**3. Compact ultra-wideband (UWB) Vivaldi antenna**

 The main object of this section is the detection of water. A compact novel shape of Vivaldi antenna as shown in **Figure 1** with dimensions of 0.17λ × 0.16λ × 0.013λ is proposed in this section [6].

A comparative study was undertaken as shown in **Table 1** for different Vivaldi antenna that are used of GPR application in the same interesting operating frequencies [8–11] to show that the proposed antenna gives compact size with higher gain from 250 MHz up to 10 GHz.

**Figure 1.**  *3D geometry of the proposed Vivaldi antenna.* 


**Table 1.** 

*Comparison of the proposed antenna with other antennas (all dimensions in cm).* 

There are a number of techniques used for improving the parameters of printed antennas with different feeding techniques. These techniques include electromagnetic bandgap (EBG), metamaterial, and defected ground structure (DGS). EBG structure has gained popularity among all the techniques reported for enhancing the parameters due to its simple structural design. The periodical shapes are etched as square, mushroom, and circular shapes in the radiator or ground plane to achieve inductive and capacitive load to create band-stop characteristics and to suppress higher mode harmonics and mutual coupling.

The basic concepts, working principles, and equivalent models of the different shapes of electromagnetic bandgap structure (EBG) are presented [36]. EBG has been used in the design of the Vivaldi antennas for improving the bandwidth and gain of proposed antenna and suppressing the higher harmonics mode and mutual coupling between adjacent elements. In addition, the proposed antenna crosspolarization is improved for the radiation characteristics [6].

#### **3.1 Vivaldi antenna geometry and principle theory**

A relatively large number of published UWB Vivaldi antennas consist of a feed line and exponential ground plane. Our proposed antenna starts as shown in **Figure 2**(**a**) from one layer of FR4 low-cost substrate with conventional exponential Vivaldi tapered slot line shape using empirical Eq.(1) [4]:

$$y = \pm \mathbf{0}.\mathbf{0}\mathbf{1}\mathbf{8}e^{\mathbf{0}.27\mathbf{x}}\tag{1}$$

where *x* and *y* are the axes of the inner and outer exponential to improve the impedance bandwidth. The first step is without any slot, **Figure 2**(**a**), the second with circular EBG-etched slot on the edge of exponential tapered slot.

The symmetrical circular electromagnetic bandgap structure (EBG) slots are etched to increase the antenna bandwidth as shown in **Figure 2**(**b**). The modified version from the first one is obtained by slotting the two arms of the Vivaldi antenna.

To improve the bandwidth of the Vivaldi antenna, symmetrical semicircular slots are etched to increase the antenna bandwidth as shown in **Figure 2**(**c**). **Figure 2**(**c**) and (**d**) shows the geometry of the proposed antenna in two different configurations. The substrate used is FR4 dielectric substrate of thickness 1 cm, a relative permittivity of 4.4 and a loss tangent of 0.02.

 Finally, dual FR4 substrates printed with Vivaldi ground plane with the same feeding line are used. The final geometry of the proposed antenna design with ground plane and feeding network is shown in **Figure 3**. This design is used to investigate a dual substrate layer of Vivaldi ground plane antenna as shown in **Figure 3**(**a**) with the same feeding network as shown in **Figure 3**(**b**). Proposed Vivaldi antenna consists of two layers from the dielectric sheet, the feeding line is sandwiched between them, and the two layers of metallic Vivaldi are mounted on *Detection of Underground Water by Using GPR DOI: http://dx.doi.org/10.5772/intechopen.83594* 

**Figure 2.** 

*The top view of Vivaldi antenna, (a) conventional Vivaldi, (b–d) modified Vivaldi antenna.* 

**Figure 3.**  *Geometry of the proposed Vivaldi antenna (a) ground plane and (b) feed line.* 

the dielectric substrate (one at the top and the other at the bottom) as shown in three dimensions of the proposed Vivaldi antenna in **Figures 1** and **7**.

#### **3.2 Antenna simulation and measured results**

The reflection coefficients of the antenna versus frequency for the four-step design of Vivaldi antennas in frequency range from 0.25 to 10 GHz are shown in **Figure 4**(**a**), and the zooming range from 0.25 to 2 GHz is shown in **Figure 4**(**b**). The final antenna design achieves improvement in antenna impedance matching all over the band. The dimensions of the proposed antennas are shown in **Table 2**.

 The surface current density distributions of the compact Vivaldi antenna is shown in **Figure 5** at different resonant frequencies 0.4, 0.5, 0.75, 1.5, 1.75, and 2 GHz. The current distribution of the proposed antenna is studied to verify the operation of the proposed Vivaldi antenna. The exponential edge is responsible for the fundamental resonant frequency of the proposed antenna at 1.75 GHz as shown in **Figure 5**. The semicircular slots are etched to create the frequencies at 0.5 and 0.75 GHz. By adding stubs with different lengths, they are affecting the resonance from 1 to 2 GHz. The highest magnitude of current (red) is related to the corresponding radiating element. The simulated antenna gain of single and dual substrate is shown in **Figure 6**(**a**). The gain gives better performance by using dual substrate layer, and it increases from 8 to 17 dBi in average over the operating band from 0.2 to 2 GHz. The average radiation efficiency is around 80% over the operating band for dual substrate, while its value is 50% for single substrate as shown in **Figure 6**(**b**).

To validate the simulated results of the proposed antennas with single and dual substrate, they are fabricated by using printed circuit board (photolithographic)

#### **Figure 4.**

*|S11| versus frequency for the design steps of the proposed antennas (a) whole operating band and (b) zoom on low operating frequency.* 


#### **Table 2.**

*Dimensions of the proposed antenna (all dimensions in cm).* 

#### **Figure 5.**

*(a–f) Surface current densities for proposed Vivaldi antenna at 0.4, 0.5, 0.75, 1.5, 1.75, and 2 GHz, respectively.* 

technology and measured by using Agilent vector network analyzer technologies "Field Fox" Microwave Analyzer N9918A 26.5 GHz. **Figure 7** shows the photo of the fabricated antenna, feeding line, and Vivaldi antenna with dual substrate. The comparison between the measured and simulated results has been performed for the proposed Vivaldi antenna with single substrate that indicates good agreement. *Detection of Underground Water by Using GPR DOI: http://dx.doi.org/10.5772/intechopen.83594* 

#### **Figure 6.**

*(a) Vivaldi antenna gain variation versus frequency and (b) radiation efficiency versus frequency of the proposed antenna.* 

**Figure 7.** 

*Photo of the fabricated antenna, (a) top view of one substrate, (b) the feeding line, and (c) 3D of dual substrate.* 

**Figure 8.** 

*Reflection coefficient versus frequency for the proposed one substrate layer of Vivaldi antenna.* 

**Figure 8**(**a**) shows the whole range of the operation presenting proposed antenna from 0.2 to 10 GHz, while **Figure 8**(**b**) shows the zoom-operating range from 0.2 to 2 GHz. The measured reflection coefficient of proposed Vivaldi antenna with dual substrate is shown in **Figure 9**. **Figure 9**(**a**) shows the whole range from 0.2 to 10 GHz, while **Figure 9**(**b**) shows the zoom-operating range from 0.2 to 2 GHz. One can notice that there is a slight difference between the measured and simulated results of the reflection coefficient due to soldering of feeding launcher and fabrication tolerances.

**Figure 9.** 

*Reflection coefficient versus frequency for the proposed two substrate layers of Vivaldi antenna.* 

**Figure 10.** 

*(a–d) 3D radiation pattern of dual layer substrate at 0.5, 1, 1.5, and 2 GHz, respectively.* 

#### **Figure 11.**

*From (i) to (iv) 2D radiation pattern at 0.5 GHz, 1 GHz, 1.5 GHz, and 2 GHz, respectively, of single and dual substrate (a) Φ°=0°, (b) Φ°=90°, of single and dual substrate, and (c) from (i) to (iv) θ=90° , ( one substrate antenna, and dual substrate antenna).* 

*Detection of Underground Water by Using GPR DOI: http://dx.doi.org/10.5772/intechopen.83594* 

**Figure 10** shows the three-dimensional radiation patterns of the proposed dual substrate Vivaldi antenna at different operating frequencies within the operating band at 0.5, 1, 1.5, and 2 GHz. The radiation patterns correspond to the axes shown in **Figure 1**. In the antenna, the radiator and the ground plane are participating to radiation. End-fire radiation pattern is an important requirement for ultrawideband GPR application system. At lower frequencies of operation, the pattern resembles a conventional dipole antenna, but at higher end of the UWB spectrum, a few ripples are observed which is due to higher-order modes.
