# Triple-Band Notched Ultra-Wideband Microstrip MIMO Antenna with Bluetooth Band
## 2 Design of an Ultra-Wideband Antenna Element
### Step 1: UWB Microstrip Monopole Antenna Element Design  
The first step in designing an UWB microstrip monopole antenna is to select the radiator shape. Figure 2 shows the design steps in designing the proposed UWB antenna. The projection plane of the cone (i.e., planar cone shape) is chosen to be the radiator shape of the proposed UWB antenna, as shown in Figure 2a. The chosen planar cone shape has a side edge length of 30 mm and the internal angle between its two-sided edges is 38 degrees. Since the required frequency for the proposed antenna is the FCC-UWB spectrum that operates from 3.1 to 10.6 GHz, in addition to the Bluetooth band at a frequency of 2.4 GHz, the electrical length of the planar cone (\(L_{Pcone}\)) is calculated at the operating frequency of 2.4 GHz, which is approximately equal to a quarter wavelength, as shown in Equations (1)-(3). \[L_{Pcone}=\frac{\lambda_{g}}{4} \tag{1}\]  
\[\lambda_{g}=\frac{C}{f_{0}\sqrt{\epsilon_{eff}}} \tag{2}\]  
\[\epsilon_{eff}=\frac{\epsilon_{r}+1}{2}+\frac{\epsilon_{r}-1}{2}\bigg{[}1+12 \frac{h}{W_{0}}\bigg{]}^{-0.5} \tag{3}\]  
where \(\lambda_{g}\) is the waveguide wavelength, \(C\) is the speed of light (\(C=3\times 10^{8}\) m/s), \(f_{0}\) is the operating frequency (i.e., \(f_{0}=2.4\) GHz), \(\epsilon_{r}\) is the relative permittivity (i.e., \(\epsilon_{r}=4.5\)), \(\epsilon_{eff}\) is the effective relative permittivity, and \(W_{0}\) is the 50-ohm feed line width. Figure 2b shows the first case (case 1) in designing the UWB microstrip antenna. In case 1, the planar cone monopole antenna with a side edge length of 19 mm is connected to the 50-ohm microstrip feeding line. The microstrip cone antenna and its feeding line are located on the top layer of the dielectric substrate. A partial ground plane layer with an etched trapezoidal slit is located at the bottom side of the dielectric substrate material. The aim of the etched trapezoidal slit at the top edge of the partial ground plane is to match the high input impedance of the monopole patch antenna and 50 \(\Omega\) microstrip feeding line. The design in Figure 2b was simulated using CST microwave studio. Figure 3 shows the reflection coefficient (\(\mid S_{11}\mid\) in dB) of the first case (for case 1) design. As shown in Figure 3, the reflection coefficient curve (case 1 curve) is not matched within the required operating  
Figure 1: Proposed UWB microstrip monopole antenna. (**a**) Antenna layer (top view). (**b**) Ground plane layer (bottom view). bandwidth (i.e., FCC-UWB spectrum). The \(1S_{11}\)  curve for case 1 oscillates between \(-5\) and \(-10\) dB. The reason behind the mismatching is that the input impedance of the planar cone antenna is very high, while the impedance of the feeding line is 50 ohms. According to the aforementioned mismatching characteristics, the next design in Figure 2c was introduced to overcome this issue. The modified design in Figure 2c consists of three connected sections at the top layer of the dielectric substrate: a cap for the planar cone section, an impedance transformer section, and the feeding line section. The cap of the cone section is the top part of the planar cone of Figure 2a with a side edge length of \(L_{P1}\) = 11.19 mm and bottom width of \(W_{P1}\) = 12.84 mm. The impedance transformer section is placed to match the high input impedance of the planar cone radiator and the 50-ohm feeding line. It is composed of a trapezoidal shape with a stepped leg. The long base of the trapezoidal shape is connected to the bottom edge of the planar cap of the cone radiator, while the small base of the trapezoidal shape is connected to the 50-ohm feeding line. As shown in Figure 3, the simulated reflection coefficient curve (for case 3) is matched within the required operating bandwidth from 2.16 to 11.96 GHz. In case 4, a series of six equal triangles is etched from the top edge of the cap of the cone radiator in order to improve the matching characteristics (i.e., reflection coefficient performance) at the frequency of 8.4 GHz.