Microstrip-fed Triangular UWB Microstrip Antenna Based on DGS

An ultra-wideband (UWB) monopole microstrip antenna is designed in this study. The design consists of a radiating triangular patch antenna (TPA) fed through a 50 Ohm microstrip transmission line (MTL) constructed over a defected ground structure (DGS). The triangular monopole well is matched to MTL with inserting a thin strip line. The performance of the proposed antenna is numerically investigated using method of moments (MoM) and verified through measurements. Based on the measured results, the antenna operates over large frequency range of 2.6-18.2 GHz at -10 dB. This makes the design suitable for UWB applications. The proposed antenna has better characteristics in terms of radiation pattern, peak gain and total efficiency across the operating ultra-wideband frequency range. Furthermore, the antenna system is suitable for near-millimetre wave applications, since the operating band reaches to 18.2 GHz.


Introduction
The ultra-wideband (UWB) systems have received considerable attention in recent years due to allowing signal transmission in a large bandwidth with a low energy level.Radar imaging, mobile and satellite communications, multimedia streaming, biomedical imaging are the most important application areas of ultrawideband systems [1][2][3][4][5].The ultra-wideband systems have been further studied especially since Federal Communications Commission (FCC) approved the unlicensed use of the frequency range of 3.1-10.6GHz for low power emitting implementations [6].Patch antennas are widely used in the wireless systems due to various advantages such as low profile, small size, light weight, conformability to mounting host and capability of integration into the arrays.Owing to these advantages, the printed antennas have been integrated to the latest wireless communications of wireless local area network (WLAN), long term evolution (LTE) and worldwide interoperability for microwave access (Wi-MAX) standards, and ultra-wideband systems.Designing an appropriate antenna is the most important part of ultra-wideband applications because it effects the whole system performance.There are several antenna structures that can be used for ultrawideband systems such as the Vivaldi antenna [7], log periodic antenna [8], spiral antenna [9] and monopole antenna [10][11][12][13][14][15][16][17][18].Monopole designs show inherently wideband behaviour as well as omni-directional radiation pattern.Therefore the printed forms of that designs can be successfully exploited for ultra-wideband systems.Over the past years, several monopole antenna structures, such as rectangular [10], circular [11], elliptical [12], conical [13], tapered [14,15], slot-loaded [16][17][18] and defected ground [19][20][21][22] configurations, have been suggested for ultrawideband applications.The popularity of defected ground structure (DGS) in antenna study has increased immensely [19][20][21][22].The DGS is basically obtained by properly modifying the ground plane of a patch antenna.Thus compact designs with high characteristics of especially wideband and gain can be achieved.In this study, an ultra-wideband triangular patch antenna (TPA) that consists of a triangular monopole is designed.The triangular monopole is positioned on a rectangular DGS.The triangular radiating monopole patch is fed through a microstrip transmission line (MTL).The TPA is modelled in HyperLynx 3D EM simulation software based on method of moments (MoM) and fabricated for measurements.The TPA with a small size of 23x31 mm2 operates across 2.6 GHz to 18.2 GHz based on measured results.

The Structure of DGS-Based TPA
The TPA structure is composed of a 14x23 mm2 triangular monopole patch fed by 50 Ohm MTL and a 17x23 mm2 rectangular DGS.The 3D geometry of the TPA model is depicted in Figure 1 and its dimensions are given in Table I.The radiating plane is placed over a 1.6 mm height substrate with 4.4 relative di-electric constant on the DGS.Therefore, the TPA has a total dimension of 23x31x1.6 mm3.In order to improve the impedance matching, a 1 mm thin strip line is inserted between triangular radiating patch and the MTL.

The TPA Design Procedure
In design process, different patch forms are assayed in order to achieve an ultra-wideband operation.To further understand the evaluation of the proposed antenna system, the designing steps are investigated in four cases: Case I, II, III and IV as illustrated in Figure 2. The simulated S 11 parameters related to the cases are accordingly shown in Fig. 3.In Case I, the design procedure is started with a 14x23 mm2 rectangular radiating patch fed by 50 Ohm MTL and a DGS.The DGS is obtained by shortening the ground plane size of 31x23 mm2 to 17x23 mm2 so as to obtain an effective radiation.Since the radiation field occurs mainly between the radiating patch and the ground edges facing to each other, modifications on these edges can yield a satisfactory results.Therefore, in Case II, the radiating edges facing to the fixed DGS are triangularly trimmed to increase the impedance bandwidth as can be seen in Figure 3. Once a better results are achieved by this way, the triangular cut's angle with the DGS is increased to further improve the impedance matching as in Case III.Note that the 50 Ohm MTL uniformly continues from the port to the input of the triangular radiating patch.Owing the radiation starting from the end of DGS, a thinner strip is needed to match the input impedance of the triangular radiating patch to the MTL.A strip is hence designed between those in Case IV which is the final form of the designed antenna system.

The Current Mechanism of The TPA
The simulated surface current distributions of the TPA at the frequency points of 2.7 GHz, 8.1 GHz and 11.9 GHz are presented in Figure 4. From the figure, the current distributions for these frequencies vary in similar to each other.As mentioned above in Section III that the radiation field comes about the facing edges, it can also be observed from the current distributions, the source current of that radiation is mostly concentrated at those edges.In addition, the majority of the current intensifies at the side edges of the matching strip line since it behaves as a resonator.This means that it does not only match the input impedance but also highly contributes to the radiation.

Fabricating a Prototype of The TPA
The prototype of the TPA is shown in Figure 5.The antenna model is etched with the use of a computer numerical control (CNC) machine on a double side 1.6 height FR4 substrate with a relative dielectric constant of 4.4 and a tangent loss of 0.017.The measurements are carried out by the agency of Keysight Technologies N5224A PNA network analyser.The measured S 11 parameters in comparison with the simulated ones are plotted in Figure 6.Based on the measured results, the TPA operates between 2.6 GHz and 18.2 GHz.It should be noticed that discrepancies between the simulated and measured curves might be attributed to the small mismatches of geometry, dielectric, thicknesses, copper cladding and feeding between the simulated models and the prototype.Figure 8 indicates the peak gain together with the total efficiency of the TPA.It is known that total efficiency of an antenna is the product of the efficiencies relevant to radiation, mismatch, conduction and dielectric.From the figure, the peak gain changes between 2.2 dBi and 4.5 dBi levels across the interested frequency band, whereas a minimum gain occurs 2.2 dBi at 2.7 GHz and maximum gain take places 4.5 dBi at 16 GHz.On the other hand, the total efficiency keeps higher than 66%.The highest and second highest total efficiencies are 94% and 93% at 2.7 GHz and 5 GHz, respectively.

Conclusion
In this work, we presented a compact ultra-wideband monopole patch antenna design operating over a large frequency range of 2.6-18.2GHz.The monopole TPA fed via a MTL is built on a half-length DGS.The whole antenna system has a small size of 23x31 mm2.A thinner strip line is inserted between the triangular monopole and the MTL to improve the impedance matching.The proposed TPA is verified by simulation and validated through measurement.The TPA has superior characteristics of larger bandwidth than the regulated ultra-wideband of 3.1-10.6GHz, near omni-directional pattern, uniform gain and high efficiency.
In conclusion, since the TPA operates up to 18.2 GHz it can be also utilized for near-millimeter wave applications.

1 Figure 1 .
Figure 1.The 3D geometry of the TPA a) perspective view, b) front view, c) side view

Figure 2 .
Figure 2. The antenna designing steps: a) Case I, b) Case II, c) Case III, d) Case IV

Figure 3 .
Figure 3. Simulated S11 plots of the antenna designing steps for cases in Figure 2

Figure 5 .
Figure 5.The photograph of the fabricated TPA

Figure 6 .
Figure 6.Measured and simulated S11 parameters of the prototyped TPA6.Radiation, Gain and Efficiency CharacteristicsSimulated 2D gain radiation patterns of the TPA at the frequency points of 2.7 GHz, 8.1 GHz and 11.9 GHz are illustrated in Figure7.The maximum gains are 2.14 dB (in direction of 180° on x-z plane), 2.2 dB (in direction of 185° on y-z plane) at 2.7 GHz; and 2.42 dB (in direction of 90°/270° on x-z plane), 1.9 dB (in direction of 320° on y-z plane) at 8.1 GHz.Likewise, they are 1.6 dB (in direction of 145°/215° on x-z plane) and 0.2 dB (in direction of 45°/315° on y-z plane) at 11.9 GHz.

Figure 7 .
Figure 7. Measured and simulated radiation gain patterns of TPA at: a) 2.7 GHz on x-z plane, b) 2.7 GHz on y-z plane, c) 8.1 GHz on x-z plane, d) 8.1 GHz on y-z plane, e) 11.9 GHz on x-z plane, f) 11.9 GHz on y-z plane (solid line measured, dot line is simulated)

Figure 8 .
Figure 8. Measured peak gain and total efficiency of TPA