211 INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, Design of Prted Log Periodic EMI Sensor Nisha Gupta and Md. Anjarul Haque Department of Electronics and Communication Engeerg Birla Institute of Technology, Mesra, Ranchi 835 215, INDIA Tel: +91-651-2275750; Fax: +91-651-2275401; E-mail: ngupta@bitmesra.ac. Abstract- This paper describes the design aspects of a prted log periodic EMI Sensor to operate a specific band of frequencies. A parametric study is conducted usg commercial software IE3D, based on Method of Moments (MoM) algorithm. The characteristics of the proposed sensor are determed terms of return loss, radiation pattern, ga and Antenna factor. A prototype with dimensions based on the simulations is built and tested. A comparison with experimental results shows good agreement. Index Terms- EMI Sensor, Prted Log periodic antenna, Antenna factor. I. INTRODUCTION The Electromagnetic Interference (EMI) noise [1] is defed as an unwanted electrical signal that produces undesirable effects a system, such as communication errors, degraded equipment performance and malfunction or non-operation. Electromagnetic terference (EMI) measurement [2-3] and its suppression are major issues most current system designs. Radio frequency Electromagnetic Compatibility (EMC) emissions tests are a common feature for EMC compliance of most electronic and electrical products. Hence an EMI sensg device is very much required for this purpose. EMI sensors have countless uses military, space, medical, and commercial applications. One particularly useful area is the test and evaluation of RF-sensitive electronic equipments used wireless devices. When measurg radiated signals, the front end of the measurement system is the sensor. The characteristic of a sensor is measured terms of Antenna Factor. Antenna factor is defed as the ratio of the cident electric field at the surface of the sensor to the received voltage at the antenna termal when termated with 50ohms load. Most commonly used EMI sensors are wire dipoles or loop antennas [4] which are effectively used up to frequency of 1 GHz. Beyond this range generally prted Log periodic antennas are a suitable option. Log-periodic antennas are generally seen as well suited structures for the design of antenna modules with theoretically frequency dependent transmission characteristics [5]. II. DESIGN OF PRINTED LOG PERIODIC EMI SENSOR Prted Log Periodic Dipole Array is a learly polarized, broadband antenna designed to operate over the desired frequency range. The choice of scalg factors, the dimensions of each element, and the center-to-center spacg of the elements play a key role obtag the desired VSWR characteristics throughout the operatg frequency range. The optimum phase relationship can be achieved by precise design of the feed and the positiong of the elements. This causes the active region, at any given frequency, to propagate RF energy towards the smaller elements leavg the elements behd it electrically dead. The reshapg of second, third, fourth and fifth element the form of triangle as shown Figure 1, improves the performance
212 INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, characteristics of the proposed Log periodic EMI sensor terms of return loss and ga compared to the conventional Log Periodic Dipole Array (LPDA). Spacg and the distance between the elements are related by the constant scale factor τ, so that L n /L n-1 = d n /d n-1 = τ Figure 1 shows the proposed structure of the prted EMI Sensor. Five elements are prted on one side of the substrate and another five on the other side of the substrate. The prted patterns of these elements are separated by dielectric thickness and are named as elements 1 to 5 startg from feed end. The transmission le consists of two strip conductors, one on either side of the dielectric substrate. By prtg one half of total elements on either side of the substrate and connectg it to the strip transmission le alternatg feed connection is obtaed. The EMI sensor is prted on a FR4 substrate with dielectric constant 4.4 and thickness equal to 1.6 mm. The dimensional parameters of the proposed sensor are as follows: Total length of the array: 206 mm. Thickness of each element: Element 1 st : 8.0 mm; Element 2 nd : 5.6 mm; Element 3 rd 3.92 mm; Element 4 th : 2.74 mm: Element 5 th : 1.92 mm. Spacg between each elements: 1 st and 2 nd : 48.25 mm; 2 nd and 3 rd : 45.03 mm; 3 rd and 4 th : 37.88 mm; 4 th and 5 th : 27.88 mm. Lengths of Each Element: 1 st Element: 173.0 mm; 2 nd Element: 115.0 mm; 3 rd Element: 81.4 mm; 4 th Element: 57.88 mm: 5 th Element: 41.42 mm. Feed Le parameters: Thickness of the Feed Le: 3 mm; Distance of feed pot to first element: 21 mm; width of the feed pot: 20 mm. and spacg between feed pot and 1 st Element: 21 mm. 3 rd Element: 3.93 mm; Height of the triangle for 4 th Element: 2.88 mm; Height of the triangle for 5 th Element: 1.43 mm. Figure 1 Proposed Log Periodic EMI Sensor III. SIMULATION AND TEST RESULTS This Sensor was simulated usg full wave IE3D commercial software based on Method of Moments (MoM) algorithm. The LPDA is designed to effectively sense the transmission the several useful band of frequencies. The simulation is carried out to obta the characteristics of the proposed sensor terms of return loss, VSWR bandwidth, pattern etc. The sensor is fabricated with the design data and the measurements are performed usg PNA series Agilent Vector Network Analyzer. The top view of the fabricated LPDA is shown Figure 2. The sensor is designed to operate at three bands of frequencies namely, 688MHz-858.59MHz, 974.46MHz-1.368GHz and 1.57GHz-2.68GHz. The experimental results are further compared with the results obtaed from the simulation for the return loss characteristics as shown Figure 3. The results show a good agreement between the two. Shaped Parameters: Height of the triangle for 2 nd Element: 5.73 mm; Height of the triangle for
213 Figure 2 Top view of Prted proposed EMI sensor Figure 4 Radiation Pattern at frequency 0.72 GHz Frequency GHz. 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8-5 -10-15 S(1,1) db -20-25 -30-35 -40 Simulated Retun Loss Measured Return Loss Figure 3 Return Loss characteristics of the proposed EMI Sensor. Next the radiation patterns of the proposed antenna are obtaed both the planes with the useful bands of frequencies namely at 720 MHz., 1.344 GHz. and 2.64 GHz. frequencies as shown Figures 4, 5 and 6 respectively. Figure 5 Radiation Pattern at frequency 1.344 GHz
214 Ga and the AF of the EMI Sensor measured at some selected frequencies are listed Table 1. It is seen that the ga of the antenna is almost constant. The constant ga of the antenna yields an antenna factor which varies learly with frequency. The variation is smooth; therefore, accurate terpolation of performance between specified frequency pots is simple. IV. CONCLUSION Figure 6 Radiation Pattern at frequency 2.64 GHz The Ga and the Antenna Factor (AF) are very important parameters of the EMI sensor. AF is defed as the ratio of cident Electric field to the received voltage. It is the most widely used parameter the EMC area. However it is one that is defitely not part of the standard antenna termology. Antenna factor reflects the use of antenna as a field measurg device or Probe. In other word AF is the factor by which one would multiply the output voltage of the receivg antenna to obta or recover the cident electric field or Magnetic field. Electric Field AF = E c /V rec (db/m) Freq. MHz Table 1 Ga and Antenna Factor of the Proposed EMI Sensor Wave Length (meter) Ga db AF= Ec/Vrec (1/m) AF (db/m) 625 0.480 13.05 4.51 13.08 850 0.353 13.91 5.56 14.90 975 0.308 14.11 6.23 15.88 1300 0.231 14.00 8.44 18.52 1600 0.188 14.20 10.15 20.13 2600 0.115 14.30 16.31 24.24 This paper presents an improved design of Log periodic EMI sensor by reshapg the prted elements. The characteristics of the sensor are determed terms of return loss, ga, radiation pattern and Antenna Factor. The results reveal that reshapg the elements help improvg the characteristics of the sensor terms of bandwidth, ga and Antenna factor. ACKNOWLEDGEMENTS The authors would like to acknowledge University Grants Commission, New Delhi for providg the fancial assistance under SAP for the development of the Antenna Laboratory, Department of Electronics and Communication Engeerg, where the simulation and measurements are carried out. REFERENCES 1. W. Ott, Noise Reduction Techniques Electronic Systems, Wiley, 1976. 2. M. Kanda, Standard Probes for Electromagnetic Field Measurements, IEEE Transactions on Antennas and Propagation, vol. 41, no. 10, pp. 1349-1364, Oct. 1993. 3. F. Krug and P. Russer, The Time-Doma Electromagnetic Interference Measurement System, IEEE Transactions on Electromagnetic Compatibility, vol. 45, no. 2, pp. 330-338, May 2003.
215 4. S. Ghosh, A. Chakrabarty, and S. Sanyal, Loaded Wire Antenna as EMI Sensor, Progress In Electromagnetics Research, PIER 54, 19 36, 2005. 5. V. H. Rumsey, Frequency Independent Antenna, New York: Academic, 1966.