This work describes a method to detect and to quantify any local or mid-range discontinuity on extended flat metal planes. Often these planes are used for antenna calibration (open area test site-OATS) or the plane could be the ground of a semi-anechoic chamber used in Electromagnetic Compatibility (EMC) testing. The measurement uncertainty of antenna calibration or EMC testing depends on the groundplane's quality, which can be accessed using this method. A vector network analyzer with time-domain option is used to determine the complex-valued input scattering parameter S̄11,F of an aperture antenna in a monostatic setup. S̄11,F contains the information desired about the discontinuities and is measured in the frequency domain with high dynamic range. But only after a linear filtering utilizing the Chirp-Z-Transform the obtained time-domain signal S̄11,T give clear evidence of local and mid-range discontinuities

Battermann, S.

Garbe, Heyno

Münter, K.

Schrader, T.

Advances in Radio Science

Institutionelles Repositorium der Leibniz Universität Hannover

Advances in Radio Science, 3, 99–103, 2005SRef-ID: 1684-9973/ars/2005-3-99© Copernicus GmbH 2005Advances inRadio ScienceMeasured Response of Local, Mid-range and Far-rangeDiscontinuities of Large Metal Groundplanes using Time DomainTechniquesT. Schrader1, K. Mu¨nter1, S. Battermann2, and H. Garbe 21Physikalisch-Technische Bundesanstalt, AG 2.21, Bundesallee 100, 38116 Braunschweig, Germany2Institut fu¨r Grundlagen der Elektrotechnik und Messtechnik, Universita¨t Hannover, Appelstraße 9a, 30167 Hannover,GermanyAbstract. This work describes a method to detect and toquantify any local or mid-range discontinuity on extendedflat metal planes. Often these planes are used for antennacalibration (open area test site – OATS) or the plane couldbe the ground of a semi-anechoic chamber used in Electro-magnetic Compatibility (EMC) testing. The measurementuncertainty of antenna calibration or EMC testing dependson the groundplane’s quality, which can be accessed usingthis method.A vector network analyzer with time-domain option isused to determine the complex-valued input scattering pa-rameter S11,F of an aperture antenna in a monostatic setup.S11,F contains the information desired about the discontinu-ities and is measured in the frequency domain with high dy-namic range. But only after a linear filtering utilizing theChirp-Z-Transform the obtained time-domain signal S11,Tgives clear evidence of local and mid-range discontinuities.1 IntroductionCalibrated antennas are used for EMC testing and in partic-ular for emissions measurements. In the calibration processof antennas the antenna factor AF is obtained, which con-verts the electrical field strength E into a voltage U acrossa 50 load resistor. For some applications the free-spaceAF is needed, for others the height-dependent AF is utilized.In theory the height-dependent AF may be obtained best onan ideal large open area test site (OATS), which can be de-scribed as a lossless metal sheet, with infinite extension andwith optical flatness in terms of a fraction of a wavelength.Real OATS are limited in size and the metal plane is con-structed from single plates made from lossy material. Theextended metal plane (groundplane) on a basement withoutor small elevation compared to the surrounding area may beassembled by different techniques. The individual parts canCorrespondence to: T. Schrader(thorsten.schrader@ptb.de)either be screwed down tightly on a supporting structure un-derneath with a conducting foil filling the gap between thesingle plates or can be assembled by welding. Choosing thelast, each welding has to be carried out very carefully duringconstruction, otherwise steps in height crossing the edges ofthe plates will occur. Spot-wise welding will lead to a dis-continuous metal plane, raised welding beads will behave asreflectors. All of the described defects can be subsumed aslocal discontinuities, adding their part to measurement un-certainty of the AF.Another part of uncertainty is caused by the finite size ofthe groundplane. All fields emanating from a source on theOATS find boundary conditions determined by the ground-plane itself. Suppose a very low-loss material, only verti-cal electrical fields and horizontal magnetic fields can existin nearest vicinity of the metal plane. Surface currents andcharges maintain the continuity of the fields at the interfaceof metal and the half space of air above. For the case ofan ideal groundplane with dimensions extended to infinitythe surface currents and charges on the metal plane are notdistorted by a sudden change of the material’s distribution.In such a case both the time-averaged and the instantaneousPoynting vector show only a uniform direction of propaga-tion of electromagnetic waves generated on the OATS. At theedge or at the end of a real metallic groundplane the bound-ary conditions for electromagnetic waves are altered quitesuddenly. This can be seen by a noticeable backward com-ponent of the Poynting vector. Therefore, special measureshave to be taken to reduce the influence of this midrange dis-continuity. The method described in this work can be used toobtain measurement data for the reflectivity of groundplaneedges, which gives rise to measurement uncertainty again.But local and mid-range discontinuities are not the onlyreasons for measurement uncertainty in calibration of anten-nas on large OATS stemming from the basic installation. Wealso have to look at the environment of such OATS, both interms of the electromagnetics (interfering signals emanatingfrom other sources) and the mechanics. For the moment weare only interested in the mechanical structure of the area100 T. Schrader et al.: Measured Response of Local, Mid-range and Far-range Discontinuitiesencompassing the OATS. With increasing distance betweenthe OATS and any trees, bushes, shelters, houses or otherreflective items and installations like e.g. fences, these far-range discontinuities will contribute less and less to the un-certainty of measurement due to a very low signal level. Butoften the surrounding area can not be kept free of reflectiveitems and the measurement setup should be able to detectfar-range reflective parts even despite long delay time of theround-trip signals. Far-range discontinuities will be subjectto another work and are not dealt with here.Yet we have not discussed the influence of e.g. antennatowers, restraints, and cabling. Usually all materials beingused as mechanical support in RF measurement show proper-ties with complex dielectric constant. In order to carry bulkyantennas the mechanics of supporting structures is designedas stiff as needed but as light as possible. Over the yearsglass reinforced plastics (GRP) and other materials with lowdielectric constant have been developed to solve the problemof contradictory demands in RF mechanical setups. But still,influence of material’s properties on the measurement datahas to be taken into account, particularly in applications likeantenna calibration. For the given purpose of antenna cal-ibration on OATS, local, mid- and far-range discontinuitiescontribute to the measurement uncertainty. Discrepancies be-come especially visible, if measurement data is compared totheoretical numbers obtained by e.g. the Method of Momentsor analytical solutions.This work describes a measurement setup to detect localand mid-range discontinuities on OATS, their position andthe order of influence. The method described can be usedsimultaneously with the construction progress of a ground-plane as an inspection tool as well as for verification in aquality assurance program.2 Theory and Measurement SetupIn order to detect the unwanted backward propagation of thePoynting vector the complex-valued input scattering param-eter S11 of an antenna is measured as a function of frequency.Two different aperture antennas have been tested in the fre-quency range from 5.85 GHz to 8.2 GHz. Among these aretwo commercially available antennas, a standard gain hornand a double ridged guide horn antenna. A 40 GHz vectornetwork analyzer (VNA) is utilized as instrumentation. Be-fore any measurement the VNA is calibrated at the end ofthe coaxial cable in 1-port reflection. For all calculationslater on, this plane is referred to as the reference plane, towhich the phase of any signal is normalized. The two differ-ent types of antennas are connected consecutively to port 1 ofthe VNA, always with the plane of the aperture on a referencelocation. Each antenna is mounted close to the groundplanewith the normal vector of its aperture parallel to the surface ofthe groundplane. The electrical field vector is always perpen-dicular to the groundplane, the magnetic field vector is paral-lel to it. Due to this arrangement we obtain a stacked antennasystem by the antenna itself and the mirror object caused bythe metallic groundplane. The radiated energy is effectivelyconcentrated in the area of concern with the Poynting vectorparallel and close to the surface of the groundplane. As theantenna is close to the metallic structure of the groundplane,the last determines the boundary conditions for the fields ra-diated in the half space above. In the ideal case a lossless,smooth and flat plane with infinite lateral dimensions guidesthe wave towards infinity without any reflection. Any distur-bance of the ideal case generates a contribution to a reflectedsignal, which will add with respect to the phase and ampli-tude to the input scattering parameter S11,F of the antenna inthe frequency domain.The measured frequency domain data is transformed to thetime-domain using the Chirp-Z-transform (Ersoy, 1997) builtinto the VNA. A Hanning window is applied during the trans-formation. After normalization the time-domain data can betreated as a reflection in arbitrary units as function of distanceto the reference plane. With time-domain gating the interest-ing band of distance can be selected and the magnitude ofdata vs. distance is stored afterwards for further calculations.For simplicity the data obtained is called S11,T throughoutthe text.Usually twice the distance between the reference plane andthe antenna aperture is the start distance for the area which islooked at, counting from the reference plane. The unambigu-ous range in time-domain and, therefore, the stop distance isobtained from the VNA and depends on the frequency rangeapplied. The Chirp-Z-transform provides an increased but fi-nite range of valid values for the abscissa, which is a distancein this application. The advantage of this particular form ofthe Discrete Fourier Transform in the kind of linear filteringis the capability to choose time and frequency sampling in-tervals at will (Ersoy, 1997).3 ResultsIn order to verify the obtained results of reflection vs. dis-tance, simple but small objects have been positioned on thegroundplane of a semi-anechoic chamber in a known dis-tance to the antenna aperture. As test objects a) a thin metalplate of 0.7 mm thickness and 70 cm×28 cm length, b) a sty-rofoam plate of about 25 cm length and 2 cm thickness and c)a small 11 mm long screw have been used consecutively. Bymovement of one object at a time over a known distance, thechange in S11,T was recorded as well as S11,T ,empty withoutany of the aforementioned additional items. Moving one ofthe items towards the antenna’s aperture or away from it, thecurvature of S11,T vs. distance will change at two spots. Atthe former place the line will drop to S11,T ,empty and at thenew location the correspondent level will be increased to thelevel of reflection caused by the individual object (Fig. 1).The other spikes stem from other local discontinuities suchas gaps between the single metal plates or access hatches forelectricity and other connections needed in a semi-anechoicchamber. Note that in frequency domain the shift of S11,Fdue to the present object is nearly not noticeable, while it canT. Schrader et al.: Measured Response of Local, Mid-range and Far-range Discontinuities 101Fig. 1. Reflection in arbitrary units vs. distance measured with a standard gain horn antenna. The difference in both curves is caused by atest object (vertical screw on the ground, 11 mm height), giving rise to reflection in 4.86 m distance to the antenna’s aperture (red spike). Thegreen curve shows data for the same measurement setup, but without the test object. The other spikes stem from local discontinuities of thegroundplane.clearly be detected in the time domain response S11,T . Butstill it is an advantage to utilize the dynamic range of fre-quency measurements, therefore, the source power level ofthe VNA of 10 dBm is adequate. What is not included inthe results yet is a correction for the site attenuation. Reflec-tions from discontinuities far away from the antenna sustaina greater attenuation than those close to the antenna. Thesignals from the test object c) may be a first estimate for theattenuation vs. distance (Fig. 2), because the reflectivity ofthe objects remains the same for all positions, but the mea-sured reflectivity level is decreasing. The attenuation follows1/r2.In principle, the input scattering parameter S11 of anantenna describes the superposition of internal reflectionsinside the antenna, reflections due to objects in front ofthe antenna weighted with the antenna pattern, and theinfluence of repeatability of coaxial connections. While thelast can not be influenced by the setup except for using aprecise PC- 7 mm connector, the influence of the antennais investigated. Besides the internal reflections, also theantenna pattern, the height and the tilt angle of the plane ofthe antenna’s aperture against the vertical direction may havean influence. To obtain the influence of different antennas,the same setup with test object c) (screw) was measuredwith each of the afore-mentioned antennas. All antennas areused with their aperture at the same location, but they differin length of the flared section and their feeding waveguide,while the reference plane for the phase is maintained fromcalibration of the VNA. Therefore, the same object at thesame position will be found in different distances from theFig. 2. Reflection (in arbitrary units) of a vertical screw with 11 mmlength as a function of distance to the aperture of a standard gainhorn antenna. The screw represents a local discontinuity. The de-creasing level of reflection is due to site attenuation and is not cor-rected in this graph. It follows 1/r2.antenna’s aperture, as can be seen in Fig. 3. What can alsobe seen is that reflections are much more pronounced usingthe standard gain horn. The reasons are a better definedradiation (and reception) pattern for a smaller solid angleand a closer match to the 50  of the feeding coaxial cable.The standard gain horn antenna turned out to be best suitedfor the given purpose, so we investigated tilt angle andheight variations with it. For this purpose we compare therelative change on the different setups.102 T. Schrader et al.: Measured Response of Local, Mid-range and Far-range DiscontinuitiesFig. 3. Reflection in arbitrary units vs. distance measured with a standard gain horn antenna (green) and a double ridged guide horn antenna(red). The reflections are caused by local discontinuities of a groundplane in a semi-anechoic chamber used for emission measurements ofElectromagnetic Compatibility. The shift vs. distance is due to different lengths of the antennas with respect to the same reference plane atthe vector network analyzer port. The apertures of both antennas are at the same location.Fig. 4. Reflection in arbitrary units of a 0.75 mm metal plate on themetal groundplane. From the two setups with the plate 3.76 m and9.19 m away from the antenna’s aperture, the local resolution canbe observed. Using frequency data in the range from 5.85 GHz to8.2 GHz, a resolution of 2 cm can be achieved, which is more thansufficient to localize any discontinuity of the groundplane.Tilting angles of ±5◦ out of the vertical direction havebeen found already critical for a 10 m distance. Althoughfor a C band standard gain horn antenna the half-power beamwidth (HPBW) in the E plane is around 25◦ (Balanis, 1997),the spread of variation in S11,T due to a varying tilt angle isequivalent to the reflection of test object c) in 10 m distance.We obtained the lowest reading for +5◦ and the highest for–5◦, while for 0◦ the result was in between the others. TheHPBW in the H plane is also about 25◦. In 10 m distancefrom the horn aperture the beam has opened to a width of2 m. Any scattering object or local discontinuity can be lo-calized on the radius within this solid angle. To localize itwithin the angular distance, a method of triangulation has tobe applied. To access larger areas a scanning procedure hasto be applied.Varying now the height of the lower edge of the antenna’saperture to the ground, we obtained the highest reflection lev-els for 2 cm height and less for 0 cm, 5 cm, 13 cm and 23 cm.We do not expect much more deviation for smaller distances.For measurement distances up to 10 m from the aperture aheight of a few cm can be chosen arbitrarily and is not a crit-ical parameter as long as a resistive contact to the ground-plane is avoided and care is taken about the alignment of theantenna without boresight displacement.The 0.7 mm thick metal plate lying flat on the ground asthe second test object was spotted with its front and backedge at 3.76 m and at 9.19 m distance from the antenna’saperture (Fig. 4). From this data the local resolution can beobtained. Using frequency data in the range from 5.85 GHzto 8.2 GHz, a resolution of 2 cm can be achieved, whichis more than sufficient to localize any discontinuity of thegroundplane.One real test object is the installation of a turntable in asemi-anechoic chamber used for EMC emission measure-ments. Manufacturers of such devices design the junctionbetween stationary and moving parts with sliding contactsto maintain a smooth transition for the RF surface currents.Applying the described procedure this property of turntablesmay be verified. Figure 5 gives an example of measurementsT. Schrader et al.: Measured Response of Local, Mid-range and Far-range Discontinuities 103Fig. 5. Reflection in arbitrary units vs. distance of a turntable usedin semi-anechoic chambers for EMC testing. To make any of thediscontinuity more visible, the 11 mm long screw was set at differ-ent locations like begin and end of the turntable. The cover plate onthe access hatch (center plate) can be closed in different orientations(here: rotation of about 90◦), which can be noticed by different re-flections of the grip holes.on such a turntable. At first the transition from the ground-plane to the rim of the turntable occurs as a spike, followedby the next transition from the rim to the turntable itself, tothe moveable center plate covering electrical installation fortest equipment and vice versa.4 ConclusionA method has been described to detect and to quantify anylocal or mid-range discontinuity on extended and flat metalplanes utilizing a vector network analyzer with time-domainoption. As a measure for the discontinuities the reflectionlevel of a stacked antenna using an aperture antenna justabove and its mirror image below the metal groundplane isused. The transmitted energy is focused by the mono-staticsetup and is travelling as a wave along the metal surface withthe Poynting vector being parallel to it. The metal planedetermines the boundary conditions for the wave which iscontinued by surface currents and charges on the metal sur-face. Any discontinuity disrupts the undistorted distributionof currents and charges and, therefore, the energy transport.The formerly unidirectional Poynting vector is superimposedwith a backward component which is then received by the an-tenna. The complex-valued input scattering parameter S11,Fcontains the superposition of the antenna’s internal reflec-tions and contributions from any object in front of the an-tenna and from the connector. It is obvious that well-matchedstandard gain horn antennas with their defined radiation andreception pattern fit best the given purpose. Although thefrequency domain signal S11,F contains all information wehave been looking for, it can not be seen very easily. After aChirp-Z-transform the time-domain signal S11,T gives clearevidence of local and mid-range discontinuities.ReferencesBalanis, C. A.: Antenna Theory, Second Edition, John Wiley andSons, NY, ISBN 0-471-59268-4, 1997.Ersoy, O.: Fourier-Related Transforms, Fast Algorithms and Appli-cations, Prentice Hall, NJ, ISBN 0-13-624412-2, 1997.

Measured Response of Local, Mid-range and Far-range Discontinuities of Large Metal Groundplanes using Time Domain Techniques

This work describes a method to detect and to quantify any local or mid-range discontinuity on extended flat metal planes. Often these planes are used for antenna calibration (open area test site - OATS) or the plane could be the ground of a semi-anechoic chamber used in Electromagnetic Compatibility (EMC) testing. The measurement uncertainty of antenna calibration or EMC testing depends on the groundplane&apos;s quality, which can be accessed using this method.  A vector network analyzer with time-domain option is used to determine the complex-valued input scattering parameter S11,F of an aperture antenna in a monostatic setup. S11,F contains the information desired about the discontinuities and is measured in the frequency domain with high dynamic range. But only after a linear filtering utilizing the Chirp-Z-Transform the obtained time-domain signal S11,T evidence of local and mid-range discontinuities

T. Schrader

K. Münter

S. Battermann

H. Garbe

Directory of Open Access Journals

Advances in Radio Science, 3, 99–103, 2005SRef-ID: 1684-9973/ars/2005-3-99© Copernicus GmbH 2005Advances inRadio ScienceMeasured Response of Local, Mid-range and Far-rangeDiscontinuities of Large Metal Groundplanes using Time DomainTechniquesT. Schrader1, K. M ünter1, S. Battermann2, and H. Garbe 21Physikalisch-Technische Bundesanstalt, AG 2.21, Bundesallee 100, 38116 Braunschweig, Germany2Institut für Grundlagen der Elektrotechnik und Messtechnik, Universität Hannover, Appelstraße 9a, 30167 Hannover,GermanyAbstract. This work describes a method to detect and toquantify any local or mid-range discontinuity on extendedflat metal planes. Often these planes are used for antennacalibration (open area test site – OATS) or the plane couldbe the ground of a semi-anechoic chamber used in Electro-magnetic Compatibility (EMC) testing. The measurementuncertainty of antenna calibration or EMC testing dependson the groundplane’s quality, which can be accessed usingthis method.A vector network analyzer with time-domain option isused to determine the complex-valued input scattering pa-rameterS11,F of an aperture antenna in a monostatic setup.S11,F contains the information desired about the discontinu-ities and is measured in the frequency domain with high dy-namic range. But only after a linear filtering utilizing theChirp-Z-Transform the obtained time-domain signalS11,Tgives clear evidence of local and mid-range discontinuities.1 IntroductionCalibrated antennas are used for EMC testing and in partic-ular for emissions measurements. In the calibration processof antennas the antenna factorAF is obtained, which con-verts the electrical field strengthE into a voltageU acrossa 50 load resistor. For some applications the free-spaceAF is needed, for others the height-dependentAF is utilized.In theory the height-dependentAF may be obtained best onan ideal large open area test site (OATS), which can be de-scribed as a lossless metal sheet, with infinite extension andwith optical flatness in terms of a fraction of a wavelength.Real OATS are limited in size and the metal plane is con-structed from single plates made from lossy material. Theextended metal plane (groundplane) on a basement withoutor small elevation compared to the surrounding area may beassembled by different techniques. The individual parts canCorrespondence to:T. Schrader(thorsten.schrader@ptb.de)either be screwed down tightly on a supporting structure un-derneath with a conducting foil filling the gap between thesingle plates or can be assembled by welding. Choosing thelast, each welding has to be carried out very carefully duringconstruction, otherwise steps in height crossing the edges ofthe plates will occur. Spot-wise welding will lead to a dis-continuous metal plane, raised welding beads will behave asreflectors. All of the described defects can be subsumed aslocal discontinuities, adding their part to measurement un-certainty of theAF.Another part of uncertainty is caused by the finite size ofthe groundplane. All fields emanating from a source on theOATS find boundary conditions determined by the ground-plane itself. Suppose a very low-loss material, only verti-cal electrical fields and horizontal magnetic fields can existin nearest vicinity of the metal plane. Surface currents andcharges maintain the continuity of the fields at the interfaceof metal and the half space of air above. For the case ofan ideal groundplane with dimensions extended to infinitythe surface currents and charges on the metal plane are notdistorted by a sudden change of the material’s distribution.In such a case both the time-averaged and the instantaneousPoynting vector show only a uniform direction of propaga-tion of electromagnetic waves generated on the OATS. At theedge or at the end of a real metallic groundplane the bound-ary conditions for electromagnetic waves are altered quitesuddenly. This can be seen by a noticeable backward com-ponent of the Poynting vector. Therefore, special measureshave to be taken to reduce the influence of this midrange dis-continuity. The method described in this work can be used toobtain measurement data for the reflectivity of groundplaneedges, which gives rise to measurement uncertainty again.But local and mid-range discontinuities are not the onlyreasons for measurement uncertainty in calibration of anten-nas on large OATS stemming from the basic installation. Wealso have to look at the environment of such OATS, both interms of the electromagnetics (interfering signals emanatingfrom other sources) and the mechanics. For the moment weare only interested in the mechanical structure of the area100 T. Schrader et al.: Measured Response of Local, Mid-range and Far-range Discontinuitiesencompassing the OATS. With increasing distance betweenthe OATS and any trees, bushes, shelters, houses or otherreflective items and installations like e.g. fences, these far-range discontinuities will contribute less and less to the un-certainty of measurement due to a very low signal level. Butoften the surrounding area can not be kept free of reflectiveitems and the measurement setup should be able to detectfar-range reflective parts even despite long delay time of theround-trip signals. Far-range discontinuities will be subjectto another work and are not dealt with here.Yet we have not discussed the influence of e.g. antennatowers, restraints, and cabling. Usually all materials beingused as mechanical support in RF measurement show proper-ties with complex dielectric constant. In order to carry bulkyantennas the mechanics of supporting structures is designedas stiff as needed but as light as possible. Over the yearsglass reinforced plastics (GRP) and other materials with lowdielectric constant have been developed to solve the problemof contradictory demands in RF mechanical setups. But still,influence of material’s properties on the measurement datahas to be taken into account, particularly in applications likeantenna calibration. For the given purpose of antenna cal-ibration on OATS, local, mid- and far-range discontinuitiescontribute to the measurement uncertainty. Discrepancies be-come especially visible, if measurement data is compared totheoretical numbers obtained by e.g. the Method of Momentsor analytical solutions.This work describes a measurement setup to detect localand mid-range discontinuities on OATS, their position andthe order of influence. The method described can be usedsimultaneously with the construction progress of a ground-plane as an inspection tool as well as for verification in aquality assurance program.2 Theory and Measurement SetupIn order to detect the unwanted backward propagation of thePoynting vector the complex-valued input scattering param-eterS11 of an antenna is measured as a function of frequency.Two different aperture antennas have been tested in the fre-quency range from 5.85 GHz to 8.2 GHz. Among these aretwo commercially available antennas, a standard gain hornand a double ridged guide horn antenna. A 40 GHz vectornetwork analyzer (VNA) is utilized as instrumentation. Be-fore any measurement the VNA is calibrated at the end ofthe coaxial cable in 1-port reflection. For all calculationslater on, this plane is referred to as the reference plane, towhich the phase of any signal is normalized. The two differ-ent types of antennas are connected consecutively to port 1 ofthe VNA, always with the plane of the aperture on a referencelocation. Each antenna is mounted close to the groundplanewith the normal vector of its aperture parallel to the surface ofthe groundplane. The electrical field vector is always perpen-dicular to the groundplane, the magnetic field vector is paral-lel to it. Due to this arrangement we obtain a stacked antennasystem by the antenna itself and the mirror object caused bythe metallic groundplane. The radiated energy is effectivelyconcentrated in the area of concern with the Poynting vectorparallel and close to the surface of the groundplane. As theantenna is close to the metallic structure of the groundplane,the last determines the boundary conditions for the fields ra-diated in the half space above. In the ideal case a lossless,smooth and flat plane with infinite lateral dimensions guidesthe wave towards infinity without any reflection. Any distur-bance of the ideal case generates a contribution to a reflectedsignal, which will add with respect to the phase and ampli-tude to the input scattering parameterS11,F of the antenna inthe frequency domain.The measured frequency domain data is transformed to thetime-domain using the Chirp-Z-transform (Ersoy, 1997) builtinto the VNA. A Hanning window is applied during the trans-formation. After normalization the time-domain data can betreated as a reflection in arbitrary units as function of distanceto the reference plane. With time-domain gating the interest-ing band of distance can be selected and the magnitude ofdata vs. distance is stored afterwards for further calculations.For simplicity the data obtained is calledS11,T throughoutthe text.Usually twice the distance between the reference plane andthe antenna aperture is the start distance for the area which islooked at, counting from the reference plane. The unambigu-ous range in time-domain and, therefore, the stop distance isobtained from the VNA and depends on the frequency rangeapplied. The Chirp-Z-transform provides an increased but fi-nite range of valid values for the abscissa, which is a distancein this application. The advantage of this particular form ofthe Discrete Fourier Transform in the kind of linear filteringis the capability to choose time and frequency sampling in-tervals at will (Ersoy, 1997).3 ResultsIn order to verify the obtained results of reflection vs. dis-tance, simple but small objects have been positioned on thegroundplane of a semi-anechoic chamber in a known dis-tance to the antenna aperture. As test objects a) a thin metalplate of 0.7 mm thickness and 70 cm×28 cm length, b) a sty-rofoam plate of about 25 cm length and 2 cm thickness and c)a small 11 mm long screw have been used consecutively. Bymovement of one object at a time over a known distance, thechange inS11,T was recorded as well asS11,T ,empty withoutany of the aforementioned additional items. Moving one ofthe items towards the antenna’s aperture or away from it, thecurvature ofS11,T vs. distance will change at two spots. Atthe former place the line will drop toS11,T ,empty and at thenew location the correspondent level will be increased to thelevel of reflection caused by the individual object (Fig. 1).The other spikes stem from other local discontinuities suchas gaps between the single metal plates or access hatches forelectricity and other connections needed in a semi-anechoicchamber. Note that in frequency domain the shift ofS11,Fdue to the present object is nearly not noticeable, while it canT. Schrader et al.: Measured Response of Local, Mid-range and Far-range Discontinuities 101Fig. 1. Reflection in arbitrary units vs. distance measured with a standard gain horn antenna. The difference in both curves is caused by atest object (vertical screw on the ground, 11 mm height), giving rise to reflection in 4.86 m distance to the antenna’s aperture (red spike). Thegreen curve shows data for the same measurement setup, but without the test object. The other spikes stem from local discontinuities of thegroundplane.clearly be detected in the time domain responseS11,T . Butstill it is an advantage to utilize the dynamic range of fre-quency measurements, therefore, the source power level ofthe VNA of 10 dBm is adequate. What is not included inthe results yet is a correction for the site attenuation. Reflec-tions from discontinuities far away from the antenna sustaina greater attenuation than those close to the antenna. Thesignals from the test object c) may be a first estimate for theattenuation vs. distance (Fig. 2), because the reflectivity ofthe objects remains the same for all positions, but the mea-sured reflectivity level is decreasing. The attenuation follows1/r2.In principle, the input scattering parameterS11 of anantenna describes the superposition of internal reflectionsinside the antenna, reflections due to objects in front ofthe antenna weighted with the antenna pattern, and theinfluence of repeatability of coaxial connections. While thelast can not be influenced by the setup except for using aprecise PC- 7 mm connector, the influence of the antennais investigated. Besides the internal reflections, also theantenna pattern, the height and the tilt angle of the plane ofthe antenna’s aperture against the vertical direction may havean influence. To obtain the influence of different antennas,the same setup with test object c) (screw) was measuredwith each of the afore-mentioned antennas. All antennas areused with their aperture at the same location, but they differin length of the flared section and their feeding waveguide,while the reference plane for the phase is maintained fromcalibration of the VNA. Therefore, the same object at thesame position will be found in different distances from theFig. 2. Reflection (in arbitrary units) of a vertical screw with 11 mmlength as a function of distance to the aperture of a standard gainhorn antenna. The screw represents a local discontinuity. The de-creasing level of reflection is due to site attenuation and is not cor-rected in this graph. It follows 1/r2.antenna’s aperture, as can be seen in Fig. 3. What can alsobe seen is that reflections are much more pronounced usingthe standard gain horn. The reasons are a better definedradiation (and reception) pattern for a smaller solid angleand a closer match to the 50 of the feeding coaxial cable.The standard gain horn antenna turned out to be best suitedfor the given purpose, so we investigated tilt angle andheight variations with it. For this purpose we compare therelative change on the different setups.102 T. Schrader et al.: Measured Response of Local, Mid-range and Far-range DiscontinuitiesFig. 3. Reflection in arbitrary units vs. distance measured with a standard gain horn antenna (green) and a double ridged guide horn antenna(red). The reflections are caused by local discontinuities of a groundplane in a semi-anechoic chamber used for emission measurements ofElectromagnetic Compatibility. The shift vs. distance is due to different lengths of the antennas with respect to the same reference plane atthe vector network analyzer port. The apertures of both antennas are at the same location.Fig. 4. Reflection in arbitrary units of a 0.75 mm metal plate on themetal groundplane. From the two setups with the plate 3.76 m and9.19 m away from the antenna’s aperture, the local resolution canbe observed. Using frequency data in the range from 5.85 GHz to8.2 GHz, a resolution of 2 cm can be achieved, which is more thansufficient to localize any discontinuity of the groundplane.Tilting angles of±5◦ out of the vertical direction havebeen found already critical for a 10 m distance. Althoughfor a C band standard gain horn antenna the half-power beamwidth (HPBW) in the E plane is around 25◦ (Balanis, 1997),the spread of variation inS11,T due to a varying tilt angle isequivalent to the reflection of test object c) in 10 m distance.We obtained the lowest reading for +5◦ and the highest for–5◦, while for 0◦ the result was in between the others. TheHPBW in the H plane is also about 25◦. In 10 m distancefrom the horn aperture the beam has opened to a width of2 m. Any scattering object or local discontinuity can be lo-calized on the radius within this solid angle. To localize itwithin the angular distance, a method of triangulation has tobe applied. To access larger areas a scanning procedure hasto be applied.Varying now the height of the lower edge of the antenna’saperture to the ground, we obtained the highest reflection lev-els for 2 cm height and less for 0 cm, 5 cm, 13 cm and 23 cm.We do not expect much more deviation for smaller distances.For measurement distances up to 10 m from the aperture aheight of a few cm can be chosen arbitrarily and is not a crit-ical parameter as long as a resistive contact to the ground-plane is avoided and care is taken about the alignment of theantenna without boresight displacement.The 0.7 mm thick metal plate lying flat on the ground asthe second test object was spotted with its front and backedge at 3.76 m and at 9.19 m distance from the antenna’saperture (Fig. 4). From this data the local resolution can beobtained. Using frequency data in the range from 5.85 GHzto 8.2 GHz, a resolution of 2 cm can be achieved, whichis more than sufficient to localize any discontinuity of thegroundplane.One real test object is the installation of a turntable in asemi-anechoic chamber used for EMC emission measure-ments. Manufacturers of such devices design the junctionbetween stationary and moving parts with sliding contactsto maintain a smooth transition for the RF surface currents.Applying the described procedure this property of turntablesmay be verified. Figure 5 gives an example of measurementsT. Schrader et al.: Measured Response of Local, Mid-range and Far-range Discontinuities 103Fig. 5. Reflection in arbitrary units vs. distance of a turntable usedin semi-anechoic chambers for EMC testing. To make any of thediscontinuity more visible, the 11 mm long screw was set at differ-ent locations like begin and end of the turntable. The cover plate onthe access hatch (center plate) can be closed in different orientations(here: rotation of about 90◦), which can be noticed by different re-flections of the grip holes.on such a turntable. At first the transition from the ground-plane to the rim of the turntable occurs as a spike, followedby the next transition from the rim to the turntable itself, tothe moveable center plate covering electrical installation fortest equipment and vice versa.4 ConclusionA method has been described to detect and to quantify anylocal or mid-range discontinuity on extended and flat metalplanes utilizing a vector network analyzer with time-domainoption. As a measure for the discontinuities the reflectionlevel of a stacked antenna using an aperture antenna justabove and its mirror image below the metal groundplane isused. The transmitted energy is focused by the mono-staticsetup and is travelling as a wave along the metal surface withthe Poynting vector being parallel to it. The metal planedetermines the boundary conditions for the wave which iscontinued by surface currents and charges on the metal sur-face. Any discontinuity disrupts the undistorted distributionof currents and charges and, therefore, the energy transport.The formerly unidirectional Poynting vector is superimposedwith a backward component which is then received by the an-tenna. The complex-valued input scattering parameterS11,Fcontains the superposition of the antenna’s internal reflec-tions and contributions from any object in front of the an-tenna and from the connector. It is obvious that well-matchedstandard gain horn antennas with their defined radiation andreception pattern fit best the given purpose. Although thefrequency domain signalS11,F contains all information wehave been looking for, it can not be seen very easily. After aChirp-Z-transform the time-domain signalS11,T gives clearevidence of local and mid-range discontinuities.ReferencesBalanis, C. A.: Antenna Theory, Second Edition, John Wiley andSons, NY, ISBN 0-471-59268-4, 1997.Ersoy, O.: Fourier-Related Transforms, Fast Algorithms and Appli-cations, Prentice Hall, NJ, ISBN 0-13-624412-2, 1997.

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Measured Response of Local, Mid-range and Far-range Discontinuities of Large Metal Groundplanes using Time Domain Techniques

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