# Planar UWB Monopole Antenna with Tri-Band Rejection Characteristics at 3.5/5.5/8 GHz
## 2 Antenna Design  
Figure 1 shows the structure of the presented planar UWB monopole antenna with triple band-notched function. The antenna was manufactured on a FR-4 substrate. The thickness of the substrate was 1.6 mm. The antenna consists of a planar circular disc monopole with an arc H-shaped slot and is fed by a 50 ohm microstrip line. Compared to straight-shaped H-shaped slot [19], the arc H-shaped slot can provide more degrees of freedom (such as \(\alpha\), L1, W2) for tuning the center frequency of the desired rejectband. The ground plane was reshaped as T-shaped defected ground structure with two narrow L-shaped slots on the top and a narrow slot on the bottom. The top corners were removed to improve the impedance bandwidth and reduce the return loss. Note that, the general length of the L-shaped slots, \(l\), can be approximately calculated by:  
\[f_{notch}=\frac{c}{4l\sqrt{\epsilon_{e}}}. \tag{1}\]where \(f_{netch}\) is the center frequency of the stopband, \(c\) is the light speed, \(\varepsilon_{e}=(\varepsilon_{r}+1)/2\) is the effective permittivity, \(\varepsilon_{r}\) is the relative permittivity. Assume the center frequency of the WLAN working frequency band is 5.6 GHz and the total length of L-shaped slot is about 7.9 mm. It is calculated that for a center frequency of WLAN of 5.6 GHz, the total length \(l\) of the L-shaped slot is about 7.9 mm; for the center frequency of the X-band of 8.1 GHz, the length of the narrow slot on the bottom of the ground plane is about 5.6 mm. The final UWB antenna design achieved dual band-notched function by adjusting the width, length and slot size of the radiating patch and the defected ground structure. Table 1 shows the optimized values of the parameters in the presented antenna. The specific parameters are studied in the next section. Figure 2 illustrates the return loss of the presented antenna structure. As shown in from Figure 2 (curve (i)), a stopband at low frequency can be excited by cutting an arc H-shaped slot on the radiation patch. And curve (ii) illustrates that two stopbands (one at the same low frequency and the other at medium frequency) are excited by cutting the arc H-shaped slot on the radiating patch and two narrow L-shaped slots on the ground plane. In addition, curve (iii) illustrates that another high frequency stopband is excited due to the narrow slot on the bottom ground plane.  
Therefore, by cutting the arc H-shaped slot on the radiating patch and three narrow slots on the ground plane, three stopband rejections were formed. And their center frequencies were 3.8 GHz, 5.6 GHz and 8.1 GHz, respectively.  
To further explain the three notched band, Figure 3 shows the distribution of current on the antenna surface at three frequencies of 3.8 GHz, 5.6 GHz and 8.1 GHz. In Figure 2(a), it can be seen that at 3.8 GHz, the current was distributed over the edges of the curved H-shaped slot on the radiation patch. In Figure 2(b), it can be observed that at 5.6 GHz, the current was distributed on the edge of the L-slot on the ground plane. Figure 2(c) reveals that the third stopband was excited mainly due to rectangular slot under the microstrip of the radiating patch.