# A Compact Size Antenna for Extended UWB with WLAN Notch Band Stub
## 2 Design Methodology of the UWB Antenna
### Antenna Design Steps  
The design methodology steps and their respective s-parameters are shown in Figure 2. The proposed work consists of two major phases. Phase 1 consists of designing a compact size UWB antenna, which is further utilized in phase 2 to achieve UWB behavior with a WLAN notch band spectrum.  
Initially, a conventional quarter-wave monopole antenna that has a rectangular radiator was designed. The commercially available CST Microwave Studio software was used to perform the antenna simulations. The antenna is CPW-fed, a potential feeding technique to achieve compact size, easy design, and broadband impedance matching. Moreover, its uni-planar structure also eases its integration with electronic circuitry. The respective length of the rectangular radiator for the desire frequency is estimated using the following equation provided in [30]:  
\[L_{\text{fo}}=\frac{\text{c}}{4\text{f}_{\text{o}}\sqrt{\text{e}_{\text{ff}}}} \tag{1}\]  
where c refers to the speed of light in the vacuum and f\({}_{\text{o}}\) is the central resonating frequency of 6.5 GHz. Furthermore, e\({}_{\text{ff}}\) is the constant representing the effective dielectric constant whose value can be approximated using Equation (2) [31]:  
\[e_{ff}\ \approx\frac{e_{r}+1}{2} \tag{2}\]  
Figure 1: The geometry of the proposed WLAN Notch UWB antenna. (**a**) Top view, (**b**) side view.  
After a slight optimization in parameters owing to the losses in the substrate material, the antenna offers a broad bandwidth of 5.4-8.2 GHz, as depicted in Figure 2b. Afterward, a slot was etched from the radiator, as shown in Figure 2a. This helps to achieve a further wider bandwidth without affecting the overall size of the antenna. The slot results in a Y-shaped radiator, which is also a potential structure to achieve wide impedance matching to cover UWB frequency band. The working principle of the Y-shaped radiator is explained in [32]. As shown in Figure 2b, the antenna now has a \(\mid\)S\({}_{11}\mid\)\(-\)10 dB impedance bandwidth of 4.8-9.4 GHz.  
In the final step in designing the proposed UWB antenna, a rectangular stub on a Y-shaped radiator is introduced, as shown in Figure 2a. The antenna operates between 5 and 14 GHz, covering more than 85% of the UWB spectrum and extended UWB spectrums of 11 and 13 GHz, as recommended by the International Telecommunication Union (ITU) [33]. Finally, to avoid the interference of the UWB antenna with sub-bands lying inside the UWB spectrum, a rectangular stub was loaded on the top side of the radiator, which notches the globally allocated WLAN band spectrum of 5.25-5.85 GHz [34]. This specific frequency band spectrum covers 4.9 GHz and 5 GHz Wireless LAN (WLAN) accessed using IEEE 802.11 communication protocols. This spectrum has a number of frequency channels, each having a dynamic bandwidth of 10 MHz, 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively. The Radio Standard Specification (RSS) provides a transmitter output power line and equivalent isotropically radiated power (EIRP) requirements, limiting the maximum EIRP peak value to 4 Watts. Therefore, the wireless signals from WLAN spectrum have the ability to interfere with the UWB spectrum. Furthermore, the notched band channels also have weather radars operating in these bands. Thus, the most congested part of the WLAN band spectrum 4.9 GHz (802.11j) and 5 GHz (802.11a/h/j) from channel 7 to channel 161 is attenuated for low interference with the UWB spectrum. Final optimization was done to achieve the maximum possible bandwidth while keeping the notch band constant. The proposed antenna offers UWB ranges from 3-14.55 GHz, having a notched band of 4.59-5.82 GHz, as depicted in Figure 2b.  
Figure 2: Proposed antenna’s (**a**) various design steps. (**b**) \(\mid\)S\({}_{11}\mid\) responses.