1. Introduction
UWB is a promising wireless technology that can operate at very low power emission levels while communicating high data rates over short distances. It has attracted much attention as a means of expanding capacity from the already heavily utilized wireless bands. The Federal Communications Commission (FCC) has allocated a bandwidth of 7.5 GHz between 3.1 GHz to 10.6 GHz for commercial UWB communication systems.
In this emerging technology, the antenna design is a challenge. Like conventional antenna design, the return loss has to remain higher than 10 dB over the frequency range of operation. For a UWB antenna, the radiation properties should be reasonably satisfactory over the bandwidth. For example, omni-directional radiation patterns are required for indoor and vehicular applications. For a more complete pattern analysis, the total field which includes the co- and cross-polarizations of the radiation should be taken into account. The time domain performance such as the group delay and impulse response should also be examined (Chen et al., 2004).
Recently, several monopoles have been proposed for various UWB applications. These antennas make use of different structures to meet the requirements of return loss and radiation pattern. A monopole comprised of a square planar structure positioned perpendicular to the ground plane has been proposed, where bevelling or a shorting post is used to optimize and achieve a broad impedance bandwidth (Ammann & Chen, 2003). A bi-arm rolled monopole (Chen, 2005) which is constructed by rolling a planar monopole has shown that it is capable of achieving broadband and omni-directional radiation characteristics within the UWB band. The UWB antenna in (Behdad & Sarabandi, 2005) consists of half of a coupled sectorial loop above the ground plane. The optimized design is able to achieve an 8.5 : 1 frequency bandwidth for the voltage standing wave ratio (VSWR) < 2.2. Its radiation patterns are relatively consistent within the frequency band. In another design (Liang et al., 2005), a printed circular disc monopole is fed by microstrip line. Its impedance bandwidth covers the UWB frequency band and the radiation patterns are nearly omni-directional. The antenna geometry in (Qiu et al., 2006) is a circular notched ring with an attached element inside the hole. This antenna has band-notched characteristics, which meet the input impedance requirement of the UWB band while avoiding interference within the 5.155.875 GHz bands occupied by existing wireless systems. A half-bowtie radiating element with a staircase-shape and a modified ground plane has been proposed (Cho et al., 2006). It has a very wide impedance bandwidth and the wireless local area network (WLAN) band is notched in the vicinity of 5 GHz. The time domain performance of these monopoles has been investigated by simulation, measurement or both.
The antenna to be presented in this chapter consists of a butterfly-shaped monopole above a ground plane. The butterfly-shaped radiator comprises two circular or elliptical wings which are connected to the two edges of a conducting plate. The wings may make different angles with the ground plane. This antenna is designed to offer a 10-dB return loss across the entire UWB bandwidth. The radiation patterns and cross-polarization performance will be investigated by simulation and validated by measurement. In addition, the group delay is examined, and the transmitted and received signals are compared with two identical antennas placed at different angles relative to each other at a far-field separation.
This chapter is organized as follows. Section 2 describes the antenna configuration and design. A parametric study of the effect of the angle between the wings of the antenna and ground plane on the return loss |
2. Antenna configuration and design
The proposed antenna is comprised of two perfectly electrically conducting (PEC) plates that are connected to the two edges of a rectangular PEC plate which is horizontal to the ground plane, as shown in Figure 1. The configuration mimics the shape of a butterfly. In general, the two PEC plates – two wings of the butterfly – can be circular, elliptical, or any shapes for impedance matching purpose. The antenna is implemented at a height
In this chapter, the design with elliptical wings has been chosen and the effect of the angle
Using Mentor Fidelity based on the FDTD method with
and 30°, the return loss |S11| is higher than 10 dB across the UWB band. The return loss bandwidth can be made much wider when
3. Analysis and discussion for return loss and radiation patterns
As a design example, the elliptical wings of the butterfly-shaped antenna are chosen to be orthogonal to the ground plane, i.e.
3.1. Return loss
The simulated and measured return losses |
3.2. Radiation patterns
Figures 4 - 6 show the co- and cross-polarized radiation patterns in the
Similarly, Figures 7 - 9 show the co- and cross-polarized radiation patterns in the
Figure 10 shows the simulated co-polarized patterns when
From the radiation patterns in the
4. Ground plane effect on the antenna performance
In the above discussions, the ground plane size for the antenna is 300 mm× 300 mm, which is much larger than the wavelength corresponding to the lower edge frequency of the UWB band. Furthermore, depending on the requirements of different applications, the ground plane size for this antenna may be varied. From the study conducted in this Section, it will be shown that the size of the ground plane will greatly affect the antenna performance in terms of |
4.1. Ground plane effect on return loss
The FDTD-based Fidelity software from Mentor was used to investigate the performance of the antenna with different square ground planes of length 25, 50, 100, 200, and 300 mm.
The return loss performance for the antennas with different ground plane sizes is displayed in Figure 11. It can be observed that for ground plane sizes larger than 100 mm × 100 mm, the return loss is quite stable with little variation. When the size is reduced to 50 mm × 50 mm and then to 25 mm × 25 mm, the impedance matching deteriorates at the lower frequencies but remains relatively unchanged at the higher frequencies. Hence, the impedance bandwidth is reduced. This is expected because for the radiating elements of a monopole mounted on a finite-size ground plane, the outer edge of the ground plane diffracts the incident radiation in all directions. This diffraction consequently alters the current distribution on the ground plane. At 3 GHz, the ground plane size of 50 mm × 50 mm corresponds to ½
4.2. Ground plane effect on radiation patterns
The effect of the size of the ground plane on the radiation patterns has been investigated across the UWB band. Figures 12
14 show the simulated radiation patterns at 3.1 GHz, 6.85 GHz, and 10.6 GHz in the
Figure 13 shows that the radiation patterns at 6.85 GHz peak at around |
An infinite-size ground plane prevents monopole radiation into the half space beneath the ground plane. In practice, a monopole antenna has to be installed on a ground plane of finite size. It has been observed that the impedance matching will be degraded significantly if the ground plane size is smaller than 1
5. Time domain characteristics
In order to evaluate the antenna performance in the time domain, a transmit-receive antenna system has been built. The antenna system consists of two identical antennas separated by a distance in the far-field region. This system is implemented using an FDTD model and becomes a two-port structure. The source pulse is fed to the input of the transmit antenna. The radiated pulse from the output of the transmit antenna travels in free space before entering the receive antenna. In this study, the antenna with
The antenna system has been simulated using the FDTD method and the group delay is calculated and shown in Figure 15. The far-field distance between the two antennas is 250 mm. In this case, the two antennas are positioned face-to-face. The group delay is defined as the change in the phase of the transmission response
Figure 16 shows the comparison of the received signals when the receive antenna is oriented at angles of 0°, 45°, and 90° with respect to the transmit antenna. The source and radiated pulses are also included. Rx 0° represents the received signal when the wings of the receive antenna are directly facing the wings of the transmit antenna, while Rx 90° represents the signal when the wings of receive antenna are perpendicular to the wings of the transmit antenna. All the received signals have been normalized with respect to the source pulse for comparison. It can be observed that the shapes of the received signals are similar and the normalized peak amplitude in the main lobe varies from 1 V to around 0.7 V for the different angles. The peak amplitude in the ripple of the received signals changes from around 0.1 V to 0.4 V. The distortion in the received signal may be caused by the larger variation in the group delay of the antenna system, especially at the higher frequencies.
6. Conclusion
The proposed butterfly-shaped monopole antenna has demonstrated good impedance and radiation performance across the UWB band. The wings of the antenna can be oriented at an angle of up to 45° with respect to the ground plane and still meet the UWB requirements. The co- and cross-polarized radiation patterns for the proposed antenna have shown stable performance across the entire UWB band in the principal planes. In addition, the radiation is omni-directional in the azimuth plane. However, at 10 GHz, the patterns are adversely affected when
A study has been conducted to investigate the effect of the size of the ground plane on the antenna performance. It has been shown that when the size of the ground plane is comparable to or smaller than 1
Based on the phase of the transmission response
Future research directions in this area may involve antenna miniaturization as well as the optimization of the current distribution on the antenna so as to reduce signal distortion. In order to avoid interference with other existing communication systems, band notching will be required, which may be achieved in the proposed butterfly-shaped monopole antenna by cutting slots on the radiators.
References
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