A compact monopole-like slot antenna and its extended band-notched design are presented for ultrawideband (UWB) applications. The proposed antenna is etched on a printed circuit board and fed by a microstrip fork-shaped feeding stub. The characteristics of the proposed antenna are investigated numerically and validated experimentally. The prototype with an overall size of 25 mm × 25 mm × 0.8 mm achieves good impedance matching, constant gain, stable radiation patterns, and consistent group delay over an operating bandwidth of 3.0–12.9 GHz (124.5%). In addition, the proposed antenna features less ground plane dependence. By adding two grounded open-circuited stubs, the proposed antenna features band-notched characteristics at 5–6 GHz while it maintains the desirable performance over lower/upper UWB bands of 3.1–4.8 GHz/6.2–9.7 GHz.
 Since the Federal Communications Commission (FCC) released the bandwidth of 3.1–10.6 GHz [FCC, 2002], the feasible design and implementation of ultrawideband (UWB) systems have become a highly competitive topic in both academic and industrial communities of telecommunications, where the antenna of ultrawide bandwidth is the key component. In particular, the antenna design for UWB applications is facing many challenges [Chen et al., 2004]. In contrast to conventional three-dimensional UWB antennas such as disc-cone antenna [Qing et al., 2005], biconical antennas, and self-complementary spiral antennas [Kraus, 1988], planar UWB antennas are more preferable for UWB system configuration and implementation especially for portable UWB devices [Chen et al., 2006].
 Among the planar UWB antenna designs, the printed slot antenna has drawn more and more attention due to their attractive merits such as wide frequency bandwidth, low profile, lightweight, ease of fabrication and integration with radio frequency (RF) circuits. A conventional narrow slot antenna shows the drawback of inherent narrow bandwidth while wide-slot (aperture) antennas show much enhanced bandwidth [Kahrizi et al., 1993; Shum et al., 1995]. This type of wide-slot antenna has been realized by etching a wide-slot on the ground plane of a printed circuit board (PCB), and a mircostrip line or coplanar waveguide (CPW) feeding structure on the same substrate. A variety of wide-slot antenna designs have been reported [Jang, 2000; Sze and Wong, 2001; Chen, 2003; Jan and Su, 2005; Qu et al., 2006; Li et al., 2006; Lin and Huang, 2006; Chen et al., 2007; Chen and Wang, 2008].
 Apart from the wide-slot (aperture) antennas mentioned above, another category of printed slot antenna, namely monopole-like slot antenna, has been reported to have wideband characteristics [Sharma et al., 2004; Latif et al., 2005; Liu et al., 2007]. Different from the wide-slot antennas, the monopole-like slot antennas do not require closed ground plane, which offers more freedom in such antenna design and the possibility to make the antenna smaller.
 In this paper, a compact monopole-like slot antenna is presented for UWB applications. The characteristics of the proposed antenna are investigated numerically and verified experimentally. After that, a band-notched design is presented by adding two grounded open-circuited stubs to the original antenna design. The location and bandwidth of the notched frequency band can be controlled by adjusting the length, width and position of the open-circuited stubs. The paper is structured as follows. Section 2 demonstrates the geometry of the proposed monopole-like slot antenna, and presents the results of the optimized antenna prototype. The parametric study of the antenna is carried out in section 3. Section 4 demonstrates the band-notched antenna design and results. Finally the conclusion is given in section 5.
2. Antenna Configuration and Results
2.1. Antenna Geometry
Figure 1a shows the geometry of the proposed antenna. The antenna is composed of a monopole-like slot and a microstrip fork-shaped feeding structure. The slot is printed on the top side of a piece of FR4 PCB with a thickness of 0.8 mm, a relative dielectric constant of ɛr = 4.4, and a loss tangent of tanδ = 0.02. A Cartesian coordinate system is oriented such that the bottom surface of the PCB in Figure 1a lies in the x-y plane. In contrast to a conventional wide-slot antenna, the monopole-like slot is not enclosed by the ground plane. The slot is surrounded by narrow ground strips, which makes the antenna compact. Two tapers are added at the corners of the upper folded ground strips to bevel the slot for enhancing impedance matching, especially at lower frequencies. The antenna is fed by a 50-Ω microstrip line with a fork-shaped feeding structure which is printed on the bottom side of the PCB and positioned symmetrically with respect to the centerline (y axis) of the slot. The fork-shaped feeding structure comprises one horizontal and two vertical stubs, which realizes more effective coupling with the slot for achieving significant bandwidth enhancement [Akhavan and Mirshekar-Suahkal, 1994; Sze and Wong, 2001]. The specified impedance matching and radiation characteristics can be achieved by selecting the dimensions of the slot, the ground plane, and the feeding stubs properly.
 The simulation and optimization of the antenna were carried out with the aid of using IE3D [Zeland Software, Inc., 2007] which is based on the Method of Moments (MOM). The optimized antenna design was prototyped using a low cost FR4 PCB (ɛr = 4.4, tanδ = 0.02) with a total size of 25 mm × 25 mm × 0.8 mm. The prototype shown in Figure 1b is with the geometric parameters of Ws = 23 mm, Ls = 16 mm, L1 = 17 mm, L2 = 10.5 mm, L3 = 6 mm, Wg = 1 mm, Lg = 8 mm, Lf1 = 7.2 mm, Lf2 = 11 mm, Wf1 = 1.5 mm, Wf2 = 0.8 mm, g = 0.8 mm, Wf = 1.5 mm, W = 25 mm, and L = 25 mm. The measurement was carried out using an HP8510 vector network analyzer and an Orbit MiDAS far-field antenna measurement system in a full anechoic chamber.
2.2. Gain and Impedance Matching
Figure 2 shows the comparison of the simulated and measured copolar peak gains and return losses of the antenna prototype. The simulated and measured results are with good agreement. The bandwidth defined by −10 dB return loss covers the frequency range of 3.0–12.9 GHz or 124.5%. Multiple nulls are observed in return loss response, which indicates that the proposed antenna operates at several dominant resonances which are generated by the folded ground strips, the feeding stubs, and the coupling between them. Flat gain response has been observed over the frequency range, which is desirable for UWB applications.
Figure 3 illustrates the measured return loss of the antenna with different bottom ground length (Lg). It is validated that the size of the bottom ground plane hardly affects the lower edge frequency of the operating bandwidth. Thus, the antenna features less ground plane dependence. This feature makes the proposed monopole-like slot antenna design flexible and conducive to practical applications where the antennas are required to be integrated into other circuits or devices.
 The evidence of the characteristic of less ground plane dependence can be found in the simulated electric current distribution of the antenna as shown in Figure 4, where the current distribution at four resonant frequencies is illustrated. It is observed that the currents are concentrated more on the upper folded ground strips, the microstrip line, and the fork-shaped feeding stubs, while the currents on the bottom ground plane are very weak. In addition, the currents on the folded ground strips are much stronger than those on the fork-shaped feeding stubs at the lower frequency (3.16 GHz) and provide the longest current path for the antenna. This presumes that the radiation at the lower operating frequencies is mainly from the folded ground strips. As a result, the lower edge frequency of the operating bandwidth of the antenna is determined by slot related parameters. At higher frequencies such as 12 GHz, the currents on the feeding stubs are much stronger and thus the radiation mainly attributes to the fork-feeding stubs. It implies that the performance of the antenna at higher frequencies is mainly controlled by the feeding stubs, and thus is more sensitive to the dimensions of the feeding stubs.
 It is observed that both the x and y components of the currents exist. Note that the x component of the currents on the upper portion of the bottom ground plane and the horizontal feeding stub are of same amplitude and out of phase with respect to the centerline (y axis). They cancel out each other in the far field zone and thus do not contribute to the copolar radiation. The radiation of the proposed monopole-like slot antenna is similar to a monopole array with two or four monopole elements oriented in the y axis, and thus the E plane lies in the y–z plane and the H plane in the x–z plane.
2.3. Radiation Patterns
 As shown in Figure 5a, good omnidirectioanl patterns in the H plane are obtained at lower frequencies (3 and 6 GHz). The patterns trend to be bidirectional at higher frequencies. At 12 GHz, a “figure eight” pattern is observed because the spacing of the fork-shaped feeding stubs is comparable to the wavelength. The radiation in the x direction is degraded due to the array factor while the radiation in the z direction is still kept maximal. The currents along the x direction on the upper portion of the bottom ground plane and the horizontal feeding stub do not contribute to the copolar radiation in the H plane but cause high cross-polarization level. The cross-polar patterns can be modeled as the radiation of four x-polarized monopoles which are of out of phase, leading to the nulls in the x axis (due to the element factor) and the z axis (due to the array factor) with the maximum in between.
 As illustrated in Figure 5b, the E plane patterns exhibit two nulls in the y direction, which is similar to that of conventional wide slot antennas or printed monopole antennas. It is also observed that all radiation patterns are relatively constant in the whole operation frequency range. The cross-polarization level is relatively high at lower frequencies, which is mainly caused by the asymmetrical current distribution on the vertical folded ground strips and the feeding stubs with respect to the x axis.
2.4. Antenna System Transfer Function and Group Delay
 In UWB communications applications, the system transfer function of a transmit-receive UWB antenna link is of interest [Chen et al., 2004; Qing et al., 2006]. The antenna system transfer function with a flat magnitude and a linear phase response (or stable group delay) is desired. The measured transfer function of a proposed antenna pair which comprises two identical proposed antenna prototypes is shown in Figure 6. The antennas are positioned face to face with a separation of 0.8 m. The flat magnitude response of around −45 dB and a constant group delay of about 18 ns at the boresight are observed over the entire UWB band of 3.1−10.6 GHz.
3. Parametric Study
 The parametric study by simulation is carried out to provide readers with more design information. After many simulations, it is found that the most sensitive parameters are L1, L2, L3, Ws, Lf1, and Lf2, whereas the remaining parameters do not show significant effect. To well understand the influence of these parameters on the impedance bandwidth, only one parameter will be varied at a time while others are kept unchanged unless especially indicated. The parameters of the optimized prototype (Ws = 23 mm, Ls = 16 mm, L1 = 17 mm, L2 = 10.5 mm, L3 = 6 mm, Wg = 1 mm, Lg = 8 mm, Lf1 = 7.2 mm, Lf2 = 11 mm, Wf1 = 1.5 mm, Wf2 = 0.8 mm, g = 0.8 mm, Wf = 1.5 mm, W = 25 mm, and L = 25 mm) are used to be the reference for parametric study. IE3D was employed to perform all the simulations.
3.1. Ground Plane Related Parametric Study
 The relevant parameters include L1, L2, L3, Ws, and Lg. Figure 7a shows the effect of varying the length of the upper folded ground strip, L1, on the bandwidth of the impedance matching of the antenna. When increasing L1 from 15 mm to 19 mm, the first resonant frequency of the antenna shifts down from 3.48 GHz to 2.99 GHz. Besides the 4th resonance, the frequencies of the other two resonances are shifted down as well. The return losses of the antenna against the width of the slot, Ws, are exhibited in Figure 7b. The lower edge of the operating frequency band is shifted down when the width of the slot tends to be larger. Increasing Ws from 23 mm to 29 mm, the first resonant frequency shifts down from 3.18 GHz to 2.80 GHz. Figure 7c illustrates the effect of the length of the horizontal folded ground strip section, L2, on the impedance matching. Similarly, a significant effect has been observed at lower band while the higher band has almost been unaffected. The frequency of the first resonance of the antenna shifts from 3.0 GHz to 4.1 GHz when L2 varies from 10.5 mm to 2.5 mm. Increasing L2 shifts down the lower edge frequency of the bandwidth so that the size of the antenna can be reduced. However, a larger L2 degrades the impedance matching at the lower band. The impedance matching at the lower band can be improved by beveling the monopole–like slot, which is realized by adding tapers at the corners of the upper folded ground strips. From Figure 7d, it is found that larger taper shows better performance while the lowest resonant frequency is slightly shifted upward as the longest current path is slightly shortened.
3.2. Fork-Shaped Feeding Structure Related Parametric Study
 The geometrical parameters of the feeding structure include the length and width of the vertical stubs (Lf1, Wf1), the length and width of the horizontal stub (Lf2, Wf2), as well as the gap (g). The effect of varying the vertical stub length Lf1 on the impedance matching characteristic is shown in Figure 8a. It can be seen that significant effect occurs at higher frequencies while the lower frequencies are unaffected. When Lf1 decreases from 8.2 mm to 6.2 mm, upper edge frequency fh is shifted from 12.10 GHz to 13.74 GHz. Figure 8b illustrates the effect of the length of the horizontal stub, Lf2, on the impedance matching. Similarly, the length of the horizontal stub shows a significant effect at higher frequencies. The width of the feeding stubs (Wf1, Wf2) and the gap (g) show a slight effect on the lower/upper edge of the operating frequency band. For brevity, the results are not shown here.
 In summary, the lower edge frequency of the operating bandwidth of the proposed monopole-like slot antenna is mainly controlled by the slot related parameters including L1, L2, Ws, while the upper edge frequency of the operating bandwidth is determined by the dimensions of the fork-shaped feeding stubs (Lf1, Lf2). Other parameters such as L3, Wg, Wf1, Wf2, and g have shown slight effect on the lower/upper edges of the operating bandwidth while can be used to optimize the antenna performance over the operating band.
4. Band-Notched Design and Results
 The band-notch characteristic of a UWB antenna will benefit a UWB system for restraining possible interferences with the IEEE802.11a and HIPERLAN/2 WLAN systems operating in the 5–6 GHz band [Cho et al., 2006; Qiu et al., 2006; Chung et al., 2007; Bahadori and Rahmat-Samii, 2007]. In this paper, a band-notched design is presented to demonstrate the superior features of the proposed monopole-like slot antenna. The band-notched characteristic is realized by adding two open-circuited stubs symmetrically on the upper portion of the bottom ground with respect to the y axis.
Figure 9a illustrates the configuration of the band-notched antenna design. The dimensions and location of the open-circuited stubs are indicated as Lbn, Wbn and d. Besides them, all other dimensions of the antenna are kept unchanged with respect to the original prototype, and no retuning is required. The implemented band-notched antenna prototype is shown in Figure 9b, where Lbn = 9.5 mm, Wbn = 0.3 mm, and d = 1.0 mm.
Figure 10 shows the measured and calculated return losses of the band-notched antenna prototype. The antenna exhibits the band-notched performance in the frequency range of 4.8–6.2 GHz, while the impedance matching degrades moderately with a maximum return loss of −7.5 dB in the frequency band of 2.7–4.8 GHz.
 The measured E and H plane radiation patterns of the band-notched antenna prototype are shown in Figure 11, the notched characteristic is obvious. The boresight gain at 5.5 GHz is about 12 dB lower than that at 7 GHz.
Figure 12 exhibits the magnitude and group delay of the antenna system transfer function of a band-notched antenna pair which is composed of two identical band-notched antenna prototypes and positioned face to face with a separation of 0.8 m. It can be seen that the band-notched antenna pair features flat magnitude of around −48 dB over the lower UWB band of 3.1–4.8 GHz, and −44 dB over the higher UWB band of 6.2–9.8 GHz; the variation is about ±2 dB except the notched 5 GHz band where a drop of about 24 dB is obtained at 5.5 GHz. The group delay is also stable at both lower and higher UWB bands with the value of around 18 ns except the large variation over the notched band of 4.8–6.2 GHz.
 The simulated electric current distribution of the band-notched antenna at 5.5 GHz is shown in Figure 13a. For comparison, the current distribution of the original antenna design is illustrated in Figure 13b as well. It is observed that the currents on the open-circuited stubs are oppositely directed to those on the folded ground strips and the fork-shaped feeding stubs. As a result, the radiation from the open-circuited stubs and folded ground strips/fork-shaped feeding stubs is canceling out each other in far field, and thus features band-notched characteristic.
Figure 14 shows the effect of the geometrical parameters of the open-circuited stubs on the band-notched characteristics. Figure 14a illustrates the effect of the length of the open-circuited stubs (Lbn) on the impedance response. It is obvious that the center frequency of the notched band is much dependent on the Lbn; shorter stubs shift up the notched band. The effect of the locations of the open-circuited stubs is shown in Figure 14b. It is found the closer the open-circuited stubs are located to the fork-shaped feeding stubs; the wider notched bandwidth is obtained while the impedance matching degrades in operating bands. Figure 14c exhibits the effect of the width of the open-circuited stubs on the impedance matching. The width of the open-circuited stubs affects both the notched frequency and the bandwidth. Narrower stubs are preferable for achieving better impedance matching in operating bands.
 In short, the required band-notched performance can be achieved by optimizing the dimensions and location of the open-circuited stubs, whereas no retuning for the original antenna design is required.
 In this paper, a compact monopole-like slot antenna and its extended band-notched design have been proposed for promising UWB applications. The characteristics of the proposed antennas have been investigated numerically and examined experimentally. The proposed monopole-like slot antenna with an overall size of 25 mm × 25 mm × 0.8 mm has achieved the features of ultrawideband impedance matching, constant gain, stable radiation patterns, and consistent group delay over the operating frequency band of 3.0–12.9 GHz. More importantly, the study has shown that the monopole-like slot antenna features the characteristic of less dependence on ground plane size, which is conducive to applications in mobile devices. Furthermore, an extended band-notched design has been proposed which has shown desirable performance over both low/high UWB bands and notched band at 5-6 GHz by adding two grounded open circuited stubs. The band-notched characteristics can be controlled by adjusting the length, width and position of the grounded open-circuited stubs, whereas no retuning of the original design is required. The parametric study has addressed the most sensitive geometric parameters of the proposed antenna, which provides useful design information for such type of antenna design and optimization.