Knowing both Tom and Yuri to be two of the best and brightest, I am sure this is going to be an interesting discussion, and only the devil is making me stir the pot a little in an impractical way. Kirchoff's Law is a derivative of Faraday's law, and if you are in a situation where Mr. Faraday's law is applicable, so is Kirchoff's law. A very gory exposition is provided by our electro-physicist buddies:
http://www.ing.uc.edu.ve/~azozaya/docs/tem2/zozaya_ajp_75_565_07.pdf Mr. Kirchoff was a very productive fellow, having deduced, often from thought experiments, similar laws named after him that apply to blackbody radiation and photons. His law that EE's love was motivated by thought experiments at DC. He also thought about the following situation. A very, very long wire in the free space of the cosmos is fed by a current source at its center. To keep the thoughts simple, toss in that the wire has no resistive loss and that the current source is injecting 1 Ampere and operating at 2MHz. Just to cover your backside you might toss in that you were in a nice part of space with no significant curvature weirdness. How much current would be measured in the wire at say 1 light year away from the injection point? You can rack your brain as to whether a numerical analysis method of moments program running on the latest NASA parallel processing behemoth would give an accurate answer, but there is no need. That wire would have emitted radiated power in each small segment along the way with the total power emitted exactly equal to the power supplied by the source as captive electrons within the wire oscillated their little hearts out. With finite current in, finite power would be radiated, and for all practical purposes most of it would have been radiated segment by segment along the wire long before Mr. Kirchoff could have been teleported a light year away to make a confirming measurement. When he did arrive at the chosen measurement point he would find nearly but never all of the injected current had disappeared. This would not have been a surprise to him but maybe to the rest of us. How could this have happened? The wire was lossless after all. And if Mr. Kirchoff had brought along the proper sensor he might have been able to detect a small but finite current travelling farther out on the wire but nothing returning from reflections off the end of the wire, especially since it has no end in the limit of his thought experiment. In fact there would be no standing waves anywhere on this wire in any practical sense because it was for all practical purposes a travelling wave antenna with no reflections coming back from the almost infinitely distant end. The electrons at his point of measurement would barely be oscillating at 2 MHz and unless he had the very latest Agilent equipment he would not be able to detect a standing wave there either despite the very distant end of the wire being unterminated. What does this have to do with the current discussion? Nothing in a practical sense. But it does point out one thing. Kirchoff's law as we EEs usually apply it is in reality a DC law. It DOES usually apply to the RF situations we encounter in hamdom--if we apply it carefully, as Yuri's reference points out. You won't find many successful designers applying it in the same manner at 50 gigahertz. And hello to Yuri, whom I have not seen eyeball to eyeball in many years. In case you don't know Yuri he is a true medical miracle. How is the Tesla Club, Yuri? Plum Island has been up for sale if there is interest. I'm staying clear of it, though. 73 Bob W2WG P.S. Don't fool around with a very long wire in space. You might find potentials (be careful how you define potential) on it produced in other ways than by your little 2 MHz current injection if the wire is long enough. -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Yuri Blanarovich Sent: Sunday, August 05, 2012 9:51 AM To: [email protected] [email protected] Subject: Re: Topband: "return" current - what is it? Here we go again, speaking of misinformation, peer review, spreading false "guru" stuff. You can not apply Kirchoff law from DC circuits to the current behavior along the STANDING WAVE RF radiator. _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
