Forum, I have made some measurements and gotten results which are at odds with my intuition. I am wondering if someone out there can shed some light on this subject.
I was interested in the losses associated with rf traveling on a twisted shielded pair cable. The scenario is that a length of this TSP cable is exposed to an rf environment (as in a test chamber during IEC 1000-4-3 testing) and then the cable penetrates a bulkhead using a grounded connector that provides excellent shield termination, and the cable continues on the other side in the pristine rf environment of a shielded control chamber, say for several meters. The question is, how much rf signal is at the final destination point vs. at the bulkhead. The concern is common mode, not differential mode. Meaning that the twisted pair can be looked at like coax, with an identical signal on both inner conductors relative to the shield. I expected losses that would be on the same order or lower than that associated with off-the-shelf coax types like RG-58. Instead my losses were dramatically higher. Following is my measurement technique. I measured the transmission line impedance of the TSP in the following way. I tied the center conductors together. I shorted the center conductors to the shield at one end, and measured the inductance, using an LCR meter. I opened the connection and measured the open circuit capacitance. The square root of the l/c ratio is the characteristic impedance. I built matching networks to get from 50 Ohms to the measured impedances which ranged from 15 - 25 Ohms for a variety of different cables. For each cable, I built two pairs of identical matching networks: 4 each: 50 to RC adapters I used an HP 4195A network analyzer, over a range of frequencies 0.1 - 500 MHz. The set up was as follows. There was 16 dB of pad coming out of the source (including the 6 dB splitter). There was 10 dB of pad at the reference and test ports. Results showed little evidence of vswr. Coming out of the source, there is the 6 dB splitter. Between one port of the splitter and the reference port, I inserted one pair of the matching networks: Splitter output connects to 50 Ohm to RC adapter connects to RC to 50 Ohm adapter connects to reference port. I connected the exact same sequence between the other splitter output and the test port. The network analyzer displayed the dB ratio of the test port signal relative to the reference signal. This would be the difference in loss between each pair of matching networks. If I had built them perfectly, the analyzer should have shown 0 dB difference. Actual differences were under 3 dB. Then I inserted the cable-under-test between the RC connections on the two matching networks in the test port side. The loss associated with cable-under-test (CUT) is the difference between the losses measured with the CUT in place and with the matching networks directly connected. The numbers I got were considerably higher than even a high loss coax such as RG-174. Because real coax uses a much thicker dielectric material than just the insulation around a TSP center conductor, my "gut feel" is that losses should be lower than for 50 Ohm coax. I expect that materials picked to be dielectrics for coax have low loss tangents relative to wire insulation, but I don't have a feel for whether the difference in loss tangents can make up for the extra thickness of the dielectric in real coax. Can any one tell me if either my test set up or my expectations are wrong, and why? Thank you. Ken Javor ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
