Abstract
Abstract
A novel dual band notched antenna is presented in this paper. First, an Ultra Wide Band (UWB) antenna structure with a rhombic radiating patch and a simple ground plane is proposed. Then, aiming at the filtering of interfering frequency bands from UWB frequency range, two pairs of joint slots are embedded in the antenna body. By the inclusion of these slots, surface current distribution is disturbed at the vicinity of the slots and as a result, 5.15–5.825 GHz Wireless Local Area Network (WLAN) band and 8.025–8.4 GHz International Telecommunication Union (ITU) band are rejected. In the next step, with the aim of comparing the presented antenna with the previously designed structures regarding different features such as size, bandwidth, gain variations and number of the filtered frequency bands simultaneously, an efficient framework is established. The proposed method is based on Analytical Hierarchy Process (AHP) that is one of the Multi Attribute Decision Making (MADM) techniques. The design process, simulation and measured results, and the AHP based method will be discussed in detail.
Keywords
Introduction
Paving the way to a more efficient utilization of communicating facilities, Federal Communication Commission (FCC) has dedicated 3.1–10.6 GHz as UWB frequency range applications. Since then, many notable efforts have been paid on thinking up various sorts of antennas with multi-functional features. Meanwhile, for UWB allocations, a vast body of antennas has been devised for which the obtained results have been reported in literature [1–5]. Some important characteristics such as small and compact size, wide bandwidth, low fabrication cost and good radiation properties highly influence the successful deployment of a new design of antenna in real-world applications. On the other hand, by screening the UWB frequency range in more detail, it would be clarified that the UWB is itself speculated to cover important frequency bands such as WLAN, Worldwide Interoperability for Microwave Access (WiMAX), and ITU frequency bands. Hence, in this context, the most challenging issue might be the possibility of interference between the forgoing frequency bands with UWB. Consequently, fostering novel and efficient designs of antennas apt to alleviate such interferences is a promising topic in communication engineering. Exploring the available literature reveals that in attempt to overcome this problem, a great number of band-notched antennas have been proposed [6–11]. However, there is a continuous motivation among the researchers to meet the requirements of industry in the best technical and economical way.
To do so, bearing new antenna designs in mind and then realizing them through both scientific analysis and expertise engineering knowledge would result in more efficient and smaller antennas. Afterwards, many engineering projects and academia research studies are still running in this issue.
Keeping the preceding discussion in mind, this survey intends to present a new design of UWB antenna with filtering characteristics. The proposed antenna is of dual band-notch categories targeting for filtering WLAN (5.15–5.825 GHz) and ITU (8.025–8.4 GHz). The antenna basic structure is composed of a rhombic radiating patch fed by a 50Ω Co Planar Waveguide (CPW) feed line and a simple ground plane. Two rectangular slots are cut from the two sides of ground plane which enhance the antenna impedance bandwidth. Subsequently, two novel slots are proposed to be cut from the radiating patch and feed line making the antenna capable of filtering WLAN from UWB. Also, two slots are launched at the top of the antenna structure aiming to disturb the antenna surface current at 8.2 GHz and notching the frequency range of ITU. To scrutinize the performance of the proposed antenna, a test case has been established in Ansoft High Frequency Structure Simulator (HFSS) where extensive electromagnetic simulation studies have been conducted. Also the proposed antenna has been fabricated and then tested to verify its real response. The obtained results certify its outperformance in designated communicating bands. The remainder of the paper is organized as follows: In Section 2, the antenna design process will be explained in detail. Section 3, addresses the performance analysis of the antenna. In Section 4, an efficient method is established to compare the presented antenna with some of the previous designs. This AHP based methodology would help in a better decision making process. Selecting the best antenna among some available candidates considering different features would be possible with the use of this method. Finally Section 5, concludes the paper.
The proposed antenna design
Figure 1 (a) illustrates the structure of the proposed antenna. As it was earlier explained, the antenna basic structure is composed of a rhombic radiating patch and also a simple ground plane. To enhance the antenna performance, two rectangular slots are cut from the two sides of the ground plane. Subsequently, two novel joint slots are proposed to be cut from the radiating patch and feed line. The censored slot in patch looks like a semicircular tube connected to simple rectangular one in feed line. Also, two mixed slots based on rectangles and semicircular tubes are launched at the top of the antenna structure. The detailed information regarding the size of overall components of the proposed antenna is demonstrated in Fig. 1(a) where the units are all in mm. The fabricated prototype of the presented antenna design is also illustrated in Fig. 1(b).
Performance analysis of the investigated antenne
VSWR curves
With the aim of interrogating the antenna performance more accurately, a test case has been established in HFSS and the design process is divided into four steps. The steps and their corresponding simulated VSWR curves are presented in Fig. 2. As it is shown, the antenna in step 1, with a simple rhombic radiating patch and a simple ground plane, would exhibit poor impedance matching at lower frequencies and hence does not fulfill UWB frequency band requirements. In step 2, two rectangular slots with dimensions of 5×1 mm2 are cut from the antenna ground plane. This revision has led to enhancing impedance matching both at lower and higher frequencies. The bandwidth enhancement process is explained using surface current distribution in Fig. 3(a). As it is depicted in this figure, after removing the rectangular slots from the ground plane, most of the current is concentrated around these slots which makes a path for the current and by producing new resonances leads to the bandwidth enhancement. The investigated design in step 2 includes the frequency range of 3.5–18 GHz. By adding some filtering elements, there might emanate some interesting characteristics in the antenna overall performance. Thus, in steps 3 and 4 the filtering elements are suitably added. The envisaged filtering elements embedded in the antenna body are two novel joint slots on the antenna radiating patch and feed line as well as two other mixed slots created by the addition of new paths on top corners of the antenna. It will be discussed in next sections that by including the filtering slots, the surface currents would flow in contradictory directions at the central frequencies of 5.5 GHz and 8.2 GHz which in turn results in notching respectively WLAN and ITU frequency bands from UWB. To validate the simulation studies, a fabricated prototype of the antenna has been measured and the corresponding VSWR waveform is denoted by “fabricated” in Fig. 2. There is a slight mismatch between the simulation and experimental results. This close agreement certifies the well performance of the proposed antenna which caters a suitable option in UWB applications. According to the measured results, the presented antenna operates in the frequency band of 3.1–18.4 GHz with WLAN (5.15–5.825 GHz) and ITU (8.025–8.4 GHz) filtered.
Filtering mechanism by embedded slots
This subsection is to study the filtering mechanism provided by the embedded slots in the proposed antenna. The surface current distribution on the slots in both radiating patch and feed line is demonstrated in Fig. 3(b). It can be seen that at 5.5 GHz, the current is flowing in contradictory directions on the slots and also on the feed line and ground plane. This intrinsic behavior leads to notching of WLAN 5.15–5.825 GHz. Also, the band stop function at ITU is addressed in Fig. 3 (c). The two slots censored at the top of the antenna, form two paths with opposite surface currents as well. For these slots, the filtering property is achieved at 8.2 GHz. As it was stated before, the inclusion of the slots in the radiating patch and feed line and also the creation of two paths at top corners of the antenna brings about the dual band notched function. It is well-recognized that the position of the notched bands is a function of the slots dimensions. Hence, by tuning the dimensions of the filtering elements, the notched bands would be tuned exactly for interfering with WLAN and ITU frequency bands.
Parametric analysis on embedded slots
In order to tailor the effect of changes in dimensions of slots on the antenna performance, a parametric analysis has been carried out regarding different parameters on the antenna. Ls as the length of slot in feed line is the first parameter considered herein. The value of this parameter is altered in three different cases and for each case the VSWR results are shown in Fig. 4. With respect to this figure, it can be inferred that when Ls is increased from 3.25 mm to 4.25 mm by a step of 0.5 mm, the first notched band’s position tends toward the lower frequencies and the second notched band remains expectedly almost constant. By fixing Ls at 3.75 mm, WLAN 5.15–5.825 GHz is completely filtered and the probable interference is fully overwhelmed. In the second analysis, two other parameters namely R1 and R2 are considered as the effective radius of the semicircular tube slots on the top corners. As declared before, the optimum values of these parameters are 6 mm and 5.5 mm respectively. However, again in three different sets of R1 and R2 values, the VSWR curves have been extracted in Fig. 5 to explore their effects on the antenna performance. Simulation results corroborates complete blockage of ITU in the case of settling R1 = 6 mm and R2 = 5.5 mm.
Radiating patterns, antenna gain and group delay evaluation
Gain of the proposed antenna is plotted in Fig. 6. With respect to the obtained results, it is clear that the antenna gain varies between acceptable values outside the notched bands. In 5.15–5.825 GHz and 8.025–8.4 GHz where WLAN and ITU are notched, gain of the antenna has sharply dropped to negative values which confirm the weak performance at the filtered frequencies. To clarify the radiating features of the proposed antenna, radiation patterns have been also plotted in Fig. 7. The patterns are studied in E and H planes at 7 and 12 GHz as sample frequencies. As expected, omnidirectional radiation patterns with low cross polarization levels suitable for UWB applications are obtained. As it is seen, co and cross components do not interfere with each other. The simulated group delay of the antenna is seen in Fig. 8. It is clear that group delay variation is less than 1 nano seconds over the entire frequency band, so, it is suitable for UWB applications.
AHP based methodology to compare the proposed antenna with previous designs
To provide a better decision making capability regarding different operational aspects of antennas and hence reaching the most suitable design, an efficient methodology is proposed. This novel methodology is based on Analytical Hierarchy Process (AHP) framework that is one of the Multi Attribute Decision Making (MADM) techniques. AHP is a powerful tool enabling the most suitable selection amongst the available options considering more than one feature. The capability of AHP in taking into account different features simultaneously is a salient merit which makes it a very useful method. To apply AHP approach to a certain problem, first, hierarchy schematic of the problem is to be clarified. The hierarchy schematic includes three levels: goal, attributes and alternatives. Goal is the purpose that is pursued in the problem. Attributes are typically the most important features influencing the overall performance of the possible alternatives, and the alternatives are the candidates that are being compared.
The hierarchy schematic is shown in Fig. 9. Here “Selecting the best antenna structure” is pursued. To investigate the presented antenna’s performance with respect to some of the previously designed antennas,a comparison is carried out. Antennas in [9–11] are selected to be compared with the present antenna regarding their size, bandwidth in UWB form, gain variation out of the notched band and the number of filtered interfering bands. The characteristics of the mentioned antennas are summarized in Table 1. Simple Additive Weighted (SAW) method is used to establish a comprehensive utility function for evaluating the alternatives. In SAW a utility value is assigned for each of the attributes namely USize, UBandwidth, UGain variation and UNotch in assessing the alternatives. The assignment of utility values for attributes has been based on intrinsic physical and expert knowledge features. Specifically speaking for an antenna, small size, wide bandwidth, low gain variations and more notched bands are preferred. A normalizing system is used to simplify the calculations. For instance, in the case of size, the antenna having the smallest size gets the most value of unity and the antenna with the biggest size has the minimum share of the unity value. Herein, the proposed utility function is shown in Equation (1). In Equation (1), WSize, WBandwidth, WGain variation and WNotch are the weights given to size, bandwidth, gain variation and number of notches respectively. Weight assignment is done according to the importance degree of each attribute.
For instance, in some applications, having small sized antenna is very important. In such cases, the attribute “size” gets higher weight value than the other attributes. It should be noted that in each case, the sum of the weights is equal to unity. Expert Choice software is speculated here to implement the AHP method [12]. Table 2 shows the obtained utility values for the four antenna structures for different weight allocations. In case 1, all the four attributes have the same weight values, meaning that the attributes are at the same level of importance. In this case, as results indicate, the antenna in this work has the most value and better performance than the other three. In case 2, for instance, size is considered twice important than the other attributes. Simulation results show that the proposed antenna exhibits better performance likewise. As shown, in case 4, where the gain variation is more important than the other attributes, antenna in [9] has exhibited better performance. In all the other cases, except case 4, the presented antenna has the best performance. The obtained utility values are plotted in Fig. 10. Based on the foregoing discussion, the proposed approach would help to determine the best possible antenna design suitable for a specificapplication.
A novel UWB antenna structure, capable of filtering WLAN and ITU interfering frequency bands is proposed. The antenna in its UWB form consists of a rhombic radiating patch and a simple ground plane. By the inclusion of two pairs of novel slots, the surface current is disturbed and the filtering property has been achieved. Also, a novel framework is proposed for the comparison of antenna structures considering their features concurrently. This method is based on AHP technique. The obtained results by comparing the proposed antenna with three others available in literature, revealed the well performance of the introduced antenna. The proposed configuration, equipped with marvelous merits such as small size, wide bandwidth, good radiation properties and the ability in canceling the interfering bands, is a suitable antenna forcommunication systems.
Footnotes
Acknowledgments
This work was supported by a research grant provided from Urmia Branch, Islamic Azad University, Urmia, Iran which is gratefully acknowledged.