transmission line model of the rectangular microstrip antenna utilizes an equivalent radiating slot of width W, and height h, to represent each of the radiating edges as graphically illustrated in Figure 5. Each slot may be represented by an equivalent admittance consisting of a conductance and susceptance: The admittances are separated by a transmission line of length, L, and characteristic impedance, Z o = 1/Y o , thereby forming the equivalent network of the microstrip antenna as illustrated in Figure 6. The Equivalent circuit provides a convenient method of input impedance calculation upon numeric evaluation of the conductance and susceptance: 6 Figure 6 – Microstrip Antenna Equivalent Circuit While the equations for the slot admittance are convenient, improved accuracy is achieved through the use of the equivalent admittance of the radiating slots available from the cavity model. Cavity Model Meticulous examination of the microstrip antenna construction discloses that the air dielectric at the sides and conductors at the top and bottom boundaries may define a resonant structure, also referred to as a resonant cavity. Cavity resonators are typically low loss structures; therefore, a mechanism must be defined to simulate the radiation loss at each edge of the microstrip conductor. Once again, the radiating slot finds application. However, in the case of the cavity model, the radiation admittance is evaluated from an electromagnetic field perspective of significant mathematical rigor which is not appropriate for this tutorial venue. Notwithstanding, the results of the cavity model are applicable and provide improved accuracy for the calculation of driving point impedance and 6 Bahl, I. J. and Bhartia, P., Microstrip Antennas, Artech House, Dedham, MA, 1980, p. 51. jB G Y + = Figure 4 – Electric Field Distribution of Rectangular Microstrip Antenna Figure 5 – Radiating Slot Equivalence to Microstrip Edges ( ) ( ) ( ) ( ) ( ) L k tan jB G j Y L k tan Y B j G Y jB G Y Z L B h k G eff o o eff o o o in o o eff o e e l e p p + + + + + + = D = ú û ù ê ë é - × = 2 24 1 120 1 2 2 Microstrip Antenna Design 6 www.cadence.com/go/awr

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