On 9/30/2012 12:02 AM, Jim Brown wrote: > YES! That is EXACTLY what I have repeated, over and over again. It has > NOTHING to do with coax. Coax, if it is not decoupled by a common mode > choke, simply ADDS to the imbalance that is already present. EVERY > transmission line needs a common mode choe at the feedpoint to decouple > the line.
I think about this as an engineer, and parts of the logic are so engrained my brain that I leave them out when explaining how things work. So, let me add this. In a perfectly balanced antenna fed by parallel wire line, and terminated in a balanced circuit at the transmitter, the current on the left wire is exactly equal to the current on the right wire at every point on the line (but varies along the line, because it's a transmission line), and at every point, the current are out of polarity, so they cancel. That line does not radiate, and it does not receive. But if ANY part of the system is out of balance -- the antenna, the line, the TX end -- the two currents are NOT equal. -- the DIFFERENCE between the two currents is the COMMON MODE CURRENT, and that line DOES radiate, and it DOES receive. That common mode current goes from the antenna to wherever the line is connected. A COMMON MODE CHOKE adds some impedance in series with that common mode circuit. If it is a GOOD choke, it is a LARGE RESISTIVE impedance. It is "brute force" solution -- by adding a high resistance in series with the common mode path, we reduce the common mode current to a very small value, which causes (almost) all of the antenna current to flow from the left side of the antenna to the left side. Please don't force me to burden the entire list with an extended discussion of all of this that I have carefully prepared and posted as part of my RFI tutorial, and as part of a Power Point on coax chokes. Go study that. THEN ask questions if you have any. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
