Implementation and investigation of circular slot UWB antenna with dual-band-notched characteristics

The design and analysis of an ultra wideband aperture antenna with dual-band-notched characteristics are presented. The proposed antenna consists of a circular ring exciting stub on the front side and a circular slot on the back ground plane. By utilizing a parasitic strip and a T-shaped stub on the antenna structure, two notched bands of 850 MHz (3.5-4.35 GHz) and 900 MHz (5.05-5.95 GHz) are achieved. The proposed antenna is fabricated and measured. Measured results show that this antenna operates from 2.3 GHz to upper 11 GHz for voltage standing wave ratio less than 2, except two frequency notched bands of 3.5-4.35 and 5.05-5.95 GHz. Moreover, the experimental results show that proposed antenna has stable radiation patterns and constant gain. A conceptual circuit model, which is based on the measured impedance of the proposed antenna, is also shown to investigate the dual-band-notched characteristics.


Introduction
Since the Federal Communications Commission (FCC) first approved rules for the commercial use of ultra wideband (UWB) in 2002 [1], the feasible design and implementation of UWB system has become a highly competitive topic in both academy and industry communities of telecommunications. Antennas are the particularly challenging aspect of UWB technology. To satisfy such a requirement, various wideband antennas have been studied [2][3][4][5]. Challenges of the feasible UWB antenna design include the UWB performances of the impedance matching and radiation stability, the compact appearance of the antenna size, and the low manufacturing cost for consumer electronics applications [6]. The UWB communication systems use the 3.1-10.6 GHz frequency band, unfortunately there are some other existing narrowband services that already occupy frequencies in the UWB band, such as the C-band (3.7-4.2 GHz) satellite communication systems and wireless local-area network (WLAN) IEEE802.11a and HIPERLAN/2 WLAN operating in the 5.15-5.825 GHz band. Therefore, UWB communication systems may cause unwanted interference with other co-existing communication systems. To overcome problems caused by this electromagnetic interference, various UWB antennas with single or multiple notch functions have been developed for UWB communication systems [7][8][9][10][11][12][13][14].
In this article, based on designs presented in [5,10], a circular slot UWB antenna with dual-band-notched characteristics is presented. The proposed antenna composed of circular ring exciting stub on the front side and a circular slot on the back ground plane. To achieve dual-bandnotched characteristics at frequency band of WLAN and C-band satellite communication systems, a parasitic semicircular annular strip and a T-shaped stub are utilized on the antenna structure, respectively. Effects of varying the parameters of the notch functions on performance of antenna have also been studied. Performance simulations of the antenna were performed with Ansoft HFSS10.0 and CST Microwave Studio 5.0 software [15,16]. A conceptual circuit model, which is based on the measured impedance of the proposed antenna, is also shown for investigating the dual-band-notched characteristics. Designed antenna with optimal dimensions was fabricated and measured. An extensive comparison between experimental and simulation results is made, which demonstrates good agreement over approximately the entire operating frequency range.
The remaining of this article organized as follows: Section 2 presents the configuration of proposed antenna, parametric study of antenna is investigated in Section 3. The simulated and measured results are discussed in Section 4, and finally the conclusion is provided in Section 5.

Antenna configuration
The geometry of the proposed circular slot UWB antenna possessing band-notched function is depicted in Figure 1, with its geometry parameters. The antenna is located on the x-y plane and the normal direction is parallel to z-axis. The proposed antenna is fabricated on a dielectric substrate of FR4 epoxy substrate with relative permittivity (ε r ) of 4.4, thickness of 1.6 mm, and ground plane size of L g × W g = 45 × 40 mm. For this antenna, the radiation element consists of a circular slot with radius of R, i.e., fed by a circular ring resonator line with inner and outer radiuses of r i and r o , respectively. This ring is connected to the 50 Ω main line with W f = 3 mm (see Figure 1a).
To overcome the unwanted electromagnetic interferences of UWB communication systems with IEEE 802.11a and HIPERLAN/2 WLAN and C-band satellite communication systems, a parasitic semicircular annular strip and T-shaped stub are utilized on antenna structure. The parasitic strip is attached to reject the frequency band of WLAN (5.15-5.825 GHz) and T-shaped stub is used to eliminate the frequency band of 3.7-4.2 GHz.

Parametric study of antenna
To achieve the desired band-notched characteristics, the effect of the parasitic annular strip and T-shaped stub parameters on the antenna's performance was studied.
The simulated VSWR of the antenna for different values of r n is plotted in Figure 2. This figure shows that by increasing the r n the second notching frequency shifts toward lower frequencies and its bandwidth corresponding increases too. Figure 3 shows the SWR curve of the proposed antenna for different values of L s . It can be observed that in this case the first notch's center frequency decreases with the increase of the length of L s .
The effect of parameter l s of T-shaped stub on the antenna's performance is depicted in Figure 4. It is clear from this figure that the first notched band shifts toward lower frequencies by increasing the length of l s .
As observed, the filtering frequencies can be tuned by varying the principal parameters of parasitic annular strip and T-shaped stub.

VSWR
The simulated and measured VSWR curves of the proposed dual-band-notched UWB antenna, whose dimensions are defined in Table 1 are depicted in Figure 5.
From this figure, we can see that the calculated bandwidth of the proposed antenna for which VSWR ≤ 2 is from 2.7 to greater than 10 GHz with two notched frequency bands of 3.5-4.4 GHz (consist of frequency band of C-band satellite communication system) and 5-6 GHz (frequency band of WLAN systems) for VSWR ≥ 2. The measured 3-dB bandwidth of fabricated antenna is from 2.3 to greater than 11 GHz with VSWR ≤ 2, and the notched bandwidths are over 3.5-4.35 and over 5.05-5.95 GHz for VSWR ≥ 2. The measured frequency range encompasses the commercial UWB band (3.1-10.6 GHz), and the notch functions reject the frequency bands of C-band satellite communication and IEEE 802.11a to overcome EMI problems of UWB systems with WLAN and C-band satellite communication systems. The correlation between the numerical and experimental results is considered to be excellent.

Radiation patterns and gain
The simulated and measured antenna gain in the entire UWB band is presented in Figure 6, which shows sharp gain decreases occur both in 3.6-4.2 and 5-6 GHz bands. However, for other frequencies outside the   The measured and simulated radiation patterns of proposed dual-band-notched UWB antenna in the Eplane (yz-plane) and H-plane (xz-plane) for three different frequencies 3.3, 7, and 10 GHz are shown in Figure  7. The patterns in the H-plane are quite omni-directional as expected. In the E-plane, the radiation patterns remain roughly a dumbbell-shape like bidirectional patterns.    impedance R a of the un-notched reference antenna connected with two parallel LC-resonant circuits in series.

Input impedance
The corresponding conceptual circuit model is shown in Figure 9.
When the proposed antenna is operating at the two desired notched frequencies, the two corresponding LCresonant circuits mentioned above will be syntonic, which leads to the input impedance to be opened. Therefore, proposed band-notched UWB antenna presents high impedance characteristics at notched frequencies.

Results and discussions
The simulated and measured results of VSWR, radiation patterns, gain, and input impedance of the proposed antenna are presented in this section. The simulations were performed using Ansoft HFSS and CST Microwave Studio, which utilize numerical methods for electromagnetic computations. The VSWR and input impedance of antenna was measured by the Agilent 8722ES network analyzer and radiation characteristics measurements were performed in anechoic chamber of antenna laboratory of Iran Research Institute for ICT (ITRC).

Conclusion
A circular slot UWB antenna with dual-band-notched characteristics at C-band (3.7-4.2 GHz) satellite communication systems and WLAN frequencies has been proposed and fabricated. The measurements show that VSWR is below 2 within the desired frequency bandwidth from 2.3 to greater than 11 GHz, whereas two notched frequency bands of 3.5-4.35 and 5.05-5.95 GHz for VSWR ≥ 2 are obtained. The stable radiation patterns and constant gain in the UWB band, with the exception of notched bands, are obtained. To discuss the mechanism of the filtering properties, a conceptual circuit model based on the dual-band-notched phenomenon has been proposed. Accordingly, the proposed antenna is expected to be a good candidate in various UWB systems.