# Wideband Ring-Monopole Flexible Antenna with Stub for WLAN/C-Band/X-Band Applications
###### Abstract  
In this paper, we designed a flexible antenna operating in the WLAN/C-band/X-band and analyzed the antenna bending characteristics. It is advantageous to have a wide bandwidth because the resonant frequency of the flexible antenna can be changed when it is bent. The proposed antenna was designed based on a ring monopole antenna with broadband characteristics. Slots and t-strip lines to the ring, stubs to the feed, and stepped structures to the ground plane were added to increase bandwidth. As a result of analyzing the characteristics of the proposed antenna when bent through the S-parameter, it was confirmed that the proposed antenna is suitable at the target frequency, even if it is bent. The size of the antenna is 0.256 \(\lambda\)\(\times\) 0.32 \(\lambda\)\(\times\) 0.0016 \(\lambda\) (32 \(\times\) 40 \(\times\) 0.2 mm\({}^{3}\)) at 2.4 GHz, and the antenna bandwidth is 15.68% (2.36 GHz-2.87 GHz) at 2.615 GHz and 111.69% (3.4 GHz-12 GHz) at 7.7 GHz.  
flexible antenna; CPW-feed; ring-monopole antenna; stub matching; slot-ring antenna; wideband antenna +
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surface. The ring-type patch has the advantage that it does not crack even when bent compared to the conventional circular or elliptical patches. Ring-type monopoles also have broadband characteristics [18; 19; 20]. Therefore, if the flexible antenna is designed as a ring-type monopole with broadband characteristics, it has the advantage of strong crack resistance. In addition, the slots and t-strip lines can be added to the ring to increase bandwidth.  
Flexible antennas were fabricated using various dielectric materials for the substrate. Although not films, there were flexible antennas proposed as substrates that could have flexible characteristics with a very thin thickness such as Rogers 5880, Rogers Ro4835 [21; 22]. However, the flexible antenna proposed as a dielectric without film properties has a disadvantage in that it is difficult to break or return to a flat state once bent. The most commonly used dielectrics for flexible antenna films are liquid crystal polymers (LCPs), polyethylene terephthalate (PET), and polyimides [23; 24; 25; 26]. The disadvantage of LCP is that it is difficult to obtain. On the other hand, polyimide is suitable for flexible antennas because it has higher electrical insulation than PET and is easy to obtain. Polyimide was selected as a dielectric in this paper because it is easy to bend, has good electrical insulation, and is easy to obtain.  
In this paper, we propose a wideband flexible ring monopole antenna. A CPW (coplanar waveguide) feeding structure is used in which the signal line and ground are on the same plane. The CPW feeding structure is suitable for flexible antennas because the antenna is thinner and easier to bend. It is also advantageous for impedance matching [27; 28; 29]. Slots and t-strip lines to the ring, stubs to the feed, and stepped structures to the ground plane were added to increase bandwidth. The antenna was designed using a commercial electromagnetic simulator HFSS. The designed antenna has a size of 0.256 \(\lambda\)\(\times\) 0.32 \(\lambda\)\(\times\) 0.0016 \(\lambda\) (32 \(\times\) 40 \(\times\) 0.2 mm\({}^{3}\)) at 2.4 GHz and was fabricated by printing copper on 0.2 mm thick polyimide. The measurement data such as S-parameters, radiation pattern, and gain were compared with the simulation results. In addition, the characteristics of the designed antenna were analyzed in terms of the degree of bending in the horizontal and vertical directions.