Conduit measured transfer impedance and shielding effectiveness (typically achieved in the RS103 and CS114 tests)

Transcription

1 Conduit measured transfer impedance and shielding effectiveness (typically achieved in the RS3 and CS4 tests) D. A. Weston K. McDougall conduitse.doc The data and information contained within this report was obtained from an independent R&D project funded by EMC Consulting Inc. The contents may be used and quoted but the source must be referenced in any publication. ) Introduction The conduits we tested comprised three types. The # type provides some shielding, #2 much better shielding and #3, the best shielding. Each conduit we tested were ½ trade size and have an inner diameter of 5/8, although other diameters are available. The #3 is manufactured from a brass spiral wound flexible conduit covered in a tin plated copper braid. The #2 is a brass spiral wound flexible conduit without braid shield and # is aluminum spiral wound with braid. The braid in these cables is fairly open weave and has an optical coverage no greater than 5%. These cables come with threaded connectors fittings at both ends and mating threaded fittings which are designed to be mounted on a panel, bulkhead or enclosure. The mating fittings can be inserted into ½ NPT threaded holes in the mating surface. The inner cable or wires are meant to be pulled through the conduit and are not typically part of the conduit construction. The cables used in the tests were m long. Two types of test were performed one is in accordance with IEC 696- and measures the transfer impedance of the conduit alone without including the transfer impedance of the fittings. The other tests are of the shielding effectiveness of the cable and fittings mounted to two soldered up copper boxes bonded to an underlying ground plane. I.e. in a typical cable configuration. 2) Cable construction As a cable with a known characteristic impedance is required for the transfer impedance test a center conductor was manufactured from a ¼ diameter solid brass rod. This rod was held in the center of the conduit by small expanded polystyrene plugs about in length separated by about 8 inches. Photo shows the center conductor rod and a wooden plug, which was used at the ends of the cable only, mounted into one of the fittings on a soldered up copper box, bonded to the ground plane and photo 2 shows the rod centered in the conduit. With the rod in the center of the conduit a coaxial cable has been constructed with a characteristic impedance close to 5Ώ.

2 Photo Plug at end of rod used to center the rod in the fitting. 2

3 Photo 2 Rod centered in conduit forming a 5Ώ coaxial cable 3) Cable shielding effectiveness The two main methods of determining the shielding of cables and connectors are shielding effectiveness (S.E.) and transfer impedance. 3. Shielding effectiveness Shielding effectiveness is defined as the ratio of electric field or magnetic field strength at a point before and after placement of a shield. For a cable this is typically defined as the voltage or current induced into the center conductors/s of the cable, by the field, with and without the shield in place. A number of disadvantages exist in the measurement and characterization of shielding effectiveness and these include: a) Shielding effectiveness is not an intrinsic electromagnetic parameter which means that its measurement is very dependent on the test set up. b) No unique definition of S.E. exists 3

4 c) No method exists for independent calibration. However data from the shielding effectiveness measurements in a companion reports show a remarkably good correlation to the shielding effectiveness derived from the transfer impedance, over the frequency range where the cable is electrically long and the frequency range where the transfer admittance predominates (MHz to MHz) Also the test set up simulates a realistic cable configuration above a ground plane. A companion report 5Hz to MHz magnetic field coupling to a typical shielded cable above ground plane configuration illustrates how a shielded cable may achieve a no higher shielding effectiveness than an unshielded cable when exposed to magnetic fields! This report also shows that from 2kHz to MHz that the SE data of the conduit and fittings and the SE derived from combined dc resistance and transfer impedance of the conduit and fittings are virtually identical, when corrected for the different current flows on the shielded versus the unshielded cables in the SE test set up. 3.2 Transfer impedance Shelkunoff showed that Surface Transfer Impedance, described in section 4, is by contrast an Intrinsic Electromagnetic Parameter and its measurement is independent of test set up. This means that any test method may be used to determine the transfer impedance and the test results should be comparable with data obtained on the same cable in a different test set up. The disadvantages of the transfer impedance test is the upper frequency limit and the fact that it does not measure effectively coupling to the cable via transfer admittance. In most set ups the connection between the shield of the cable and the driveline is Ω and this maximizes the transfer impedance coupling and minimizes the transfer admittance coupling. Many consider the transfer impedance the most important factor and this is true at low frequency, but not necessarily true at high frequency. The upper frequency for most coaxial and triaxial cable test set ups is MHz, although GHz is achievable. The method used to obtain the data described in this test is the IEC 96- and is useable up to GHz, and theoretically beyond. However the limit we have achieved is GHz. The cable transfer impedance test induces a current directly into the Cable Under Test (CUT), which is part of the test fixture. The predominant form of coupling is porpoising and magnetic field penetration through the apertures, although some coupling of the E field occurs due to the transfer admittance of the cable. The conduit contains an over braid in which large apertures do exist although no apparent apertures exist in the underlying conduit. However as the conduit is constructed as a helical spiral a contact impedance almost certainly exists at the joint in the spiral, as discussed later. Also in the IEC 96- test only the transfer impedance of the cable is tested and not the connectors/fittings at either end of the cable or the bulkhead mounted connectors/fittings 4

5 The major weakness in using the transfer impedance to derive the S.E. is the impact of cable configuration. For example the current flow in a shielded cable connected to a ground plane at either end is typically higher than an unshielded cable terminated with an higher impedance at either end or disconnected from ground at one end. Thus the S.E is lower than predicted using the transfer impedance. 3.3 E field shielding effectiveness test method on an electrically long cable The test method described in reference uses a capacitive injection probe to create a high electric field between the probe and the cable. The cable connected to a soldered up copper box at either end, and the capacitive injection probe is shown in photo. This capacitive injection more realistically simulates a high level E-field incident vertically on the cable and the source would typically be from a high gain antenna and somewhat localized, as the MIL-STD-46D/E RS3 test. The test method maximizes aperture coupling through the cable transfer admittance and at the same time sets up a current down the length of the cable which ensures both transfer impedance coupling, through the magnetic field aperture coupling and porpoising coupling. Although the test set up is not standardized we have achieved reproducible results using the same type of shielded cable and injection probe in a number of tests. As the shielding effectiveness test compares the induced levels with and without the shield and is made with the cable electrically long, i.e. multiple wavelengths long, the measurement is virtually independent of cable length. This test method does include the total SE of the cable and of the connectors/fittings at either end of the cable and the mating bulkhead mounted connectors/ fittings This test method was used from 2MHz to 8GHz 5

6 Photo Capacitive injection probe on the unshielded rod (reference measurement). 3.4 H field shielding effectiveness test method In addition to the E field test two magnetic field injection probes were used to inject a current into either the unshielded rod (the reference measurement) and the coaxial cable manufactured form the conduit and rod. One injection probe covered the frequency range of 2kHz to MHz and the second probe from MHz to 2MHz. These probes are used in the MIL-STD-46D/E CS4 test and exhibit an insertion loss within specified minimum and maximum values. A companion report entitled 5Hz to MHz magnetic field coupling to a typical shielded cable above a ground plane configuration covers the magnetic field shielding effectiveness of the test set up cable configuration. This report shows that as the current flow in the shielded cable is very much higher than the unshielded cable the magnetic field SE is close to zero at low frequency. As the current probes exhibit some series impedance the ratio of current flow in shielded versus unshielded cable is not so high. In the CS4 test the input power is calibrated and the current on the cable is monitored. The test specifies that the forward power into the probe is maintained at the calibrated level, or the maximum current level for the applicable 6

7 limit, whichever is less stringent. This means that the current flow in the cable can be lower and in the case of the unshielded center rod compared to shielded cable it is much lower. The test data presented in section 6 shows both the SE for unequal injection currents (as in the CS4 test) as well as for equal currents. Photo 2 shows the test set up with one of the injection probes around the conduit which is connected to a soldered up copper box at either end with a copper tube soldered to one of the two boxes. The copper tube contains the shielded cable used to measure the voltage induced into the cable. Photo 2 Injection probe around the conduit. 4) Transfer impedance test set up The test method used is described in reference 2. The method is also included in IEC 96 Amendment 2, however IEC 96- contains a major error in the transfer impedance equation on page 4 and does not provide the background information that reference 2 does. The test set up for the conduit shielded cable is shown in figure 4. and in photo 3. In this test the copper box containing the 5Ώ load is isolated from the underlying ground plane. The injection circuit is designed to have a characteristic impedance of 5Ω and the Cable Under Test (CUT) is also designed to have a characteristic impedance of 5Ω. 7

8 Conduit cable transfer impedance test set up PVC Insulation 9.5mm 3mm Shielded Room Wall mm Injection Braid 4m Copper Box Soldered up Copper Pipe 5/8" OD Fittings Braid over conduit Spacer Completely Soldered Copper Box Fittings Center rod Center rod SMA Cable SMA connector S/A Shielded Cable Soldered SMA Connector (Far End) 5Ω Load.5m Braid Injection Wire Clamped Spacer (Near End) 5Ω Termination Clamped Signal Generator conduit Zt.skf Figure 4. Transfer impedance test set up 8

9 Photo 3 Cable transfer impedance test set up in anechoic chamber The current is injected via a 4mm wide braid located above and insulated from the conduit shield and returns to the signal generator via the conduit shield. This injection circuit also has a 5Ω characteristic impedance. The voltage developed across this segment of the conduit is also measured between the center conductor of the CUT and its shield. This transferred voltage is used in calculating the transfer impedance of the cable when measured with a 5Ω load at the near end and open circuit at the far end of the CUT. When measured with a 5Ω load at the far end, the measured voltage is multiplied by two to obtain the transferred voltage. As the transfer impedance is specified in Ohms/m and the test section is.5m the transferred voltage is again multiplied by two to obtain the transfer impedance. In the IEC 96- test set up the CUT is extended either side of the length into which the current is injected, in our case.5m. The voltage developed at the far end of the injection cable is also measured to ensure that the correct value of injection current is sued in the test data. Reference 2 recommends that a measurement be made of the insertion loss of the CUT and that a correction be made in the transfer function and this correction was performed. 9

10 5) Transfer impedance test results The far end transfer impedance of the.5m length of the #3 conduit was measured as described in section 4. After correction for the 5Ώ load, the measurement cable attenuation, the injection current and after normalization to the m length the measured transfer impedance in Ohms/m is shown in figure 5.. At low frequency the transfer impedance should be the same as the dc resistance of the conduit. The dc resistance of the m length of conduit was mώ and the transfer impedance from 5kHz to khz was measured at 9.5mΏ thus validating the transfer impedance measurement at low frequency. #3 Conduit, without fittings, Zt Transfer impedance ( Ω /m) Frequency (MHz) Figure 5. Transfer impedance of #3 conduit 6) Cable shielding effectiveness with and without the effect of the transfer impedance of the conduit fittings. The conduit with center rod forms a coaxial cable with a 5Ώ characteristic impedance. At one end of the cable the measuring equipment provides the termination and this has a characteristic impedance of 5Ώ. At the other end of the conduit a 5Ώ load is used as the termination. As the cable and load impedances are matched a value of the SE can be derived from the transfer impedance measurement from the following equation:

11 . 2 Log 2 RL Zt total The SE measurements, made with the two current probes and the electric field probe, from MHz to GHz are plotted in figure 6.. These measurements include the transfer impedance of the m length of conduit plus the fittings at each end. When making SE measurements it is important to check that the test set up is not limiting the maximum level of SE which can be measured. To ensure an adequate margin between the measured cable SE and the limit imposed by the test set up the center conductor of the rod was disconnected from the center conductor of the shielded cable in the copper pipe. This ensured that any voltage induced by common mode current flow on the copper pipe to the shielded room wall would be measured but that the voltage induced into the center conductor of the conduit cable would not be measured. The maximum attenuation limited by the test set up is also plotted in figure 6. and is at least 2dB higher than the cable measured SE. Conduit plus two fittings, Shielding effectiveness Conduit shielding effectiveness Limit on SE measurement 2 SE (db) Frequency MHz Figure 6. #3 Conduit measured SE from MHz to 8GHz compared to the limit on SE measurement (test set up capability).

12 The predicted SE of the conduit alone, calculated from the transfer impedance data, is shown in figure 6.2 as well as the measured SE using the magnetic field and electric field probes. The SE measurements include the transfer impedance of the fittings. This additional transfer impedance can explain the approximately db difference in SE from 5MHz to MHz however the much lower attenuation from 2kHz to 5MHz is due to another mechanism which is explained in detail in the companion report on the web site entitled 5Hz to MHz magnetic field coupling to a typical shielded cable above a ground plane configuration. The SE measurements using the injection probes are made on the shielded and unshielded cables with a constant input power level to the probe, just as in the CS4 test. This means that the current flow on the shielded conduit, terminated to ground with low impedances, is higher than on the unshielded rod which is terminated at both ends in 5Ω and the value of SE is lower than achieved when the current is kept constant. Thus the measured SE from 2kHz to 2MHz can be used to predict the performance of the conduit in a CS4 test. SE of #3 Conduit without fittings, from Zt, and SE with fittings Conduit without fittings, from Zt With fittings Shielding effectiveness (db) f (MHz) Figure 6.2 Measured SE of #3 conduit plus fittings (typical for the CS4 test) compared to SE of conduit alone from the transfer impedance measurement. 2

13 The input level was injected at a constant power level was measured. The current flow on both the conduit and the unshielded rod was measured with a Fischer current probe, useable from khz to 25MHz. If the measured SE is corrected for the difference in these two currents from 2kHz to 5MHz then the cable SE and the cable plus fittings SE track more closely and this is shown in figure 6.3. Comparison of SE cable alone and cable with fittings SE from cable Zt SE (db) SE with fittings corrected for I 3 SE with fittings 2.. Frequency (MHz) Figure 6.3 #3 Cable plus fittings SE with corrected for equal currents in the unshielded and shielded measurement. The dc resistance of the fittings increase the overall transfer impedance of the cable to some extent. The low frequency predicted SE can be calculated based on the cable transfer impedance added to the dc resistance of the fittings. If this calculated SE is plotted against the corrected SE then a better correlation exists, as shown in figure 6.4, and the SE and the correction for the induced current are validated. 3

14 Low frequency SE, corrected for injection current and cable and fittings dc resistances 9 SE from cable Zt 85 SE (db) 8 Meas ured SE corrected for I Calculated SE from cable Zt and dc res is tance correction Frequency (MHz) Figure 6.4 Measured and calculated SE from the #3 cable transfer impedance and fittings dc resistance. Figures 6.5 and 6.6 show the transfer impedance and the shielding effectiveness derived from the transfer impedance for conduit #2, constructed from a brass spiral without braid. We see that at low frequency the brass spiral with and without braid are similar. Above about MHz frequency the transfer impedance of the spiral without braid increases and so the shielding effectiveness, as shown in figure 6.6, increases. The high transfer impedance of the # cable caused a great deal of concern initially. It was tested on three separate occasions with the test set up torn down and replaced after each test. The results were within the 4dB expected due to measurement error. After the final test on the # conduit the #3 conduit was immediately re-tested with the same set up and the results were consistent with the previous test on #3. The dc resistance of the # conduit was 3mΩ and that for the #3 was 9mΩ for the.5m length and so this easily explains the db difference in shielding effectiveness at low frequency. However we have never seen transfer impedance as high as 8Ω at 7MHz and S.E. of only 25dB with a braid shielded cable. It was noticed that whereas with #3 and #2 conduit the joints were very stiff it was seen that the joints in the # cable were loose and the cable drooped where the outer insulation was bared back. As the conduit is wound in a spiral the contact impedance of the joint appears down the length of the cable. Thus the longitudinal current generated in the transfer impedance test has to jump as many as 25 of these joints over the.5m test length. The optical coverage of the braid is only approximately 5% and the braid may not be well attached to the end fittings, as these are designed to cover the conduit only. Although initially surprising the test results on the # conduit did indeed show a very poor shield compared to #3 and #2 and this is almost certainly the result of the high contact impedance due to loose joints. 4

15 #2 Conduit Zt Transfer impedance ( Ω /m) Frequency (MHz) Figure 6.5 Transfer impedance of #2 conduit. #2 Conduit SE Shielding Effectiveness (db) Frequency (MHz) Figure 6.6 Calculated SE from the cable transfer impedance of #2 conduit. 5

16 # Conduit Zt 9 Transfer Impedance ( Ω ) Frequency (MHz) Figure 6.7 Transfer impedance of # conduit. # Conduit SE 8 Shielding Effectiveness (db) Frequency (MHz) Figure 6.8 Calculated SE from the cable transfer impedance of # conduit. 6