**3. Basics of polarization agile antennas**

Basically, any polarizations can be expressed in the sum of two orthogonal polarizations. For example, the electric field propagating along the *z*-axis can be expressed as follows:

$$E(z,t) = i\_x E\_x \sin\left(2\pi ft - kz\right) + i\_y E\_y \sin\left(2\pi ft - kz + \Phi\right) \tag{3}$$

where *ix*, *iy*, *Ex*, *Ey*, and φ are the unit vectors of the *x* and *y* direction, the amplitude of *x* and *y* component of the electric field, and the phase difference between the components, respectively. Eq. (3) expresses elliptical circularly polarized waves in general. When the electric field has only the *x* or *y* component, the radio wave becomes a linearly polarized wave. When φ = 0 or *π*, the radio wave also becomes a linearly polarized wave. On the other hand, the radio wave becomes a circularly polarized wave when φ = ±*π*/2 and *Ex* = *Ey*.

**Figure 3** shows basic configurations of several types of polarization agile antennas. The antenna shown in **Figure 3a** is a linear polarization switchable antenna, and it consists of a switch and dual-polarized antenna which radiates horizontal and vertical polarizations. The polarization modulated signals can be excited by simply switching the horizontal and vertical polarizations.

The antenna shown in **Figure 3b** is a circular polarization switchable antenna. A 90-degree hybrid is placed between a switch and dual-polarized antenna. As the input signal fed to one of the input ports of the hybrid is divided into two signals with the phase difference of *π*/2, a circularly polarized wave is excited. By switching the input ports of the hybrid, the antenna switches right-handed and left-handed circular polarizations (RHCP and LHCP).

#### **Figure 3.**

*Basic configurations of polarization agile antenna. (a) Linear polarization switchable antenna; (b) circular polarization switchable antenna and (c) linear/circular polarization switchable antenna.*

**Figure 3c** shows a basic configuration of a polarization agile antenna which switches four polarizations of ±45° linear polarizations, RHCP and LHCP. The antenna consists of a phase shifter and dual-polarized antenna. When the phase shift value φ = 0 or *π*, the antenna excites ±45° linear polarizations. Furthermore, when φ = ±*π*/2, the antenna excites circular polarizations.
