Chang-Fu Chen, Xiao-Jian Tian, and Yang Tang
For ultrawideband (UWB) communications technologies to gain momentum worldwide, compact, high-performance antennas will be needed. These antennas must be capable of covering the broad instantaneous bandwidth required for UWB communications, while at the same time providing notches at key frequencies already occupied by other wireless services to minimize interference during transmission and reception. Such an antenna design was developed by the authors based on standard microstrip circuitry, using a C-shaped slot in the circuitry to eliminate interference from the existing wireless-local-area-network (WLAN) band. The antenna was fabricated on low-cost FR-4 printed-circuit-board (PCB) material with a thickness of 0.8 mm and studied theoretically and experimentally. Measured results on the prototype antenna reveal good UWB performance with an eff ective band-notch function.
UWB technology has been proposed for many low-power indoor applications,1 including in sensor and radar systems as well as for personal-area-network (PAN) communications systems. Over short distances, UWB systems can support high data rates with extremely low transmit power levels. Radar and sensor systems typically operate with pulsed signals at low repetition rates, while UWB communications employ pulses at much faster repetition rates, so as to not interfere with existing narrowband communications systems . A number of different antenna types have been developed for UWB use, including planar monopoles and dipoles.2-7 However, due to the wide bandwidth covered by the UWB frequency band and overlapping wireless communications bands, such as the WLAN band, special characteristics such as a notched band across the WLAN frequency range is desirable for UWB antennas in order to reduce interference between UWB and WLAN systems. As a result, numerous UWB antennas with a notched function have been developed for UWB communication systems.8-11 To achieve these antenna designs, different slot shapes have been proposed, such as adding an inverted-L shaped, V-shaped, or hybrid-shaped slot12-15 to a planar monopole.
To provide a suitable frequency notch at the WLAN operating frequency band, this report examines the use of a C-shaped slot in a planar monopole (microstrip) antenna. The bandwidth of this UWB antenna is 3.0 to 13.2 GHz. It is designed with a partial ground while the patch is achieved with two-step ladder shapes. A C-shaped slot was inserted into the radiation patch to achieve a notched frequency feature at the WLAN frequency band. Parametric studies were carried out to analyze the antenna, and the results of these studies will be presented.
Figure 1 shows the geometry of the experimental antenna, with all of the dimensions shown in millimeters. The antenna is shown in the x-y plane and the normal direction is parallel to the z-axis. The experimental UWB antenna was designed using a two-step ladder-type patch and a partial ground plane to obtain UWB frequency coverage. A C-shaped slot was used to achieve a relatively wide notch band. To better understand the notch characteristics, the effects of varying the slot's radius and angle were studied. The patch was fed by a 50-Ω microstrip transmission line, and the width of the feed line was chosen to be 1.4 mm for good impedance matching. The antenna was printed on low-cost FR-4 dielectric substrate material with relative dielectric constant (er) of 4.4 and thickness of 0.8 mm. The dimensions of the UWB microstrip antenna are 24 x 32 mm.
The band rejection of the proposed antenna was achieved by means of adding the C-shaped slot to the radiating patch. Figure 2 offers a view of the simulated distribution of induced current in the radiation element at 5.5 GHz. As can be clearly seen, the band-notched characteristic is due mainly to the induced current flowing in the C-shaped ring of the radiation patch, in such a way that the C-shaped slot prevents the patch from radiating.
Simulated results when using the High-Frequency Structure Simulator (HFSS) commercial electromagnetic (EM) simulation software from Ansoft show that variations of parameters R and α can be of great importance when making adjustments to the antenna's band-notched feature. Here, R is the radius of the C-shaped slot while α is the angle. It is known that the resonant bandnotched frequency is mainly determined by the radius of the slot. It can be observed that the value of R influences the band-notched frequency greatly, as shown in Fig. 3. The band-notched frequency varies from 7.0 to 4.0 GHz as the value of R varies from 3.5 to 5.5 mm. It can also be observed that the band-notched frequency becomes lower when the value of R is larger. In this study, R was chosen to be 4.0 mm so that the band-notched frequency would be located at 5.5 GHz. The bandwidth of the notch is greatly affected by the angle of the C-shaped slot structure. It can be seen that the value of a greatly influences the band-notched characteristic, as shown in Fig. 4. It is apparent that the bandwidth of the notched band becomes wider as the angle, α, grows smaller and the resonant frequency becomes higher. For optimization purposes, the value of α was chosen as 280phi.The width of the C-shaped slot also impacts the notch-band characteristics, but with less influence that the other parameters.
The optimized UWB antenna design was fabricated and measured. The simulation and the experimental studies of the antenna were obtained using HFSS simulation software and a model 37269C microwave vector network analyzer (VNA) from Anritsu Co., respectively. The simulated and measured return loss characteristics are presented in Fig. 5. It can be seen that there is a certain frequency offset of the notch band between measured and simulated results. This may due to manual processing errors. By introducing the C-shaped slot on the radiation element, the sharp frequency band-notched characteristic is obtained very close to the desired frequency at 5.5 GHz. A prototype of the antenna fabricated on FR-4 is shown in Fig. 6.
Figure 7 presents the peak gain of the proposed antenna throughout the frequency range with and without the C-shaped slot. As depicted in Fig. 7, the gain decreases at the notched frequency band around 5.2 GHz. The gain of the antenna is over 2.5 dB. Figure 8 shows the x-z plane and x-y plane radiation patterns at (a) 4 GHz, (b) 5 GHz, and (c) 7 GHz. It is apparent that these radiation patterns are stable across the operating bandwidth, and the polarization planes are similar to each other at 4, 5, and 7 GHz.
In conclusion, a novel planar monopole antenna was designed and fabricated for UWB communications applications, incorporating a C-shaped slot to provide specific notch-band characteristics. By understanding the necessary parameters for the C-shaped slot, a notch-band characteristic from 5.5 to 5.8 GHz was achieved within the wider operating bandwidth of the UWB antenna.
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