Monopole Crescent Elliptical Antenna with Band-Notched Characteristics for UWB Applications*
In 2002, the Federal Communications Commission(FCC) allowed ultra wideband (UWB) communicationsfor short-range peer-to-peer high speed communication.The spectrum from 3.1 to 10.6 GHz has been allocated for unlicensed UWB measurement and communication applications with equivalent isotropically radiated power less than ?41.3 dBm/MHz. Antenna
designs for UWB systems are very demanding. Due to the inherently ultra-wide operating bandwidth from 3.1to 10.6 GHz, circuit designs for UWB radio systems are much more challenging than for conventional narrowband
systems. The systems must produce broad operating bandwidths for impedance matching, highgain transmissions in the desired direction, stable
transmission patterns and gains, consistent group delays, high transmission efficiency, and low profiles. Various studies have been devoted to evaluating theperformance of UWB antennas[2-6]. Planar, monopole, and dipole antennas have been proposed for UWB applications[5-11]. Although some new antennas[8,9] have been shown to provide very low voltage standing wave ratio (VSWR) over extremely wide frequency ranges, they will likely interfere with
existing systems. Thus, UWB antennas with bandnotched characteristics have been developed to reduce the interference. However, none of the studies of band-notched antennas[9,12-14] show how to control the notched band with a simple band-notched structure.
1 Antenna Configuration
The notched band antenna consists of a crescentshaped elliptical monopole radiator and a reflecting ground plane. A T-shaped stub is added to provide the
broadband radiation pattern and the band-notched characteristic.
The monopole may be a circular, elliptical, square, rectangular, or hexagonal planar antenna with a hole. The characteristics of monopoles with various circular holes were experimentally examined by Qiu et al.
Their results demonstrated that the modified antennas still offer a broad impedance bandwidth and acceptableradiation patterns. Their conclusions are based on the principle that the current is concentrated on the outer
edge of the planar monopole. However, the various outside shapes of the antenna affect the impedance bandwidth, while the inside resonator shape influences
the band-notched characteristics. Therefore, the hole shape should either be similar to the original antenna shape or be carefully designed so as not to influence the original input impedance characteristics as shown in Fig. 1. The effects of the hole radius in Fig. 1b on the return loss, S11, expressed as the antenna impedance calculated by using computer simulationtechnology (CST), are shown in Fig. 2. The results show that the impedance bandwidth is only slightly influenced by the hole size. An analysis of the band-notched structure leads to the T-shaped stub design which, with the crescent shape, has two advantages compared to other stubs[13-15] as follows:
(1) The antenna can be easily extended to other
band-notched designs without changing the dimensions of the original shape.
(2) The T-shaped stub has a simple geometry with
fewer parameters, which reduces the computational effort for optimization.
The structure of the band-notched antenna with the T-shaped stub is shown in Fig. 3 with a crescentshaped
elliptical monopole radiator and a reflecting ground plane.
2 Parametric Study and
The antenna geometry, especially the size of the T-shaped stub, affects the bandwidth W and the central frequency f of the notched band. The design of
the T-shaped stub has two parameters, the top length and the height of the T-shaped stub. These two parameters greatly affect the bandwidth and central frequency. The other antenna design parameters will just improve the band-notched characteristics. The antenna in Fig. 3 has a wide bandwidth, while the T-shaped stub provides the band-notched characteristics. As shown in Figs. 4 and 5, the band-notched central frequency is dependent on t and h1 with larger heights of the T-shaped stub reducing the central frequency.Longer lengths of the T-shaped stub also reduce the central frequency. The band-notched bandwidth of the antenna is also greatly influenced by the length of the T-shaped stub with longer lengths reducing the band-notched bandwidth. Thus, the central frequency
is mainly dependent on the height and the length of the T-shaped stub, while only the length t strongly affects the bandwidth. Table 1 lists simulationresults for W and f for various h1. Table 2 lists the simulation results for W and f for various t.
Thus, a rectangular ground plane was chosen withdimensions of 75 mm×75 mm×1 mm. The radiating element was a 0.5-mm thick copper sheet placed vertically
above a finite-sized ground plane with a Sub-miniature-A connector.
3 Measurement Results and
3.1 Impedance bandwidth
An antenna was built to verify the simulation results with a =30 mm, b=20 mm, R=8 mm, h = 0.4 mm, w = 0.5 mm, t =5.02 mm, and 1 h = 4.91mm. The VSWR
or return loss was measured with an Agilent N5230A vector network analyzer. The radiation patterns were measured in a far-field anechoic chamber. Figure 6 shows that the simulation results agree well with the measured VSWR. The results show the input impedance is well matched with the VSWR (below 2:1)
bandwidth covering the entire UWB bandwidth (3.1- 10.6 GHz) .
3.2 Group delay characteristics
For UWB systems, and especially impulse-based systems, the shape of the transmitted electrical pulse should not be distorted by the antenna. Thus, a stable group delay response is desirable, which requires a highly linear phase response with respect to frequency. The group delay shown in Fig. 7 was obtained by taking the first derivative of the phase measured by using
an N5230A vector network analyzer. The observed variations are less than 400 ps with an average of 1.5 ns for the frequency range from 2 to 12 GHz, for the
traveling time of the propagating waves between a pair of the current antennas 40 cm apart. Therefore, a UWBpulse template (within 3.1-10.6 GHz) transmitted or received by the antenna will retain its basic shape
without severe distortion.
3.3 Field distribution and radiation patterns
In addition to the radiation patterns, the antenna gain was also measured. The measurement was performed by using a standard horn antenna as a reference gain antenna. The distance between the transceivers was 1 m. The antenna radiation patterns at 4.0 GHz, 5.3 GHz,
and 7.0 GHz are shown in Fig. 8. The radiation patterns show an omni-directional pattern in the UWB except for the exempted band. This pattern comes from3.3 Field distribution and radiation patterns In addition to the radiation patterns, the antenna gain was also measured. The measurement was performed by using a standard horn antenna as a reference gain
antenna. The distance between the transceivers was 1 m. The antenna radiation patterns at 4.0 GHz, 5.3 GHz, and 7.0 GHz are shown in Fig. 8. The radiation patterns show an omni-directional pattern in the UWB except for the exempted band. This pattern comes from the design of the tapered slot between the monopole and the ground board plane, which is part of the antenna, being responsible for forming a directional pattern in the x-z plane. As a result, the antenna forms a wide beam in the direction along the slot with shallow
nulls observed perpendicular to the slot. The effect can also be explained from the surface current of the monopole component. The simulated surface current
distribution for the antenna is shown in Fig. 9. The surface current is mainly distributed along the tapered slots at lower frequencies, but is mainly on the
T-shaped stub in the band-notched frequency. The measured peak antenna gain is shown in Fig. 10.
A crescent-shaped monopole antenna with good band-notched characteristics was developed for UWB communications in the 3.1-10.6 GHz band. The key
configuration design parameters are analyzed in detail.
Tests of a sample antenna show that the design produces a wide working bandwidth of 3.1-10.6 GHz with VSWR<2 dB while avoiding interference from existing wireless systems in the
5.11-5.47 GHz or