Compact Antennas for Wireless Communications and Terminals: Theory and Design

Chapter 7Circularly Polarized Antennas ¹

 

 

 

When polarization matching is not necessarily assured for a given orientation of linearly polarized transmitter and receiver antennas, it is preferable to use circular polarization. This is the case, for example, with low Earth orbit or unstabilized satellites, where the antennas are randomly oriented. Similarly, radio-frequency identification (RFID) reader antennas usually present circular polarization in order to enable communication with RFID tags, whatever their orientation. In radar technology, circular polarization is used to minimize interference from rain.

In communications between ground stations and satellites, the Faraday effect causes left and right circularly polarized (or CP) waves crossing the ionosphere to propagate at slightly different speeds. Since a linear polarization can be decomposed into two CP components, the effect of a relative phase shift, induced by the Faraday effect, is to rotate the orientation of a wave's linear polarization. Therefore, low depolarization will be observed if a pure circular polarization is used.

First, we will present the characteristic parameters of circular polarization (axial ratio – AR, polarization ellipse). Then, we will focus on different CP antenna topologies:

– planar microstrip CP antennas that can be categorized in two ways, according to the number of feed points: single- and double-feed antennas;

– sequential arrays, which enable a significant widening of the operating band ...