In recent applications of telecommunications and remote sensing at microwave frequencies, the exploitation of UWB antennas is steadily growing.1 UWB radio can use the frequency band from 3.1 to 10.6 GHz. UWB systems also need to be compatible with other already existing standards.2


To allow such a wider bandwidth in UWB systems, the Federal Communications Commission (FCC) imposes severe power restrictions. By doing so, UWB devices can make use of an extremely wide frequency band while not emitting enough energy to interfere with narrower band devices nearby, such as Wireless LAN and Hyper LAN devices at approximately 5 GHz. There are also some unlicensed bands in Japan (4.900 to 5.091 GHz) and the United States (5.150 to 5.825 GHz).3

A similar need also arises from multi-band communication systems. A multi-band system deals with a wide frequency band and needs to filter out non-service bands to avoid interferences. Recently, in order to meet these requirements, several UWB antennas with band rejection characteristics have been proposed.4–6

The band reject operation is achieved when the length of the embedded slot is approximately one-half wavelength at the desired rejection frequency. In this case, destructive interference for the excited surface currents in the antenna will occur, which causes the antenna to be non-responsive at that frequency.7 However, as mentioned in the literature,8,9 the notch of those antennas can exhibit only a single narrow frequency notch band and cannot provide a satisfactory rejection bandwidth and skirt characteristics for most of the systems requiring the good band rejection response.

In order to solve this problem, by embedding two horizontal resonant slots and adjusting the mutual coupling values between them, the band rejection characteristics of a wideband patch antenna has been improved. This technology allows a wideband antenna to be a multi-band antenna, and many systems to be compactly designed without additional components such as notch filters.

Wideband Antenna Design

The structure of the proposed wideband bow-tie monopole antenna is shown in Figure 1. It is composed of a bow-tie main patch fed electromagnetically and a parasitic semicircular ground patch. Typically, a bow-tie antenna is fed at its center, the same as for a dipole antenna.10

The bow-tie patch needs to be fed by direct or proximity coupling. A proximity coupling from an embedded feed line within the substrate gives better bandwidth than a direct coupling.11 The antenna, with a width W = 22 mm and height H = 22 mm, is constructed on a substrate with a thickness of 0.64 mm, a dielectric constant of 6.15 and a loss tangent of 0.0028. It is fabricated and mounted on a rectangular finite (80 x 80 mm) ground plane. A 50 Ω SMA connector, centrally mounted from the back of the ground plane, is used to excite the antenna.

The other antenna dimensions used in this study include: BL = 4 mm, BH = 13 mm, WM = 3.4 mm, WF = 0.8 mm, HM = 6.5 mm, R = 15.0 mm and g = 4.5 mm. The proposed wide bandwidth is achieved by inserting a semicircular shaped parasitic ground patch between the bow-tie patch and the ground plane. The width W of the bow-tie patch is one-half wavelength at the lower resonance frequency, and the widths WM and WF of the feed line are important parameters for impedance matching.

Band Rejection Antenna Design

The aforementioned band reject operation is analyzed as a transmission line when the length of the embedded slot in the radiator is one-half wavelength at the desired rejection frequency.5 These slots can be modeled as a one-half wavelength short stub. In the proposed antenna, the physical length of the embedded slots varies with their position. In addition, the VSWR at the rejection frequency is increased by locating the slot at the edge of the bow-tie patch near the parasitic ground patch, because there is greater surface current distribution near the edge.

The improved band rejection characteristics are achieved by controlling the mutual coupling between the slots with the same notch frequency. Figure 2 shows the geometry of the proposed band rejection antenna with two horizontal resonant slots. Here, the width of a slot is 0.3 mm. As shown in Figure 3, these slots will be used as notch resonators for band rejection, filter-like characteristics. The conceptual equivalent circuit model for a band rejection antenna has two one-half wavelength notch resonators, coupling and antenna resistance (Ra). EM simulation results were obtained using HFSS™, a full-wave simulator.

Considerable enhancement of the coupling between slot resonators and improvement of the magnitude difference of two notch peaks are observed when the slots are closely placed to each other, as shown in Figure 4. The band rejection characteristic is improved by inserting additional small slots in the center of the band-notch slots. The return losses for various heights of inserted slots, when gs is 0.4 mm, are shown in Figure 5.

The coupling decreases as the size of the inserted slot becomes larger. The rejected center frequency barely moves when the gap between resonators is varied, but its movement depends on the height of the inserted slot. Although the rejection characteristics show good performance without the inserted slots, gs = 0.4 mm was chosen in order to apply two factors for controlling the mutual coupling at the antenna.

As shown in Figure 6, the rejection frequency of the antenna can be controlled by the resonance frequencies of the two slots and the rejection characteristic such as bandwidth and flatness is improved by two factors (gs, HC). The design parameters of the antennas are shown in Table 1.

The proposed antenna, with the dimensions of type-B, was fabricated and tested. The measured return losses with and without the notch slots are shown in Figure 7. The measured results were obtained using an Anritsu 37397C vector network analyzer.

The rejection band of 4.96 to 5.51 GHz is created by inserting the two notch slots, without any degradation of the required performance in the normal operating frequency band of 2.70 to 6.90 GHz for a return loss below –10 dB (VSWR < 2.0). The average gain of the proposed antennas, measured in the x-y plane, is shown in Figure 8 for a frequency range of 2.7 to 6.9 GHz.

The gain of the antenna with two slots has two rejection peaks and is reduced at the broad rejection bandwidth, compared to the antenna using one slot. It is clear that the proposed antenna provides the improved band rejection characteristics with the broad rejection bandwidth and sharp skirt characteristic, compared to the antenna with one notch slot.

Figures 9 and 10 show the radiation patterns at 2.70, 5.10 and 6.90 GHz. The frequencies 2.70 and 6.90 GHz are within the radiation band, while 5.10 GHz is in the rejection band. The radiation patterns of the operating frequency band are nearly the same as those of the reference antenna, which is the same antenna except without a slot. On the other hand, within the rejection band the antenna radiation gain is greatly reduced.

Conclusion

A method to improve the band rejection characteristics of a wideband patch antenna by embedding two horizontal resonant slots and adjusting their mutual coupling has been presented. To show the method, a wideband bow-tie shaped antenna with improved band rejection characteristics has been proposed and implemented. A broad rejection band, two-pole response and improved notch skirt are obtained. The proposed technique could be useful to improve and/or control the band rejection characteristics of wideband antennas for many applications including UWB and multi-band systems.

Acknowledgment

The authors would like to thank Amotech Co. Ltd. and the Kangwon Institute of Telecommunications and Information (KITI) for measurement and other support.

References

1. W.S. Chen and M.K. Hsu, “The Design of a Finite Ground Plane Cross Semi-elliptic Monopole Antenna for UWB Applications,” Microwave Journal, Vol. 49, No. 5, May 2006, pp. 192–204.

2. G.R. Aiello and G.D. Rogerson, “Ultra-wideband Wireless System,” IEEE Microwave Magazine, Vol. 4, No. 2, June 2003, pp. 36–47.

3. I.J. Yoon, H. Kim, H.K. Yoon, Y.J. Yoon and J.H. Kim, “Ultra-wideband Tapered Slot Antenna with Band Cut-off Characteristic,” Electronics Letters, Vol. 41, No. 11, May 2005, pp. 629–630.

4. T. Dissanayake and K.P. Esselle, “Design of Slot Loaded Band-notched UWB Antennas,” 2005 IEEE Antennas and Propagation Society International Symposium Digest, Vol. 1B, pp. 545–548.

5. W.Y. Choi, J.H. Jung, K.H. Chung and H.H. Choi, “Compact Wideband Printed Monopole Antenna with Frequency Band-stop Characteristic,” 2005 IEEE Antennas and Propagation Society International Symposium Digest, Vol. 3A, pp. 606–609.

6. Y.W. Rho, K.H. Kim and J.H. Choi, “Design of a Microstrip-fed Ultra-wideband Monopole Antenna Having Band Rejection Characteristic,” 2005 IEEE Antennas and Propagation Society International Symposium Digest, Vol. 2B, pp. 556–559.

7. W.S. Lee, D.Z. Kim, K.J. Kim and J.W. Yu, “Wideband Planar Monopole Antennas with Dual Band-notched Characteristics,” IEEE Transactions on Microwave Theory and Techniques, Vol. 54, No. 6, June 2006, pp. 2800–2806.

8. S.D. Moon, K. Choi and H.Y. Hwang, “A Wideband Bow-tie Monopole Antenna with Improved Band Rejection Characteristic,” 2006 KEES Korean Conference, Vol. 16, No. 1, pp. 203–207.

9. S.D. Moon, K. Choi and H.Y. Hwang, “A Wideband Bow-tie Monopole Antenna with Improved Band Rejection Characteristic,” The Journal of the Korea Electromagnetic Engineering Society, Vol. 17, No. 12, December 2006, pp. 1199–1205.

10. F. Soldovieri, A. Brancaccio and G. Leone, “Characterization of Ultra-wideband Bow-tie Antennas for Ground Penetration Radar Systems,” Microwave Journal, Vol. 49, No. 8, August 2006, pp. 186–194.

11. L. Le Coq, S. Von der Mark, M. Drissi and J. Citerne, “Printed Bow-tie Antenna Fed by Electromagnetic Coupling,” 1999 IEEE Antennas and Propagation Society International Symposium Digest, Vol. 4, pp. 2710–2713.



Soo Deok Moon received his BS and MS degrees in electrical and electronic engineering from Kangwon National University in 2005 and 2007, respectively. His research interests include microwave antennas and RF systems.







Hee Yong Hwang received his BS degrees in biology and electronic engineering from Seoul National University in 1988 and 1992, respectively, and his MS and PhD degrees in electronic engineering from Sogang University in 1995 and 1999, respectively. From March 2000 to February 2001, he was with the department of electronic engineering, Sogang University, as a research professor. From March 2001 to April 2002, he was with the department of ECE, University of Maryland, College Park, MD, as a Visiting Researcher. Since March 2003, he has been with the department of electrical and electronics engineering at Kangwon National University, Chuncheon, Korea, where he is an associate professor. His research interests include microwave passive and active components and RF systems.