Broadband Koch Fractal Boundary Printed Slot Antenna for ISM Band Applications

A new broadband radiating slot antenna with fractal shape is modeled, fabricated and experimentally studied. The presented slot antenna is examined for first three iterations. Optimization of iteration factor (IF) and iteration angle (IA) have been done for each iteration order (IO) to enhance the impedance bandwidth significantly. All the antennas are fed with a simple microstrip line. Bandwidth achieved with Antenna 1 (IO=1, IF=0.35 and IA=600) is 1550 MHz which is five times more than that of the square slot antenna. The performance of the proposed fractal slots is also compared with the rotated slot antenna. The experimental data validates the reported analysis with a close agreement.


Introduction
Recent advancements in the wireless communication systems field require compact size and broadband microstrip antennas [1].Printed circuit antennas have been thoroughly enquired in the last two decades.Among them slot antennas have played an important role for a variety of radar and satellite communication applications.The broad bandwidth, less radiation from feeding network, good isolation and less interaction via surface waves are the advantages of slot antennas.Detailed description, mathematical modeling and analysis of a microstrip slot antenna have been thoroughly studied [2].Slot antennas with different shapes such as square [3], circle, ellipse, triangle and E-shape are reported for wide bandwidth.A printed wide-slot microstrip antenna feeding with a microstrip line having a fork-like tuning stub is mentioned for bandwidth improvement [4] and [5].Bandwidth enhancement is achieved by a rotating the slot in the center of a square [6].A slot and a monopole antenna printed on a single substrate is suggested for broadband operation [7].The antenna structure mainly composed of a rectangular slotted patch and a metallic reflector is proposed by Chen et al for wideband operation [8].A microstrip-fed reconfigurable eccentric annular ring slot antenna with switchable polarization is demonstrated by Sim et al [9].A two slot antenna for long-term evolution usable on the metal body handsets is studied by Chang et al [10].
Fractal concept is best depicted and yielded by employing an iterative procedure that results to self-similar and self-affine structures.Fractals display space-filling attributes that can be utilized to cut down the classical antenna elements size, such as dipoles and loops, and thus subdue some of the restrictions of small antennas [11].Fractal patches are studied for multiband or wideband applications [12]  Here, the authors introduce a microstrip line-fed koch fractal radiating slot antenna for bandwidth enhancement.The analysis is carried out for different IF and IA, Koch fractal slot is used.In this work using fractals the path of the current distribution has been changed.This led to the change of lower edge of the operating frequency.Such a significant change in bandwidth is because of the change in the lower and upper edges of the working frequency band of the wide slot antenna.For a slot antenna and with the rotation of the slot, always the secondary resonant frequency is dominating one.On the other hand, in a fractal slot antenna, with the increase of iteration order the dominant changes are from second resonance frequency to first resonance frequency.The proposed antenna is analyzed for linear polarization.There is a considerable enhancement of impedance bandwidth with this fractal slot antenna when compared with the square slot and rotated slot.FDTD analysis for the first two iterations is presented.The measured data agrees well with the theoretical results.

Antenna Configuration
In order to generate two modes with close resonant frequencies to enhance the impedance bandwidth, initially a square slot antenna is introduced.A Koch curve is generated as indicated in Fig. 1.The straight line is divided into three equal sections.The central section is replaced by two lines at an angle (IA).This process is repeated up to IO three to get fractal shapes.

Results and Discussion
To observe the performance, all the antennas are fabricated.Fig. 3 shows the prototype of fabricated antennas.All the simulations are carried with the Finite Element method based HFSS software and are presented in Fig. 4.
In order to achieve significant enhancement of bandwidth, optimization of IF is done for the slot antennas.As depicted in the Fig. 6, with the increase of IO and as shown in the Fig. 7, with the change of IF for the same IO 3 there is a change in the calculated bandwidth.The shift of the primary resonant frequency is upward for Antennas 1 and 2, where as for Antenna 3 it is downward in Figure 6.The reason behind this is with the increase of IO, electrical length of the slot also increases and so lower frequencies become more dominant [19].The comparison of Antenna 2 measured reflection coefficient with FDTD result is given in Fig. 8.As the number of IO increases, the antenna becomes more compact and the fabrication process becomes a bit difficult.So IO is limited to only three.The reflections coefficients of the entire intended slot antenna are measured with HP 8719A (130MHz -13.5GHz)Network Analyzer.The radiation patterns are measured in standard anechoic chamber of a defense organization.Electric current distribution of Antenna 3 at 1.8, 3, 4 GHz frequencies is indicated in Fig. 9. Strong current distribution is observed along the fractal boundary at 1.8 and 3 GHz frequencies, whereas at 4 GHz frequency weak current distribution is noticed.The comparison between numerical results and measured data is given in Fig. 8. E and H plane patterns of Antenna 3 are measured at four different frequencies as depicted in Fig. 5. Table 1 summarizes the simulated and measured results of the fractal slot antenna.From the simulation results it can be inferred that Antenna1 with IO=1, IF=0.35 and IA=60 0 gives more bandwidth (equal to 1550 MHz).From the Table 1 it is observed that while the IO increases the bandwidth decreases, resonance band is varying where the new lower band frequencies are added.Note that from the literature a printed slot antenna is a bi-directional radiator, so the measured E and H plane patterns are similar on both sides of the antenna.The performance of the proposed antenna is compared with reference antennas in Table 2.

Figure 4 .
Figure 4. Simulated reflection coefficients of the fractal slot antennas.

Figure 5 .
Figure 5. Measured E-Plane and H-Plane Co-polarization and Cross-polarization patterns of Antenna 3 at different frequencies

Figure 7 .
Figure 7. Measured reflection coefficients of the fractal slot antennas of iteration three with different IF and IA.
band square Koch fractal slot antenna has been investigated in this paper.Several fractal slot antennas are proposed by varying the IF, IA and IO.For every iteration order the IF and IA are optimized to get better bandwidth.It shows that with the increase of the IO, electrical length of the slot increases, which results lower frequency components become more dominant.The presented slot Antenna 1 generates the highest 10 dB return loss bandwidth which is about 57.5 %.Measured results are validated with the FDTD full wave analysis for Antenna 2. The proposed slot antennas are well suited for 2.4 GHz ISM band wireless applications.

Table 1 :
Summarized results of the square slot, rotated slot and fractal slots

Table 2 :
Comparison of the proposed antenna with reference antennas.