Comparison between recently proposed antennas and this antenna
Since the first Report and Order by the Federal Communications Commission (FCC) authorized the unlicensed use of ultra wideband (UWB) which must meet the emission masks on February 14, 2002 , both industry and academia have paid much attention to R&D of commercial UWB systems. Among UWB system design, the UWB antenna is the key component. Recently, a considerable amount of researches have been devoted to the development of the UWB antenna for its enabling high data transmission rates, low power consumption and simple hardware configuration in communication applications such as radio frequency identification devices, sensor networks, radar, location tracking, etc. Nowadays, the planar printed antenna fed with a microstrip line or a coplanar waveguide (CPW) has received much attention due to its high radiation efficiency and compact size and can be easily integrated with the other circuit. However, compared to the microstrip-fed antennas, the CPW-fed antennas are very good candidates since the feed line and slots are on one side of the substrate . In order to obtain ultra wideband, the different optimum metal radiation patch geometries have been developed, like fork shape , elliptical shape , square shape , spade shape , circle shape , or made some modifications about the radiation patch . Besides, over the designated frequency band, there exist some narrow bands for other communication systems, such as WiMAX operating in the 3.3 to 3.6 GHz band, and WLAN operating in the 5.15 to 5.825 GHz. They may cause communication interference with the UWB system. To solve this problem, it is desirable to design antennas with band notched characteristics to minimise potential interference. Several UWB antennas with frequency band notched function have been reported recently. The reported antennas are generally embedded with a half-wavelength structure such as a ω-shaped slot , a U-shaped slot , a C-shaped slot , or a V-shaped slot . But most reported antennas were designed with only one notched band, mainly discussed on WLAN frequency band 5.15 to 5.825 GHz. UWB antennas with dual notched band were recently reported. In , the dual notched bands were formed by two nested C-shaped slots embedded in the bevelled patch. A pair of asymmetrical spurlines on the feedline was used to achieve dual notched band in . A recently reported antenna has been designed by making use of two split resonant rings (SRR) to obtain dual band-notched characteristics . Nevertheless, the geometry of the SRR structure is relatively complex.
In this paper, a CPW-fed novel planar ultra-wideband antenna with dual band-notched characteristics is introduced. In order to obtain ultra wideband, some modifications about the antenna are introduced. Simply by etching a U-shaped slot in the radiating element, single band-notched characteristic from 5 to 6 GHz can be easily obtained. By etching two nested U-shaped slots in the radiating patch, dual band-notched characteristics for the proposed UWB antenna can be created to reduce the potential interferences between UWB system and narrowband systems. Details of the antenna design and simulation are presented in order to demonstrate the performance of the proposed antennas. Sec. 2 presents the details of the antenna structure and the design procedure. The antennas with single and dual band-notched characteristics were designed in Sec. 3. Sec. 4 analyses the radiation patterns and makes a comparison between recently proposed antennas and this antenna. Finally, the conclusion is presented in Sec. 5.
2. UWB antenna design
It is well known that the rectangular patch antenna has a narrow-band characteristic. To improve its operating bandwidth, we shape the bottom of the patch into an arc. In practical applications, the size of ground plane is finite and the direction of maximum radiation tilts somewhat upwards from the horizontal plane. To reduce this beam tilting, the ground plane of the proposed antenna is designed to have not rectangular but rounded shape. The arc-shaped patch and tapered ground plane make good broadband impedance matching of the antenna possible. To further expand the impedance bandwidth, a right-angle triangle on the upper corners of the patch and a small fan angle on each side of the ground plane near the feeding line are removed respectively. The improvement process steps are shown in Fig. 1.
A comparison of the simulated reflection coefficient (S11) of the antenna with a full rectangular radiating patch, with the modification of the bottom side, with the cuts on the upper corners, and with the small angle on each side of the ground plane is plotted in Fig. 2 when all the dimensions are the same. It can be seen that the first line demonstrates the rectangular patch antenna has a narrow-band characteristic. The second line shows the impedance bandwidth is broadened when the bottom of the patch is shaped into an arc. On the basis of this, the third line about the upper corners modification indicates the bandwidth is further expanded compared with the second line (the main advantages that this modification is reducing the lowest frequency). The fourth line demonstrates the small angle on each side of the ground plane near the feeding line will affect the characteristic impedance of the CPW line and also explains the proposed antenna has a very wide impedance bandwidth.
The final geometry of the proposed compact band-notched antenna is depicted in Fig. 3. The antenna (referred to as antenna A) is printed on a FR4 substrate with size 28 mm × 30 mm, thickness of 1.6 mm, relative permittivity of 4.4 and loss tangent tan δ = 0.02. The proposed antenna is located in x-y plane and the normal direction is parallel to z-axis. The center strip and gap of the CPW line are 3.6 mm and 0.3 mm to achieve 50-Ω port characteristic impedance. By the improvement steps mentioned above the second paragraph, good impedance matching can be achieved across a wide bandwidth. We optimize the antenna geometrical parameters for S11 < -10 dB over the entire frequency range (3.1-10.6 GHz). The optimisation procedure is done by using Ansoft HFSS software. The final antenna geometry parameters are obtained as L1 = 28 mm, L2 = 30 mm, L3 = 13.5 mm, L4 = 5 mm, L5 = 1.5 mm, L6 = 10.8 mm, L7 = 5.4 mm, L8 = 12.9 mm, R = 15 mm, g = 0.7 mm, f = 3.6 mm, c = 0.3 mm, t = 0.3 mm, H = 1.6 mm. By selecting the optimal parameters, the proposed antenna can be tuned to operate within the UWB band.
3. Band-notched UWB antenna design
3.1. Single band-notched UWB antenna design
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. Such as IEEE 802.11a in U.S.A (5.15 GHz to 5.35 GHz, 5.725 GHz to 5.825 GHz) and HIPERLAN/2 in Europe (5.15 GHz to 5.35 GHz, 5.47 GHz to 5.725 GHz). 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. The simple and commonly used approach is to incorporate slots into the antennas’ main radiator. Fig. 4 shows a schematic diagram of the UWB planar antenna (referred to as antenna B) with filtering property operating in the 5 to 6 GHz band.
The band-notched operation is achieved by using a U-shaped slot in the radiating patch of antenna A. Note that when the band-notched design applied to antenna A, there is no retuning work required for the previously determined dimensions. The notch frequency given the dimensions of the bandnotched feature can be given as
Where is the total length of the U-shaped slot, is the effective dielectric constant, and is the speed of the light. We can take (1) into account in obtaining the total length of the U-shaped slot at the very beginning of the design and then adjust the geometry for the final
design. From (1), we can known the length of the U-shaped slot (L = L9 + 2L10) is a crucial parameter to determine the center frequency of the stop band. In this design, the U-shaped slot was chosen to be nonresponsive close to about 0.33λ at the center frequency of the desired notched-band, where λ is the wavelength at the center frequency of the rejection band, the final design parameters of the U-shaped slot are L9 = 8.6 mm, L10 = 4.6 mm, w = 0.2 mm.
The impedance performance of the designed antenna was simulated using electromagnetic software Ansoft HFSS 11 which is based on finite element analysis. The simulated VSWR is shown in Fig. 5. The result demonstrates that a stop-band can be obtained by inserting a U-shaped slot in radiating patch. Compared to antenna A design, the single band-notched UWB antenna successfully blocks out the 5 to 6 GHz band and still performs good impedance-matching at other frequencies in the UWB band. Fig. 6 shows the simulated current distributions at specific frequencies. In Fig. 6(a) and (c), at frequencies 3.5 and 7.5 GHz, the current distributions mainly flow along the transmission line; while around the U-shaped slot the current is small. The simulated current distribution at 5.5 GHz is shown in
Fig. 6(b). It reveals that the currents mainly concentrate over the area of the slot in the radiation patch. The impedance nearby the feed-point changes acutely making large reflection at the desired notched frequency. Fig. 7 also shows the surface current around the slot of antenna B at the notch frequency at 5.5 GHz. At 5.5 GHz, current is around the U-shaped slot. The impedance is nearly zero at the top of slot and the impedance is very high nearby the antenna feeding. In this case, the high impedance at the feeding point leads to the desired impedance mismatching near the notch frequency at 5.5 GHz .
By tuning various parameters, we can control the resonance frequency and quality factor to confirm the stop band. L9, L10 and w are key parameters to tune when the others are fixed. The effects of the lengths (L19, L10) and width (w) of the U-shaped slot on band-notched performance of the antenna are analyzed below. Fig. 8 illustrates the VSWR for different values of L9, It is obvious that when L9 becomes larger, the central frequency is reduced, and the edge of low frequency and the bandwidth are nearly unchanged. Fig. 9 shows the band-notch characteristics with different L10, the behavior is similar to changing L9. When L10 is increasing from 4.1 mm to 5.1 mm, the edge of low frequency and the bandwidth is nearly unchanged. However, the central frequency of the notched band is decreased. In Fig. 10, the central frequency increases as w increases. The bandwidth is shortened when the width (w) of the slot decreases from 0.3 mm to 0.1 mm. The antenna gain of the antenna B is shown in Fig. 11. The figure indicates that, the proposed antenna has good gain flatness except for in the stop-band. The antenna gain variations are less than 4 dB throughout the desired UWB frequency band, and a sharp gain drop of about 7 dB occurs at 5.5 GHz.
To further explain the principle of the band-notched structures and how the center frequencies and bandwidths can be controlled an LC circuit model has been extracted for the proposed antenna A as shown in Fig. 12. Approximately, the radiating element of UWB antenna can be seen as several RLC parallel cells in series. The equivalent circuit input impedance can be expressed as
To simplify the calculation, only the real part is considered to determine the values of components with the formula
Fig. 13 shows the equivalent circuit model for radiating element of UWB antenna B. Le and Ce indicate the inductor and capacitor values of the slot resonator. When the current propagates along the edge of the slot, an inductance should be introduced to the model. The narrow slot is equivalent to a capacitor. Increasing the length of the slot or decreasing the slot width is similar to increasing the capacitor value in the parallel LC circuit. On the other hand, increasing the length of slot will also lead to the increase of the inductor value . Next we discuss bandwidth adjustment. For the parallel RLC resonator circuit as shown in Fig. 13, the input admittance is given by
Where RR represents the radiation resistance of the monopole element of antenna A. For a frequency near the resonant frequency 
In this case it is obvious that Yin is similar to the input admittance of a parallel R and 2C circuit near 0 GHz. Then the -3dB bandwidth (BW) of the parallel RLC resonant circuit approximately equals twice the -3dB bandwidth of the parallel R and 2C circuit as shown in below 
Where FBW represents the fractional bandwidth. Based on the above analysis we have a clear method for controlling the centre frequency and bandwidth of the notch. Increasing the length of the slots, which is similar to increasing the inductor value and the capacitor value, has the effect of decreasing the centre frequency and bandwidth . Simulated results shown in Figs. 8 and 9 prove this conclusion. Fig. 10 shows the simulated VSWR for various width of the slot. It can be seen that, increasing the slot width is equivalent to decreasing the capacitor value, which ultimately leads to the rise of centre frequency and bandwidth of the notch band.
3.2. Dual band-notched UWB antenna design
Besides WLAN systems, IEEE 802.16 WiMAX from 3.3 to 3.6 GHz also operates in the UWB band. Dual notched bands are realized by using two nested U-shaped slots in the radiating patch (antenna A), which can yield band-notched characteristics centered at 3.4 GHz and 5.5 GHz. Fig. 14 shows the geometry and configuration of the UWB antenna (referred to as antenna C) with dual band-notched characteristics. It is note that, since the mutual coupling exists between the exterior and interior U-shaped slots, the simulation values are not exactly equal to the prediction ones . The optimized design parameters are L11 = 5 mm, L12 = 11 mm, L13 = 5.1 mm, L14 = 10 mm, w1 = 0.3 mm.
Fig. 15 shows the simulated VSWR against frequency for the proposed antenna C. Results of the reference antennas without band-notched characteristics and with single band-notched characteristic are also shown for comparison. It is seen that the proposed antenna exhibits two notched bands of 3.3 to 3.5 and 5 to 6 GHz, while maintaining wideband performance from 1.8 to 15 GHz for VSWR < 2, covering the entire UWB frequency band. It is concluded that the notch bands for antenna C are indeed created by the two nested U-shaped slots. The simulated gain is shown in Fig. 16 and indicates a gain variation from 0.5 to 4.2 dB in the frequency range of 1.8 to 15 GHz. As can be expected, Sharp gain decreases occur both in 3.3 to 3.5 and 5 to 6 GHz bands. However, for other frequencies outside the rejected bands, the antenna gain is nearly constant in the entire UWB band. Fig. 17 shows the axial ratio against theta (spherical coordinate) for the proposed antenna C. It is observed that the whole axial ratio is greater than 3 dB, so the proposed antenna C is a linearly polarized antenna.
4. Radiation patterns and comparison
From the UWB applications point of view, the UWB antennas are usually required to have an omnidirectional radiation in the plane orthogonal to the radiating element. The radiation patterns at specific frequencies 3.2 GHz, 6 GHz, 9 GHz, 10.6 GHz and 13.5 GHz are simulated and illustrated in Fig. 18. The quasi-omnidirectional and symmetrical patterns in H plane in the whole band are obtained, which are necessary in a UWB communication system. The interference of high modes in high frequency is reduced. As a result, the proposed antenna can be used in several applications to compensate the drawback of the antennas at the whole frequency band from 3.1 to 10.6 GHz.
It is note that, the radiation patterns of antenna A and antenna B are not depicted. From the analysis by the Reference , the proposed band-notched structures including the single band-notched and dual band-notched ones have little influence on the radiation patterns of the UWB antenna. Tab. 1 summarizes the performances and compares this design to recently published UWB antennas with stop-band characterization. From Tab. 1, we can see that some of the antennas have only one notched band; others which have two notched bands, but the operation frequency band is not wide. When comparing the results to the performance of some UWB antennas presented in Tab. 1, the following features of the implemented antenna can be highlighted: wide impedance bandwidth, good band-notch characteristic, compact in size and good gain flatness.
|Relative bandwidth||Stop-band bandwidth|
|Mehdipour, A. ||Rogers RT Duroid 3003||embedding a ω-shaped slot||3.0 - 11||114 %||5.4 - 5.95||35 × 35||---|
|Nikolaou, S.||liquid crystal polymer||inserting a U-shaped slot||3.0 - 12||120 %||5.0 - 6.0||38.7 × 26.88||< 4.2|
|Yu, F.||FR4||embedding a C- shaped slot||3.0 - 23||153.8 %||5.0 - 6.0||28 × 30||0.2 - 3.9|
|Kim, Y.||FR4||inserting a V-shaped slot||2.8 - 10.6||116.4 %||fc = 5.25||22 × 31||0.8 - 5.0|
|Chu, Q.-X.||FR4||embedding two nested C-shaped slots||3.1 - 10.6||"/> 91.3 %||fc1 = 3.4|
fc2 = 5.5
|26 × 30||2 - 5|
|Luo, J.||FR4||embedding an E-shaped slot in the patch and a U-shaped slot in the ground||2.87 - 10.91||116.6 %||3.49 - 4.12|
5.66 - 6.43
|35 × 14||----|
|This work||FR4||embedding two nested U-shaped slots||1.8 - 15||"/> 157.1 %||fc1 = 3.4|
fc2 = 5.5
|28 × 30||0.5 - 4.2|
In this paper, a CPW-fed novel planar ultra-wideband antenna with dual band-notch characteristics was presented. One right-angle triangle was simulation removed from both two upper corners of the rectangular UWB antenna combined with an arc-shaped bottom and with a tapered arc-shaped ground plane. The simulation result of the VSWR demonstrates that the antenna has very broadband impedance which covers the frequency range of 1.8 to over 15 GHz. By embedding two nested U-shaped slot in the radiating patch, two frequency band notches between the bandwidth of 3.3 to 3.5 GHz and 5 to 6 GHz for WiMAX and WLAN have been obtained. Stable radiation patterns and constant gain in the UWB band are also obtained. Simulated and analyzed in detail in this paper, the proposed antenna could be a good candidate for UWB communication applications.
The authors would like to thank the reviewers for their professional comments and suggestion. They also thank CEO Lazinica Aleksandar from InTech for his valuable suggestions in revising this paper and Prof. Viktorija Zgela for his assistance in improving the manuscript quality. They are also grateful to their family for selfless help.