Ultrawideband (UWB) communications technology makes use of broad expanses of frequency spectrum, transferring information at high data rates using low-energy pulses. While UWB signals are designed not to interfere with existing narrowband communications systems within the same frequency range, they are subject to interference from narrowband communications signals.
To overcome that interference problem, an UWB antenna was designed with a bandwidth of 3.1 to 11.5 GHz and with two notches; thus, the antenna doubles as a band-reject filter. The notches are designed to prevent interference from larger signals in such applications as WiMAX systems and wireless local-area networks (WLANs). The experimental antenna was fabricated on low-cost FR4 printed-circuit-board (PCB) material to demonstrate the effectiveness of the design.
A Look at UWB
Although UWB technology has existed for some time for radar applications, the Federal Communications Commission (FCC) increased interest in the frequency range from about 3.1 to 10.6 GHz for short-range, high-data-rate commercial communications.1 Many components have been designed for UWB use, including antennas based on various topologies.2-13
For UWB applications, planar antenna structures appear to offer advantages over other configurations for their simple structures, ease of fabrication, small size, low profiles, and low cost. UWB communications systems provide many benefits, such as enabling high data rates, increased security, low power consumption, and simple hardware requirements in practical applications.2 For UWB systems to be effective, however, antennas for those systems must meet several requirements, including small size; omnidirectional radiation patterns; high, stable gain across a wide frequency range; and compatibility with other required components, such as filters and amplifiers.
Of course, signals occupying the UWB frequency range must coexist with many well-established narrowband communications systems, such as IEEE 802.11a and HIPERLAN/2 WLAN systems operating in the 5- to 6-GHz range. In some European and Asian countries, WiMAX service occupies the frequency range from 3.3 to 3.6 GHz. UWB antennas have typically functioned with the addition of filters to suppress narrowband signals that might interfere with UWB operation. As an alternative approach, portions of the UWB spectrum can be notched out by developing antennas with band-notch characteristic.10-17
Building the Two-Notch Antenna
To demonstrate this design approach, an UWB antenna with two notched frequency bands was developed. Two techniques were used to achieve the notches. In one, a T-shaped slot was formed in the radiating patch on the current path of the antenna. In the other, a U-shaped parasitic strip was added above the antenna ground plane. The first notch is intended to reduce interference in the frequency band from 3.3 to 4.1 GHz. The second notch is to eliminate interference within the frequency band of 5.1 to 6.0 GHz.
CST Studio three-dimensional (3D) electronic design automation (EDA) software from Computer Simulation Technology was used in the design and simulation of the antenna.
Figure 1 shows the basic antenna structure. It’s composed of an octagonal-shaped radiator with radius (r) of 8.2 mm. This radiator is interconnected with the microstrip transmission line with a line width, Wf, of 3 mm to achieve a characteristic impedance of 50 Ω; the transmission has a line length (Lf) of 13.8 mm. As may be apparent from Fig. 1b, the antenna has a partial ground plane with length (Lg) equal to 13.4 mm to achieve the desired impedance bandwidth required for UWB applications. The width (W) of the antenna is 30 mm and the length (L) is 30 mm.