TO radiate the electromagnetic energy in a very short period

Transcription

1 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH Improved Feeding Structures to Enhance the Performance of the Reflector Impulse Radiating Antenna (IRA) Majid Manteghi, Member, IEEE, and Yahya Rahmat-Samii, Fellow, IEEE Abstract This paper considers the improvement of the feeding structure of the reflector impulse radiating antenna (IRA). Full-wave analysis and measured results of the orthogonal cross-coplanar plate reflector IRA shows that the aperture fields are not uniform. The arm angle is varied as an optimization factor and it is shown that the arm angle of 70 has the maximum radiation efficiency. The termination load and the arm tapering effects are studied using simulation and measurement results. Furthermore, the effect of radius of circle of symmetry is studied and it is shown that a greater circle provides higher gain. A combination of transverse electromagnetic (TEM) horn antenna and the conical coplanar TEM transmission line is investigated to avoid tiny structure at the focal point and make the connection between the coaxial cable and the feeding arms more convenient. It is shown that a small triangle does not degrade the antenna performance but helps to excite the antenna by a coaxial cable. Finally a combination of the Vivaldi antenna and the coplanar transmission line is introduced to improve the antenna performance. The simulation results for the new antenna show that the antenna efficiency is improved to 45% at the frequency band between 2 GHz to 6 GHz in comparison to the 20.9% for the traditional design and 29.7% for the tapered design. The calculated far-field results of all these antennas are used to radiate a 0.5 ns impulse. The radiated impulse from the Vivaldi fed reflector IRA is 3.55, 2.41, and 2.12 db higher than the same radiated impulses from the reflector IRA fed by a 45 traditional feeding arms, 70 traditional feeding arms, and 70 tapered feeding arms, respectively. Index Terms Aperture antennas, impulse radiation antennas (IRAs), method of moments (MoM), ultrawide-band antennas. Fig. 1. Schematic of the traditional reflector IRA with both the conically symmetric structure parameters ( ; ; ) and the equivalent longitudinally symmetric structure parameters (a; b ; b ; D; F). parabolic reflector [1], [3]. The boresight far-field can be calculated from the aperture field distribution by [8] (1) I. INTRODUCTION TO radiate the electromagnetic energy in a very short period of time the impulse radiating antennas (IRAs) are required [1] [5]. One of the most commonly used IRA consists of a parabolic reflector fed by a self-reciprocal transverse electromagnetic (TEM) transmission line to realize a reflector IRA [6], [7]. The spherical mode that propagates through the TEM feed is converted to the plane wave by the parabolic reflector. The reflector rim usually is a circle that lies on the circle of symmetry of the TEM feed therefore half of the accepted power by the antenna radiates outside of the parabolic reflector aperture and does not contribute to the highly directive radiated field by the Manuscript received March 9, 2005; revised September 14, The authors are with the Antenna Research and Measurements Laboratory, Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA USA ( majid@ee.ucla.edu; rahmat@ee.ucla.edu; Digital Object Identifier /TAP where indicates the aperture and is the speed of light. For an antenna with a uniform aperture field the electric far-field at boresight is Equation (1) shows that the electric field in the far zone at boresight is proportional to the time derivative of the aperture field. Schematic of the traditional reflector IRA is shown in the Fig. 1. The antenna consists of a parabolic reflector with a focal length of and diameter of. A pair of crossed conical coplanar TEM transmission lines is used to realize the feeding structure. The feeding arms originate at the focal point and terminate at the rim of the parabolic reflector through the termination loads. The conically symmetric TEM feeding structure parameters are related to the equivalent (2) X/$ IEEE

2 824 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH 2006 longitudinally symmetric structure parameters through a stereographic projection [9], [10] as (3) In (3), defines the radius of the so called circle of symmetry [6]. Once the input impedance of the infinitely long coplanar TEM feeding structure is known, a hybrid calculation method can be used to calculate and ( and ) for a given (the angle between the feeding arms and the axis) [10]. In this method, first the propagating spherical mode through the TEM feeding structure is calculated numerically and then using them, the aperture field and finally the far-field patterns are calculated [11]. It has been shown that the maximum aperture efficiency can be obtained by choosing a proper set of, and [11] [13]. The maximum aperture efficiency that has been achieved with this method occurs at and [11]. This simulation method does not take into the account the interaction effects and multiple reflections between the feeding structure and the parabolic reflector. Furthermore, the mismatches and the end effects of the feeding structure are not included. All the models used for calculations and optimizations are based on the classical feeding structure which originally has been chosen for since they have a closed form of analytic solution [3]. It has been shown that the is not the best design for the feeding structure. In addition, the circle of symmetry of this self-reciprocal structure may not be the best place to locate the rim of the reflector. Due to the end effects, the connection between the feeding arms and the reflector should change to increase the aperture efficiency. Moreover, to achieve a low reflection coefficient over a broader frequency range and to reduce the blockage effect at lower input impedances the feeding structure at the focal point should change. A method of moments (MoM) based software, hybrid EFIE and MFIE iterative (HEMI), is employed to calculate the current distribution on the antenna body as well as the far-field patterns [15]. In Section II, the best angle between the feeding arms and the -axis is calculated using the MoM simulation results to increase the frequency domain gain of the antenna. The feeding structure is modified to increase the aperture efficiency in frequency domain. A comparison study of the measured and simulation results between the modified antenna and the original design is provided in Section III. In Section IV, the feeding structure is modified at the focal point to make the coaxial cable-feeding arms connection more convenient. The Vivaldi fed reflector IRA is introduced in Section V. Different variation of Vivaldi antennas are investigated to find the best design in terms of radiation efficiency. Section VI provides a Fig. 2. The 57 cm diameter reflector IRA with F=D = 0:4 mounted at the spherical near-field chamber at UCLA. comparison study between different design in terms of radiation efficiency and amplitude of the radiated impulses. Conclusion is presented in Section VII. II. NON-PERPENDICULAR FEEDING ARMS The original design of the reflector IRA utilizes a parabolic reflector with a cross conical coplanar feeding structure. We constructed the reflector IRA (Fig. 2) using a 57 cm diameter parabolic reflector with focal length of 23 cm based on the IRA-4 design with nonfloppy arms in [13], [14]. All the measured and simulated data of this antenna are presented as the traditional IRA in this paper. The calculated current distribution and the measured aperture-fields and the far-field pattern at 4 GHz are shown in the Fig. 3. The current distribution for design has more concentration around the -axis in comparison to the -axis. Therefore, the beam-width in the E-plane is wider than the beam-width in the H-plane. The angle of the feeding arms with respect to the -axis,, should change in order to achieve more uniform aperture fields. The calculated current distribution at 4 GHz for four different angles of feeding arms are presented in Fig. 4. It can be observed from this figure that as increases the current density around the -axis increases. The antenna gain at boresight versus frequency is a good measure to find the best value for. Fig. 5(a) illustrates the antenna gain at ten different frequency from 1 GHz to 10 GHz for,60,65,70, and 75. To make the difference more visible, gain of the traditional antenna with is subtracted from gain of the other antennas and the results are presented in Fig. 5(b). This figure shows that the maximum gain versus frequency can be achieved around.

3 MANTEGHI AND RAHMAT-SAMII: IMPROVED FEEDING STRUCTURES 825 Fig. 3. Measured and calculated results for the traditional reflector IRA at 4 GHz. (a) Calculated current distribution, (b) measured holographically projected aperture field, (c) measured elevation-azimuth far-field pattern, and (d) calculated and measured E-plane and H-plane far-field patterns. been suggested in [12], [13]. The calculated far-field patterns for,60,65,70, and 75 at 4 GHz are presented in the Fig. 6. It can be seen from these figures that increasing broadens the H-plane beam-width and reduces the E-plane beam-width. These calculations show that is a good value for the arm angle to obtain more uniform aperture field and similar beam width in the E-plane and H-plane. Fig. 4. Calculated current distribution at 4 GHz for different arm angles: (a) ' =60, (b) ' =65, (c) ' =70, and (d) ' =75. It means that at the antenna has a more uniform aperture field distribution in the entire frequency band of interest as has been shown in [11] with a different simulation method. Our simulation results for constant and shows that increasing produces higher input impedances. If a 200 input impedance is desired, one has to change and to have fatter arms which increases the blockage therefore has III. END EFFECT OF THE FEEDING ARMS Fig. 7 shows the current distribution on the feeding arms of a traditional reflector IRA at different frequencies. These arms are terminated to the parabolic reflector by 200 loads, which are used as low frequency matching circuits. This figure demonstrates that the current density on the coplanar feed decreases with distance from the focal point. Also, there is a standing wave effect at the end of the feeding arms. Furthermore, the calculated currents show that the current density is higher at edges of the excitation arms and has lower density along the middle of each arm. The current distributions on the feeding arms are calculated for different termination loads to study the standing wave. Fig. 8 shows the calculated current distribution on the feeding arms at 4 GHz for different termination loads: 0, 150, 300, 600, and the open circuit. The standing waves are stronger for 0 termination load and do not vary rapidly by changing the resistive termination load. Different termination loads have been tried [16] and it has been our observation that there is no resistive termination load that can match the feeding arms to the reflector and remove the standing waves. It means that there is

4 826 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH 2006 Fig. 5. (a) Calculated gain versus frequency for different arm angles. (b) Difference between gain of the reflector IRA with various arm angles with the gain of the traditional ' =45 arm angle fed reflector IRA. Fig. 6. Far-field pattern of the reflector IRA with different arm angles at 2 GHz. (a) ' =45, (b) ' =60, (c) ' =65, (d) ' =70, and (e) ' =75. some stored energy around the arm-reflector junction. Due to the stored energy at the junction, the observed impedance has both the real and imaginary parts. Since the physical structure does not change with frequency, even if the imaginary part of the impedance observed at the junction is known, it is not easy to match the feeding arms to the reflector for the entire frequency band. Furthermore, the reactive energy is distributed around the junction and as a result the imaginary part of the impedance is a distributed reactance and can not be compensated with lumped elements at the junction point. Our calculations show that the standing waves on the traditional feeding arms cannot be removed by any combination of termination lumped loads. As a practical way to reduce the stored energy around the junction, the end parts of the feeding arms are tapered. The current distributions on the tapered arms at different frequencies are presented in the Fig. 9. The antenna gain gives a good measure of the antenna performance and to study the effect of shaping of the feeding arms. Fig. 10 demonstrates the antenna gain versus frequency for both the traditional and the tapered feeding arms [Fig. 10(a)] and the difference between gain of these two antenna [Fig. 10(b)]. This figure shows that the antenna gain is improved by tapering and the average gain improvement in the entire frequency band is 0.30 db. The other parameter that controls the antenna gain is the location of the arm-reflector junction. The circle of symmetry of the feeding structure can be relocated from the rim of the reflector. The radius of this circle was varied as a parameter to control the antenna gain. Fig. 11 shows the antenna gain for different cases at 10 frequency points from 1 GHz to 10 GHz. As one can see from this figure, gain of the antenna reduces when the radius of circle of symmetry decreases. Gain of the antenna starts to reduce rapidly when the outside edge of the feeding structure moves inside the reflector. The radiated fields associated with the outer layer of the feeding structure induce a current on surface of the reflector 180 out of phase with the current generated with the spherical mode inside the TEM feeding structure. The

5 MANTEGHI AND RAHMAT-SAMII: IMPROVED FEEDING STRUCTURES 827 Fig. 7. Calculated current distribution on the feeding arms with ' =70 at different frequencies. Fig. 8. Calculated current distribution on the feeding arms with ' =70 for different termination loads at 4 GHz. Fig. 9. Calculated current distribution on the double tapered feeding arms with ' =70 at different frequencies. part with out of phase current can be removed from the reflector body to reduce its destructive effect in the antenna gain [14]. As radiated fields associated with the outside edge of the TEM feeding structure do not see the parabolic reflector, gain of the antenna does not change dramatically. Gain of the antenna is calculated for three different cases (Fig. 11): circle of symmetry is located on the reflector rim, outside edge of the feeding arms lie on the reflector rim, and outside edge of the feeding arms are located inside the reflector. This figure shows that the reflector IRA with greater circle of symmetry has higher gain. Due to the arm-reflector junction, the radius of the circle of symmetry can not exceed the radius of the reflector rim. To achieve the maximum value for radius of this circle, the tapering method should change. The inner edge of the newly proposed feeding arm is a straight line from the focal point to the rim of the reflector. The tapering point at the outer edge is located a focal length further the focal point of the parabolic reflector to achieve the same time delay for the reflection from this discontinuity and the reflection from the parabolic reflector (Fig. 12). For this configuration (4)

6 828 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH 2006 Fig. 10. (a) Calculated gain of the reflector IRA with the double tapered and the traditional feeding arms. (b) Difference between gains of the antenna with the double tapered feeding arms and the traditional feeding arms. Fig. 11. Calculated gain of the reflector with the double tapered feeding arms and different arms angels: circle of symmetry is located on the reflector rim ( ), outside edge of the feeding arms lie on the reflector rim ( ), and outside edge of the feeding arms are located inside the reflector ( ). Fig. 12. Schematic of the reflector IRA with the single tapered feeding structure. Fig. 13. Arm-reflector junction of the reflector IRA showing the single tapered feeding arms and the termination load. which can be reduced by avoiding sharp variation in the arm width at the tapering point. Fig. 15 shows the calculated current distribution on the reflector, measured aperture field, measured far-field, calculated and measured far field at E-plane and H-plane for the single side tapered feeding structure at 4 GHz. Both the calculated surface current and the measured aperture field have a more uniform distribution. Symmetric aperture field radiates symmetrically. The beam-width in E-plane and H-plane are very close. The calculated far-fields have a good agreement with the measured ones. The gain versus frequency of the single side tapered feeding structure is compared to the traditional reflector IRA and the double side tapered feed in Fig. 16. This figure shows that the single side feeding structure has a similar performance with the double side tapered design. and the input impedance is decided by choosing at a given. Fig. 13 shows the arm-reflector junction and the low power lumped resistive termination load at the junction. The tapered junction has lower coupling with the reflector in comparison to the traditional feeding arm and as a result the stored energy is reduced. The current distributions on the feeding arms at different frequencies are presented in Fig. 14. There are still some standing waves associated with the tapering discontinuity IV. FEEDING STRUCTURE AT THE FOCAL POINT Performance of the reflector IRA at higher frequencies is very sensitive to the feeding structure design at the focal point. Due to the conical property of the coplanar TEM feeding structure, the width of each arm goes to zero at the focal point. This makes the design more difficult when the coaxial cable with a nonzero diameter connects to the feeding arms at the focal point. Furthermore, the dielectric breakdown limits the minimum feature

7 MANTEGHI AND RAHMAT-SAMII: IMPROVED FEEDING STRUCTURES 829 Fig. 14. Calculated current distribution on the reflector IRA with the single tapered feeding arms at different frequencies. Fig. 15. Measured and calculated results for the modified reflector IRA (fed by tapered arms with ' =70) at 4 GHz. (a) Calculated current distribution, (b) measured holographically projected aperture field, (c) measured elevation-azimuth far-field pattern, and (d) calculated and measured E-plane and H-plane far-field patterns. size and minimum distance between the arms with opposite potentials for high power applications. It means that the upper frequency and the maximum operating power of the antenna are limited by design of the feeding arms at the focal point. A combination of a TEM horn antenna ( and ) and a pair of conical coplanar TEM transmission lines feed the parabolic reflector to realize the reflector IRA. Fig. 17 shows the TEM horn antenna and the conical coplanar TEM transmission lines. The proposed feeding structure originates at the focal point as a TEM horn antenna and gradually tapers to a pair of coplanar transmission lines. The connection between a coaxial cable with a TEM horn antenna is much more convenient in comparison to the same connection with a pair of coplanar transmission lines. Due to the implementation difficulties, the TEM horn antenna at the focal point was made using another technique. Two triangles with a vertex located at the focal point were placed under each connected pairs of the feeding arms. The free edge of each triangle is cut to reduce the blockage loss (Fig. 18). The triangle should be big enough to make the connection at the focal point more convenient and small enough not to increase the blockage loss. Furthermore, our simulation shows that a big triangle can deteriorate the input impedance. The gain of the reflector IRA with the TEM horn-coplanar transmission lines is calculated for

8 830 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH 2006 Fig. 16. (a) Calculated gain of the reflector IRA with the traditional coplanar feeding arms, double tapered feeding arms, and single tapered feeding arms. (b) Difference between gain of the reflector IRA with double tapered and single tapered feeding arms with the traditional feeding arms. Fig. 17. Schematic drawing of (a) TEM horn antenna (side view), (b) single tapered feeding arms (side view), (c) TEM horn antenna (top view), and (d) single tapered feeding arms (top view). Fig. 18. Combination of the TEM horn antenna at the focal point and the single tapered feeding arms. different triangle sizes and for this case a triangle with has been chosen. Fig. 19 compares the reflection coefficient and gain of the tapered coplanar TEM fed reflector IRA with the TEM horn-coplanar TEM fed reflector IRA for different frequencies. These figures show that this combination does not affect the antenna gain and input impedance. V. THE VIVALDI FED REFLECTOR IRA All the former modifications were based on the conical coplanar TEM feeding structure. There is an analytical solution for the input impedance, the aperture field, and the far-field of the orthogonal conical coplanar TEM feeding structure fed reflector IRA [2]. It seems that most of the designers tried to improve the same structure which is compatible with the analytical solution. The traditional feeding structure has a low reflection coefficient in the entire frequency range but low radiation efficiency. A combination of the Vivaldi antenna and the TEM coplanar transmission line is introduced as a feeding structure to improve the radiation efficiency of the reflector IRA. Many different types of tapering for the Vivaldi part with arm angle have been simulated and the input impedance and the radiation efficiencies have been studied [17]. The calculated current distributions on the parabolic reflector at various frequencies for the optimum Vivaldi fed reflector IRA are presented in Fig. 20. These figures show that the Vivaldi feeding structure has a more uniform illumination on the parabolic reflector in comparison to the traditional design. The calculated current distributions on the feeding structure at different frequencies are presented in Fig. 21. The current distribution is more concentrated at edges of the feeding arms as is expected. The input impedance and the reflection coefficient in a 200 system of this antenna are presented in Fig. 22. This figure shows that this feeding structure has a low reflection coefficient over an ultra wideband frequency range. The ultra wideband low reflection coefficient comes from the nature of the tapered Vivaldi antenna. Fig. 23 presents the E-plane and H-plane far-field patterns of the Vivaldi fed reflector IRA at different frequencies. At low frequencies the beam-widths in H-plane and E-plane are very similar (lower than 6 GHz) and the radiation efficiency is high. Our simulation results shows that the E-plane phase center of the Vivaldi feeding structure

9 MANTEGHI AND RAHMAT-SAMII: IMPROVED FEEDING STRUCTURES 831 Fig. 19. Calculated (a) gain and (b) scattering parameter of the reflector IRA in a 200 system with regular feeding arms and combination of the TEM horn antenna and single tapered feeding arms. Fig. 20. Calculated current distribution on the parabolic reflector of the Vivaldi fed reflector IRA at different frequencies. may move away from the reflector focal point with frequency. As a result, the first side-lobe in the E-plane is merged with the main beam at higher frequencies. Therefore, the radiation efficiency decreases at higher frequencies. VI. COMPARISON BETWEEN DIFFERENT DESIGNS One can compare the radiation performance of different types of reflector IRA s by comparing their radiation efficiencies calculated from the realized gain as where is the gain, represent the realized gain, and shows the return loss. The antenna diameter is given by and is the wavelength. Fig. 24(a) shows the radiation efficiency of the traditional 45 coplanar TEM fed reflector IRA, the 70 coplanar TEM fed reflector IRA, the tapered coplanar TEM fed reflector IRA, and the Vivaldi fed reflector IRA. The radiation efficiency of the (5) traditional 45 coplanar TEM fed reflector IRA is around 21% with a low variation in most part of the entire frequency range. Changing the arm angles from 45 to 70 increases the antenna efficiency to 27% and using the tapered structure with the same arm angle (70 ) improves this average to 30%. The Vivaldi fed reflector IRA has the best performance at low frequency and increases the antenna efficiency to 45% but decreases at higher frequencies. If the antenna is excited with an impulse, the average efficiency may help to compare the antenna performances. This average is calculated from the antenna efficiencies in the frequency range 0.1 GHz to 10 GHz and show 20.93%, 26.96%, 29.7%, and 36.08% for the traditional 45 coplanar TEM fed reflector IRA, the 70 coplanar TEM fed reflector IRA, the tapered coplanar TEM fed reflector IRA, and the Vivaldi fed reflector IRA, respectively. All these antennas are excited with a differentiated Gaussian pulse [Fig. 25(a)] and the radiated impulses are compared in the Fig. 25(b). Clearly other pulses can be used as excitation. The radiated field in the frequency domain is normalized to the accepted power for each single frequency step then it is multiplied by the Fourier transform of the input signal. Then the time domain signal is calculated by inverse Fourier transformation of these signals. Finally, all time domain radiated fields are normalized to the maximum of the radiated field of the Vivaldi fed reflector IRA. Our simulation results show that the radiated field from the tapered coplanar fed reflector IRA is 2.12 db lower than the radiated field from the Vivaldi fed reflector IRA. This number for the 70 coplanar fed reflector IRA is 2.41 db and for the traditional 45 coplanar fed reflector is 3.55 db. Since the main goal of using the reflector IRA is in impulse applications, the level of radiated impulse for a given input power is a better measure of antenna performance in contrast to antenna gain or efficiency. The Vivaldi fed reflector IRA increases the level of the radiated impulse by 3.55 db in comparison to the traditional reflector IRA. VII. CONCLUSION The reflector IRA was studied with various feeding structure. The traditional 45 coplanar TEM fed reflector IRA was investigated as a reference and the arm angle was varied as an optimization factor. The antenna radiation efficiency was chosen as a measure to compare the antenna performances. It was shown that the best arm angle for the coplanar TEM feeding structure

10 832 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH 2006 Fig. 21. Calculated current distribution on the feeding arms of the Vivaldi fed reflector IRA at different frequencies. Fig. 22. (a) Calculated input impedance and (b) calculated return loss of the Vivaldi fed reflector IRA. Fig. 23. Calculated H-plane and E-plane far-field patterns of the Vivaldi fed reflector IRA at different frequencies. is 70. The antenna with the same feeding arm and 70 arm angle was studied and the current distribution on the parabolic reflector and the feeding arms were presented. The calculated far-field patterns show similar beam widths in the E-plane and the H-plane for the new design. The arms were tapered to increase the antenna efficiency and performance. The next parameter to investigate was the radius of the circle of symmetry of the tapered coplanar TEM transmission line feeding arms. Different radii of the circle of symmetry were studied and the simulation results show that a greater circle provides higher gain. The tapering was modified to increase the radius of the circle of symmetry greater than the radius of the reflector rim. The antenna with the new tapered feeding structure was constructed and the measured far-field supported the method of moments results. The simulation results show that the new tapering improves gain of the antenna. All the former modifications were based on the conical coplanar TEM transmission line feeding structure. A combina-

11 MANTEGHI AND RAHMAT-SAMII: IMPROVED FEEDING STRUCTURES 833 Fig. 24. (a) Calculated radiation efficiency of the reflector IRA with different feeding structure. (b) The frequency spectrum of a typical impulse (0.5 ns pulse duration). Fig. 25. (a) Input differentiated Gaussian pulse. (b) Radiated impulse from the reflector IRA with different feeding structure. The radiated fields are normalized to their own accepted power and again normalized to the radiated field from the Vivaldi fed reflector IRA. tion of the Vivaldi antenna and the coplanar transmission line was introduced as an alternative design to improve the antenna performance especially at lower frequencies. The simulation results show that the antenna efficiency increased up to 45% at frequencies between 2 GHz to 6 GHz and start to decrease at 6 GHz. A comparison has been performed between simulation results of the reflector IRA with different feeding structure by averaging the antenna efficiency over the entire frequency band. These results show that the Vivaldi fed reflector IRA has the highest average efficiency among all simulated antennas. Also these antennas were excited with a differentiated Gaussian pulse and the amplitude of the calculated radiated pulse at the boresight increased for the Vivaldi fed reflector IRA by 3.55 db in comparison to the traditional design. The real part and imaginary part of the input impedance of the Vivaldi fed reflector IRA have low variations around 200 and 0, respectively, in the entire frequency band which is appropriate for the impulse application. Furthermore, the Vivaldi feeding configuration provides a more rigid structure at the focal point which is suitable for high power application. There were many different models of the Vivaldi structure simulated and investigated for this application but it may need additional studies for the optimum design. REFERENCES [1] R. H. DuHamel et al., Frequency independent conical feeds for lens and reflectors, in Proc. IEEE Int. Antennas Propagat. Symp. Dig., vol. 6, Sep. 1968, pp [2] C. E. Baum, Radiation of impulse-like transient fields, in Sensor and Simulation Notes #321, Nov [3] C. E. Baum and E. G. Farr, Impulse radiating antennas, in Ultra-Wideband/Short-Pulse Electromagnetics, H. L. Bertoni, L. Carin, and L. B. Felson, Eds. New York: Plenum, 1993, pp [4] E. G. Farr, C. E. Baum, and C. J. Buchenauer, Impulse radiating antennas, part II, in Ultra Wideband/Short-Pulse Electromagnetics 2. New York: Press, 1995, pp [5], Impulse radiating antennas, part III, in Ultra Wideband/Short-Pulse Electromagnetics 3, C. E. Baum, L. Carin, and A. P. Stone, Eds. New York: Plenum Press, 1997, pp [6] C. E. Baum, Radiation from self reciprocal aperture, in Sensor and Simulation Notes #357, Apr [7] E. G. Farr and C. E. Baum, Radiation from self-reciprocal aperture, in Electromagnetic Symmetry, C. E. Baum and H. N. Kiritikos, Eds. Bristol, U.K.: Taylor and Francis, 1995, ch. 5. [8] M. Manteghi and Y. Rahmat-Samii, On the characterization of a reflector impulse radiating antenna (IRA): Full-wave analysis and measured results, IEEE Trans. Antennas and Propagat., vol. 54, no. 3, pp , Mar [9] E. G. Farr and C. E. Baum, Prepulse associated with the TEM feed of an impulse radiating antenna, in Sensor and Simulation Notes #337,C. E. Baum, Ed. Albuquerque, NM: Phillips Laboratory, Mar [10] M. J. Baretela and J. S. Tyo, Selective trimming of impulse radiating antenna apertures to increase prompt radiated field, in Sensor and Simulation Notes #461, C. E. Baum, Ed., [11] J. S. Tyo, Optimization of the TEM feed structure for four-arm reflector impulse radiating antennas, IEEE Trans. Antennas Propag., vol. 49, no. 4, pp , Apr [12] L. H. Bowen, E. G. Farr, C. E. Baum, T. C. Tran, and W. D. Prather, Experimental results of optimizing the location of feed arms in a collapsible IRA and a solid IRA, in Sensor and Simulation Note 450, Nov [13], Results of optimization experiments on a solid reflector IRA, in Sensor and Simulation Note #463, Jan [14] J. S. Tyo, E. G. Farr, L. H. Bowen, and L. L. Altgilbers, IRA variations useful for flexible feed arms, in Sensor and Simulation Note #472, Mar [15] R. E. Hodges and Y. Rahmat-Samii, An iterative current-based hybrid method for complex structures, IEEE Trans. Antennas Propag., vol. 45, no. 2, pp , Feb [16] D. V. Giri and C. E. Baum, Temporal and spectral radiation on boresight of a reflector type of impulse radiating antenna (IRA), in Ultra Wideband/Short-Pulse Electromagnetics 3, C. E. Baum, L. Carin, and A. P. Stone, Eds. New York: Plenum Press, 1997, pp [17] M. Manteghi and Y. Rahmat-Samii, A novel vivaldi fed reflector impulse radiating antenna (IRA), in Proc. IEEE Int. Symp. Antennas and Propagation, Washington, DC, Jul. 3 8, 2005, pp

12 834 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 3, MARCH 2006 Majid Manteghi (S 01 M 05) received B.S. and M.S. degrees from the University of Tehran, Tehran, Iran, in 1994 and 1997, respectively, and the Ph.D. degree in electrical engineering from the University of California, Los Angeles (UCLA), in He worked as a Research Assistant in the Microwave Laboratory, University of Tehran, from 1994 to 1997, where he designed microstrip patch antennas, arrays, traveling wave antennas, handset antennas, Base Transceiver Station (BTS) single and dual polarized antennas, reflector antennas, and UHF transceiver circuits and systems. From 1997 to 2000, he worked in the telecommunication industry in Tehran where he served as the head of an RF group for a GSM BTS project. In fall 2000, he joined to the Antenna Research, Analysis, and Measurement Laboratory (ARAM) of the University of California, Los Angeles. He is currently a Research Engineer with the Electrical Engineering Department of UCLA. His research area has included ultrawide-band impulse radiating antennas, miniaturized patch antennas, multiport antennas, dual frequency dual polarized stacked patch array designs, and miniaturized multiband antenna for MIMO applications. Yahya Rahmat-Samii (S 73 M 75 SM 79 F 85) received the M.S. and Ph.D. degrees in electrical engineering from the University of Illinois, Urbana-Champaign. He was a Guest Professor with the Technical University of Denmark (TUD) during summer He was a Senior Research Scientist at NASA s Jet Propulsion Laboratory, California Institute of Technology, Pasadena, before joining the University of California, Los Angeles (UCLA) in Currently, he is a Distinguished Professor and the Chairman of the Electrical Engineering Department, UCLA. He has also been a Consultant to many aerospace companies. He has been Editor and Guest Editor of many technical journals and book publication entities. He has authored and coauthored more than 660 technical journal articles and conference papers and has written 20 book chapters. He is the coauthor of Impedance Boundary Conditions in Electromagnetics (Washington, DC: Taylor & Francis, 1995) and Electromagnetic Optimization by Genetic Algorithms (New York: Wiley, 1999). He is also the holder of several patents. He has had pioneering research contributions in diverse areas of electromagnetics, antennas, measurement and diagnostics techniques, numerical and asymptotic methods, satellite and personal communications, human/antenna interactions, frequency selective surfaces, electromagnetic bandgap structures and the applications of the genetic algorithms. On several occasions, his work has made the cover of many magazines and has been featured on several television newscasts. Dr. Rahmat-Samii is a Member of Sigma Xi, Eta Kappa Nu, Commissions A, B, J, and K of the United States National Committee for the International Union for Radio Science (USNC/URSI), Antennas Measurement Techniques Association (AMTA), and the Electromagnetics Academy. He was elected as a Fellow of the Institute of Advances in Engineering (IAE) in Since 1987, he has been designated every three years as one of the Academy of Science s Research Council Representatives to the URSI General Assemblies held in various parts of the world. In 2001, he was elected as the Foreign Member of the Royal Academy of Belgium for Science and the Arts. He was also a member of UCLA s Graduate council for a period of three years. For his contributions, he has received numerous NASA and JPL Certificates of Recognition. In 1984, he received the coveted Henry Booker Award of the URSI which is given triennially to the Most Outstanding Young Radio Scientist in North America. In 1992 and 1995, he was the recipient of the Best Application Paper Prize Award (Wheeler Award) for papers published in the 1991 and 1994 IEEE ANTENNAS AND PROPAGATION. In 1999, he was the recipient of the University of Illinois ECE Distinguished Alumni Award. In 2000, he was the recipient of IEEE Third Millennium Medal and AMTA Distinguished Achievement Award. In 2001, he was the recipient of the Honorary Doctorate in physics from the University of Santiago de Compostela, Spain. In 2002, he received the Technical Excellence Award from JPL. He is the winner of the 2005 URSI Booker Gold Medal to be presented at the URSI General Assembly. He was also a Member of the Strategic Planning and Review Committee (SPARC) of the IEEE. He was the IEEE AP-S Los Angeles Chapter Chairman ( ) and his chapter won the Best Chapter Awards in two consecutive years. He was the elected 1995 President and 1994 Vice-President of the IEEE Antennas and Propagation Society. He was one of the Directors and Vice President of the Antennas Measurement Techniques Association (AMTA) for three years. He was appointed an IEEE Antennas and Propagation Society Distinguished Lecturer and presented lectures internationally. He is listed in Who s Who in America, Who s Who in Frontiers of Science and Technology, and Who s Who in Engineering. He is the designer of the IEEE Antennas and Propagation Society logo that is displayed on all IEEE ANTENNAS AND PROPAGATION publications.