# Extremely Low-Profile Monopolar Microstrip Antenna with Wide Bandwidth
###### Abstract  
In this paper, we propose a new monopolar microstrip antenna for a high-speed moving swarm sensor network. The proposed antenna shows an extremely thin substrate thickness supported with an omni-directional radiation pattern and wide operation frequency bandwidth. First, to achieve the low-profile monopolar microstrip antenna, the symmetrical center feeding network and the gap-coupled six arrayed patches which form a hexagonal microstrip radiator were utilized. The partially loaded ground-slots under the top patches were employed to improve the radiation performance and adjust the impedance bandwidth. Second, to obtain the broad bandwidth of the low-profile monopolar microstrip antenna, the degenerated non-fundamental TM\({}_{\text{Q2}}\) modes, that is, even and odd TM\({}_{\text{Q2}}\) modes, were carefully analyzed. To verify the feasibility of the degenerated TM\({}_{\text{Q2}}\) mode operation, the parametric study of the proposed antenna was theoretically investigated and implemented with the optimized parameter dimensions. Finally, the fabricated antenna showed a 0.254 mm-thick substrate and demonstrates 1.5-wavelength resonant monopolar radiation with broad impedance bandwidth of 855 MHz and its fractional bandwidth of 15.24% at the resonant frequency of 5.57 GHz.  
low-profile; broad bandwidth; degenerated modes; ground slot; conical beam pattern +
Footnote †: journal: Sensors  
0000-0001-5295-3000  
0000-0002-0  
microstrip antenna is to utilize the equivalent magnetic loop current density of the radiator edge side, which is mathematically equal to the electric line current density. Representatively, it is divided into a method that utilizes a 1.5-wavelength over-mode resonance rather than a fundamental resonance [24; 25; 26] and a method that utilizes a metamaterial-based zeroth order resonant mode [27; 28; 29; 30]. This method may show a disadvantage in that a single resonant mode supports only a single operation bandwidth, the thickness of the substrate is thick and the bandwidth is relatively narrow. And to overcome the narrow bandwidth issues, a design method of adding a slit and through-hole vias on the radiator [7; 10; 31; 32; 33], stacking multi-layered substrates for multiple resonance modes [9] and adding stubs in same substrate layer for degenerated mode with adjunct frequency with 90-degree phase delay [34; 35; 36; 37; 38]. Unfortunately, these technologies can have improved bandwidth, but the thickness of the antenna dielectric substrate is still thick, and the design method is complicated.  
In this study, we have focused the theoretical analysis and implementation of the extremely thin monopolar microstrip antenna with broad operation bandwidth. The final goal of this study is to maximize the operating bandwidth of the low-profile monopolar antenna using two adjacent frequencies with the degenerated TM\({}_{02}\) mode using a single resonator on a very thin single substrate. In other words, the proposed antenna can provide a wide bandwidth with a simple design and a thin thickness by using the degenerated mode of a single resonator on a single substrate. To verify this new physical phenomenon, that is, the degenerated TM\({}_{02}\) modes, we designed the proposed antenna based on the verified antenna model previously presented in [31]. In addition, the parametric studies of the proposed antenna were investigated and optimized to control the degenerated modes of the proposed antenna. The following sections will show details of the proposed antenna configuration, electrical and radiation characteristics and electric field distribution for the operation principle.