Compact Antennas for Wireless Communications and Terminals: Theory and Design

Chapter 6Feeding Circuits for Microstrip Antennas ¹

 

 

 

6.1. Introduction

Even if it is effectively susceptible to radiation, an antenna is only usable if there is a feeding circuit enabling an acceptable transfer of guided energy toward the antenna. The optimum excitation of the antenna, or optimum coupling of the antenna to the feeding circuit, is a problem found equally in the excitation of microwave cavities and waveguides [GAR 02]. There are essentially three mechanisms for exciting printed antennas (Figure 6.1):

– direct coupling (by coaxial probe or printed line);

– proximity coupling;

– slot coupling.

The choice of feed type depends on constraints, such as the simplicity of the feeding circuit, the isolation between the excitation circuit and the antenna, or the eventual arrangement into an array of the radiating element. Whichever type of excitation is chosen, there are some general rules for optimizing the circuit between the feed and the antenna:

– define the mode of the antenna that needs to be excited;

– define the field distribution near to the excitation;

– determine the component(s) of the excitation field (E_x,y,z or H_x,y,z) that will effectively contribute to the coupling (the excitation/antenna field distributions must correspond);

– determine the position for the excitation, which allows for optimum matching (this position does not necessarily correspond to a maximum field under the patch);

– take account of the parasitic effect of feed (degradation in the ...