Vic et al, The currents on a particular wire of a two wire feedline, will always be the vector sum of differential current and common mode current. This is not the same thing as forward and reflected, which are both expressed in the differential current. The tuner will be able to control the differential current "leaving" the tuner. It will NOT be able to control the common mode current. The common mode current can occur because the TERMINATION of the feedline is flawed at the antenna or a balanced to coax junction. Or common mode current can occur simply because it is induced by the antenna, because the feedline is a conductor in the vicinity of the antenna and the relationship of the physical placements of antenna and feedline favor induction.
The tuner IMHO is least likely to be the major malefactor in a case of unbalanced currents on a feedline. What occurs out at and toward the feedpoint has been mildly astounding in cases I have helped solve. I have had that problem personally, and had to deal with beating a common mode issue on a balanced line that was putting 900 volts RF across a balun blocking path at QRO and destroying the balun in the process. The common mode path of a balanced feedline is a big antenna, and THE ENTIRE LENGTH of the feedline should be modeled as a single conductor along with the antenna, just to see what is being induced. This includes grounding in the shack if your tuner has a grounded center point which effectively grounds common mode current. Other tuners will present anything from a brute force common mode block to a resistive path to ground. From the tuner's treatment of incoming common mode, its direct grounding, or resistive path to ground, or open circuit to ground, needs to be put in the model. I had to do all of that to dig my way out of my problem. Note, you can't buy broadband RF common mode blocks for balanced line at Radio Shack. Nor anywhere else, for that matter, that I've been able to find. 73, Guy. On Fri, Apr 6, 2012 at 12:24 PM, Vic K2VCO <k2vco....@gmail.com> wrote: > I am not sure I know how to do this. I have a device that I made > <http://www.qsl.net/k2vco/sniffer/rfsniffer.html> > to measure current but it gave me anomalous results when I tried it on the > matchbox. I > found that when I measured the currents about 4 inches from the matchbox they > were equal, > but when I measured them two feet away they were significantly different. So > there is > something I don't understand about this process. > > > On 4/6/2012 9:16 AM, Geoffrey Mackenzie-Kennedy wrote: >> Vic, >> >> Before you write off "truly balanced tuners" in favour of the T-network + >> balun <g>, it >> would be interesting to measure and compare the current in each wire of the >> feeder (while >> transmitting a carrier) when using the Matchbox and then the T-network + >> balun? >> >> 73, >> Geoff >> LX2AO >> >> >> On April 05, 2012 18:28 +0200, Vic K2VCO wrote: >> >> >> <snip> >> >>> But here is the surprising part: I expected that the Matchbox would provide >>> better >>> balance, which would reduce radiation and pickup from the feedline. To test >>> this, I >>> coupled a grid-dip oscillator to the feedline about 20 feet from the tuner. >>> >>> To my great surprise, the signal from the oscillator was much weaker with >>> the T-network + >>> balun than with the Matchbox! >>> >>> Signals the same, 'noise' weaker. So much for the need for 'truly balanced' >>> tuners! >> > > -- > Vic, K2VCO > Fresno CA > http://www.qsl.net/k2vco/ > ______________________________________________________________ > 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 ______________________________________________________________ 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
