Robert it is difficult to validate your models without knowing more about them but they are suspect when they fail to uphold empirically derived practice that has existed for longer than field solvers. You certainly stick your neck out when you say:
"CONCLUSION: Round is always better." I can think of numerous exceptions, exceptions that may prove the rule. In fairness many of the exceptions are caused effects not considered by your models but are real world, nevertheless. ( I thought this discussion was about best practice.) For instance if one is terminating in a, PCB or just a tab, round tube requires a round to rectangular conversion. How may that conversion effect corona? As you said nature doesn't like 'pointy' stuff. It would be folly to characterize the problem as purely one of inductance. We like to model lightning as pulse but we know lightning is often a damped sinusoid. Where does the sinusoid come from? We know that if hit a LC circuit with an impulse function, the circuit will ring at its natural frequency. Could this be the source of the sinusoid? If we suspect there are resonance effects, then clearly we must address capacitance and L/C ratios and transmission line effects as well. Clearly your 2D field solver isn't going to do that. Finally in your models, isn't there going to be significant energy below 10 MHz? My AM broadcast radio certainly thinks there is whenever lightning strikes nearby. Why do you show reactance for ribbon and resistance for tube? Is that an error of missing 'j's or is this truly apples and oranges? Best regards, Fred Townsend DC to Light [email protected] wrote: Flat rectangular is NOT better than a round tube if each have the same circumference. The round tube has lower inductance than a flat ribbon conductor of comparable circumference. Makes sense, because Nature abhors 'pointy' stuff. This statement is based upon a comparison analyzing the two structures using finite element analysis, femm 4.2. Given: solid ribbon copper conductor, 1 inch wide and 120 mils thick copper tube, 0.75 inch outside diameter and 120 mils wall thickness The two circumferences are approximately the same. Rectangle 2 * (0.12+1) = 2.24 inches Tube pi * 0.7 = 2.20 inches Which is a better conductor? Assume infinitely long, straight conductors. Assuming most of lightning energy is significant between 1MHz to 100MHz, calculate each conductor's characteristics at 10MHz and at 100MHz using 2D finite element analysis. Method: place each conductor in a 24 inch diameter metal 'tube' to provide return current and represent infinity. Note: I also used single conductors in free space with current return at infinity. Values changed, but the conclusions did not change. Use a 12 inch length to reference values per ft. Mesh was set to be fine near the surfaces of the conductors, so that even with hgih frequency currents inside the conductors were accurately represented. Skin depth was more than 3 nodes. >From results, the current as a function of depth into the conductors matched expected values. Plots of current/eddy currents verified mesh was of suficient density for these calculations. TABLE RESULTS: Ribbon - Solid mesh 69,406 10MHz 210 nH/ft 0.00347 W 13.2j ohm/ft 83 milliohm/ft 100MHz 210 nH/ft 0.0115W 132j ohm/ft 0.15 ohm/ft Tube - Hollow mesh 90,233 100MHz 203 nH/ft 0.00239W 12.8 ohm/ft 69 mOhm/ft 100MHz 203 nH/ft 0.00732 128 ohm/ft 0.12 ohm/ft It was interesting to note that with a wall thickness of more than 10 mils, at these frequencies the metal was doing nothing but physically supporting the outside layer. CONCLUSION: Round is always better. Robert As a rule of thumb, a conductor has an (self)inductance proportionally inverse with it's circumference (if fact the shortest way the magnetic field lines will take). Big flat conductors always perform better then round ones, as they have the highest circumference per kilo. Litz and silver(gold) coated conductors do contribute to the real part of the impedance (=resistance) properties only. Litz by increasing the conductive surfaces so reducing the resistance increase caused by the skin effect. Regards, Ing. Gert Gremmen [email protected] <mailto:[email protected]> <mailto:[email protected]> www.cetest.nl Kiotoweg 363 3047 BG Rotterdam T 31(0)104152426 F 31(0)104154953 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> <mailto:[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. 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