Band-notched inverted-cone monopole antenna for compact UWB systems R. Gayathri, T.U. Jisney, D.D. Krishna, M. Gopikrishna and C.K. Aanandan A microstrip-fed planar ultra-wideband monopole antenna with band- notch characteristics is proposed. The antenna size is compact (18  30 mm) and operates over an extremely wide band of 3–16 GHz. The proposed design consists of an inverted cone as the radiating patch and a tapered ground plane. Wideband matching is obtained by properly shaping the ground. A pair of symmetrically placed quarter wave slot resonators is embedded in the ground plane for rejecting the 5– 6 GHz WLAN band. Results indicate a stable and omnidirec- tional radiation pattern with an average gain of 3 dBi and a sharp reduction in gain at notched frequencies. Moreover, measured group delay and transmission characteristics show excellent transient response. Introduction: There has been considerable interest in ultra-wideband (UWB) antennas ever since the FCC allocation of the 3.1– 10.6 GHz fre- quency band for commercial applications [1]. Emerging technologies demand planar, compact and highly efﬁcient radiators. Broadband planar monopole antennas have been popular owing to their attractive merits, such as ultra-wide frequency band, good radiation properties, a simple structure, ease of fabrication and good pulse handling capabili- ties. The extremely wide impedance band is ensured by the exponential or stepped impedance transformer arrangement between the ground and the patch [2–4]. The planar inverted cone monopole antenna shows excellent wideband characteristics since it belongs to the biconical family of antennas. However, they are large in size [5, 6] and fail to retain its wide band matching when the size is reduced for catering to UWB applications such as USB dongles, which are typically designed on PCBs with a width less than 20 mm. In this Letter a very compact microstrip-fed planar inverted cone antenna (18  30 mm), which oper- ates over an extremely wide band of 3.0–16 GHz, is proposed. It is the introduction of a U-shaped section engraved in the ground plane of the proposed antenna which improves its impedance bandwidth. For UWB systems to coexist with standards like IEEE 802.a and HIPERLAN/2, a band rejection is necessary in UWB RF front ends in the 5.15 – 5.825 GHz sub-band. Antenna topologies have been widely reported where half wavelength slot resonators are embedded in the radiating patch to introduce the desired band notch. For compact antennas, accommodating a half wavelength slot resonator in the radiating patch would not only be a challenge due to the space con- straints imposed, but it also leads to perturbation in the radiation and impedance characteristics of the antenna in the rest of the band. Instead, in the proposed design, the band-notched action is obtained using a pair of symmetrically placed quarter wavelength slot resonators etched in the ground plane. Antenna design: The topology of the proposed monopole antenna is illustrated in Fig. 1. The antenna is printed on FR4 substrate (1 r ¼ 4.4, thickness h ¼ 1.6 mm) and fed by a 50 V microstrip line. The height of the inverted cone radiating element (a p þ b p ) is a quarter wave- length at the lower edge of the operating band ( f l ) given by (1). The 5.2/ 5.8 GHz WLAN frequencies are notched by means of the symmetrically placed slot resonators in the ground plane. The quarter wavelength slot resonators are open at one end and their length (l s ) is deduced as in (2). However, a sharper notched band is observed as the slots are located closer to the microstrip feed-monopole transition. It is the destructive interference of the surface currents that causes the antenna to be non- radiative at the notched frequency, f notch : a p þ b p ¼ 75 f l ﬃﬃﬃﬃﬃﬃﬃ 1 eff p ð1Þ l s ¼ 75 f notch ﬃﬃﬃﬃﬃﬃﬃ 1 eff p ð2Þ where f l and f notch are in GHz and 1 eff is the effective permittivity of the substrate deﬁned as: 1 eff ¼ ð1 r þ 1Þ 2 ð3Þ The width of the antenna is given as: W ¼ 2a p ð4Þ The tapering of the ground edges and dimensions of the U-shaped curve are optimised for UWB operation using Ansoft HFSS simulation soft- ware. In order to ensure the usefulness of the proposed UWB antenna, its time domain responses must be measured. It indicates the quantity of pulse distortion and far-ﬁeld phase linearity. In a UWB system, the phase of the radiated ﬁeld should vary linearly with frequency which means that a stable group delay response is desired. front view L d ground plane side view y x a l s b u a u l g t s d s b g b p a p W Fig. 1 Geometry of proposed antenna Results and discussions: The prototype of the proposed antenna, with width W ¼ 18 mm, was fabricated and measured using R&S ZVB20 VNA. The simulated and measured VSWRs of this antenna with and without the slot resonators are plotted in Fig. 2 and show good agree- ment. For the antenna without the slot resonators, the VSWR character- istics reveal UWB behaviour with a 2:1 VSWR bandwidth from 3 to 16 GHz. When the quarter wave slot resonators are introduced, the VSWR is high (’7) at 5.5 GHz. The value of VSWR is above 4 in the 5–6 GHz band and is only slightly affected in the passband except at frequencies above 14 GHz. 10 8 6 4 2 4 6 8 10 12 14 16 frequency, GHz VSWR simulated (without notch) simulated (with notch) measured (without notch) measured (with notch) Fig. 2 VSWR of proposed antenna b p ¼ 6.4 mm, a p ¼ 9 mm, a ¼ 86 0 , d ¼ 1.7 mm, b u ¼ 2.6 mm, a u ¼ 3 mm, b g ¼ 8.2 mm, l g ¼ 5.1 mm, l s ¼ 8.3 mm, t s ¼ 0.1 mm, d s ¼ 0.2 mm, L ¼ 30 mm, W ¼ 18 mm The gain of the antenna is measured with reference to a standard wideband horn antenna and presented in Fig. 3. An average gain of 2.5 dBi is noted throughout the operating band except at the notched fre- quency. The HFSS simulations have shown that the proposed antenna design on low loss substrates has a radiation efﬁciency of more than 90% in the pass band. The group delay of S 21 is measured between two identical antennas in the face-to-face and side-by-side orientations, with a distance of 30 cm between them, and is given in Fig. 3. The results indicate that the variation in group delay is within 0.4 ns across the whole UWB except at the notched band. This proves that the proposed antenna has linear phase and hence will introduce minimum distortion in the time domain response. The measured radiation patterns of the UWB antenna at 3.3, 5.5, 7 and 9.5 GHz are plotted in Fig. 4 and are compared with the simulated patterns. It can be observed that the antenna pattern is omnidirectional in the x-z plane. ELECTRONICS LETTERS 25th September 2008 Vol. 44 No. 20