What usually happens under those conditions may not damage the balun but does cause lots of RFI: the balun turns non-linear!
When the magnetic flux saturates the core it heats and its magnetic properties change dramatically. That produces a non-linear transfer function. Non-linearity is the hallmark of a good mixer! So your balun starts making all sorts of unintentional RFI that wasn't at the output of the transmitter. Under such conditions the core is absorbing a lot of power too. Had you touched it, you'd probably have found it noticeably warm, perhaps even hot, possibly hot enough to blister your finger. At the extreme, the core will heat and crack, ruining it and the balun. But that doesn't always happen. Another temporary failure mode is arcing. That may be more obvious since you may hear it if the balun is nearby. Most baluns are not made to handle really large RF voltages. There isn't enough spacing between leads and terminals for that. The insulation on the wires isn't rated for high voltages. So, when you have it at a voltage loop and crank up the power, it's easy for an arc to occur across adjacent leads. If it's through the air between metallic conductors usually no harm is done. Often it's through the insulation of adjacent wires or across a PCB. In those cases a carbon trail is formed that will have to be removed before the balun will work properly again. The bottom line is that modern baluns are NOT designed to be used in systems with a wide range of impedances. You can get away with using a balun in a multiband doublet, Windom fed with parallel lines, G5RV, etc., only when those antennas happen to provide a reasonable impedance at the balun: neither too low (which would produce high RF currents) or too high (which would produce high RF voltages). Many years ago many of us did use baluns in such systems quite successfully. They used air core coils. Air does not saturate magnetically. They had large enough spacing to avoid arcing at high RF voltages. Such baluns commonly measured a foot square, or larger, by nearly a foot deep. They were generally mounted on the wall well away from anything that might draw an unintentional arc from the coils - such as the operator! There are times when size does matter! Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Markowski Sent: Tuesday, May 15, 2007 6:08 AM To: Elecraft Subject: [Elecraft] OT: balun rating exceeded The following happened to me a few years ago as a brand new ham in my pre-Elecraft days and I wonder if anyone can explain to me the details. I apparently had high swr and exceeded the rating on my balun. It suffered accordingly but didn't completely fail. The result was when I transmitted cw on 40m an internal fuse would blow in the stereo which was on in the next room room. It took two go rounds before I caught onto the cause and effect... A new & better balun cured the problem. At the time I had a Zepp (135') and 4:1 balun at the house entrance. I assume the ruined balun was emitting all sorts of RF that the stereo amplifier picked up, tried to amplify, and couldn't. But I don't understand exactly what would make it draw that much current. Can someone enlighten me? By the way, since then I put up a dipole, halfwave for 160m, about 200' behind the house. That's what I should have begun with. Thanks! Mike ab3ap _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
