2172 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 Communications A Planar UWB Antenna With a Broadband Feeding Structure Wee Kian Toh, Zhi Ning Chen, and Xianming Qing Abstract—A planar antenna with a broadband feeding structure is pre- sented and analyzed for ultrawideband applications. The proposed antenna consists of a suspended radiator fed by an n-shape microstrip feed. Study shows that this antenna achieves an impedance bandwidth from 3.1–5.1 GHz (48%) for a reflection of coefficient of , and an average gain of 7.7 dBi. Stable boresight radiation patterns are achieved across the entire operating frequency band, by suppressing the high order mode resonances. This design exhibits good mechanical tolerance and man- ufacturability. Index Terms—Broadband antennas, microstrip patch antennas, sus- pended plate antennas, ultrawideband (UWB) antennas. I. INTRODUCTION A low profile and embeddable unidirectional antenna is required for certain ultrawideband (UWB) [1] communication, imaging, localiza- tion, and radar applications. The lower and upper UWB spectrums are 3.1–4.8 GHz (43%) and 6.0–10.6 GHz (55%), respectively. The ex- isting broadband directional antennas, such as the Vivaldi [2], log-pe- riodic, cavity-backed, waveguide, horn, and dish antennas, cover the entire 3.1–10.6 GHz band (109%). However, they are electrically large, and have a high profile in the direction of wave propagation. Omni- and bi-directional antennas, suchas the planar monopoles [3]–[5], disc cone [6], and slot antennas [7], have a low gain and back radiation pattern, therefore they are not suitable for sectorial or unidirectional commu- nication. Also, it is a challenge to maintain a stable radiation pattern across the whole frequency band, since the radiation aperture is fre- quency dependent. To lower the Q-value for increasing the impedance bandwidth of the patch antenna, the dielectric substrate is replaced by air, the patch is usually suspended at a height of , where is the free space wavelength. However, when the height of the patch antenna is increased, the high inductance [8] from a long feeding probe makes it difficult to achieve impedance matching. To broaden the impedance bandwidth, notches and slots have been employed, such as the E-shaped [9], U-slot patches [10], and planar electric dipole with shorted patches [11], to compensate the large input inductance. Furthermore, other broadband feeding techniques such as the aperture coupled feed [12], L-probe feed [13], center slot feed [14], suspended probe feed [15], and folded feed [16] have been used. Although a broadband impedance matching is achieved, the radiation patterns of such patch antennas at the higher frequency are degraded by the occurrence of high order modes. Consequently, the co-polarization Manuscript received January 15, 2008; revised January 15, 2008. First pub- lished May 02, 2009; current version published July 09, 2009. The authors are with the RF & Optical Department, Institute for Infocomm Research, Singapore 138632, Singapore (e-mail: wktoh@ieee.org). Color versions of one or more of the figures in this communication are avail- able online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2009.2021968 radiation patterns in the E-plane are squinted, while the cross-polar- ization radiation levels in the H-plane are increased. Therefore, the boresight gain and half-power beamwidth are inconsistent across the broad operating bandwidth. This feature limits the applications of patch antennas in UWB systems requiring stable radiation patterns, such as the beacon or requestor for location systems. In this communication, a planar unidirectional UWB antenna with non-squinting radiation patterns and a relatively constant gain profile ( 1.5 dBi) is presented. Using a single patch radiator and a simple n-shape feeding structure, broadband impedance matching is achieved. II. A NTENNA STRUCTURE The schematic diagram of a single-element planar antenna is shown in Fig. 1. It consists of a radiator and a feeding structure. The radiator measures 22 30 mm, and is positioned at a height of . The n-shape microstrip feeding line is excited by a 50 SMA con- nector, while the other end is connected to the radiator through a ver- tical strip. Thewidth of the feeding line is 5 mm wide (89 ), suspended at a height of , using Styrofoam material with a relative permittivity of . The antenna is designed to operate in the lower UWB band of 3.1 to 4.8 GHz. At 3.0 GHz, the free space wavelength . The radiator structure measures , and the ground plane is . The copper plates are 0.5 mm thick. The 7.5 mm wide (70 ) feeding line is optimized to improve the impedance matching. A gradual 50-70-89 impedance transforma- tion provides a broadband impedance matching between the feeding line and the patch. III. R ESULTS All the simulations in this communication were conducted using a full-wave simulator IE3D. The antenna was subsequently pro- totyped for experimental verification. The reflection coefficient measurement was taken using a HP8510C vector network analyzer. A broad impedance bandwidth covering from 3.1–5.1 GHz (48%) for is shown in Fig. 2. The simulated resonances are in good agreement with those of the measurement. The measured radiation patterns at 3.0 , 4.0, and 5.0 GHz are shown in Figs. 3(a)–(c). The maximum H-plane cross-polarization levels grad- ually increase with increasing frequency, and peaked at . At 5.0 GHz, the increase in cross-polarization levels is due to the oc- currences of high order mode resonances. Nonetheless, the radiation patterns remain stable, and there is no squinting from 3.0–5.0 GHz. Fig. 4 shows the maximum co-polarizations gain profile, which is also directed at the boresight, and the maximum cross-polarization on the E- and H-planes. An average gain of 7.7 dBi is achieved with 1.5 dBi of gain fluctuation. As the H-plane cross-polarization levels increase, the co-polarization gain decreases. Fig. 5 shows the 3-dB beamwidth plot for both the E- and H-planes, varying from 40 to 65 and 70 to 90 respectively. Fig. 6 shows the simulated boresight gain and reflection coefficient characteristics for the antenna, when the ground plane is reduced from infinity to 40 40 mm. The measured gain profile on a 100 100 mm ground plane agrees well with that of the simulation, comparing Fig. 4 and Fig. 6. Both readings show a peak gain of 9.4 dBi at 3.75 GHz, and a decreasing gain of 6.5 dBi at 5.0 GHz, with less than 0.4 dBi of differences. The impedance matching remains unchanged, when the ground plane is varied from infinity 0018-926X/$25.00 © 2009 IEEE Authorized licensed use limited to: Phan Phuong. Downloaded on October 6, 2009 at 04:45 from IEEE Xplore. Restrictions apply. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 2173 Fig. 1. Schematic diagram antenna. to 60 60 mm ( at 3.0 GHz). The corresponding peak gain varies from 8.0 dBi for an infinitely large ground plane, to 9.5 dBi for a 100 100 mm ground plane, and 8.0 dBi for a60 60 mm ground plane. The 80 80 mm ground plane provides the most stable gain performance. IV. A NALYSIS The lower and higher resonant frequencies of this antenna are mainly determined by two components, (a) the length of the radiator , and (b) the height of the vertical strip. The resonant frequencies are estimated by (1) Fig. 2. Measured and simulated reflection coefficient . where is in GHz, and are in mm. Fig. 7(a) and (b) depict the instantaneous current distribution and vector plot of a cycle, at 3.4 and 4.6 GHz, respectively. The two current minima Authorized licensed use limited to: Phan Phuong. Downloaded on October 6, 2009 at 04:45 from IEEE Xplore. Restrictions apply. 2174 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 Fig. 3. Measured radiation patterns at (a) 3.0 GHz, (b) 4.0 GHz, and (c) 5.0 GHz. of the current distribution on the patch depict the half-wavelength resonant frequencies. At the lower resonance, the vertical strip is considered as part of the radiating element. The dash-lines depict the half-wavelength resonant. Therefore, the effective radiating length is 43 mm, and . The difference between the estimated and measured lower resonating frequencies is 2.5%. At the higher resonance, the radiation on the vertical strip is substantially weaker than that of at the lower resonance. Therefore, the effective radiating length is 30 mm, and . The difference between the estimated and measured higher resonating frequencies is 5.2%. It is validated that (1) could be used as design guidelines to estimate the resonating frequencies for the proposed antenna. These guidelines are not applicable when the height of the patch is raised from to , when the two resonances and are far apart from each other; where the coupling between the n-shaped microstrip line and patch is reduced. A parametric study of this antenna, by varying the length and height of the n-shaped feeding line, and height of the patch Fig. 4. Measured maximum gain on the E- and H-planes. Fig. 5. 3-dB beamwidth of the radiation patterns on the H- and E-planes. Fig. 6. Simulated reflection coefficient and boresight gain performances for various ground plane dimensions. were conducted. The results are shown in Figs. 8 and 9. When is in- creased from 13–20 mm (53.8%), the impedance bandwidth is reduced to 3.1–4.4 GHz (17.3%). The boresight gain is reduced, and begins to squint after 4.3 GHz. This is due to the reduced coupling between the n-shaped microstrip line and patch. When is extended from 26–30 mm (15.3%), there are no changes to the gain performance, no squinting, and the impedance matching remains at . Authorized licensed use limited to: Phan Phuong. Downloaded on October 6, 2009 at 04:45 from IEEE Xplore. Restrictions apply. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 2175 Fig. 7. Instantaneous current distribution and vector plot of a cycle. (a) 3.4 GHz; (b) 4.6 GHz. Fig. 8. Maximum gain, boresight gain, and reflection coefficient plots for con- figurations A (original), B (extended patch height), and C (extended n-shaped feed line), using simulator. Fig. 9. Maximum gain, boresight gain, and reflection coefficient plots for con- figurations D ,E , and F , using simulator with infinite ground plane. Fig. 9 shows the impedance bandwidth and gain profile when height ( 20%), using infinite ground plane. It can be seen that there are minimal changes to the impedance matching and gain profile. Therefore this design has a good mechanical tolerance due to the low Q value of the antenna. Furthermore, from Fig. 7(b), it is seen that at the higher resonance, the current at the top radiator does not have any cross-polarized component; hence beam squinting effect from higher order modes at the higher frequency has been reduced. V. C ONCLUSION A planar UWB antenna design using a broadband feeding technique has been presented and discussed. This simple feeding structure cou- pled with the radiator has suppressed high order mode resonances at higher frequencies. Therefore it achieved a non-squinting broadside ra- diation patterns across the broad operating bandwidth of 48%. The two resonances have been explained, and a simple formula has been pro- vided to estimate the resonating frequencies. This design has a high mechanical tolerance. REFERENCES [1] First Report and Order Federal Communications Commission (FCC), Feb. 2002. [2] P. J. Gibson, “The Vivaldi aerial,” in Eur. Microw. Conf., Oct. 1979, pp. 101–105. [3] N. P. Agrawall, G.Kumar, and K. P.Ray, “Wide-band planar monopole antennas,” IEEE Trans. Antennas Propag., vol. 46, no. 2, pp. 294–295, Feb. 1998. [4] Z. N. Chen, “Broadband rolled monopole,” IEEE Trans. Antennas Propag., vol. 51, no. 11, pp. 3175–3177, Nov. 2003. [5] Z. N. Chen, M. Y. Chia, and M. J. Ammann, “Optimization and comparison of broadband monopoles,” Proc. IEE Microw. Antennas Propag., vol. 150, no. 6, pp. 429–435, Dec. 2003. [6] X. Qing, Z. N. Chen, and M. Y. W. Chia, “UWB characteristics of disc cone antenna,” in Proc. Int. Workshop on Antenna Technol. IWAT, Mar. 2005, pp. 97–100. [7] Y. F. Liu, K. L. Lau, and C. H. Chan, “Experimental studies of printed wide-slot antenna for wide-band applications,” IEEE Antennas Wire- less Propag. 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Hoboken, NJ: Wiley, Feb. 2006. [15] N. Herscovici, “A wide-band single-layer patch antenna,” IEEE Trans. Antennas Propag., vol. 46, pp. 471–474, Apr. 1998. [16] Z. N. Chen and W. K. Toh, “Dual broadband folded antenna embedded into access-point decices,” presented at the EuCAP 2006, Nov. 2006. Authorized licensed use limited to: Phan Phuong. Downloaded on October 6, 2009 at 04:45 from IEEE Xplore. Restrictions apply. . man- ufacturability. Index Terms—Broadband antennas, microstrip patch antennas, sus- pended plate antennas, ultrawideband (UWB) antennas. I. INTRODUCTION A low profile and embeddable unidirectional antenna is required. ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 Communications A Planar UWB Antenna With a Broadband Feeding Structure Wee Kian Toh, Zhi Ning Chen, and Xianming Qing Abstract—A planar antenna. applications of patch antennas in UWB systems requiring stable radiation patterns, such as the beacon or requestor for location systems. In this communication, a planar unidirectional UWB antenna with non-squinting