In order to predict the immunity of a generic missile (GENEC), not only the electronic system but also the enclosure has to be taken into consideration. While a completely closed metallic missile enclosure shows a high electric shielding effectiveness, it is decreased substantially by apertures which could not be avoided by different reasons. The shielding effectiveness of the generic missile could be investigated by means of a hollow cylinder equipped with different apertures. Numerical simulations and measurements of this hollow cylinder will be carried out and analyzed

Fisahn, Sven

Garbe, Heyno

Advances in Radio Science

S. Fisahn

H. Garbe

Directory of Open Access Journals

Adv. Radio Sci., 5, 63–67, 2007www.adv-radio-sci.net/5/63/2007/© Author(s) 2007. This work is licensedunder a Creative Commons License.Advances inRadio ScienceProtective properties of a missile enclosure against electromagneticinﬂuencesS. Fisahn and H. GarbeLeibniz Universit¨ at Hannover, GermanyAbstract. In order to predict the immunity of a generic mis-sile (GENEC), not only the electronic system but also theenclosure has to be taken into consideration. While a com-pletely closed metallic missile enclosure shows a high elec-tric shielding effectiveness, it is decreased substantially byapertures which could not be avoided by different reasons.The shielding effectiveness of the generic missile could beinvestigated by means of a hollow cylinder equipped withdifferent apertures. Numerical simulations and measure-ments of this hollow cylinder will be carried out and ana-lyzed.1 IntroductionModern electronic systems are of particular importance un-der many aspects like security, medicine, economy, trafﬁc,communication and armed forced. Civil and military sys-tems consist of several electronic components and subsys-tems (e.g. microprocessor boards). Malfunctions of thesedevices could lead to unexpected consequences, since theelectroniccomponentshavegotsafetycriticalfunctionalities.Susceptibility investigations of different electronic systemshave shown that electromagnetic threats like electromagneticpulses (EMP), ultra wideband (UWB) and high power mi-crowave pulses (HPM) can cause failures or damages. Incase of a time critical system like an aircraft, even a fail-ure without any physical damage of an electronic component(e.g. self reset of a microcontroller) could lead to the destruc-tion of the complete system due to a crash. In order to predictthe immunity of a missile against different electromagneticthreats, not only the electronic system but also the enclosurehas to be taken into consideration. Whilea completely closedmetallic missile enclosure shows a high electric shielding ef-Correspondence to: S. Fisahn(ﬁsahn@ieee.org)fectiveness, it will be decreased substantially by apertureswhich could not be avoided by different reasons.In this paper, the protective properties of a missile en-closure are investigated by means of a simpliﬁed enclosuremodel, which is described in Sect. 2. In order to obtain theﬁeld distribution inside the missile enclosure, numerical ﬁeldcalculations were carried out in frequency domain. The re-quirements of these simulations will be presented in Sect. 3,as well as the simulation results and their evaluation. Af-ter this, several measurements of this the missile model wereperformed in time and frequency domain. The measurementsetup and the evaluation of the measurements will be de-scribed in Sect. 4. In the following conclusion, the results ofthe numerical ﬁeld calculations and the measurements willbe compared to each other and evaluated as well.2 Modelling conceptIn this contribution, the investigation of the protective prop-erties of a missile enclosure against electromagnetic inﬂu-ences is performed on the basis of a simpliﬁed enclosuremodel. Assuming that the conductivity of the enclosure wallsis sufﬁciently, only aperture penetration affects the couplingof electromagnetic ﬁelds to the interior of the missile enclo-sure. Thus, the model consists of a metallic hollow cylin-der equipped with an aperture at the front side. Such anopening could be necessary by different reasons e.g. for theoptical component of a target tracking system. The geo-metrical dimensions of the hollow cylinder are l=1 m andDcyl=100 mm. Furthermore, a monopole ﬁeld probe is posi-tioned inside the hollow cylinder, in order to detect the ﬁeldsinside the cylinder.Published by Copernicus Publications on behalf of the URSI Landesausschuss in der Bundesrepublik Deutschland e.V.64 S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic inﬂuences  10  Figure 1. Simulation model Fig. 1. Simulation model.  11 0 0.5 1 1.5 2 2.5 3 3.5 410-410-2100102Impedance Voltage Drop f / GHzU / V Figure 2. Calculated impedance voltage drop at the antenna base of the monopole  Fig. 2. Calculated impedance voltage drop at the antenna base ofthe monopole.3 Numerical ﬁeld calculationsSimulations have been carried out in frequency domain withthe numerical ﬁeld calculation program CONCEPT-II. Theaim of these calculations is the investigation of the couplingmechanism of the electromagnetic ﬁelds from the surround-ings to the interior of the missile enclosure. Additionally,the simulation results could be used in order to validate themeasurement results.3.1 Simulation modelThe simulation model consists of an ideal conducting metal-lic hollow cylinder with the geometric dimensions l=1m and  12  Figure 3. Calculated distribution of electric field strength at 1 GHz Fig. 3. Calculated distribution of electric ﬁeld strength at 1 GHz.  13  Figure 4. Calculated distribution of electric field strength at 2 GHz Fig. 4. Calculated distribution of electric ﬁeld strength at 2 GHz.Dcyl=100 mm, as mentioned in Sect. 2. The cylinder isequipped with an aperture (diameter Da=50 mm) at the frontside and a monopole probe positioned in a distance of 0.25 mwith respect to the aperture. The monopole is implementedby an ideal conducting wire, which lengths is lm=50 mm andconnected to the enclosure with a 50  resistor. The structureis illuminated by an incident electromagnetic plane wave asshown in Fig. 1. It should be mentioned that the monopoleis orientated parallel to the vector of the electric ﬁeld of theplane wave. The impedance voltage drop which is equivalentto the antenna base voltage of a ﬁeld probe in measurementscan calculated numerical by means of this simulation modelas well as the ﬁeld distribution inside and outside the missileenclosure.3.2 Simulation resultsAll simulations have been performed in the frequency rangebetween 10 MHz and 4 GHz at discrete frequencies. The stepsize was 10 MHz, thus 400 different frequencies have beentreated within this investigation. The magnitude of the elec-tric ﬁeld strength of the incident plane wave is E=1 kV/m.Figure 2 shows the calculated impedance voltage drop atthe antenna base of the monopole. While the voltage drop issmaller than 1 V in the frequency range below 1.5 GHz, itincreases rapidly at 1.8 GHz. In the frequency range above1.8 GHz, the voltage drop is several times larger than 1 Vfor frequencies, but shows more or less strong variations.Adv. Radio Sci., 5, 63–67, 2007 www.adv-radio-sci.net/5/63/2007/S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic inﬂuences 65  14  Figure 5. Measurement setup inside the GTEM cell Fig. 5. Measurement setup inside the GTEM cell.In order to ﬁnd an explanation for this behaviour, the dis-tribution of the electric ﬁeld strength has been examined fortwo different frequencies. The calculated magnitude of ﬁeldstrength in the x-z-plane (with respect to the coordinate sys-tem in Fig. 1) is illustrated in Fig. 3 respectively in Fig. 4.While the magnitude is almost equal to the electric ﬁeld inz-direction, only the magnitude is presented. Although abso-lute ﬁeld strength values are not given, both ﬁgures show theactive principle very well.As it could be seen in both ﬁgures, the incident wave prop-agates from the right side to the left one. While the wavepropagation outside the hollow cylinder is nearly unaffecteddue to its presence at 1 GHz, ﬁeld disturbances occur in frontof the aperture at 2 GHz. Furthermore, the magnitude of theﬁeld strength inside the enclosure is negligible at the lowerfrequency, whereas wave propagation could be found insidethe hollow cylinder at the higher frequency.This frequency dependent behaviour is affected by the ge-ometry of the simulation model, because the missile modelforms a cylindrical waveguide. The cut-off frequency of acertain mode is the characteristic quantity of a waveguide.This quantity is given by Eq. (A1) in case of a transverseelectrical mode (TEmn-mode) respectively by Eq. (A2) incase of a transverse magnetic mode (THmn-mode). Ta-ble 1 gives an overview about cut-off frequencies of the dif-ferent modes which are able to propagate in a waveguidewith D=100 mm. The fundamental mode is the TE11-modeand the correspondent cut-off frequency is approximately1.75 GHz.Taking into consideration that the enclosure forms a cylin-drical waveguide, the different ﬁeld distributions can be ex-plained easily. The incident electromagnetic wave couples tothe waveguide because of aperture penetration at both inves-tigated frequencies. While the cylindrical waveguides oper-ates at 1 GHz below the frequency cut-off, there is no wavepropagation possible and the electromagnetic ﬁelds inside  15  Figure 6. Different apertures and terminations, monopole probe Fig. 6. Different apertures and terminations, monopole probe.Table 1. Cut-off frequencies of a metallic tube with D=100 mm.Mode Bessel n-th Function Cut-offfunction root Value frequencyTE11 J01(x) 1 1.841 1.757 GHzTM01 J0(x) 1 2.405 2.295 GHzTE21 J02(x) 1 3.054 2.914 GHzTE11 J00(x) 1 3.832 3.657 GHzTM11 J1(x) 1 3.832 3.657 GHzthe metallic tube are dumped exponentially. The things aredifferent in the higher frequency range due to the fact thatthe waveguide operates above its frequency cut-off. As il-lustrated in Fig. 4, the wave excited by aperture penetrationpropagates from the right side to the left one. The metalliccap on the left side forms a short termination of the waveg-uide, thus the wave is reﬂected completely. The reﬂectedwave propagates in backward direction and reaches the aper-ture, where it is reﬂected partly, as well as the radiated. Thus,not only standing waves occur inside the enclosure, but alsoﬁeld disturbances are affected in front of the hollow cylinder.4 MeasurementsMeasurements of the hollow cylinder have been preformedwith continuous wave signals in the frequency domain. Theaim of these measurements is the investigation of differentaperture sizes and terminations.4.1 Measurement setupA GTEM cell is a well suited environment in order to gene-rate deﬁned electromagnetic ﬁelds. Thus, the measurementshave been performed in a GTEM 1250. Therefore, the metal-lic hollow cylinder is placed parallel to the ground planeof the cell as illustrated in Fig. 5. The aperture is orientedwww.adv-radio-sci.net/5/63/2007/ Adv. Radio Sci., 5, 63–67, 200766 S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic inﬂuences  16 0 0.5 1 1.5 2 2.5 3 3.5 4-80-60-40-20020Metallic Tube terminated with Shortf / GHzS21 / dB  10mm30mm50mm100mm Figure 7. Variation of the aperture size, tube terminated with short  Fig. 7. Variation of the aperture size, tube terminated with short.  17 0 0.5 1 1.5 2 2.5 3 3.5 4-80-60-40-20020Metallic Tube with Aperture (D=30mm)f / GHzS21 / dB  openshortload Figure 8. Variation of the termination (aperture size Da = 30 mm)  Fig. 8. Variation of the termination (aperture size Da=30 mm).towards the cell port and the open ending towards the ab-sorbing cones. The monopole probe inside the metallic tubeis positioned in vertical direction with respect to the groundplane, thus it measured the electric ﬁeld in z-direction. Itslength is lm=5 mm and it is positioned in a distance of 0.25 mto the aperture.The metallic tube can be equipped with different apertureson one end and terminations on the other. Figure 6 showsthe different apertures, which are realized due to metallicdiscs with centric holes. A metallic support ﬁxes the differ-ent apertures at the front side of the tube. Possible termina-tions of the open ending of the tube are open, short and load.The load emulates the loading of the cylinder with a printedcircuit board (PCB) and is implemented by absorbing foamwith cylindrical shape. A network analyzer serves as powersource as well as receiver. Therefore, one port of the net-work analyzer is connected to the cell port and the other tothe monopole probe.  18 0 0.5 1 1.5 2 2.5 3 3.5 4-80-60-40-20020Metallic Tube with Aperture (D=50mm)f / GHzS21 / dB  openshortload Figure 9. Variation of the termination (aperture size Da = 50 mm)     Fig. 9. Variation of the termination (aperture size Da=50 mm).4.2 Measurement resultsAll measurements have been performed with CW (continu-ous wave) signals in the frequency range from 10 MHz upto 4 GHz at 400 discrete frequencies. While the measuredquantity is the transfer function between the cell port and themonopole, the frequency response of the monopole has tobe taken into consideration. Thus, the S21-parameter denotesthe corrected transfer function in the following.The inﬂuence of different terminations is shown in Fig. 8for an aperture size of D=30 mm. The magnitude of the mea-sured S21-parameter is less than −50 dB in the frequencyrange below 1.75 GHz and nearly independent from the ter-mination. At approximately 1.75 GHz, there is an abrupt riseof this parameter. While the termination with short shows al-ternating minimums and maximums in the higher frequencyrange, the deviation is smaller for a termination with open.Furthermore, the termination with load has got a lower mag-nitudethantheterminationopen, becausetheabsorbingfoamdumps the resonance effects.The rapid increase of the transfer function at approxi-mately 1.75 GHz is caused by the geometrical dimensionsof the metallic tube, because the tube forms a cylindricalwaveguide. The cut-off frequency of the fundamental modeis 1.757 GHz with respect to Eq. (A1) and Table 1. Be-low 1.757 GHz, the electromagnetic ﬁeld excited at the aper-ture is an evanescent ﬁeld and dumped exponentially. Anincrease of the aperture size affects of a slow rise in thefrequency range between 1.3 GHz and 1.75 GHz, thus theabrupt increase at the cut-off frequency is more ﬂat (seeFig. 7). Nevertheless, the magnitude is larger in the upperfrequency range than in case of a smaller aperture, as shownin Fig. 9. Above the cut-off frequency of the fundamentalmode, transverse electrical waves (TE11-mode) can be ex-cited at the aperture. These waves propagate towards the ter-mination, where they are reﬂected more or less depending onAdv. Radio Sci., 5, 63–67, 2007 www.adv-radio-sci.net/5/63/2007/S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic inﬂuences 67the termination. The reﬂected waves propagate towards theaperture, where the waves will be partially reﬂected respec-tively radiated to the surroundings. It is quite obvious thatmetallic tube operates as a cavity resonator, thus the mag-nitude of transfer function reaches values larger than 0 dBat certain frequencies. The termination with load effects adumping of the resonances.5 ConclusionsIn this paper the shielding properties of a missile enclosurewas examined by numerical ﬁeld calculations as well as bymeasurements. Under assumption of a sufﬁcient conductiv-ity of the enclosure walls, the coupling to the inside is af-fected by aperture penetration. Therefore, the missile enclo-sure was emulated by means of a metallic hollow cylinder,which includes an aperture in the top cap. Furthermore, amonopole ﬁeld probe was positioned inside the cylinder inorder to detect the excited electric ﬁeld inside.The simulations point out the active principle very well.The metallic hollow cylinder behaves like a cylindricalwaveguide. Below the ﬁrst cut-off frequency, only evanes-cent ﬁelds are present in the waveguide. In the higherfrequency range, the fundamental mode (TE11-mode) orother higher order modes are able to propagate. The calcu-lated voltage at the antenna base of the monopole increasesrapidly. While the enclosure is ideal conductive, the hollowcylinder forms a cavity with the high quality factor. The res-onances will be dumped only by losses due to radiated emis-sions at the aperture.The measurements show results, which are very similarto the numerical calculations ones. Below the ﬁrst cut-offfrequency, only evanescent ﬁelds are present in the waveg-uide. In the higher frequency range, the fundamental mode isable to propagate and the ﬁeld strength increases rapidly. Thevariation of the aperture shows that a growing aperture sizeincreases the ﬁeld excitation in the higher frequency rangeabove 1.75 GHz.The different terminations of the waveguides inﬂuence thedumping of the standing waves. While a cylinder terminatedwith a metallic cap (short) shows the strongest resonance be-haviour, an open ending effects a higher dumping due to ad-ditional radiated emissions. The most realistic setup, whichincludesaPCBmodelthatconsistsofabsorbingfoam, showsthe smallest resonance effects.Appendix ACut-off frequencies of cylindrical waveguidesThe characteristic quantity of a waveguide is the cut-off fre-quency fc of a certain mode. In case of a cylindrical waveg-uide it is given for transverse electrical modes (TEmn-modes)byf TEc,mn =c0π√µrεr·x0mnD, (A1)where m, n are integers and D is the diameter of the metallictube. x0mn is the n-th root of the deviation of Bessel functionJ0m(x). εr and µr are the relative permittivity respectivelypermeability of the material inside the waveguide and c0 isthe speed of the light. In case of transverse magnetic modes(TMmn-modes), the cut-off frequency is dependent to the n-th root xmn of the of Bessel function Jm(x):f TMc,mn =c0π√µrεr·xmnD. (A2)Acknowledgements. This investigation is part of the study “Protec-tion of Electronic Systems to electromagnetic Inﬂuences, Phase II”,commissioned by the Armed Forces Scientiﬁc Institute for Protec-tion Technologies and NBC Protection (Munster, Germany). Theresponsibility for the content is with the authors.ReferencesCollin, R. E.: Field theory of guided waves, IEEE Press, New York,1991.Groh, C., K¨ arst, J. P., Koch, M., and Garbe, H.: TEM Waveguidesfor EMC Measurements, IEEE Transactions on ElectromagneticCompatibility, 41(4), 440–445, Nov., 1999.Nitsch, D., Camp, M., Sabath, F., ter Haseborg, J. L., and Garbe, H.:Susceptibility of Some Electronic Equipment to HPM Threats,IEEE Transactions on Electromagnetic Compatibility, 46(3),380–389, Aug., 2004.www.adv-radio-sci.net/5/63/2007/ Adv. Radio Sci., 5, 63–67, 2007

Protective properties of a missile enclosure against electromagnetic influences

Institutionelles Repositorium der Leibniz Universität Hannover

Adv. Radio Sci., 5, 63–67, 2007www.adv-radio-sci.net/5/63/2007/© Author(s) 2007. This work is licensedunder a Creative Commons License.Advances inRadio ScienceProtective properties of a missile enclosure against electromagneticinfluencesS. Fisahn and H. GarbeLeibniz Universita¨t Hannover, GermanyAbstract. In order to predict the immunity of a generic mis-sile (GENEC), not only the electronic system but also theenclosure has to be taken into consideration. While a com-pletely closed metallic missile enclosure shows a high elec-tric shielding effectiveness, it is decreased substantially byapertures which could not be avoided by different reasons.The shielding effectiveness of the generic missile could beinvestigated by means of a hollow cylinder equipped withdifferent apertures. Numerical simulations and measure-ments of this hollow cylinder will be carried out and ana-lyzed.1 IntroductionModern electronic systems are of particular importance un-der many aspects like security, medicine, economy, traffic,communication and armed forced. Civil and military sys-tems consist of several electronic components and subsys-tems (e.g. microprocessor boards). Malfunctions of thesedevices could lead to unexpected consequences, since theelectronic components have got safety critical functionalities.Susceptibility investigations of different electronic systemshave shown that electromagnetic threats like electromagneticpulses (EMP), ultra wideband (UWB) and high power mi-crowave pulses (HPM) can cause failures or damages. Incase of a time critical system like an aircraft, even a fail-ure without any physical damage of an electronic component(e.g. self reset of a microcontroller) could lead to the destruc-tion of the complete system due to a crash. In order to predictthe immunity of a missile against different electromagneticthreats, not only the electronic system but also the enclosurehas to be taken into consideration. While a completely closedmetallic missile enclosure shows a high electric shielding ef-Correspondence to: S. Fisahn(fisahn@ieee.org)fectiveness, it will be decreased substantially by apertureswhich could not be avoided by different reasons.In this paper, the protective properties of a missile en-closure are investigated by means of a simplified enclosuremodel, which is described in Sect. 2. In order to obtain thefield distribution inside the missile enclosure, numerical fieldcalculations were carried out in frequency domain. The re-quirements of these simulations will be presented in Sect. 3,as well as the simulation results and their evaluation. Af-ter this, several measurements of this the missile model wereperformed in time and frequency domain. The measurementsetup and the evaluation of the measurements will be de-scribed in Sect. 4. In the following conclusion, the results ofthe numerical field calculations and the measurements willbe compared to each other and evaluated as well.2 Modelling conceptIn this contribution, the investigation of the protective prop-erties of a missile enclosure against electromagnetic influ-ences is performed on the basis of a simplified enclosuremodel. Assuming that the conductivity of the enclosure wallsis sufficiently, only aperture penetration affects the couplingof electromagnetic fields to the interior of the missile enclo-sure. Thus, the model consists of a metallic hollow cylin-der equipped with an aperture at the front side. Such anopening could be necessary by different reasons e.g. for theoptical component of a target tracking system. The geo-metrical dimensions of the hollow cylinder are l=1 m andDcyl=100 mm. Furthermore, a monopole field probe is posi-tioned inside the hollow cylinder, in order to detect the fieldsinside the cylinder.Published by Copernicus Publications on behalf of the URSI Landesausschuss in der Bundesrepublik Deutschland e.V.64 S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic influences 10  Figure 1. Simulation model Fig. 1. Simulation model. 11 0 0.5 1 1.5 2 2.5 3 3.5 410-410-2100102Impedance Voltage Drop f / GHzU / V Figure 2. Calculated impedance voltage drop at the antenna base of the monopole Fig 2. C lculate imped nce vol age drop at the anten a base ofthe monopole.3 Numerical field calculationsSimulations have been carried out in frequency domain withthe numerical field calculation program CONCEPT-II. Theaim of these calculations is the investigation of the couplingmechanism of the electromagnetic fields from the surround-ings to the interior of the missile enclosure. Additionally,the simulation results could be used in order to validate themeasurement results.3.1 Simulation modelThe simulation model consists of an ideal conducting metal-lic hollow cylinder with the geometric dimensions l=1 m and 12  Figure 3. Calculated distribution of electric field strength at 1 GHz Fig. 3. Calculated distribution of electric field strength at 1 GHz. 13  Figure 4. Calculated distribution of electric field strength at 2 GHz Fig. 4. Calculated distribution of electric field strength at 2 GHz.Dcyl=100 mm, as mentioned in Sect. 2. The cylinder isequipped with an aperture (diameter Da=50 mm) at the frontside and a monopole probe positioned in a distance of 0.25 mwith respect to the aperture. The monopole is implementedby an ideal conducting wire, which lengths is lm=50 mm andconnected to the enclosure with a 50  resistor. The structureis illuminated by an incident electromagnetic plane wave asshown in Fig. 1. It should be mentioned that the monopoleis orientated parallel to the vector of the electric field of theplane wave. The impedance voltage drop which is equivalentto the antenna base voltage of a field probe in measurementscan calculated numerical by means of this simulation modelas well as the field distribution inside and outside the missileenclosure.3.2 Simulation resultsAll simulations have been performed in the frequency rangebetween 10 MHz and 4 GHz at discrete frequencies. The stepsize was 10 MHz, thus 400 different frequencies have beentreated within this investigation. The magnitude of the elec-tric field strength of the incident plane wave is E=1 kV/m.Figure 2 shows the calculated impedance voltage drop atthe antenna base of the monopole. While the voltage drop issmaller than 1 V in the frequency range below 1.5 GHz, itincreases rapidly at 1.8 GHz. In the frequency range above1.8 GHz, the voltage drop is several times larger than 1 Vfor frequencies, but shows more or less strong variations.Adv. Radio Sci., 5, 63–67, 2007 www.adv-radio-sci.net/5/63/2007/S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic influences 65 14  Figure 5. Measurement setup inside the GTEM cell Fig. 5. Measurement setup inside the GTEM cell.In order to find an explanation for this behaviour, the dis-tribution of the electric field strength has been examined fortwo different frequencies. The calculated magnitude of fieldstrength in the x-z-plane (with respect to the coordinate sys-tem in Fig. 1) is illustrated in Fig. 3 respectively in Fig. 4.While the magnitude is almost equal to the electric field inz-direction, only the magnitude is presented. Although abso-lute field strength values are not given, both figures show theactive principle very well.As it could be seen in both figures, the incident wave prop-agates from the right side to the left one. While the wavepropagation outside the hollow cylinder is nearly unaffecteddue to its presence at 1 GHz, field disturbances occur in frontof the aperture at 2 GHz. Furthermore, the magnitude of thefield strength inside the enclosure is negligible at the lowerfrequency, whereas wave propagation could be found insidethe hollow cylinder at the higher frequency.This frequency dependent behaviour is affected by the ge-ometry of the simulation model, because the missile modelforms a cylindrical waveguide. The cut-off frequency of acertain mode is the characteristic quantity of a waveguide.This quantity is given by Eq. (A1) in case of a transverseelectrical mode (TEmn-mode) respectively by Eq. (A2) incase of a transverse magnetic mode (THmn-mode). Ta-ble 1 gives an overview about cut-off frequencies of the dif-ferent modes which are able to propagate in a waveguidewith D=100 mm. The fundamental mode is the TE11-modeand the correspondent cut-off frequency is approximately1.75 GHz.Taking into consideration that the enclosure forms a cylin-drical waveguide, the different field distributions can be ex-plained easily. The incident electromagnetic wave couples tothe waveguide because of aperture penetration at both inves-tigated frequencies. While the cylindrical waveguides oper-ates at 1 GHz below the frequency cut-off, there is no wavepropagation possible and the electromagnetic fields inside 15  Figure 6. Different apertures and terminations, monopole probe Fig. 6. Different apertures and terminations, monopole probe.Table 1. Cut-off frequencies of a metallic tube with D=100 mm.Mode Bessel n-th Function Cut-offfunction root Value frequencyTE11 J ′1(x) 1 1.841 1.757 GHzTM01 J0(x) 1 2.405 2.295 GHzTE21 J ′2(x) 1 3.054 2.914 GHzTE11 J ′0(x) 1 3.832 3.657 GHzTM11 J1(x) 1 3.832 3.657 GHzthe metallic tube are dumped exponentially. The things aredifferent in the higher frequency range due to the fact thatthe waveguide operates above its frequency cut-off. As il-lustrated in Fig. 4, the wave excited by aperture penetrationpropagates from the right side to the left one. The metalliccap on the left side forms a short termination of the waveg-uide, thus the wave is reflected completely. The reflectedwave propagates in backward direction and reaches the aper-ture, where it is reflected partly, as well as the radiated. Thus,not only standing waves occur inside the enclosure, but alsofield disturbances are affected in front of the hollow cylinder.4 MeasurementsMeasurements of the hollow cylinder have been preformedwith continuous wave signals in the frequency domain. Theaim of these measurements is the investigation of differentaperture sizes and terminations.4.1 Measurement setupA GTEM cell is a well suited environment in order to gene-rate defined electromagnetic fields. Thus, the measurementshave been performed in a GTEM 1250. Therefore, the metal-lic hollow cylinder is placed parallel to the ground planeof the cell as illustrated in Fig. 5. The aperture is orientedwww.adv-radio-sci.net/5/63/2007/ Adv. Radio Sci., 5, 63–67, 200766 S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic influences 16 0 0.5 1 1.5 2 2.5 3 3.5 4-80-60-40-20020Metallic Tube terminated with Shortf / GHzS 21 / dB  10mm30mm50mm100mm Figure 7. Variation of the aperture size, tube terminated with short Fig. 7. Variation of the aperture size, tube terminated with short. 17 0 0.5 1 1.5 2 2.5 3 3.5 4-80-60-40-20020Metallic Tube with Aperture (D=30mm)f / GHzS 21 / dB  openshortload Figure 8. Variation of the termination (aperture size Da = 30 mm) Fig. 8. Variation of the termination (aperture size Da=30 mm).towards the cell port and the open ending towards the ab-sorbing cones. The monopole probe inside the metallic tubeis positioned in vertical direction with respect to the groundplane, thus it measured the electric field in z-direction. Itslength is lm=5 mm and it is positioned in a distance of 0.25 mto the aperture.The metallic tube can be equipped with different apertureson one end and terminations on the other. Figure 6 showsthe different apertures, which are realized due to metallicdiscs with centric holes. A metallic support fixes the differ-ent apertures at the front side of the tube. Possible termina-tions of the open ending of the tube are open, short and load.The load emulates the loading of the cylinder with a printedcircuit board (PCB) and is implemented by absorbing foamwith cylindrical shape. A network analyzer serves as powersource as well as receiver. Therefore, one port of the net-work analyzer is connected to the cell port and the other tothe monopole probe. 18 0 0.5 1 1.5 2 2.5 3 3.5 4-80-60-40-20020Metallic Tube with Aperture (D=50mm)f / GHzS 21 / dB  openshortload Figure 9. Variation of the termination (aperture size Da = 50 mm)     Fig. 9. Variation of the termination (aperture size Da=50 mm).4.2 Measurement resultsAll measurements have been performed with CW (continu-ous wave) signals in the frequency range from 10 MHz upto 4 GHz at 400 discrete frequencies. While the measuredquantity is the transfer function between the cell port and themonopole, the frequency response of the monopole has tobe taken into consideration. Thus, the S21-parameter denotesthe corrected transfer function in the following.The influence of different terminations is shown in Fig. 8for an aperture size of D=30 mm. The magnitude of the mea-sured S21-parameter is less than −50 dB in the frequencyrange below 1.75 GHz and nearly independent from the ter-mination. At approximately 1.75 GHz, there is an abrupt riseof this parameter. While the termination with short shows al-ternating minimums and maximums in the higher frequencyrange, the deviation is smaller for a termination with open.Furthermore, the termination with load has got a lower mag-nitude than the termination open, because the absorbing foamdumps the resonance effects.The rapid increase of the transfer function at approxi-mately 1.75 GHz is caused by the geometrical dimensionsof the metallic tube, because the tube forms a cylindricalwaveguide. The cut-off frequency of the fundamental modeis 1.757 GHz with respect to Eq. (A1) and Table 1. Be-low 1.757 GHz, the electromagnetic field excited at the aper-ture is an evanescent field and dumped exponentially. Anincrease of the aperture size affects of a slow rise in thefrequency range between 1.3 GHz and 1.75 GHz, thus theabrupt increase at the cut-off frequency is more flat (seeFig. 7). Nevertheless, the magnitude is larger in the upperfrequency range than in case of a smaller aperture, as shownin Fig. 9. Above the cut-off frequency of the fundamentalmode, transverse electrical waves (TE11-mode) can be ex-cited at the aperture. These waves propagate towards the ter-mination, where they are reflected more or less depending onAdv. Radio Sci., 5, 63–67, 2007 www.adv-radio-sci.net/5/63/2007/S. Fisahn and H. Garbe: Protective properties of a missile enclosure against electromagnetic influences 67the termination. The reflected waves propagate towards theaperture, where the waves will be partially reflected respec-tively radiated to the surroundings. It is quite obvious thatmetallic tube operates as a cavity resonator, thus the mag-nitude of transfer function reaches values larger than 0 dBat certain frequencies. The termination with load effects adumping of the resonances.5 ConclusionsIn this paper the shielding properties of a missile enclosurewas examined by numerical field calculations as well as bymeasurements. Under assumption of a sufficient conductiv-ity of the enclosure walls, the coupling to the inside is af-fected by aperture penetration. Therefore, the missile enclo-sure was emulated by means of a metallic hollow cylinder,which includes an aperture in the top cap. Furthermore, amonopole field probe was positioned inside the cylinder inorder to detect the excited electric field inside.The simulations point out the active principle very well.The metallic hollow cylinder behaves like a cylindricalwaveguide. Below the first cut-off frequency, only evanes-cent fields are present in the waveguide. In the higherfrequency range, the fundamental mode (TE11-mode) orother higher order modes are able to propagate. The calcu-lated voltage at the antenna base of the monopole increasesrapidly. While the enclosure is ideal conductive, the hollowcylinder forms a cavity with the high quality factor. The res-onances will be dumped only by losses due to radiated emis-sions at the aperture.The measurements show results, which are very similarto the numerical calculations ones. Below the first cut-offfrequency, only evanescent fields are present in the waveg-uide. In the higher frequency range, the fundamental mode isable to propagate and the field strength increases rapidly. Thevariation of the aperture shows that a growing aperture sizeincreases the field excitation in the higher frequency rangeabove 1.75 GHz.The different terminations of the waveguides influence thedumping of the standing waves. While a cylinder terminatedwith a metallic cap (short) shows the strongest resonance be-haviour, an open ending effects a higher dumping due to ad-ditional radiated emissions. The most realistic setup, whichincludes a PCB model that consists of absorbing foam, showsthe smallest resonance effects.Appendix ACut-off frequencies of cylindrical waveguidesThe characteristic quantity of a waveguide is the cut-off fre-quency fc of a certain mode. In case of a cylindrical waveg-uide it is given for transverse electrical modes (TEmn-modes)byf T Ec,mn =c0pi√µrεr· x′mnD, (A1)where m, n are integers and D is the diameter of the metallictube. x′mn is the n-th root of the deviation of Bessel functionJ ′m(x). εr and µr are the relative permittivity respectivelypermeability of the material inside the waveguide and c0 isthe speed of the light. In case of transverse magnetic modes(TMmn-modes), the cut-off frequency is dependent to the n-th root xmn of the of Bessel function Jm(x):f TMc,mn =c0pi√µrεr· xmnD. (A2)Acknowledgements. This investigation is part of the study “Protec-tion of Electronic Systems to electromagnetic Influences, Phase II”,commissioned by the Armed Forces Scientific Institute for Protec-tion Technologies and NBC Protection (Munster, Germany). Theresponsibility for the content is with the authors.ReferencesCollin, R. E.: Field theory of guided waves, IEEE Press, New York,1991.Groh, C., Ka¨rst, J. P., Koch, M., and Garbe, H.: TEM Waveguidesfor EMC Measurements, IEEE Transactions on ElectromagneticCompatibility, 41(4), 440–445, Nov., 1999.Nitsch, D., Camp, M., Sabath, F., ter Haseborg, J. L., and Garbe, H.:Susceptibility of Some Electronic Equipment to HPM Threats,IEEE Transactions on Electromagnetic Compatibility, 46(3),380–389, Aug., 2004.www.adv-radio-sci.net/5/63/2007/ Adv. Radio Sci., 5, 63–67, 2007

Protective properties of a missile enclosure against electromagnetic influences

Abstract

Similar works

Full text

Available Versions

Directory of Open Access Journals

Institutionelles Repositorium der Leibniz Universität Hannover