Comparative Study of a Bowtie Antenna in THz Region

Transcription

1 Comparative Study of a Bowtie Antenna in THz Region 1 Abhishek Kumar, 2 Ajay A Bharadwaj, 3 Darshan S Patil, 4 Harshith Raj 1,2,3,4 Department of ECE, Sir M Visvesvaraya Institute of Technology Abstract THz technologies are expanding into much broader applications in recent years. THz range offer the possibility for wireless transmission of very high data rates. There is great potential in high data wireless applications in environments with difficult cabling and size/weight constraints. Bowtie antenna are suitable for use in the terahertz region. In the following paper; first a bowtie antenna was designed, following which a comparative study has been performed by varying substrate dimensions to observe the effect of variation in substrate parameters. The design has been simulated on the HFSS software and the corresponding performance characteristics have been found. The main objective of this study was to arrive at a bowtie configuration which would give us best receiving performance. Our design frequency was 330GHz. The design frequency was chosen so that it would be able to replace a bolometer for IR radiation detection and detect the presence of water. Keywords:Bowtie antenna, THz range antenna I. INTRODUCTION The essence of Modern-day Communication Systems is being able to transmit and receive signals between any two places or people. A signal is an electric current or electromagnetic field used to convey data/information from one place to another. Now, the devices that are used to transmit and receive these electromagnetic pulses are known as antennas. Antennas can be split up into numerous different types based on their applications, properties and shape. This research project is based on the Bowtie antenna and its operation and application in the frequency range of 0.1THz 10THz and specifically for the frequency of 330 GHz. A bow-tie antenna consists of two wire triangles, or two triangular plates, arranged similarly to a bow-tie, which is fed at the gap between the triangles. It is a wideband antenna used mostly in the very-high frequency and ultra-high frequency ranges.the bow-tie antenna which has been designed can be used to replace a bolometer in earth sensors which are used for navigation purposes in geostationary satellites. While the satellite is orbiting around the earth at approximately 35,800 kilometers, the earth acts as a black body, emitting radiation in the form of infrared waves. These infrared waves indicate heat energy and the infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz THz. At this point it is important to note that the temperature in outer space in the vicinity of the satellite will be around 3-4 degrees kelvin. But, the temperature of the earth is around 298 degree kelvin. So, the infrared radiation coming from the earth will be large compared to outer space. The bolometer is responsible for receiving these infrared waves. A bolometer is a highly sensitive instrument used for detecting heat or electromagnetic radiation. This instrument was invented by Samuel Pierpont Langley, an American astronomer, in A bolometer was initially used in combination with a telescope to quantify infrared radiation. It consists of an absorptive element made up of a thin metal layer. The absorptive element is connected to a thermal reservoir via a thermal link. When a radiation strikes the absorptive element, its temperature is increased above the reservoir s temperature due to the absorption of radiation. This change in temperature can be directly related to an increase on the resistance of the bolometer which in turn leads to an increase in the voltage (as V=IR). The two sets of bolometer kept at a specified distance from each other will help us in determining the position of the satellite with respect to the earth. The amplitude of the voltage signal will give us the position and orientation of the satellite with respect to the earth, and the team responsible for its navigation will use these values to best position the satellite. As the bowtie antenna has been designed within the infrared frequency range, it may be able used in the place of the bolometer. Another application of the 330 GHz bowtie antenna would be the detection of water in the ground, on other planets, and can even be used to detect the presence of absorbed water in vegetation and everyday electronics. This is because water has the property of absorbing radiations of this particular frequency (330GHz and 660GHz). Say that a signal ranging from 0.1THz - 10THz is transmitted towards a target at which one would like to detect the presence of water. When the reflected signal is received, it is noticed that in the plot Frequency Vs Radiation power, there is a dip at 330GHz (this dip in the graph indicates thepresence of water at the target).the above-mentioned applications are those of 74

2 the silicon lens bowtie antenna which will be designed and analyzed following the comparative study. It is a receiving type antenna which is excited with a plane wave.it is important to first understand how the substrate thickness and dimensions affect the radiation plot or performance of the antenna. Following which, the arrangement can be mounted on a silicon lens. HFSS (high frequency structural simulator) is the software used to design antennas. It is an industry standard simulation design tool for 3-D electromagnetic field simulation. It is essential for the design of highfrequency and high-speed components. This paper is basically a comparative study between various alternate designs of the bowtie antenna. The dimensions of the substrate have been varied and the different radiation patterns have been generated and observed. A bow-tie antenna is a broadband antenna and is simple to design and fabricate. It is used applications which requires larger bandwidth. It is most commonly used in UHF range. It is possible to realize the design and structure of a bow-tie antenna for THz applications. They are commonly used for UHF television antennas where several elements may be arrayed and placed in front of a flat screen reflector. II. ANTENNA DESIGN The antenna is designed and simulated on HFSS(high frequency structrural simulator) software.the antenna model has been designed using a quartz ( r =3.78) substrate. It is placed on a rectangular substrate of various thickness in the order of millimeters. The antenna elements are assigned a finite conductivity of 5.8X10 7 siemens/m which is same as the conductivity of copper.the structure of the antenna is shown in Fig. 1. Fig. 1-3D representation of basic bow-tie arrangement The antenna has been designed at frequency f 0 = 330GHz. The physical parameters have been calculated : The value arm length (L) should be close to ¼ wavelength of the medium, taking into account the dielectric constant of the quartz medium (ε r =3.78). Where, The wavelength of the medium can be calculated by: Main lobe is the radiation lobe containing the direction of maximum radiation. The lobes adjacent to the main 75 λ = c f ε r λ Wavelength of the substrate medium ε r relative permittivity of the substrate c The velocity of light f The designed frequency On calculating, we obtain the value of λ (wavelength) as mm. Accordingly, the value of arm length would be close to mm. We have taken it as 0.1mm. The value of width (w) of the bowtie is also taken to be close to ¼ wavelength, so we take it as 0.1mm also. The feed length (x) must be close to ½ wavelength of the medium and is taken as 0.2mm. We shall assume the feed width (y) to be 0.02mm and the feed gap to be 0.03mm. These calculated dimensions have been listed: Table I -Mentions the values of the physical parameters of the bow-tie arrangement PARAMETERS Arm length (L) Width (w) Feed length (x) Feed width(y) Feed gap (gap) DIMENSIONS 0.1mm 0.1mm 0.2mm 0.02mm 0.03mm III. COMPARATIVE STUDY The comparative study is performed to obtain the parameters in such a way that the radiation pattern has least side lobes and back lobes and other desirable features for good reception. Such as front to back ratio, radiated power and beam area, directivity, and beam area.the antenna is being designed as a receiving antenna. It needs to have low directivity. This is because, the reception area needs to be high when being used for long distance applications. If it has high directivity, then it would only receive the incoming signals ata small-range of angles, which is not suitable for long distance applications such assensors used in satellites. Low directivity implies higher beam area (or beam solid angle) defined as the integral of the normalized power pattern over a unit sphere. Hence, our antenna s performance will be better if it has low directivity and higher beam area. Front to back ratio is the ratio of received-signal strength when the antenna is rotated 180. It is desired that front to back ratio is high. Max U is nothing but the maximum amount of radiated power per unit solid angle. Radiated power is simply the power radiated by the antenna. Antenna efficiency is directly proportional to radiated power. Hence, it is desirable to have a higher value of radiated power.

3 lobe are called side lobes. These side lobes represent unwanted radiation in undesired directions. In transmitting antennas, excessive side lobe radiation wastes energy and may cause interference to other equipment or to the main lobe. This even means that classified information may be picked up by unintended receivers. In receiving antennas, side lobes may pick up interfering signals, and increase the noise level in the receiver. In the case of an antenna array, the side lobes might interfere with the main lobes of adjacent antennas. Hence, the side lobes must be minimized. The designed parameters of the bow-tie antenna are kept constant. The initial designed values are taken as reference from table. Only the physical parameters of the substrate of the antenna have been varied and the results have been observed on the HFSS for different variations. The parameters that have been varied one at a time and are namely the thickness of substrate (t) and substrate dimensions(l*w). A. Variation in substrate dimension Keeping the thickness of the substrate constant at 0.3mm, the substrate dimension has been varied and the output was observed for different variations. The values have been summarized in table mm x 0.8mm On setting the dimensions of substrate at 0.8mm x 0.8mm, we observe that the directivity is of an average value. The radiated power is slightly high. But the front to back ratio is quite less. From the Fig. 2 it is observed that the side lobes are diminished. On decreasing the dimensions of substrate to 0.6mm x 0.6mm, we observe that it has the least front to back ratio, directivity and radiated power. The beam area is higher when compared to the other values due to the directivity being less. The 3D radiation polar plot is observed in Fig.2.2. Fig D polar radiation plot when substrate dimensions are 0.6mm x 0.6mm mm x 0.7mm Now the dimensions are kept at 0.7mm x 0.7mm. This results in a very high front to back ratio, which is desired. The directivity is optimum value. Hence it can be concluded that by keeping the dimensions at 0.7mm x 0.7mm the optimum values are obtained as shown in Fig 2.3. Fig D polar radiation plot when substrate dimensions are 0.8mm x 0.8mm mm x 0.6mm Fig D polar radiation plot when substrate dimensions are 0.7mm x0.7mm Table II - Compares antenna parameters for different substrate dimensions Antenna Substrate Dimensions Parameters 0.8mm x 0.7mm x 0.6mm x 0.8mm 0.7mm 0.6mm Max U(W/sr) E E E-010 Directivity Radiated Power(W) E E E-010 Front to Back Ratio Beam Area(dB) B. Variations in Substrate Thickness The substrate thickness is now varied and the pattern is observed and antenna parameters are calculated using HFSS for different variations. We are keeping the substrate dimensions fixed at 0.7mm x 0.7mm. 1. t = 0.2mm 76

4 For a thickness of 0.2mm there is a very large back lobe resulting in very low front to back ratio. The directivity is found to be slightly less when compared to other variations. The Fig.3.1shows the pattern for the above thickness. Fig D polar radiation plot when substrate thickness is 0.2mm 2. t = 0.3mm For thickness 0.3mm we obtain a very high front to back ratio. The side lobes are also less. The directivity and radiated power are larger when compared with other variations. Hence, for 0.3mm we obtain best values of antenna parameters. Fig D polar radiation plot when substrate thickness is 0.2mm 3. t = 0.4mm When the thickness is further increased to 0.4mm the directivity is decreased. Hence it is observed that 0.3mm thickness will have the highest directivity. Hence it is preferred to fix thickness to 0.3mm. The Fig. 3.3 shows the pattern for 0.4mm. Fig D polar radiation plot when substrate thickness is 0.4mm The table 3 shows the antenna parameter values for different substrate thickness. It summarizes the comparative study for variation in thickness of the substrate. Table III -Compares antenna parameters for different substrate thickness Antenna Substrate Thickness Parameters 0.4mm 0.3mm 0.2mm Max U(W/sr) E E E-010 Directivity Radiated E E E-010 Power(W) Front to Back Ratio Beam Area(dB) IV. CONCLUSION By the above comparative study, it is observed that for the substrate dimensions as 0.7mm x 0.7mm and thickness 0.3mm, the most optimum parameter values are obtained. This has the best front to back ratio, directivity, radiation intensity and other parameters. The table 4 mentions the antenna parameters of the optimum design. We have thus arrived at a configuration which gives the best performance characteristics for the bowtie antenna in the terahertz region. Table IV - Mentions the antenna parameters of the optimum design. Antenna Parameters Parameter Values Max U(W/sr) E-009 Directivity Radiated Power(W) E-009 Front to Back Ratio Beam Area(dB) V. BEHAVIOR OF ANTENNA AT FREQUENCIES 0.1THZ AND 1THZ The same designed antenna parameters have been taken and simulated for frequency 100GHz. The Fig 4.1 shows the pattern at 0.1THz. At 0.1THz the directivity is extremely low and there is a high possibility of receiving unwanted signals. This enables us to use the antenna at 0.1THz but with higher noise levels. The noise levels are high because the signal can be received at almost all directions. 77

5 Fig D radiation pattern at 0.1THz The model was simulated with solution frequency of 1THz and the Fig. 4.2 shows the 3d pattern of radiation. The directivity value is found to be very high. This allows us to use the model at frequency 1THz with high directivity. This means we can receive only at particular directions Fig D radiation pattern at 1THz VI. FUTURE WORK Now that the behavior and radiation plot of the bowtie antenna has been studied, the future work would be to develop the bowtie antenna with the use of the silicon lens. This silicon lens bowtie can be used in place of the bolometer and can be used for water detection purposes. An array of these bowties can be designed such as to increase the overall bandwidth of the antenna which can lead to the utilization of the antenna in a wide range of applications. This will allow us to use this arrangement for pure THz applications at around 10THz or even greater. The THz range antennas have a wide range of applications in our life. Since the size is in the order of nm (Nano-meters), it can replace the existing antennas in many small/portable devices such as smartphones etc. Also, the THz range can be used in weapon detection, or locating criminals who are hiding in a small region by using the property that water absorbs 330GHz radiation. By careful consideration, it is possible to design a bowtie array for the above given purposes. It is also possible to go even further in the existing study and try to find an even more efficient configuration by testing the antenna for different dimensions of the bowtie itself or by varying the substrate. This effort was not possible in this study due to the time constraints. But it is an important aspect of further development of this study. The bowtie antenna certainly has the potential and flexibility to be further developed for a very wide range of applications and we look forward to dealing with these designs in the future. REFERENCES [1] Hairui Liu, Junsheng Yu, Peter Huggard, and Byron Alderman. A Multichannel THz Detector Using Integrated Bow-Tie Antennas [2] K. H. Sayidmarie and Y. A. Fadhel. A PLANAR SELF-COMPLEMENTARY BOW-TIE ANTENNA FOR UWB APPLICATIONS [3] K.D Prasad, SatyaPrakashan (1999), Antenna & Wave Propagation [4] Yi Huang, Kevin Boyle,Wiley, 13-Oct- 2008,Antennas from Theory to Practice [5] J.I.Chakravarthy, P.SaleemAkram, Dr.T.VenkataRamanaDesign of Bowtie Antenna for Wideband Applications [6] M. Abri, H. AbriBadaoui, H. Dib, A. S. E. Gharnaout Bi-Band Bow-Tie Antennas Array Design Using A Simple Equivalent Transmission Line Model 78