[Elecraft] OT: balun rating exceeded
Ron D'Eau Claire
rondec at easystreet.com
Tue May 15 12:35:03 EDT 2007
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: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] 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: Elecraft at mailman.qth.net
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
More information about the Elecraft
mailing list