Hi Mick, There are some other things to consider. The center conductor will slowly migrate through a foam dielectric, this process will be accelerated with heat. Most baluns will be at least a little bit warm when they are operating, sometimes they can be quite hot. The migration of the center conductor will certainly cause an impedance change, and it may eventually lead to a short circuit. I would use coax with a solid dielectric, Teflon is preferable over other materials. Winding data for using multiple cores of the smaller toroids is in a previous cookbook on K9YC's website. http://k9yc.com/CoaxChokesPPT.pdf There is a lot of good information in that document, it's worth reading in its entirety. Dennis
the thing with winding torroids as a choke or transformer! induction wise the windings do not have to be pulled tight. looping your coax through a torroid is fine. the only reason for drawing them tight is dependent on the space you have for it magnet wire is usually drawn pretty tight and spacing observed and adjusted to get the inductance values needed.
The "ugly balun" has already been looked into by K9YC, he found that it is a poor performer, under some circumstances it can be worse than no balun at all. I suggest that you look into his documents, there are links to them elsewhere in this thread.
Your statements apply accurately to low frequency AC chokes and transformers. Things are much more complicated at RF. Take a look at the work that has been done by K9YC, he explains it all. He found that magnet wire is a poor choice for RF chokes. A brief quote from K9YC's Choke Cookbook: "Enameled copper pairs have much greater loss than other paired lines. This is because the magnetic fields produced by currents in very closely spaced pairs used as transmission line cause the current to be concentrated in the side of the conductors closest to each other. This mechanism, which is strongly related to skin effect, is called proximity effect, and is what causes differential current to flow on the inside of the coax shield. Just as skin effect forces current to the skin of the conductor, proximity effect forces it to only one half of the skin! Proximity effect rises rapidly as the center-to-center spacing approaches the conductor diameter, which is the case with enameled wire. As can be seen from the table of measured transmission line data, the enameled pairs have significantly higher loss (and greater dissipation) than other paired cables. It’s also possible for the enamel to be scraped by the ferrite core during winding, shorting to the core at multiple points and significantly degrading choke performance. For both reasons, I no longer recommend chokes with enameled wire."
I agree with you I was just stating that coax does no need to wound tightly to make an effective choke.
Seems like a good point after looking at the article you linked, and also looking at the article THAT article linked. Maybe it's another case of "This might work for my purposes, but not for yours". Considering there's no one single "Ideal" operating setup, but a large set of variables that are going to change for each operator based on a million different things.
