**4. Monopole antenna for communication systems**

Monopole antenna is usually a one-quarter wavelength-long conductor mounted above a ground plane or the earth as shown in **Figure 1**. Based on the image theory behind the ground plane, the monopole image is located. The monopole antenna and the monopole image form a dipole antenna. Monopole antenna is half a dipole that radiates electromagnetic fields above the ground plane. The impedance of 0.5 λ monopole antenna is around 37 Ω. The beam width of a monopole, 0.25 λ long, is around 40°. The directivity of a monopole, 0.25 λ long, is around 3 dBi to 5dBi. Usually in wireless communication systems, very short monopole antenna is employed. The impedance of 0.05 λ monopole antenna is around 1 Ω with capacitive reactance. The beam width of a monopole, 0.05 λ long, is around 45°. The directivity of a monopole, 0.05 λ long, is around 3 dBi.

Inverted monopole antenna is shown in **Figure 2a**. Loaded monopole antenna is shown in **Figure 2b**. Monopole antennas may be printed on a dielectric substrate as part of wearable communication devices.

**Figure 1.** *Monopole antenna.*

#### *Advanced Radio Frequency Antennas for Modern Communication and Medical Systems*

**5.2 Radiation from small dipole**

The length of a small dipole is small compared to its wavelength and is called elementary dipole. The current along a small dipole is uniform. We can compute the electromagnetic fields radiated from the dipole in spherical coordinates by using the potential function given in Eq. (1). The electromagnetic field at a point P(r, θ, φ) is

fields are the dipole near fields. In the near field, the waves are standing waves and the energy oscillates in the antenna near zone and is not radiated to the free space. The real part of the pointing vector equals to zero. At r >> 1, the dominant

component of the field varies as 1/r as written in Eq. (5). These fields are the dipole

The antenna radiation pattern represents the radiated fields in space at a point P (r, θ, φ) as a function of θ and φ. The antenna radiation pattern is three dimensional. When φ is constant and θ varies, we get the E plane radiation pattern. When φ varies and θ is constant, usually θ = π/2, we get the H plane radiation pattern.

The dipole E plane radiation pattern is given in Eq. (2) and presented in

At a given point P(r, θ, φ), the dipole E plane radiation pattern is given in

*lβI*0j j sin *θ* 4*πr*

j j *E<sup>θ</sup>* ¼ *η*<sup>0</sup>

*<sup>r</sup>* , <sup>1</sup> *<sup>r</sup>*<sup>2</sup> , <sup>1</sup>

ð Þ*<sup>r</sup>* <sup>3</sup> and is written in Eq. (4). These

*<sup>r</sup>* and sinθ. Wave

*<sup>r</sup>*3. For r < < 1,

(2)

listed in Eq. (3). The electromagnetic fields in Eq. (3) vary as <sup>1</sup>

far fields. In the far fields, the electromagnetic fields vary as <sup>1</sup>

the dominant component of the field varies as <sup>1</sup>

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

impedance in free space is given in Eq. (6).

**6. Dipole radiation pattern**

**6.1 Dipole E plane radiation pattern**

**Figure 4**.

Eq. (7).

**Figure 4.**

**7**

*Dipole E plane radiation pattern.*

**Figure 2.**

*(a) Inverted monopole antenna. (b) Loaded monopole antenna.*
