# A Low-Profile Wideband Monocone Antenna Using Bent Shorting Strips
## 3 Antenna Analysis  
The fractional impedance bandwidth for the proposed antenna is about 146.5%, measured over the -10 dB reflection coefficient range of 810-5250 MHz, as shown in Figure 3. To analyze the effects of the bent shorting strips, parasitic sleeves, and circular sleeves, a comparison among the reflection coefficients of reference 1 (only monocone), reference 2 (monocone with bent strips), reference 3 (monocone with bent strips and circular sleeves), and the proposed antenna is conducted, as shown in Figure 3. For reference 1, a -10 dB impedance bandwidth of 1680 MHz (1820-3500 MHz) is obtained. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameters** & \(R_{1}\) & \(R_{2}\) & \(R_{3}\) & \(R_{4}\) & \(H_{1}\) & \(H_{2}\) \\ \hline
**Value (mm)** & 50 & 35 & 51 & 1.4 & 26 & 9 \\ \hline
**Parameters** & \(H_{3}\) & \(H_{4}\) & \(H_{5}\) & \(H_{6}\) & \(W_{1}\) & \(W_{2}\) \\ \hline
**Value (mm)** & 5 & 10 & 15 & 2 & 10 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of the proposed antenna. Figure 2: The geometry of the proposed antenna design. (**a**) 3-D view of the proposed antenna, (**b**) 3-D view of the parasitic and circular sleeves, and (**c**) side view of the proposed antenna. To expand the low frequency limit, the shorting strips are employed such that an additional resonance mode is generated below the fundamental mode [13; 21]. We adopted the bent shorting strip instead of a straight shorting strip to improve the impedance matching at around 1.2 GHz, as well as to minimize the antenna height. The simulated -10 dB impedance bandwidth of reference 2 is 2340 MHz (810-3150 MHz). However, the impedance bandwidth of reference 2 is not enough to cover the requirements of the present wireless communication systems. To improve the impedance matching characteristics at the mid-and upper-frequency bands, we employed the inner and outer circular sleeves on the ground plane [14; 15]. Two -10 dB impedance bandwidth regions of 3500 MHz (810-4310 MHz) and 740 MHz (4440-5180 MHz) are obtained as shown in Figure 3. It can be seen that the circular sleeves can improve the impedance matching characteristics in the mid- and upper-frequency bands. However, the -10 dB impedance bandwidth of reference 3 is still not satisfied in the frequency ranges between 4310 and 4440 MHz. To improve the -10 dB impedance matching characteristics at 4310-4440 MHz and further expand the high frequency limit, parasitic sleeves are mounted on the ground plane [14]. Consequently, the simulated -10 dB impedance bandwidth of the proposed antenna becomes 4440 MHz (810-5250 MHz). This result indicates that the existence of parasitic sleeves effectively improves the impedance matching characteristics in the mid- and upper-frequency bands. To investigate the operational principle of the antenna, the current distributions of the proposed antenna of reference 1, reference 2, and proposed antenna at 0.91 GHz, 2.63 GHz, 3.41 GHz, 4.07 GHz, 5.03 GHz, and 5.45 GHz are analyzed, as shown in Figure 4. For reference 1, the currents are mainly concentrated at the center of the ground plane and shows that \(TM_{01}\) resonance mode operates mainly. At 2.63 GHz, strong currents are excited at the center of the ground plane, revealing that the resonance at this frequency is generated by the monocone antenna itself. For reference 2, the currents are strongly excited at the center of the ground plane and along the bent shorting strips from the monocone hat to the ground plane. Particularly, at 0.91 GHz, the current distribution shows that \(TM_{41}\) resonance mode is the principle operating mode. Therefore, the bent shorting strips primarily affect the antenna performance in the low-frequency regime. For the proposed antenna, the currents flowing through the monocone antenna and shorting strips are coupled to the inner, outer, and parasitic sleeves. Particularly, at 3.41 GHz, 4.07 GHz, and 5.03 GHz, the currents are strongly induced on these additional structures (inner, outer, and parasitic sleeves).