Novel Compact UWB Planar Monopole Antenna Using a Ribbon-shaped Slot

Ultra-Wideband (UWB) is a wireless communication technology that can be utilized for precise indoor positioning system. UWB is low power-consuming and resistant to complex multipath environments, thanks to a short pulse signal. Since the main desired characteristics of the UWB antennas for radio-frequency localization systems are wide bandwidth, omnidirectional radiation pattern, and low profile for simple integration with printed circuit boards, various planar monopole antennas for UWB applications were proposed to meet the requirements. In order to satisfy these conditions, in this paper, a novel compact UWB planar monopole antenna operating in 3.1 GHz – 10.8 GHz is designed on FR-4 substrate. The overall size of the antenna is 12.5 × 12.5 × 1 mm3, which is 0.129 λ0 × 0.258 λ0 × 0.01 λ0 in free space at the lowest frequency. The transmission line is designed based on a coplanar waveguide with ground (CPWG) and vias in the CPWG are employed to eliminate non-radiating phenomenon at some specific frequencies. The shape of the radiator is modified from the hexagonal shape and a ribbon-shaped slot inside the radiator is adapted to improve the operating frequency range. The proposed UWB planar monopole antenna is fabricated and measured. The proposed antenna provides good antenna performance from 3.1 to 10.8 GHz and also the radiation patterns at various frequencies are omnidirectional pattern.

sufficient accuracy and reliability in indoor environments [4]- [7]. Note that UWB short pulse signals can increase the transmission speed, overcome multipath fading and frequency selective fading, and offer the high security and resolution for precise indoor positioning systems [8]- [10].
In general, for UWB-based indoor positioning systems, the main desired characteristics of UWB antennas are wide bandwidth, omnidirectional pattern, and compact size [11]. Recently, some researchers proposed UWB antennas that satisfy the aforementioned characteristics in many different ways [12]- [26]. Various planar monopole antennas for UWB systems were studied for miniaturization by employing an electromagnetic-bandgap [12], a metamaterial [13], a partial ground plane [14], and two triangular and one circular sector slots [15]. In addition, various shaped compact UWB planar antennas were proposed, including a symmetrical hexagonshaped radiator [16], a z-shaped radiator [17], a compact tapered-shape slot [18], a bow tie-shaped antenna [19], and so on [20]- [26].
We propose a novel compact UWB planar monopole antenna using a new shape to yield good antenna performance in the frequency range of 3.1 GHz -10.8 GHz. Our proposed UWB antenna is developed by the following three-step procedure: 1. Basic design of a hexagon-shaped radiator 2. Modification of a hexagon-shaped radiator to improve matching performance 3. Insertion of a ribbon-shaped slot to increase the bandwidth In this work, the transmission line is made of a coplanar waveguide with ground (CPWG). The proposed antenna performance is investigated in both frequency and time domains such as reflection coefficient, radiation pattern, gain, efficiency, and fidelity factors. The main contributions of our work include compact size, elimination of radiation efficiency deterioration, and comprehensive analysis of antenna performance. The remainder of this paper is organized as follows. We first present the UWB planar monopole antenna operating the frequency range from 3.1 GHz to 10 GHz. And then, vias are employed in the CPWG to prevent unwanted electromagnetic phenomenon at 3.6 GHz and 10.2 GHz. Next, we propose the UWB planar monopole antenna operating the frequency range from 3.1 GHz to 10.8 GHz by inserting a slot inside the radiator. Moreover, the effects of the position and the shape for the slot are investigated. The proposed UWB planar monopole antenna is fabricated and its experimental results are presented. Finally, concluding remarks are provided.

A. UWB Planar Monopole Antenna
As alluded previously, in this work, the UWB planar monopole antenna is designed for compact size, broad bandwidth, and omnidirectional pattern. Fig. 1 (a) shows the planar monopole antenna, which is adopted by a hexagonshaped radiator [16]. The lowest operating frequency (3.1 GHz) is considered to initially design the hexagon-shaped radiator. The lowest operating frequency (3.1 GHz) is considered to initially design the hexagon-shaped radiator. The lowest frequency of the planar monopole antenna can be approximately calculated by equating its area to an equivalent cylindrical monopole antenna [27]: 2πrl = area of the radiator (1) where co is speed of light in free space, l is the height of the radiator, and r is the equivalent radius. Note that the effects of the substrate and the feed gap are not considered in the above design equation [28]. In this work, we choose the height (l) of the hexagon-shaped radiator as 8 mm and its area as 77.2 mm 2 (its equivalent radius r of 1.54 mm). Since the substrate effect cannot be considered analytically, we adjust the feed gap by the EM simulator (HFSS) such that the lowest operating frequency of the radiator is 3.1 GHz. Fig. 1 (a) shows our hexagon-shaped radiator with the feed gap of 2.6 mm. The hexagon-shaped radiator is transformed to a modified shaped radiator, where both sides are partially cut into an oval shape to have a concave shape and both top and bottom shapes are the same as that of both sides in order to improve the reflection coefficient performance, as illustrated in Fig. 1 (b). The CPWG consists of the spacing of 0.2 mm and the width of 1 mm for a given 50 Ω impedance as shown in Fig  For the modified shaped antennas, two types of CPWGs (without vias and with vias) are considered. It is observed that the reflection coefficient of the hexagon-shaped monopole antenna is larger than -10 dB from 9 GHz to 9.8 GHz. For the two modified shaped antennas, the reflection coefficients are less than -10 dB from 3.1 GHz to 10.1 GHz, different from the hexagon-shaped antenna. Fig. 2 (b) illustrates that radiation efficiency is significantly deteriorated at 3.6 GHz and 10.2 GHz for the hexagon-shaped antenna and the modified shaped monopole antenna without vias. In addition, Fig. 2 (c) shows that the imaginary values of the impedance for these antennas are rapidly changed near the same frequencies. To further investigate how the antennas operates at these frequencies, we examine surface current density distribution for the considered antennas at the corresponding frequencies. As shown in Fig. 3, the surface current densities are concentrated on the transmission line for the antennas based on the CPWG without vias and lower current flows on the radiator, which leading to poor radiation efficiency. On the other hand, for the antenna based on the CPWG with vias, nonradiating phenomenon does not occur and thus radiation efficiency is significantly improved compared to the nonvias counterparts. Therefore, the CPWG with vias is selected for the transmission line of the UWB monopole antenna in this work. Note that the return loss is not larger than 10 dB at high frequencies (> 10.1 GHz) for the designed UWB planar antenna up to now. Therefore, the operating bandwidth of the UWB planar antenna should be further increased and this will be discussed in the following subsection.

B. UWB Planar Monopole Antenna with a Ribbonshaped Slot
In this subsection, a ribbon-shaped slot is applied inside the designed modified shaped planar monopole antenna to satisfy the entire UWB band (3.1 GHz -10.6 GHz). A ribbon-shaped slot that becomes narrower from both ends of the radiator toward the center is formed and both ends of the slot have a concave cut shape to minimize reflection coefficient. Figure 4 shows how the reflection coefficient performance of the antenna changes depending on the position of the ribbon-shaped slot. According to Fig. 4 (b), the -10 dB S11 bandwidth is improved due to the additional resonance at 10.2 GHz, when the position of the ribbonshaped slot is -2 mm. Also, we analyzed how the performance changes when a rectangular slot is applied to the antenna based on the minimum and maximum widths of the ribbon. Fig. 5 (a) displays the rectangular slot with minimum and maximum widths of the ribbon and Fig. 5 (b) demonstrates that the reflection coefficient for the ribbonshaped slot antenna near 10.2 GHz is significantly small compared to the other rectangular slot antennas. As shown in Fig. 6, it is illustrated that the ribbon-shaped slot resonates at 10.2 GHz and thus the operating bandwidth is extended to cover the whole UWB band. Fig. 7 shows the final design schematic of the proposed UWB planar monopole antenna, consisting of the modified shaped radiator (which is transformed from a hexagonal shape for bandwidth enhancement), the CPWG with vias (which can eliminate undesired non-radiating phenomenon), and the ribbonshaped slot (which extends the operating bandwidth by resonance at 10.2 GHz).

III. Fabrication and Measurement
The designed UWB planar monopole antenna is fabricated and measured, as shown in Fig. 8. The simulated and measured results of the reflection coefficient for the proposed UWB antenna is shown in Fig. 9. The measured result is generally in agreement with the simulated result. Somewhat discrepancies between the measurement result and simulation result may be caused by errors of fabrication and measurement. It is observed that the measured reflection coefficient of the fabricated UWB antenna is below -10 dB from 3.1 GHz to 10.8 GHz. The measured radiation patterns for the cross polarization and the co polarization at various frequencies are also shown in Fig. 10. Omnidirectional radiation patterns can be observed, similar to the simulated radiation patterns. Fig. 11 show the simulated and measured gains of the proposed antenna and it is observed that its maximum measured gain is 4.12 dBi at 7 GHz. Fig. 12 provides the simulated and measured radiation efficiencies and some discrepancies are caused by the fabrication and measurement errors. Also, the time-domain behaviors of the proposed antenna are investigated by CST EM simulations [17], [19]- [20], [23], [29]- [30]. The antenna fidelity factor can be obtained by calculating with the cross-correlation   between the input pulse and the radiated E-fields. Fig. 13 (a) demonstrates the antenna fidelity factor of 0.8-0.9 values for all angles. The system-fidelity factor is calculated by crosscorrelation between the transmitted pulse and the received pulse. To verify the system-fidelity factor, the two same proposed antennas are placed in 1 meter away from each other and the two cases of face-to-face and side-by-side are considered. Fig. 13 (b) and (c) show the normalized transmitting pulse and received pulse for the two cases. The system-fidelity factors for the face-to-face and the side-byside are 91.8% and 82.6% respectively. Fig. 14 shows the phase and group delay of S21 between the two proposed antennas placed in 1 meter from each other. The measured phase and group delay are in good agreement with the simulated results. Finally, Table 1 shows comparison between the proposed UWB planar monopole antenna and the previous UWB antennas in literature. Note that the λ0 is the wavelength at the lowest frequency of the operating bandwidth in free space. As shown in the table, the UWB antenna proposed in [25] is the smallest size, but the authors did not provide the time-domain analysis such as fidelity factors and group delay, which are important characteristics for UWB antennas discussed in [30]. Although the proposed UWB antenna is not the smallest size, our antenna is comprehensively analyzed in both frequency domain and time domain and is sufficiently compact compared to other UWB antennas.
This article has been accepted for publication in IEEE Access. This is the author's version which has not been fully edited and content may change prior to final publication.  This article has been accepted for publication in IEEE Access. This is the author's version which has not been fully edited and content may change prior to final publication.

IV. CONCLUSION
In this paper, a novel compact UWB planar monopole antenna is proposed. The proposed UWB antenna consists of a modified shaped radiator from a hexagonal shape and the CPWG with vias. The modified shaped radiator is designed to improve the -10 dB S11 bandwidth from 3.1 GHz to 10.1 GHz and vias in the CPWG are used to avoid the unwanted electromagnetic field phenomenon at specific frequencies. Also, a ribbon-shaped slot which resonates at 10.2 GHz is applied inside radiator to extend the operating frequency band. The overall size of the proposed antenna is 12.5 ×25 × 1 mm 3 , (0.129 λ0 × 0.258 λ0 × 0.01 λ0). The proposed UWB antenna is fabricated and measured. Experimental results indicate that -10 dB S11 bandwidth is from 3.1 GHz to 10.8 GHz and the radiation pattern is omnidirectional pattern in the frequency range of interest. The proposed UWB planar antenna has compact size, omnidirectional radiation pattern, and good performance in both frequency and time domains in the UWB band and thus it is highly suitable for UWB indoor positioning systems.