## 2 Single Antenna Design  
Figure 1 shows three geometrical designs of a single antenna, which are: conventional patch, inset-fed patch, and inset-fed and two-slot patch antennas, from left to right, respectively. All three geometries use the microstrip printed circuit board (PCB) technique, such as in Figure 1d. The entire circuit is based on the design of a signal processing circuit for an IR-UWB radar application and is constructed on an FR-4 four-layer stack-up substrate with a thickness of 1.6 mm. Nevertheless, because this antenna is developed for a wideband application, the second and third layers on the radiated region (antenna portion) are eliminated. This approach allows the top and bottom layers to be considered as a radiated patch and a ground patch of the antenna, respectively. By this removal, the highest thickness of 1.6 mm of the PCB is utilized for the antenna portion to acquire more bandwidth than other thinner substrates [15]. Firstly, we propose a conventional rectangular single-patch structure that is optimized at a center frequency of 7.5 GHz. The width of the feed line is chosen to be 0.4 mm. The antenna is printed on an FR-4 substrate with a dielectric constant (\(\varepsilon_{r}\)) of 4.3, loss tangent (\(tan\delta\)) of 0.02, and a thickness of 1.6 mm. The substrate dimension of the three  
Figure 1: Geometries of the single antennas: (**a**) conventional patch; (**b**) inset-fed patch; (**c**) inset-fed and two-slot; (**d**) side view of the single antenna. single antennas is 18 mm \(\times\) 20 mm, which equals one-fourth of the maximum area for the antenna region. This conventional patch is represented in Figure 1a. More particularly, the integrated circuit employs a chip to supply an RF signal for the antenna. The power from the pin of this chip directly transmits through a 50 Ohm grounded coplanar waveguide (GCPW) with the width of the signal line being 0.4 mm. Hence, the small feed line is retained to save the footprint for further designing a microstrip line array in restricted space. Additionally, to expand the bandwidth, lossy elements are composed of an inset-fed geometry and two horizontal symmetric slots along the feed line. The input impedance decreases monotonically as the inset feed point moves from the edge toward the center. Furthermore, the inset feed is helpful for controlling the input impedance by moving near or far from the center. This is effective in designing an antenna array with a co-operating feed network. The slot length (Slot-L) and slot distance (Slot-D) are configured to construct a small patch within the conventional patch that resembles E-shaped geometry [16]. This inset-fed antenna and the inset-fed and two-slot patch antenna are presented in Figure 1b,c to control and expand the bandwidth. Figure 2 compares the simulated reflection coefficients (S11) of the three antennas. The simulation was performed by an ANSYS Electronics Desktop simulator. The star-dash blue, dash-dot red lines, and diamond-dot black represent the S11 of the conventional antenna, the inset-fed patch antenna, and the inset-fed and two-slot patch antenna, respectively. As expected, the third microstrip antenna reflection coefficient bandwidth extends from 6.98 GHz to 8.05 GHz, which is approximately 14 percent FBW, almost three times that of a conventional patch antenna. Figure 3 shows parametric studies of S11 by varying the slot length (Slot-L), slot distance (Slot-D), the slot width (Slot-W), and the inset-fed length (inset-L). Among them, the slot length and slot distance are the most important parameters to widen the bandwidth by introducing the second resonance. As indicated in Figure 3a, the resonance of the higher frequencies can be controlled by the length of the slot. Through fixing the slot distance at 5 mm and increasing the slot length, the second pole at higher frequencies becomes closer to the first pole. In principle, the slot length is a critical factor in determining the E-shaped resonant frequencies of the patch antenna.