In this letter, a simple monopole antenna with variable band-notched characteristic for ultra wide band (UWB) function is proposed. Two L-shaped quarter-waveguide resonators coupled to the ground plane with two shorting tracks at the sides of the antenna are used to generate stop-band performance around 5.5 GHz (WLAN). The proposed antenna is fabricated on the substrate FR4 (relative permittivity of 4.7) and has a compact size of 16 × 28.5 × 1.6 mm. The designed antenna has a good impedance matching in 3.1 - 11.4 GHz frequency range with VSWR < 2, except the band 5 - 5.85 GHz.
Ultrawideband (UWB) communication systems have received great attention in the wireless world due to their merits such as high data rate, small emission power and low cost for short range access and remote sensing applications. However, interference between the existing narrow band wireless systems and UWB systems has been a concern due to the inherently ultra-wide operating frequency range for UWB communication. For instance, Wireless Local Area Network (WLAN) is operated at the frequencies of 2.4 GHz (2400 - 2484 MHz), 5.2 GHz (5150 - 5350 MHz), and 5.8 GHz (5725 - 5825 MHz). To overcome this problem, various UWB antennas with a band-notched function have been developed not only to mitigate the potential interference but also to remove the requirement of an extra bandstop filter in the system [1,2].
The common method to achieve the band-notched function is incorporating slots into the antenna’s main radiator, such as a U-shaped slot [2,3], a V-shaped slot [
In this letter, a novel planar UWB monopole antenna with variable frequency band-notch function is proposed. The present technique for creating filtering function has not yet been studied in the previous proposed antennas. The notched band, covering the 5 - 5.85 GHz WLAN band, is provided by a pair of L-shaped strips on the ground plane connected to the main radiation patch from the sides of the antenna using two strips.
The paper is organized as follows. Section 2 gives a brief description of the antenna configuration. Section 3 presents the proposed antenna design method and results of simulation using Ansoft HFSS. Section 4 reports on experimental results and Section 5 concludes the findings of this paper.
The base configuration of the antenna is refers to [
al loss of 0.02, a width W of 16 mm and a length L of 28.5 mm. The radiation patch has the shape of an inverted trapezoid but the length of the smaller parallel side is exactly equal to the width of the feed-line. This forms a smooth transition between the feed-line and the patch so the overall impedance matching is enhanced [
The notch creates a capacitive load that neutralizes the inductive nature of the patch [
In this section, the antenna covering the UWB band is first described. Then the new band notched structure is investigated. The effects of changing the geometric parameters of the proposed antenna on impedance matching and bandwidth are discussed. The proposed antenna structure is simulated using the Ansoft High Frequency Structure Simulator (HFSS) software.
The UWB antenna design features a gap (slot) between the radiation patch and the ground plane which introduces a coupling capacitance and plays an important role in obtaining UWB behavior. The size of the gap opening defines the impedance matching [
The UWB system, operating between 3.1 - 11.4 GHz causes interference to the existing wireless communication systems, for example the WLAN operating in 5.15 - 5.85 GHz. The band rejection filter employed in UWB RF front-ends avoids the interference but gives complications to the UWB system. To overcome this difficulty, UWB antenna with a band rejected characteristic is required.
The band rejection function of the proposed antenna is achieved by printing two small L-shaped strips on the bottom side of the substrate and properly tuning the dimensions of the strips to determine the center frequency and bandwidth of the rejected band. The open circuited L-shaped strips introduced on the bottom side of the substrate are shunt connected to the main radiation patch from the sides of the antenna through two silver strips. The L-shaped strips act as resonator and introduce capacitive coupling to offer series resonance band stop function. Since the resonator has an impedance zero at its resonant frequency. The main line is effectively shorted at fr and thus no power is delivered to the radiation patch. It is to be noted that capacitive coupled transmission line inductor is less than quarter wavelength at the resonant frequency [
To estimate the center frequency at which the rejected bands are achieved, one may use the following formulas:
where
is the center frequency of the rejected band. We have a clear method for controlling the center frequency and bandwidth of the notch. Increasing the length of the L-shaped strips (increasing Lp2) has the effect of decreasing the center frequency and increasing the bandwidth. Also decreasing the distance of the Lshaped strips from ground plane (increasing L4) has the same
effects.
In this study, The center frequency of the stopband is varied by adjusting the length of Lp2, L4 and Lp3. Therefore, following (1) and fine adjusting from experiments for the desired center frequency of the rejection at 5.5 GHz, the total lengths of the folded strips are found to be 8.91 mm.
band of frequencies except for the notched band that we can observe an attenuate of the gain at this band. It means that the antenna does not radiate at the stop band.