Comparative Analysis on Antenna Balun and Feeding Techniques of Step Constant Tapered Slot Antenna

This paper represents the performance analysis of the different shapes of antenna balun and feeding techniques for step constant tapered slot antenna. This work also addresses the benefits of antenna balun (circular and rectangular) along with two types of feeding techniques (Microstrip line L-shape and Microstrip line I-shape). The performance of the antenna for each technique is thoroughly investigated using Computer Simulation Technology (CST) Microwave Studio software simulation under the resonant frequency of 5.9 GHz. Results demonstrate that the proposed model is an effective tool for improving antenna performance. Moreover, an extensive comparison has been carried out between the two different shapes, with and without antenna balun and between two feeding techniques focusing on return losses, gain, directivity, and voltage standing wave ratio (VSWR).


Introduction
Recently, different types of antenna have been manifested in the applications of ultra-wideband wireless communication [1]- [8]. Among the various types of antenna, tapered slot antenna (TSA) is one of the most important, which exhibits a wide operational bandwidth, improves gain, and identical radiation pattern. A typical Tapered Slot Antenna (TSA) consists of a tapered slot cut in a thin film of a metal plate with an electrically thin substrate on one side of the film [9] [10] [11] [12]. Numerous research work has been conducted for improving the performance of the TSA antenna and utilizing this antenna in wireless communications, remote sensing, biosensing, ground-penetrating radar (GPR) system, etc. [13]- [20]. Due to the technological revolution, wireless communication is increasing at an exponential rate and the efficiency of the wireless system depends on these antennas that can efficiently radiate and accept UWB signals [21] [22] [23] [24] [25]. Tapered Slot Antenna was first introduced in 1979, and the basic three types of TSA antennas are linearly tapered slot antenna (LTSA), constant width slot antenna (CWSA), and Vivaldi antenna (VA) [26] [27] [28] [29] [30]. Tapered Slot Antennas are a traveling-wave antenna, whose voltage or current distribution can be represented by one or more traveling waves. TSAs contain high directivity and narrow beam width because of the traveling wave properties [31] [32] [33]. Additionally, it has almost symmetric E-plane and H-plane radiation patterns over a wide frequency band as long as antenna parameters like shape, total length, dielectric thickness, and dielectric constant are selected appropriately. TSAs have a lot of benefits over conventional antenna such as broadband operation, low sidelobes, planar footprints, and simple in fabrication [10] [31] [33]. It also shows a good match at both sides i.e. input side (transition from the feed line to slot line) and the radiation side (transition from the antenna to free space) of the antenna, then a TSA has a large bandwidth. At high operating frequencies (greater than 10 GHz), tapered slot antennas are also suitable, where a long electrical length corresponds to a considerably short geometrical length. Despite the above benefits, TSA has some limitations such as only linear polarization can be obtained with conventional geometries [10] [11] [12]. This paper will describe the effect of with and without antenna balun and two different feeding techniques for the step constant tapered slot antennas (STSAs) and will find the optimum results.
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Antenna Design
TSAs can be classified into three basic taper profiles such as linear tapered slot antenna(LTSA), constant width slot antenna (CWSA), and non-linear taper slot antenna (i.e., exponentially tapered slot antenna or Vivaldi) [34]. Many taper profiles exist for normal TSA [34]. Each antenna differs from one another only in the taper profile of the slot. Only the Vivaldi and linearly tapered profiles have been thoroughly studied over the past few years. But this thesis paper works on a step-constant tapered antenna. The basic architecture of the proposed step constant tapered slot antennas is shown in Figure 1.
The following parameters are used to design the proposed step-constant tapered slot antenna. All the antenna parameters are calculated thoroughly and listed in Table 1.  Step length (SL) 8.0 Slot width (S) 0.4113 Step width (SW) 3.9 Slot length 5.5 Total length (L) 70.5 Circular Balun Radius (R) 2.5 Total width (W) 58.7 Substrate Thickness (h) 1.5

Antenna Balun
Antenna balun plays an important role to design an antenna. The antenna balun may be different in shape. They may be circular, rectangular, square, etc. In this paper Figure 2 shows the structure of an antenna with and without a balun. In this design, circular and rectangular shape baluns are used. Between the two shapes balun, circular shape balun provides better results. The radius of the circular balun is 2.5 mm.

Feeding Techniques
A feed line is used to excite/radiate the antenna by direct or indirect contact.
Step constant tapered slot type antennas can be fed by different methods. In this work, two types of feeding techniques such as Microstrip line L-shape and Microstrip line I-shape are analyzed. For fabrication Microstrip line feed is one of the simplest techniques as shown in Figure 3. It is easy to model. But, the downside of this technique is, in case of increasing substrate thickness the surface wave and spurious feed radiation increases that limit the bandwidth [35] [36] [37].

L-Shape Feeding Technique
In the L-shape feeding technique, the feeding microstrip-line is placed on the right side of a substrate, which is shown in Figure 4. In one direction, the strip is 0.4 mm wide and 7 mm long in the beginning part. Moreover, the strip is 0.4 mm wide and 10 mm long in the second part. At the beginning end of the microstrip-line, a waveguide port is connected between the microstrip-line and the substrate. The feeding position and the width of the feeding line should be optimized in such a way that, optimum simulation results can be achieved.

I-Shape Feeding Technique
In this type of feeding technique, a conducting strip is directly connected to the slot line and the microstrip line is placed on the bottom side of the substrate, which is shown in Figure 5. The conducting strip width is thick as compared to    Figure 6 illustrates the L-shape feeding technique. In this technique, the feeding microstrip-line has four segments. At first segment, the strip is 0.4 mm wide and 3.5 mm long horizontally. The strip is 0.2 mm wide and 3.5 mm long in the second part. In the third segment, the length and width are the same as the second segment but this is aligned in the vertical position. Finally, triangular shape balun is connected with the vertically aligned segment.

Simulation Results
The simulated results on with and without antenna balun and three types of feeding techniques are bringing together in the following figures.

Return Loss
Return loss is one of the key factors for designing as an antenna. Return loss calculates the effectiveness of antenna power which is radiated from the source to the antenna [35] [36] [37]. Figure 7 represents the return loss for with and without antenna balun and L and I-shape feeding respectively. For circular shape, the balun antenna provides the minimum return loss and for the I-shape feeding technique the return loss is increasing with the increase in step size but it is not the same for the L-shape and L-shape with balun feeding techniques. Therefore, it will be effective to take the optimum result among the three feeding techniques.
After observing Figure 7 it is clear that the return loss for circular shape balun has a better result than two others and L-shape feeding is greater than the I-shape feeding technique at the same operating frequency of 5.9 GHz. Where the value of circular shape balun is −46 dB, rectangular shape −24 dB and without balun −26.64 dB respectively and the L-shape feeding is −46.22 dB, I-shape feeding is −25.15 dB at 5.9 GHz and L-shape with balun feeding is −49 dB at 4.6 GHz.

Voltage Standing Wave Ratio (VSWR)
Voltage Standing Wave Ratio is defined as the ratio of the peak amplitude of a standing wave to the minimum amplitude of a standing wave. The minimum value of VSWR is 1.0 [30]. The following figures show the VSWR for different balun and three feeding techniques. Figure 8 displays the voltage standing wave ratio for with and without balun and all feeding techniques. The VSWR is minimum for circular balun and L-shape feeding is 1.00.
In this paper, different step size STSAs are analyzed with and without balun and with three feeding techniques and it represents optimum results for 15-step size antenna for circular shape balun and L-shape feeding. Table 2 represents the performance of the STSA. After analyzing the performance of the antenna with and without balun, it concluded that because of using balun the antenna provides better performance than without using a balun. Between two balun circular balun gives better results. So, antenna balun decreases the return losses of antennas and enhances the directivity as well as antenna gain.
The performance parameters of the L shape feeding antenna are presented in Table 3. After analyzing Table 3 it is observed that the results of the performance parameter for different step size are increasing to a specific step size (5 steps to 15 steps) and after that if we increase the step size (15 steps to 25 step) the results of the performance parameter of STSA are decreasing. The optimum results achieved for 15-step size STSA. Table 4 displays that, the simulation results for different step size antenna for I-shape feeding, which are increasing with the increase in step size and it is approximate to 15-step size of L shape feeding technique. The best simulation result for I-shape feeding is less than the L-shape feeding technique.

Gain and Directivity
The ratio of the radiation intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna radiated isotopically is defined as gain (IEEE 1993, p. 547). The directivity of the proposed model is shown in Figure 9.

Radiation Pattern
The radiation pattern is usually a graphical representation of the radiation properties including power flux density, radiation intensity, field strength, and polarization as a function of angle [35] [36] [37]. The radiation pattern for the different types of antenna patterns is illustrated in Figure 10.
For L-shape, feeding technique the gain is 10.21 dB, Sidelobe level of −8.9 dB, and Angular width of 35.5 deg. is observed in the radiation pattern. At the same time, for I-shape feeding the gain of 9.83 dB, Sidelobe level of −9.1 dB, and Angular width of 37.1 deg. is observed in the radiation pattern.
After analyzing the above figures and comparison Table 5, it is established that the use of antenna balun is necessary for better performance of antenna which provides best return loss, gain, directivity, and VSWR than without balun. Comparing with and without balun, we conclude that the antenna with circular balun has better radiation performance among others.
After analyzing Table 6, it is established that the L-shape feeding techniques provide the best return loss, gain, directivity, and VSWR than others. Comparing the feeding techniques, we conclude that the L-shape feeding has better radiation performance.

Conclusion
This paper has examined the performance of step constant tapered slot antenna with and without balun along with three types of feeding techniques and comparisons are made among them in terms of S11 parameter, VSWR, directivity, gain, and sidelobe level. The optimal quantity and perfect matching of the antenna have been found by varying the key parameters such as width and position of the feeding considering the fifteen steps constant antenna. Additionally, an extensive simulation has been done considering the effect of return loss, radiation pattern, VSWR. Numerical results tell that the selection of balun and feeding techniques is an effective tool because it affects the antenna parameter and the proposed antenna works well at the 5.9 GHz operating frequency.