Design of a Triple Band-notched UWB Planar Monopole Antenna

In this paper, a novel design of a compact planar ultra wide band (UWB) monopole antenna with triple band notched characteristics has been presented. The antenna has a unique shape and has been fed through a 50 Ω microstrip feedline. The strength of the antenna lies in three band notched properties for WiMax system at 3.3 3.7 GHz, WLAN IEEE 802.11a at 5.15 5.85 GHz and X band satellite communication at 7.25 8.395 GHz. The notched bands have been achieved by incorporating an inverted Uslot etched on the radiator patch, an inverted C-slot also on the radiator patch and a U-slot on the microstrip feed line. In order to achieve FCC defined impedance bandwidth of 7.5 GHz, a rectangular cut in the ground plane has been introduced along with staircase structure in the bottom edge of the radiator. The proposed antenna is having a compact size of 30 X 30 X 1.6 mm 3 and exhibits a large bandwidth from 2.8 GHz to 10.7 GHz. The antenna has been successfully simulated and fabricated. The radiation pattern is omni-directional in H-plane and the E-plane exhibits dipole like radiation pattern. The gain of the antenna is stable across the whole operating frequency band except at three notched bands.

for instance, WiMax (wireless interoperatibility for microwave access) for some Asian and European countries operating at 3.3 -3.7 GHz, WLAN (wireless local area network) IEEE 802.11a operating at 5. 15 -5.85 GHz and X band satellite communication operating at 7.25  GHz. It is thus essential to design an antenna which is not only compact and planar but also has multiband filtering capability to protect the UWB based applications from possible interference from existing narrow band services. While integrating a bandstop filter with UWB antenna may increase the complexity and cost of fabrication [2], however, antenna design incorporating band notched characteristics is a simpler way to solve the interference problem.A number of designs have been reported till now for multi band notched functions with in-built structures or design topology to avoid EM interference.
Broadly, various techniques of band rejection capability involves etching slots of various shapes and sizes on radiating patch, microstrip lines or ground plane [3]- [11], embedding stubs in the radiator patch or in the vicinity of feedline [12]- [14], use of metamaterial [15]- [17] or electromagnetic band gap structure [18]- [20]. Using these techniques, a number of antenna designs have been reported in which single band notch [21]- [22] , dual band notch [23]- [24], triple band notch [25]- [26] and quadruple band notch [27] have been reported the most. It is observed that in most of the reported designs the antenna size is relatively large and the notched bands are quite wide thereby resulting in reduction in useable bandwidth for UWB communication. Moreover, the mutual coupling among each slot or each parasitic strip leads to a more complicated design procedure requiring tedious simulations and long simulation time to achieve design goals [28].
In this paper, a compact printed unique lamp post shaped UWB monopole antenna having triple band notched characteristics has been proposed. The notched bands have been achieved by etching a U-slot and a C-slot on the radiator patch and a U-slot on the microstrip feedline. In order to achieve wideband matching of the transmission with the radiator, a rectangular cut has been introduced in the partial ground plane just below the feedline. The stepped structure at the lower edge of the radiator also results in improvement in the characteristics of the high frequency band.
The band notched functions of the antenna has been achieved by etching three slots at different locations. The length of the slots is taken to be one half of the guided wavelength at the respective notched band frequency [29]. The length of each slot can be calculated using the expression (1) and (2) as under:

B. Results and Discussion
The final design of the compact planar monopole antenna as shown in Fig  The radiation pattern and gain of compact antenna is measured using standard Horn antenna. The normalized radiation pattern at 3 GHz, 6.5 GHz and, 9.5 GHz in the E-plane and H-plane are shown in Fig. 12. The measured radiation pattern at the pass band frequencies shows that the antenna is able to retain its omni-directional behavior in H-plane at lower frequencies while there is little variation at higher frequencies compared to simulated response. The E-plane radiation pattern is bi-directional in nature at lower frequencies, however, there is slight variation at higher frequencies in comparison to simulated response. The overall response of antenna is in agreement with a conventional monopole antenna. The radiation pattern at higher frequencies deteriorates because the equivalent radiating area changes with frequency over UWB [30].  The group delay of the antenna is shown in Fig. 15. It is seen that the group delay is constant over the entire operating band except at the notched frequencies where the group delay is more than 2 ns.