**4. Printed quasi-Yagi antenna with size reduction for water detection**

 Nowadays, quasi-Yagi printed antenna is extensively used in modern radar systems due to some advantages as high directivity, good radiation efficiency, affordable, low profile, and easy fabrication [33-35, 37]. However, the disadvantage of these antennas is narrow bandwidth, which achieves about 10%. So, the microstrip-fed quasi-Yagi antenna was initially introduced in 1991 [38] to improve the bandwidth of planar printed quasi-Yagi antennas, and many designs have been reported in [39]. A quasi-Yagi antenna based on microstrip-to-slot-line transition structure was presented in [40]. Modified wideband microstrip-to-coplanar strip-line (CPS) balun was used in quasi-Yagi antenna designs for increasing the antenna bandwidth [41]. Approximately 48 and 38.3% bandwidths were achieved by using the microstrip-to-coplanar strip-line transition structures in [42] and [43], respectively. However, the antenna bandwidths are still restricted by the delay line used in the balun structures. Coplanar waveguide feeding or ultra-wideband balun was presented to improve the bandwidth in some designs [44]. A broad bandwidth of 44% was obtained in [45]. However, the asymmetric nature of the printed quasi-Yagi antenna deteriorates the unidirectional radiation patterns. An ultra-wide band balun feeding structure in which the balun was realized via holes was used in quasi-Yagi antenna for wideband in [46]. Slot and CPS-fed feeding structures were also used in planar printed quasi-Yagi antenna to increase the bandwidth. The maximum available bandwidth of these techniques is about 55%. To improve the bandwidth of quasi-Yagi antenna by modifying the driver to a tapered driver or bowtie driver, rapid developing technology of remote sensing and radar has led to the ultra-wide band (UWB) electronic systems.

#### **4.1 Antenna design and geometry**

 **Figure 12** shows the geometric structure and parameters of the proposed planar quasi-Yagi antenna. This antenna is printed on commercial thick FR4 substrate and a thickness of 9.5 mm. The antenna consists of a microstrip-line-to-slot-line transition structure, a meandered driver T-shaped dipole and two meandered parasitic strips on top layer. The feeding system is printed on the other substrate side with lengths *Lf* and *SD* with a circular resonator. The circular resonator is used to match the input impedance of the antenna to a 50 Ω feeding line. The dimension of the substrate width and length are 72 × 70 cm<sup>2</sup> . For matching the antenna, a λ/4 slot line ended with a circular slot of diameter *LD* is used.

The traditional printed quasi-Yagi antenna started to resonate from 90 MHz as shown in **Figure 13**(**a**). Folded stub ground plane and driven dipole that is etched in order to reduce the size of the printed quasi-Yagi antenna, as shown in **Figure 13**(**b**), are employed. The ground plane width and the driver dipole length are equal. The antenna shown in **Figure 13**(**b**) has 80 MHz as the lowest frequency. Since the ground plane has reduced size, the bandwidth of the antenna is reduced. A driver meander

**Figure 13.** 

*The compact printed quasi-Yagi antenna design steps [37].* 

**Figure 14.** 

*Simulated |S11| of the design steps as shown in Figure 11 [37].* 

dipole is etched to increase the electrical size of the antenna (**Figure 13**(**c**)). The antenna started to operate from 62.5 to 185 MHz. Two meandered stubs are symmetrically extended from its ground plane as shown in **Figure 13**(**d**). The reflection coefficients |S11| of the antenna design procedure are shown in **Figure 14**.

As shown, the antenna consists of a circular balun feeding which takes the form of a curved microstrip line step transition, in addition to a printed dipole, a ground plane, and two parasitic strips. The larger dipole is located at a distance S away from the ground plane which has a greater length than the larger dipole itself, so it can act *Detection of Underground Water by Using GPR DOI: http://dx.doi.org/10.5772/intechopen.83594* 

**Figure 15.** 

*The surface current distribution of the printed quasi-Yagi antenna at (a) 50, (b) 100, and 150 MHz [37].* 

as a reflector. On the second size of the substrate, the two dipoles are located, their line length is 2.6 cm, and the two parasitic spacing was optimized using the readingmade software package HFSS ver.14 in order to improve the antenna performance which means that it has a wide bandwidth, stable radiation, moderate gain, and high front-to-back ratio.

In quasi-Yagi antenna design, metallic strip is always used as a director. To improve the directivity and impedance matching in the high-frequency band, the two metallic strips are used. The simulated surface current distributions of the proposed antenna at 50, 100, and 150 MHz are shown in **Figure 15**. The directors have weak surface current as shown in **Figure 15**, while the two parasitic strips have a large magnitude of surface current at 50 MHz. the largest value of surface current at resonant 100 MHz takes place at larger parasitic strip and the driven dipole. At 150 MHz, the current concentrates on the meandered driven dipole. Compared to **Figure 15**, the surface currents on the metallic strips are enhanced, which means that the effects of the parasitic strips as directors improved the antenna performance at the high-frequency band.

#### **4.2 Antenna parameter study**

 The antenna structure was optimized to operate at 100 MHz center frequency. The reflection coefficient against frequency for different values of *L1* is shown in **Figure 16**(**a**). As the value of *L1* is increased (in steps from 40 to 55 cm), the resonance frequency decreased. At *L1* = 50 cm, the antenna provides the largest beamwidth. One can say that the resonant frequency of the lower band is mainly determined by the length of the larger strip. The dipole in this antenna acts as director of quasi-Yagi antenna. Also, simulation was done to see the effect of *L*<sup>2</sup> on the antenna bandwidth. **Figure 16**(**b**) shows S11 against frequency for different values of *L2*, which shows that as *L2* increased from 20 to 35 cm, the highest frequency almost did not change, while the lower resonance (50 MHz) slightly changed. When the ground plane circular slot *LD* was changed from 4 to 5.5 cm, impedance matching was affected, while the lower and higher frequency did not change (**Figure 17**(**a**)). One can conclude that the diameter of the slot affects the feeding impedance. The effect of the feeding length *Lf* was studied as shown in **Figure 17**(**b**). *Lf* was varied from 15 to 16.5 cm which caused noticeable changes in the operating frequency band and the value of the reflection coefficient. The length *Lf* highly affects the impedance matching, and the optimized length of the feeding is 16 cm. **Figure 18**(**a**) shows the reflection coefficient of the antenna as a function of the spacing between the driver and director (*T* and *T1*).

Increments of the spacing decreased the coupling effect between the parasitic element and the dipole induced significant changes in the reflection coefficient in the operating band region from 50 to 150 MHz, but negligible changes occur in the bandwidth.

**Figure 16.**  *Effect of the length (a) L1 and (b) L2 on the simulated reflection coefficient [37].* 

**Figure 17.**  *Effect of the length (a) LD and (b) Lf on the simulated reflection coefficient [37].* 

 In the high-frequency region of 100 MHz band, the antenna performance is affected mainly by the spacing between the driver and the director. The quasi-Yagi antenna and the T-dipole were designed to operate at 50 and 150 MHz, which shows that a suitable choice of this spacing is very important for the wideband operation of the proposed antenna.

The reflection coefficient *S11* against frequency is shown in **Figure 19** for different values of the parameter D (balun circle) and *SD* (length from the feeding). The lower resonance and impedance matching are varied as D increases from 4 to *Detection of Underground Water by Using GPR DOI: http://dx.doi.org/10.5772/intechopen.83594* 

**Figure 18.**  *Effect of the length (a) T1 and (b) Ton the simulated reflection coefficient.* 

**Figure 19.**  *Effect of the length (a) D and (b) SD on the simulated reflection coefficient.* 

 5.5 cm. As the value of *SD* is changed from 15 to 18 cm, the behavior of S11 is changed significantly, so the choice of *SD* is very important for the operation of the antenna.

#### **4.3 Ground-penetrating radar antenna system**

 FMCW GPR system is used for the detection of underground water in the frequency range from 50 to 150 MHz. **Figure 20**(**a**) shows the radar system which requires a high-gain antenna to obtain acceptable scanning resolution. We used laboratory measurement and EM simulation in order to investigate the electrical and physical properties of the sand and fresh water. The simulated parameters depend on the Debye dispersive model inherent in HFSS software package. **Figure 20**(**b**) shows the study of the ground effect on the radiation characteristics of the antenna S11 projection. The distance *K* between the antenna and the ground surface was increased from up to 100 cm. The volume of the sand layer was 300 × 200 × 200 cm3 . **Figure 21** shows the reflection with and without the sand layer. It was found that in order to keep *S11* very close from the case of free space, *K* should not be less than 50 cm. **Figure 22** shows both the gain and radiation efficiency. It is clear that the gain was increased by about 1.5 dBi compared to the case of free space, which may be attributed to the increase in directivity at certain frequencies, while it remains unchanged in other frequencies. The antenna radiation efficiency, as indicated from **Figure 22**, is reduced. The threedimension radiation pattern of the proposed antenna in both cases is also studied at three different resonant frequencies 50, 100, and 150 MHz, respectively, as shown in **Table 3**, while the 2D radiation patterns at xy-plane (*E*Ф*, E*θ) at Ф = 0° and yz-plane

#### **Figure 20.**

*(a) The GPR antenna system for water detection and (b) the effect of K on proposed antenna reflection coefficient [37].* 

**Figure 21.**  *|S11|of the receiver antenna in different cases at K = 50 cm.* 

**Figure 22.** 

*|S11| comparison between measured and simulated reflection coefficient of the proposed antenna [37].* 

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

#### **Table 4.**

*The 2D EФ and Eθ (phi = 90° and theta = 90°) at frequencies 50, 100, and 150 MHz with and without sand layer, black; without sand, red; with sand layer, solid line (EФ) and dash line (Eθ), respectively* 

**Figure 23.**  *The antenna gain and efficiency with and without sand layer [37].* 

 (*E*Ф*, E*θ) at θ = 90 ° of the original design are plotted in **Table 4** at the same frequencies. The two-dimensional radiation patterns at 50, 100, and 150 MHz with and without sand are platen in **Table 4**. A slight change took place in the E-plane and H-plane radiation patterns keeping good directivity within the whole frequency band. The bock lobes are below −5 dB. Deterioration takes place in the H-plan (YZ plane) as the frequency increases especially in the high-frequency bands (e.g., 500 MHz).

#### **4.4 Fabrication and measurements**

 A photolithographic technique on FR4 substrate with 100 μm copper thickness was used to realize the proposed antenna. A 50 Ω SMA launcher was used as a transition between the microstrip antenna and the coaxial line, which was not included in the simulation process. Rohde and Schwarz ZVA67 VNA was used to measure S11. The antenna gain and radiation efficiency are also tested as shown in **Figure 23** in both cases namely free space and in the presence of sand layer. The antenna gain is also tested as shown in **Figure 23** in both cases namely free space and in the presence of sand layer. The proposed antenna performance is investigated with and without sand layer as shown in **Figure 20(a)**. The sufficient distance that keeps the antenna reflection coefficient near from free space is almost about 50 cm as shown in **Figure 20(b)**. **Figure 23** shows that the antenna radiation efficiency is increased at present of sand layer by 5%. While the antenna gain is reduced by about 1.5 dBi in the present of sand layer. The gain in the present of sand layer increases at certain resonant frequency due to increase in directivity and has value less than free space on the other resonant frequencies. The optimized antenna reflection coefficient is measured, and there is a good agreement with the simulated results. The measured bandwidth extends from 56 to 150 MHz for the −6 dB reflection coefficient, while the simulated bandwidth extends from 45 to 140 MHz. This bandwidth completely covers the specification for operation.

The slight difference between the measured and simulated reflection coefficient could be from misalignment between curved microstrip line and the circular slot of the balun and effect of the SMA connector, in addition to some fabrication tolerances.
