**8. Antenna impedance**

Antenna impedance determines the efficiency of transmitting and receiving energy in antennas. The dipole impedance is given in Eq. (16).

$$\begin{aligned} R\_{rad} &= \frac{2W\_T}{I\_0^2} \\ \text{For a Dipole: } R\_{rad} &= \frac{80\pi^2 l^2}{\lambda^2} \end{aligned} \tag{16}$$

#### **8.1 Loop antennas for wireless communication systems**

Loop antennas are compact, low-profile, and low-cost antennas. Loop antennas are employed in wearable wireless communication systems. The loop antenna is referred to as the dual of the dipole antenna, see **Figure 8**. A small dipole has magnetic current flowing (as opposed to electric current as in a regular dipole), the loop antenna fields are similar to that of a small loop. The short dipole has a

**Figure 8.** *Duality relationship between dipole and loop antennas.*

**Figure 9.** *Photo of loop antennas.*

capacitive impedance and the impedance of a small loop is inductive. A photo of loop antennas is shown in **Figure 9**.

Disadvantages of microstrip antennas:

*Introductory Chapter: Novel Radio Frequency Antennas DOI: http://dx.doi.org/10.5772/intechopen.93142*

• Limited power handling up to 50 W.

in Eq. (18).

**Figure 11.**

**Figure 12.**

**13**

*Microstrip antenna shapes.*

*Rectangular microstrip antenna.*

• High feed network losses at high frequencies.

• Narrow bandwidth (usually 0.5–5%). However, wider bandwidth may be

The electric field along the radiating edges is presented in **Figure 11**. The magnetic field is perpendicular to the electric field according to Maxwell's equations. At the edge of the strip (X/L = 0 and X/L = 1), the magnetic field drops to zero, because there is no conductor to carry the electromagnetic current, it is maximum in the patch center. The electric field is at maximum level (with opposite polarity) at the patch edges (X/L = 0 and X/L = 1) and zero at the patch center. The ratio of electric to the magnetic field is proportional to the impedance that we measure when we feed the patch. Microstrip antennas may be fed by a microstrip line or by a coaxial line or probe feed. By varying the location of the antenna feed point between the patch center and the patch edge, we can get a 50 Ω impedance or any other desired impedance. Microstrip antenna may have any arbitrary shape such as square, triangle, circle, ring, rectangular, and fractal shape as presented in **Figure 12**.

The antenna dimension W is given by Eq. (17). The antenna bandwidth is given

2*f* ffiffiffiffiffiffi

ffiffiffiffiffiffi

*<sup>ϵ</sup>eff* <sup>p</sup> (17)

*<sup>ϵ</sup>eff* <sup>p</sup> (18)

*<sup>W</sup>* <sup>¼</sup> *<sup>c</sup>*

*BW* <sup>¼</sup> *<sup>H</sup>*

achieved by using multilayer structure and novel technologies.
