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THz Rectangular Patch Microstrip Antenna Design Using Photonic Crystal as Substrate
Aditi Sharma, Vivek K. Dwivedi, and G. Singh Department of Electronics and Communication Engineering Jaypee University of Information Technology, Solan-173 215, India

Abstract In this paper, the eects of two dimensional photonic band gap crystals (electromagnetic crystal) substrate on the performance of a rectangular microstrip patch antenna at THz (0.640.8 THz) frequency are simulated. Photonic crystal has been used as substrate material for high gain and highly directional resonant antenna. The bandwidth and gain of the designed antenna is 13.36% and 3.852 dB respectively which is very interesting result for this small dimension antennas. The simulation has been performed using CST Microwave Studio, which is a commercially available electromagnetic simulator based on nite dierence time domain technique. 1. INTRODUCTION

The growing demand of wireless applications has presented RF engineers with continuing call for low cost, power ecient, and small size system designs. Depending on the application at hand and required system characteristics, such as data rate, environment, range, etc., the system parameters such as operating frequency transmitted power, and modulation scheme may vary widely. However, independent of the applications, compactness, wide bandwidth, high eciency, ease of fabrication, integration and low cost are always sought in wireless systems [1]. THz region occupies a large portion of electromagnetic spectrum located between the microwave and optical frequencies and normally dened as the band from 0.1 to 10 THz. There are many forms of the communication system for sending message from one distant place to another. The basic motivation behind each new form is either to improve the transmission delity, to increase the data rate so that more information could be sent, or to increase the transmission distance between the relay stations. The frequencies higher than microwaves oer many advantages for technology, including wider communication bandwidths, improved spatial directivity and resolution with system compactness. At these frequencies communication with high data rates are possible, although greater bandwidths [2]. THz communication link is most likely secure communication for short-distance, point-to-point, and demanding high information data rate (multi-Mb/s to Gb/s). The eective range for free-space transmission of THz signals is also a concern. Among the practical advantages of using THz region for satellite communication system is the ability to employ smaller transmitting and receiving antennas. This allows the use of smaller satellite and a lighter launch vehicle. One of the most important components of the wireless systems is their antenna. Using carrier frequencies above 300 GHz, oscillator and amplier sources with approximately 10% fractional bandwidth would enable very high data rate (> 10 GB/sec) wireless communications with high security protection. A photoconductive antenna is an alternative THz source because of its compactness and wide tunability at room temperature. However, the photoconductive antenna has the signicant disadvantage of low output power. This is mainly due to high impedance inherent to photomixer. When an antenna with moderate input impedance is connected to a photomixer, the power transferred from photomixer to the antenna is poor due to the severe impedance mismatching. Microstrip antennas, because of their ease of fabrication/integration as well as compactness, are highly desirable for these systems. The substrate of microstrip antennas plays a very important role in achieving desirable electrical and physical characteristics. In conventional microstrip patch antenna, the antenna placed on a dielectric substrate radiates more eciently into the dielectric substrate than the air-side. If we replace the substrate with the photonic crystal substrate, whose forbidden gap encompasses the antenna excitation frequency, in the manner that there are no surface modes, the power previously radiated into the substrate will be reected towards the air-side. Photonic crystal is a new class of periodic dielectric structures has been developed in which electromagnetic wave propagation in any direction is completely prohibited for all frequencies within a stop band [3]. A photonic band gap material or photonic crystal is an articial material made of periodic implants within a surrounding medium. Electromagnetic propagation through such a medium is aected by the scattering and diraction properties of the periodic elements. Brown and Parker [4] proposed

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increasing performance of planar antennas on a dielectric substrate, which suer large radiation loss into the substrate with only 23% of the power radiated in the air. In the microwave and millimeter-wave integrated circuits, the control of the radiation from a microstrip antenna is of great importance. In such circuits, the antennas are mounted on a semiconductor substrate, which enhance the performance and functionality of the circuit. But most of the power from the antenna on a dielectric substrate is radiated into the substrate. If a thin substrate is used to overcome the loss due to this trapping, another problem arises. A 180 degree phase shift comes from the reection at the bottom conductor, causing the radiation cancel out at driving point. These problems can be solved, if the antenna is mounted on a 2-D photonic crystal, from which the radiation will be fully reected in all directions [58]. By reducing or eliminating the eects of these electromagnetic inhibitors with photonic crystals, a broadband response can be obtained from inherently narrowband antennas [913]. It is also useful to a reduction in pattern side lobes resulting in improvements in the radiation pattern front-to-back ratio and overall antenna eciency. Agi and Malloy [14] have experimentally and computationally studied the integration of a microstrip patch antenna with a two-dimensional photonic crystal substrate [15]. The aim of this paper is to demonstrate a gain enhancement method for the rectangular microstrip patch antennas through the use of photonic band gap materials made of planar arrays of circular blocks (circular air implants) within the dielectric layers. The high gain is due to the excitation of stronger leakywave elds. The organization of the paper is as follows. The Section 2 concern with geometrical conguration of the rectangular microstrip patch antenna. The Section 3 discusses the simulated results. Finally, Section 4 concludes the work.
2. ANTENNA CONFIGURATION

Figure 1 shows the geometrical conguration of the proposed rectangular microstrip patch antenna. The substrate material of this antenna is two dimensional photonic band gap crystals. The photonic band gap crystal composed of stacked layers of cylindrical air gap of 15 m diameter at 200 m distance between the two consecutive air gaps in the dielectric substrate. The dimension of substrate material is 1000 1000 m2 and 200 m thickness with dielectric permittivity 9.1. The radiating patch has dimensions 600 460 m2 . In this model of rectangular microstrip patch antenna, we have used microstrip strip line feeding [16, 17]. The dimension of the strip line is 270 50 m with the width 36 m.

Figure 1: Geometrical conguration of the rectangular microstrip patch antenna with photonic crystal as substrate. 3. RESULTS AND DISCUSSION

Figure 2 shows the frequency versus return loss and reveals that the return loss is below 12.56 dB at frequency 720 GHz which is a center frequency of the operation range. The 10 dB impedance bandwidth as calculated from the Fig. 2 is 13.36%. The complete eld expression for the microstrip structure is in terms of a continuous plane-wave spectrum. With planar material gratings, there may exist, three dierent propagating waves. Space waves and surface waves are similar to those in

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conventional microstrip structures. Leaky waves are due to the periodic nature of the planar grating structure. The surface wave (bound wave) is a slow wave with normalized phase constant /k0 (k0 the free-space wave number) that increases with frequency. When the frequency increases to a point that the condition /k0 /q 1 holds, this bound wave becomes a leaky wave which is a fast wave for 1 space harmonic with a complex propagation constant. The far-zone radiated elds are due to the combination of space wave and leaky waves. The energy carried by bounded surface waves that propagate laterally is considered a loss. Antenna directivity, gain, and eciency are determined by the energy distribution among these three propagating waves. Antenna eciency can be maximized by partial elimination of the bound surface wave within certain directions. For high-eciency antennas, directivity and gain are determined by the energy distribution between the space and leaky waves. Leaky-wave radiation pattern is highly directive in contrast to the low directivity of the space wave. To achieve high-antenna gain, it is necessary to excite a strong leaky wave.
-5 -10

Return Loss (dB)

-15 -20 -25 -30 -35 600 683.84 GHz

650

700

750

800

Frequency (GHz)

Figure 2: Frequency versus return loss of rectangular microstrip patch antenna at THz frequency using photonic crystal as substrate.

E-Plane H-Plane

E-Plane H-Plane

(a)

(b)

Figure 3: E and H plane far eld radiation pattern of (a) gain and (b) directivity of proposed rectangular microstrip patch antenna at frequency 720 GHz.

E and H plane far eld patterns for gain and directivity at frequency 720 GHz is shown in Fig. 3. The radiation eciency, gain and directivity from Fig. 3 are 54.9%, 3.852 dB and 6.456 dBi at 36 m strip feed line width. Signicant gain enhancement is found (3.82 dB) in the E plane. It is observed that the gain decreases and the beam angle increases with increasing angle. As the

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angle approaches 90 , the leaky-wave eld is much weaker than the space-wave eld and no gain improvement is observed (the H -plane pattern is shown in Fig. 3(a)). This observation is generally true for antennas on a thin photonic band gap substrate. The results of the directivity patterns are shown in Fig. 3(b). A particular plane with the strongest leaky-wave elds is determined by the arrangement of substrate conguration. The antenna location relative to the air blocks has a signicant eect on the antenna gain. The thicker the air blocks are, the stronger the leaky wave and the higher the gain. The eects of varying the width of micro strip feed line on radiation eciency and gain is observed and shown in Fig. 4, which reveals that as the width of the strip line increases, the radiation eciency and gain of the proposed microstrip patch antenna decreases continuously. At a point near to 30 m the radiation eciency and gain is maximum, but at this point the return loss is very high, which will aect the antenna performance. As we reached the width of 36 m, the radiation eciency and gain is good and the return loss decreases to approximately 30 dB at the center frequency of operation. The experimental results of the rectangular microstrip patch antenna on photonic crystal substrate at THz frequencies are still now not reported.

Figure 4: The eects of variation of width of microstrip strip feed line on the gain and radiation eciency of rectangular microstrip patch antenna. 4. CONCLUSION

This paper explored a novel concept in the development of wideband rectangular microstrip patch antennas using 2-D photonic crystals as substrate at THz frequencies. We demonstrated that the gain of a printed circuit antenna can be greatly enhanced with photonic band gap materials as a substrate. It was found that signicant gain enhancement is achieved by exciting strong leaky waves through proper designs of the planar periodic material structure. Photonic crystals were realized to reduce and, in some cases, eliminate surface waves, which leads to an increase in directivity, bandwidth and radiation eciency. Using photonic crystals or we can say air gaps, promising results have been obtained by simulation through CST Microwave Studio. We will report the theoretical results as well as optimize the geometrical parameters to obtain the optimum electrical parameter of this proposed rectangular microstrip patch antenna at THz frequency in another future communication.
REFERENCES

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