# Antennas for Licensed Shared Access in 5G Communications with LTE Mid- and High-Band Coverage
## 4 Wideband Antenna for LSA Bands and LTE Mid and High Bands
### Design Procedure and Working Principle  
In this design approach, our target is to design a more compact antenna than the previous two antennas (Ant#1 and Ant#2) and also remove the nulls from the radiation patterns. The four main steps of the evolution of this antenna are shown in Figure 13. The predicted reflection coefficients for each step are shown in Figure 14.  
The working principle of the antenna can be easily described by the design steps. The design of Ant#3 started with a simple planar antenna, where an L-shaped strip UOQP is imposed on the middle element MNO, as shown in Figure 13 (Step 1). This antenna has a \(-6\) dB reflection coefficient bandwidth of 1.58 GHz (1.73-3.31 GHz) and resonated at 2.66 GHz, as shown in Figure 14 (Step 1 curve). Therefore, the antenna is unable to cover the whole LTE mid-band since the lower frequency limit of the LTE mid-band is 1.42 GHz. The effective length of the antenna is \(l_{ILE}+w_{14}/2+l_{9}+w_{15}/2+g_{1}+w_{16}=32.7\) mm, which is equal to a quarter-wavelength at 2.66 GHz. In Step 2, an L-shaped bent plate is loaded onto the antenna for tuning the lower-frequency limit, together with the operating bandwidth. First, an I-shaped vertical plate with a thickness of 0.4 mm is added to the edge PQ, which is aligned with the edge point Q,  
Figure 14: Reflection coefficients for each step in the evolution of the wideband antenna (Ant#3). Figure 13: The four steps of the evolution of the wideband antenna (Ant#3). as shown in Figure 13 (Step-2). The length and height of this vertical plate are \(l_{9}+l_{10}\) and \(h_{2}\), respectively. Later, a top plate with a size of \(l_{9}\) mm \(\times\)\(w_{t}\) mm \(\times\) 0.4 mm is loaded onto the top of the vertical plate, which is also aligned with the edge point Q, as shown in Figure 13 (Step-2). This antenna resonates at 1.82 GHz and had a \(-6\) dB reflection bandwidth of 1.46 GHz (1.46-2.92 GHz), as shown in Figure 14 (Step-2 curve). Our next aim is to shift the lower-frequency limit further and generate a second resonant mode in the high band. In Step 3, first, an I-shaped patch OR with a size of \((l_{11}+l_{12})\times w_{16}\) is coupled to the L-shaped strip UQP, maintaining a gap of \(g_{1}\) and \(g_{2}\) along the y- and x-directions, respectively. Second, a triangle-shaped element with a base and height of \(l_{11}\) and \(w_{17}\), respectively, is added to the bottom-right corner of the element OR. At the end of Step 3, a strip with dimensions of \(g_{2}\times w_{18}\) is added to the left of the element UQ. The predicted reflection coefficient (Step 3 curve in Figure 14) shows that a second resonance appears at 3.86 GHz, with a dip in magnitude of \(-13\) dB, and the lower-frequency limit shifts from 1.46 to 1.43 GHz. At the second resonance (3.86 GHz), the antenna resonates in the half-wavelength mode since the effective length of the U-slot antenna POR fed by MNO is \(l_{ILA}+w_{14}+2(l_{11}+l_{12}+g_{2})=39.7\) mm. The \(-6\) dB reflection bandwidths in the first and second operating bands are 1.67 GHz (1.43-3.1 GHz) and 300 MHz (3.68-3.98 GHz), respectively. The top plate over the element PQ and coupled element OR not only improves the operating bandwidth but also reduces the nulls in the x-y plane radiation patterns at frequencies close to the upper limit of the LTE mid band. Although it improves the x-y plane radiation pattern nulls within the frequency band of 3.7-4 GHz, there is still room for improvement. Finally, to expand the reflection bandwidth of the antenna at the second resonance in order to cover the high band and to fine-tune the lower-frequency limit of the mid band, an L-shaped loop NSTS with a strip width of \(w_{23}\) is integrated into the right side of the middle element in Step 4, as shown in Figure 13. The reflection coefficient curve (Figure 14 (Step 4 curve)) shows that the lower-frequency limit further shifted from 1.43 to 1.41 GHz and the \(-6\) dB reflection coefficient bandwidth of the antenna is 2.59 GHz (1.41-4 GHz) with two resonances at 1.81 and 3.64 GHz.