150Hz to 1MHz magnetic field coupling to a typical shielded cable above a ground plane configuration

Transcription

1 150Hz to 1MHz magnetic field coupling to a typical shielded cable above a ground plane configuration D. A. Weston Lowfreqcablecoupling.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. 1) Introduction A typical cable configuration is a short length of shielded cable attached to a conductive enclosure at one end and a conductive enclosure at the other with both enclosure connected to a ground plane. One practical example is a 19 rack with front panels to which shielded cables are connected,or shielded cables external to a vehicle or ship routed above the surface. Another example is in Satellites which often consist of a Bus on which many electronics units are mounted. The Bus often forms a ground plane and the units are connected to each by shielded cables. Other reports in this series have described the coupling to electrically long cables using an E field injection probe above 100MHz. Of equal interest is either the magnetic field coupling of a magnetic field source or the magnetic field component of a plane wave to cables which are electrically short. Electrically short means that is the frequency is low enough or the cable short enough so that the cable is much less than a wavelength in length. This report shows that when calculating the magnetic field shielding effectiveness (SE) of a cable and connectors based on merely the combined dc resistance or measured transfer impedance of the cable and connectors, a massive over estimation of SE can occur. This is due to the very large difference in induced current from the incident magnetic field in the shielded versus unshielded cables. In the example of a conduit used here the predicted SE at low frequency based on the transfer impedance is 70dB whereas the actual SE of the configuration is 0dB! 2) Cable configuration The basic cable configuration is shown in figure 1. The dimensions are those for the 2cm diameter conduit described in another report. The shielding effectiveness is often calculated in db as 20 log the ratio of the two load impedances divided by the combined transfer impedance of the cable and connectors. i.e Log Zt total 1

2 In the above calculation we assume that the impedance of the ground plane is relatively low, which may not be the case when enclosures are bonded together using ground braid or the front panels are bonded together via the cabinet. Figure 2 shows the coupling to the unshielded center conductor, termed the reference measurement in the conduit shielding effectiveness measurements. 1.0m H field cable Is Ground plane Figure 1 cable configuration 1.0m H field Unshielded wire Ic Ground plane Figure 2 Unshielded center conductor (used as reference in SE measurements) 3) Predicted shielding effectiveness The low frequency transfer impedance of the conduit and its fittings (terminations) has been proven to be equal to the dc resistance and the dc resistance is 40mΏ. Assuming the same current is induced into the shielded conduit as induced into the unshielded cable the magnetic field shielding effectiveness based on the conduit transfer impedance and the two 50Ώ load resistors can be predicted using: 2

3 2 50Ω 20 Log = 68dB 0.04Ω (See reference 1 for an explanation) The cable above a ground can be modeled as a loop receiving antenna. When the incident magnetic field couples to the loop a current is induced. This current is limited by the impedance of the loop made up of its inductive reactance, dc resistance and the termination impedances. At low frequency the inductance and therefore the impedance of the cable is low and this is especially true for the low inductance and low resistance conduit. When the magnetic field couples to the unshielded center conductor the current is limited by the two 50Ώ load resistors and is much lower than the current in the shield, which is terminated to ground at both ends through very low impedances. Table 1 compares the current induced into the cable shown in figure 1 compared to the current induced into the center conductor in figure 2, due to a 1A/m incident magnetic field. Table 1 Current in cable configuration 1 versus 2. Frequency (MHz) Current in shield of cable (figure 1) Is Current in center conductor of unshielded cable (figure 2) Ic mA 1.6µA mA 11 µa mA 22 µa mA 55 µa mA 110 µa mA 550 µa 3

4 mA 1.1mA mA 5.5mA mA 11mA As frequency increases the transfer impedance of a shielded cable either reduces due to the skin depth, typically from 20kHz to 2MHz or stays constant and begins to increase, typically above 2MHz, due to porpoising coupling and aperture coupling. See reference 1 for an explanation of these coupling modes. The transfer impedance of the conduit described in a companion report is reasonably constant up to 100MHz and then begins to increase. If we assume that the transfer impedance equals the dc resistance of the cable and fittings up to 1MHz (a reasonable assumption based on previous measurements) then the shielding effectiveness of the cable to an incident magnetic field can be found from: 2 50Ω I c 20 Log and this is plotted in figure Ω I s Low frequency shielding effectiveness of a short cable above ground 3.50E E+01 SE (db) 2.50E E E E+01 Cable SE 5.00E E E E E E E+00 Frequency (MHz) Figure 3 Cable low frequency magnetic field shielding effectiveness based on dc resistance and currents on the shielded versus unshielded cable. 4

5 4) Conduit measured results The measured results on a conduit, presented in the report Conduit measured transfer impedance and shielding effectiveness, indeed shows a very low level of shielding effectiveness from 20kHz to 10MHz. The injection method used over this frequency range was a magnetic field injection probe which has some insertion loss (i.e. series equivalent impedance) and therefore the shielding effectiveness is not as low as shown in figure 3, which assumes coupling from a uniform low impedance magnetic field. When the measured shielding effectiveness is corrected for the difference in the measured current on the shield versus the measured current induced into the center conductor then the shielding effectiveness is an almost constant 70dB from 20kHz to 10MHz. Very close to the predicted 68dB, based on the ratio of load impedance to conduit and fittings transfer impedance. At 10MHz to 50MHz the shielding effectiveness is the same regardless of the use of an inductive magnetic field injection probe or a capacitive E field injection probe. Thus the magnetic or electric coupling results in approximately the same level of shielding effectiveness in a 1m long cable above 10MHz. Above 50MHz and up to 8GHz the capacitive injection probe is used and we see the effects of cable resonances as it becomes electrically long, as described in the companion report. Reference 1 Electromagnetic compatibility: Principles and applications D. A. Weston, Printed by Marcel Dekker, NY. 5