Thank you for replying. For corona, round IS better. Rounding the edges of a ribbon cable are better than square shape. As empirical evidence, Molecularly 'rounding' is better. -- while at HP, silver plating *and* polishing the plating resulted in higher Q for the cavity resonators.
You correctly point out that I concentrated on a single value, inductance. Not the whole picture. But, at least with one foot of conductor at 100MHz with 1/4 wavelength in air of 30 inches, we are starting to get close to transmission line effects. But, also, as you point out, most energy is in lower than 10MHz. Most of the energy I've seen in ESD ievents [miniature lightning] is in the 1-3MHz ranges. After changing the structures for 'better' grounding the lower frequencies were reduced, moving the peak out to 20-30MHz. >From my experience in trying to model a structure to control the design, ringing relates to the tank circuit you have left. Oops! left the j's off those terms, but at least they are below inductance. What caught my eye doing this, is the large disparity between resistive losses and extremely high reactive impedance. Didn't realize that it was going to be so big. So that just verifies my next rule: A connection is only a connection if it is wider than long, else it's definitely an inductor. Robert > 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 > > m...@sfo.com 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 >>> >>> >>> >>>g.grem...@cetest.nl <mailto:g.grem...@cetest.nl> >>> >>>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 <emc-p...@ieee.org> 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. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
