There is some sort of popular folk theorem that open wire feed line doesn’t carry common mode currents. It is obviously wrong. Both wires high, both wires low, and that is common mode. Any RF that excites both dipole elements high against ground will cause common mode currents on the feed line. “Balance” does not fix that.
wunder K6WRU Walter Underwood CM87wj http://observer.wunderwood.org/ (my blog) > On Jul 16, 2020, at 1:14 AM, Victor Rosenthal 4X6GP <k2vco....@gmail.com> > wrote: > > Re open-wire fed antennas: > > If the antenna is well-balanced and fed via a true balanced antenna tuner > (preferably link-coupled) then there shouldn't be a problem with common mode > currents. > > Yes, you have to worry about nearby objects unbalancing the antenna, which is > less of a problem with a choked coax-fed dipole. But being able to use a very > simple single antenna from (for example) 7 to 28 mHz. with relatively good > efficiency is advantageous. > > My experiments with baluns seem to indicate that just using a current balun > to go from an unbalanced transmitter or tuner to a balanced line works poorly > unless you cancel out the reactance with a balanced network on the antenna > side of the balun. All these tuners with "balanced" outputs provided by a > balun (often a voltage balun) do not work well at all. > > My best result with balanced lines has been with the Johnson Matchbox. It's a > shame that there doesn't seem to be an equivalent available today (and it > wouldn't be easy to procure the parts to build one). > > 73, > Victor, 4X6GP > Rehovot, Israel > Formerly K2VCO > CWops no. 5 > http://www.qsl.net/k2vco/ > On 16/07/2020 6:26, Jim Brown wrote: >> On 7/15/2020 1:36 PM, CUTTER DAVID wrote: >>> It's all about size. Bigger core helps, >> Dave, >> No, it is NOT about size. It is about design of the entire antenna system, >> including the antenna, the feedline, and other parts needed to make the >> SYSTEM work. The principal characteristic of a common mode choke is the >> resistive component of its common mode impedance at the operating >> frequency(ies) where it will be used. Further, dissipation in the choke >> occurs at least as much in the WIRE that is wound around the core as in the >> core itself. >> There is another fundamental error in many antenna systems that ONLY looks >> at matching to the transmitter at the transmitter, ignoring the match >> between the antenna and the transmission line, using high impedance, >> parallel wire line, and using a random center-fed or off-center fed >> horizontal wire on all bands. Yes, the transmitter can be made to supply >> power to the feedline, yes, it will get to the antenna, and yes, it will >> radiate. But it may not receive all that well due to common mode current on >> the line from noise sources in our own homes and those of our neighbors. >> THAT is the problem with using a decades-old design for a world where there >> was 20 dB less noise than most of us face today. >> so a core that is OK for ssb >>> and cw might be undersized for AM or some data modes. Just like linear >>> amplifiers. >> So it is NOT the size of the core, it's the design of the antenna system. >> HFTA author and retired ARRL Antenna Book and Handbook editor Dean Straw, >> N6BV, published an excellent piece in QST 6-8 years ago called "Don't Blow >> Up Your Balun," in which he pointed out the differential mode dissipation in >> chokes, which can be extremely high if the choke is at a very high current >> point in a mismatched line. When he passed it to me for review, I noted that >> these losses were in addition to the common mode dissipation, and he revised >> the piece to reflect that. >> you can't >>> label something poorly designed because it doesn't pass the BOK test. >> BOK? >> 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 > Message delivered to wun...@wunderwood.org ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
