**2. Fundamental of antenna**

#### **2.1 Introduction of dipole antenna**

Dipole antenna is one of the simple antenna that demonstrates the fundamental concept of antenna and it is a foundation of many practical antennas [9]. In **Figure 1**, dipole antenna is configured with two symmetric conductive arms carrying radio frequency current. Its length is required to be half wavelength (0.5λ) for maximum response and a half wavelength corresponds to approximately 6 cm (in air) in the 2.4 GHz ISM band. The current across the dipole generates the electromagnetic wave radiation propagating from the dipole arms.

#### **2.2 Ground plane**

A good conductive and reflective ground plane reduces the half wavelength dipole antenna to be quarter wavelength antenna [11]. This ground plane plays the same role of one of the arms and becomes a part of the antenna. This ground plane is considered as a mirror. In an optical mirror, if an object is placed in front of a mirror, a virtual image is generated with the same size and the same distance behind the mirror. In this case, if a signal source is placed above the ground plane, a virtual image of the source is generated with same current flowing direction and same phase shown in **Figure 2**, therefore, a quarter wavelength antenna and a ground plane form a half wavelength antenna. A well-designed ground plane should be very much larger in its dimensions than the half wavelength itself [11]. The active antenna measurement such as the over-the-air (OTA) measurement is a good method to indicate the overall antenna performance in the complete products compared to passive antenna measurement [12]. If the ground plane is significantly small, poor radiation performance is predicted in active antenna measurement.

#### **2.3 Effect on length**

An electric field with wavelength λ1 propagates toward to a dipole antenna with length L equals to 0.5λ1. This induces a sinusoidal current distribution shown in

**51**

**Figure 2.**

desired frequency.

to zero, as shown in **Figure 3(b)**.

*condition with a virtual image under the ground plane.*

*Planar Antenna Design for Internet of Things Applications*

**Figure 3(a)**, given that the current distribution on the dipole is uniform. If the incident wave has wavelength λ2 which is much longer than the dipole length, for example L = 0.1λ2, current distribution is induced in triangular shape. The maximum current occurs at the center feed point and decreased linearly toward two ends

*Signal source and ground plane effect [11]. (a) Actual condition above a ground plane. (b) Equivalent* 

For a dipole with uniform current distribution, the radiation resistance Rrad in

 ( \_ L λ) 2

The radiation resistance of **Figure 3(b)** is much smaller than that of **Figure 3(a)** based on Eq. (1). Low radiation resistance is an indication of inefficient radiation. Most of the power is not radiated by the antenna when the length of antenna is not designed based on the wavelength of incident signal. This poor radiating condition occurs when the antenna operates not equal to its resonant frequency. Matching network is usually used to maximize the power transfer from the radio transceiver to the antenna [10]. Matching network sometimes is used to tune the operating frequency back to the desired value if the resonant frequency is little shifted to

(1)

Rrad = 80 π<sup>2</sup>

free space is given by the following well-known equation [9, 10]:

*DOI: http://dx.doi.org/10.5772/intechopen.92456*

**Figure 1.** *Configuration of a half wavelength dipole antenna [10].*

*Planar Antenna Design for Internet of Things Applications DOI: http://dx.doi.org/10.5772/intechopen.92456*

#### **Figure 2.**

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

magnetic wave radiation propagating from the dipole arms.

Dipole antenna is one of the simple antenna that demonstrates the fundamental concept of antenna and it is a foundation of many practical antennas [9]. In **Figure 1**, dipole antenna is configured with two symmetric conductive arms carrying radio frequency current. Its length is required to be half wavelength (0.5λ) for maximum response and a half wavelength corresponds to approximately 6 cm (in air) in the 2.4 GHz ISM band. The current across the dipole generates the electro-

A good conductive and reflective ground plane reduces the half wavelength dipole antenna to be quarter wavelength antenna [11]. This ground plane plays the same role of one of the arms and becomes a part of the antenna. This ground plane is considered as a mirror. In an optical mirror, if an object is placed in front of a mirror, a virtual image is generated with the same size and the same distance behind the mirror. In this case, if a signal source is placed above the ground plane, a virtual image of the source is generated with same current flowing direction and same phase shown in **Figure 2**, therefore, a quarter wavelength antenna and a ground plane form a half wavelength antenna. A well-designed ground plane should be very much larger in its dimensions than the half wavelength itself [11]. The active antenna measurement such as the over-the-air (OTA) measurement is a good method to indicate the overall antenna performance in the complete products compared to passive antenna measurement [12]. If the ground plane is significantly small, poor radiation performance is predicted in active antenna measurement.

An electric field with wavelength λ1 propagates toward to a dipole antenna with length L equals to 0.5λ1. This induces a sinusoidal current distribution shown in

**2. Fundamental of antenna**

**2.2 Ground plane**

**2.3 Effect on length**

**2.1 Introduction of dipole antenna**

**50**

**Figure 1.**

*Configuration of a half wavelength dipole antenna [10].*

*Signal source and ground plane effect [11]. (a) Actual condition above a ground plane. (b) Equivalent condition with a virtual image under the ground plane.*

**Figure 3(a)**, given that the current distribution on the dipole is uniform. If the incident wave has wavelength λ2 which is much longer than the dipole length, for example L = 0.1λ2, current distribution is induced in triangular shape. The maximum current occurs at the center feed point and decreased linearly toward two ends to zero, as shown in **Figure 3(b)**.

For a dipole with uniform current distribution, the radiation resistance Rrad in free space is given by the following well-known equation [9, 10]:

$$\mathbf{R}\_{\text{rad}} = \mathbf{80} \,\pi^2 \left(\frac{\text{L}}{\lambda}\right)^2 \tag{1}$$

The radiation resistance of **Figure 3(b)** is much smaller than that of **Figure 3(a)** based on Eq. (1). Low radiation resistance is an indication of inefficient radiation. Most of the power is not radiated by the antenna when the length of antenna is not designed based on the wavelength of incident signal. This poor radiating condition occurs when the antenna operates not equal to its resonant frequency. Matching network is usually used to maximize the power transfer from the radio transceiver to the antenna [10]. Matching network sometimes is used to tune the operating frequency back to the desired value if the resonant frequency is little shifted to desired frequency.

**Figure 3.** *(a) Sinusoidal current distribution for L = 0.5*λ *1. (b) Triangular current distribution for L = 0.1*λ *2 [9].*
