822 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 50, NO. 4, NOVEMBER 2008 Analysis of Corona Discharge Interference on Antennas on Composite Airplanes Huan-Zhan Fu, Yong-Jun Xie, and Jun Zhang, Member, IEEE Abstract—Static electrification of the airframe can often cause electromagnetic interference on aircraft radios. Triboelectric charging, occurring when an aircraft is operated in precipitation, raises the aircraft potential until corona discharges occur from points of high dc field on the aircraft. These corona discharges generate noise that is coupled into antenna systems installed on the aircraft. The characteristics of electrostatic accumulation on the surface of carbon fiber composite airplanes are studied in this paper. The differences in electrostatic accumulation between car- bon fiber composite airplanes and metal airplanes are also demon- strated. Based on the E fields radiated by an electrostatic discharge (ESD) current, a model of ESD in composite airplanes is estab- lished. As is shown, the induced currents excited by the ESD can successfully predict the interference to the airborne antennas in the frequency and time domains, providing a reference for composite airplane design. Index Terms—Antenna, composite airplane, corona discharge, electrostatic discharge (ESD), interference. I. I NTRODUCTION U NDER all-weather conditions, electrostatic accumulation happens on the surface of an airplane when in flight. As a result, electrostatic discharge (ESD), breaking the raised poten- tial, occurs on the tip of the airplane where the electric field is intense, such as the end of an airfoil, the end of an empennage, and so on. It is shown in [1]–[3] that these discharges can produce serious interference in airplane and receiving antennas, and disable com- munication and navigation systems. The discharge form, accom- panied by a series of short pulses, generates considerable RF en- ergy, and produces serious interference at low frequencies (LF) and high frequencies (HF) due to the energy of the ESD. Since the hazard of ESD on airplanes and the interference in antennas are clear, it is worthwhile paying attention to these problems. A variety of early work [4]–[8] studied the generation of ESD interference and its elimination in aircraft. Some researchers studied the characteristics of ESD and devised techniques for the elimination of electrostatic interference in aircraft [9]. The noise generated by corona discharge [3], as well as its interfer- ence to airborne antennas [10], has also been studied in some early work. Alternately, some researchers used the geometrical theory of diffraction (GTD) to compute the surface current and Manuscript received December 27, 2007. Current version published November 20, 2008. This work was supported by the University of Ministry of Education, China, under Grant NECT-04-0950 to the Program for the New Century Excellent Talents. The authors are with the National Laboratory of Antennas and Microwave Technology, Xidian University, 710071 Xi’an, China (e-mail: laycko@tom.com; yjxie@xidian.edu.cn; zhangjun1982@163.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TEMC.2008.2004598 charge density induced on aircraft [11]. The form of the dis- charge current pulse, numerical models, and the fields radiated by ESD were studied in [12]–[14]. As early as 1966, Yee first presented the equivalent electric dipole model [13], and in 1991, Wilson introduced a relatively simple dipole model of corona discharge to predict radiation fields [14]. Preceding studies show that these electrostatic discharges occur on metal aircraft. With the wide application of composite materials in aviation, ESD should also be considered under composite conditions. In this paper, corona discharge interference to antennas on carbon fiber composite airplanes is discussed. The corona discharge current waveform produced by corona discharge in a composite airplane is given in Section II. In Section III, the model of ESD in a composite airplane is established to analyze the characteristics of corona discharge and its effects on antennas. In Section IV, the induced current density on the surface of a carbon fiber composite airplane and the radiation patterns of corona discharge are studied. Based on the electric fields radiated by an electrostatic corona discharge current, the interference to the airborne antennas is analyzed both in the frequency and time domains, and the induced currents excited by the corona discharge can predict the influence on the equipment of receiving antennas in composite aircraft. Finally, in Section V, conclusions are drawn on the various results of this study. II. C ORONA D ISCHARGE D IFFERENCES B ETWEEN C OMPOSITE AND M ETAL A IRCRAFTS A. General Triboelectric charging associated with impinging precipita- tion particles during low-altitude flight of composite airplanes generally charges the airplanes to a high potential. As charge accumulates on a composite aircraft, the potential will increase until a threshold is reached above which corona discharge break- down occurs in regions of high electric field at the aircraft ex- tremities. This breakdown occurs as a series of discrete pulses of short duration and rapid rise time, and therefore, produces noise over a broad spectrum. The pulses can be very adequately ap- proximated by a decaying exponential with zero rise time. The time-varying corona discharge current pulses generated during the discharge cycle are double exponential in nature and can be represented by the following equation [12]: I (0,t)=KI p  e −αt − e −βt  ,t≥ 0 where K =1, α =4× 10 6.8 , β =4.76 × 10 7.6 , I p =0.05. For composites, the corona discharge occurs as a series of discrete impulses, but much different from corona discharge 0018-9375/$25.00 © 2008 IEEE FU et al.: ANALYSIS OF CORONA DISCHARGE INTERFERENCE ON ANTENNAS ON COMPOSITE AIRPLANES 823 TABLE I R ESISTIVITY C LASSIFICATION R ELATED TO E LECTROSTATIC D ISCHARGE for metals. The following section discusses a single corona discharge current pulse produced by a corona discharge at an airfoil terminal. B. Corona Differences Between Composite and Metal Airplane The electrostatic distribution on a composite airplane is asym- metric. Similar to the metal material case, it mostly concentrates on the positions where the curvature is large, such as the nose of the fuselage, the airfoil tips, or points inboard of the dischargers on the wing. The most common charging mechanism on the surface of an airplane is triboelectric charging. This mechanism involves electron or ion transfer upon contact, due to the frictional local- ized heating of microscopic contact areas on solid surfaces. The localized microscopic regions of material are melted, allowing increased charge mobility. Distinctions among various levels of material resistivity are based on surface and volume resistance. Categories with corresponding ranges of surface and volume resistivity are given in Table I [15]. The electrostatic mobility of a material gets stronger as the surface or volume resistance trails off. The electrostatic charge that can be developed on an airplane is a function of its relative position in the triboelectric series, and of its resistivity. Those materials in the series that also have a higher resistivity can accumulate significant charge because a longer time is required for charge to bleed off such materials. However, under certain circumstances, even conductive mate- rials, such as metals, can be charged. Compared with metal, electrostatic accumulation capacity on composite airplanes is much larger. In addition, the attenuation time of an electrostatic discharge is longer, so it is impossible to totally discharge. Sim- ilar to metal materials, the accumulated electrostatic charge on a composite airplane is allowed to transfer to positions where brushes are located, and to be bled off with corona discharge. The time-domain representation of the positive corona dis- charge current pulses produced on a composite airplane is shown in Fig. 1. Fig. 2 shows the corona discharge current pulses in the frequency domain. This breakdown occurs as a series of discrete impulses of short duration. Compared with metal, as a primary interference source, the rise time of positive corona discharge current pulses on a com- posite aircraft is longer (100 ns at sea level), and the loss time is longer, with a time of 2000 ns. Besides, I P reaches a peak of about 19 mA, which is larger than on a metal aircraft. From Fig. 2, we note that the frequency spectrum is less than 30 MHz and the spectrum energy mostly centralizes around 1 MHz. Dif- ferent series of corona discharge current pulses have the same spectrum range. Fig. 1. Corona discharge current pulses in the time domain. Fig. 2. Corona discharge current pulses in the frequency domain. III. S IMULATION M ODEL The composite airplane here is a certain type of unmanned airplane made of carbon fiber composite material. The relative permittivity is ε r = 439.81 − j227.19 and the relative perme- ability is µ r =1.0. Fig. 3 shows the model of the composite air- plane. The belly and tail-cap antennas are monopoles that have center frequencies of 150 MHz and 1 GHz, respectively. The lengths of the belly and tail-cap antennas are 0.5 and 0.075 m, and the wingspan of the composite airplane is 10 m. In the HF range, the size of the airplane wingspan is about ten times longer than the radiation wavelength, so we use the method of multi- level fast multipole algorithm (MLFMA) to simulate the corona discharge radiation field. But, in the LF range, the method of moment is used to calculate the interference on antennas. Electrostatic charge accumulates at the tip of the airplane because the curvature of the tip is large, and the accumulated electrostatic charge is of high density. The E fields radiated by an ESD current are given by E = σ/S.Asisshown,with the increase of electric potential at the surface of a composite 824 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 50, NO. 4, NOVEMBER 2008 Fig. 3. Model of the composite airplane. airplane, the air molecules around are easier to ionize and corona discharges occur at the tip of the airfoil. According to the theory of corona discharge, dischargers can be installed on an aircraft to permit current to be discharged. In an airplane, the corona discharge model could be equivalent to an electric dipole [13]. So, the effect of corona discharge to the belly and tail-cap antennas can be computed as the effect generated by an electric dipole model. At the tip of the left and right composite airfoil, the electric dipole models can be located to simulate the excitation of an ESD current, as shown in Fig. 3. Further, corona discharge characteristics and its interference to antennas are illustrated. IV. C ORONA D ISCHARGE C HARACTERISTICS AND I TS E FFECTS IN C OMPOSITE A IRCRAFT A. Corona Discharge Characteristics in Composite Aircraft 1) Induced Current Density: Surface current density can be induced on the airframe surface owing to corona discharge to the aircraft. Sampling the frequencies between 0.1 and 200 MHz and computing the electric fields radiated by an ESD current, the current density that is induced on the surface of the airframe is shown. Fig. 4(a) illustrates the induced current density on the back surface of a composite airplane and (b) shows it on the belly. The more bright the color is, the greater the induced current density. As is shown in Fig. 4, the induced current density was mainly concentrated at the nose of the airplane, the edge of the airfoil, and the tail of the composite airplane. Besides, the maximum current destiny distribution positions induced on the edge of airfoil are quite repetitive and predicable. It should be noted that sensitive instruments should not be installed at these positions. 2) Corona Discharge Far-Field Radiation Pattern: At dif- ferent frequencies, for example, 30, 60, and 100 MHz, the corona discharge far-field radiation pattern is different, as is shown in Fig. 5. It is shown that the far radiation field remains largely symmetric, and the electric fields radiated by corona discharge are concentrated mostly at the nose and the tail of the airplane at 30 MHz. The directions with strong radiation increase with the increase of the frequency. The higher the frequency is, the more complex the corona discharge far-field radiation pattern. Fig. 4. Distribution of the induced current density on the surface of the airplane. Fig. 5. Corona discharge far-field radiation patterns. (a) 30 MHz. (b) 60 MHz. (c) 100 MHz. FU et al.: ANALYSIS OF CORONA DISCHARGE INTERFERENCE ON ANTENNAS ON COMPOSITE AIRPLANES 825 Fig. 6. Electric noise field at the belly antenna location in the frequency domain. B. Corona Discharge Interferences on Antennas To investigate the interference generated by the discharging of corona produced on the airplane, an airplane model (antennas were mounted as indicated in Fig. 3) was established to predict the noise generated in aircraft antennas. The airborne antennas were installed at the belly and the tail-cap of the composite airplane. When corona discharges happen at the tip of the airfoil, the electric radiation fields could have effects on the antennas. According to the results in the time domain for the corona discharge current pulses that were shown in Fig. 1, we can get the counterpart in the frequency domain by Fourier transformation, as shown in Fig. 2. By setting up the equivalent excitation source, we can simulate the electric noise field at the belly and tail-cap locations, and the currents induced in the antennas. 1) Electric Noise Field E: The electric noise field at the belly antenna location in the time domain and the frequency do- main are shown in Figs. 6 and 7, respectively. In Fig. 6, E is in- tense when the frequency is under 10 MHz. When the frequency is higher than 10 MHz, the electric noise field decreases gradu- ally. From Fig. 7, we can draw the conclusion that the E field has a change from 113 to 86 V/m. The E field in the time domain at the belly antenna location changes slowly. At about 3 × 10 −6 s, the value of the E field increases to a maximum of 113 V/m and it falls to the minimum of 86 V/m at 7 × 10 −6 s.Figs.8 and 9 show the electric noise field at the tail-cap antenna location in the frequency and time domains. Compared with [3] and [9], the value of the frequency domain E fields at the belly and tail- cap locations in the composite airplane are much higher than that in the KC-135(707) aircraft. As shown in Figs. 6 and 8, the form of time-domain E field is similar to the ESD form at the high electric potential in [14]. But, the value of the maximum noise field in that paper is higher than that shown in Figs. 6 and 8. 2) Induced Noise Current I: By computing the induced cur- rents at the belly and tail-cap antennas and using an inverse Fig. 7. Electric noise field at the belly antenna location in the time domain. Fig. 8. Electric noise field at the tail-cap-antenna location in the frequency domain. Fig. 9. Electric noise field at the tail-cap antenna location in the time domain. 826 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 50, NO. 4, NOVEMBER 2008 Fig. 10. Belly antenna noise current in the time domain. Fig. 11. Tail-cap antenna noise current in the time domain. Fourier transformation, the time-domain current induced in the antennas can be illustrated in Figs. 10 and 11. Figs. 12 and 13 show the noise currents in the frequency domain. In the com- posite condition, the induced current is concentrated within 10 MHz. The induced currents on the belly and tail-cap an- tennas are lower than the ESD current generated at the tip of the airfoil. With the increase of frequency, the induced currents become lower and appear oscillatory from 10 to 300 MHz. In the time domain, the currents at the belly and tail-cap antennas induced by corona discharge generated at the tip of airfoil reach to the maximum of 59 and 1.7 µA, respectively, at the time of 20 ns, and become lower quickly as time passes. Compared with [3], the value of induced current on the belly antenna in the composite airplane is ten times the current on the belly antenna in 431-1 aircraft. The value of tail-cap antenna noise current is smaller than that induced at the tail-cap of the 353-1 aircraft and the 431-1 aircraft. Fig. 12. Belly antenna noise current in the frequency domain. Fig. 13. Tail-cap antenna noise current in the frequency domain. V. C ONCLUSION The aforesaid study has indicated that the characteristics of corona discharge in composite aircraft are different from the characteristics that occur in metal aircraft. For a composite air- plane, an electric dipole model was used to simulate the charac- teristics of corona discharge at the tip of the airfoil. The induced current density and the radiation pattern of corona discharge were illustrated in this paper. Besides, the electric noise fields at belly and tail-cap antenna locations have been analyzed. Corona interference on antennas in the frequency and time domains has been predicted. The results shown can predict the interference on the airborne antennas, thus decreasing the risk of corona dis- charge and providing reference for composite airplane design and the distribution of interior airborne equipments. FU et al.: ANALYSIS OF CORONA DISCHARGE INTERFERENCE ON ANTENNAS ON COMPOSITE AIRPLANES 827 R EFERENCES [1] A. Vassiliadis, “A study of corona discharge noise in aircraft antennas,” Stanford Res. Inst., Menlo Park, CA, Tech. Rep. 70, SRI Project 2494, Contract AF 19(604)-3458, Jul. 1960 [2] R. L. Tanner and J. E. Nanevicz, “Precipitation charging and corona- generated interference in aircraft,” Stanford Res. Inst., Menlo Park, CA, Contract AF 19(604)-3458, Tech. Rep. No. 73, SRI Project 2494, Apr. 1961. [3] R. L. Tanner and J. E. Nanevicz, “An analysis of corona-generated inter- ference in aircraft,” Proc. IEEE, vol. 52, no. 1, pp. 44–52, Jan. 1964. [4] A. Curtis, “Discussion on ‘radio range variations’ by R. H. Marriot,” Proc. 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Antennas Propag., vol. AP-26, no. 1, pp. 77–81, Jan. 1978. [12] S. K. Nayak and M. J. Thomas, “A novel technique for the computation of radiated EMI due to corona on HV transmission lines,” in Proc. Rec. IEEE EMC Int. Symp. [C], 2003, pp. 738–742. [13] K. S. Yee, “Numerical solution of initial boundary value problems in- volving Maxwell’s in isotropic media,” IEEE Trans. Antennas Propag., vol. AP-14, no. 4, pp. 302–307, May 1966. [14] P. F. Wilson and M. T. Ma, “Fields radiated by electrostatic discharges,” IEEE Trans. Electromagn. Compat., vol. 33, no. 1, pp. 10–18, Feb. 1991. [15] IEEE Guide on Electrostatic Discharge (ESD): Characterization of the ESD Environment. Washington, DC: Amer. Nat. Standards Inst., Dec. 23, 1992. Huan Zhan Fu received the Bachelor’s degree in electronic engineering in 2006 from Xidian Univer- sity, Xi’an, China, where, since then, he has been working for a Master’s degree. He has been focusing on the electromagnetic inter- ference on composite aircrafts. His current research interests include the area of techniques for the elim- ination of corona on aircraft. Yong-Jun Xie received the B.S., M.S., and Ph.D. degrees in electronic engineering from Xidian Uni- versity, Xi’an, China, in 1990, 1993, and 1996, respectively. From 1998 to 1999, he was with the University of Texas at Dallas, Dallas, as a Postdoctoral Research Associate. From 1999 to 2001, he was with Duke University as a Postdoctoral Research Associate. In 2004, he was supported by the Program for the New Century Excellent Talents in the University of Ministry of Education, China. Currently he is a Pro- fessor at Xidian University. His research interests include electromagnetic the- ory, microwave technology, and mobile telecommunication. Jun Zhang (M’02) received the Bachelor’s degree in electronic engineering in 2005 from Xidian Univer- sity, Xi’an, China, where, since 2006, she has been working toward the Master’s degree. She has been focusing on the electromagnetic in- terference on composite aircraft. Her current research interests include the area of techniques for the elim- ination of corona on aircraft.