On 1/9/2013 10:59 AM, David Gilbert wrote:
More importantly, a balun with a ruined core is going to look bad for SWR
Please delete the word "balun" from this discussion, and use the words "unknown black box" instead. That's because the word "balun" is used to describe at least a half dozen circuit components, most of which are very different from each other.
A so-called "current balun" is really a common mode choke formed by either placing a lot of ferrite beads on coax, or by winding multiple turns of coax around/through a ferrite core. The most common failure mode of such a choke is overheating of the coax, causing it to deform, and in the extreme, to short (between center and shield).
A common mode choke fails when it's choking impedance is too low to limit common mode current to a value that does not limit dissipation to a safe value. Excessive common mode current is caused by high common mode voltage, which results from antennas that are unbalanced (especially "Windom" style antennas, and, of course, by running higher power.
Many years ago, before I had learned about what was inside an unknown black box called a balun at the feedpoint of a dipole, I fried one by tying both sides of the feedline together to feed it as a top-loaded long wire, and running 100W for a weekend 160M contest.
73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
