On Sun, 9 Jan 2005 15:06:59 +0000, Ian Jackson
wrote:
But how good is the efficiency when the balun/transformer is working
between completely the wrong impedances (which will be most of the
time)? My understanding (and limited experience) is that most of your
power simply heats up the ferrite.

Hi Ian,

In a current BalUn it is the Common Mode current that is absorbed by
the BalUn as a choke (one purpose of two purposes to use it). The
current BalUn is designed to add R rather than X to the Common Mode
path (although, any additional X that comes down the pike is not
shrugged off). Thus the product of that current and that resistance
is heat. However, if you have a sufficient R, it snubs the I before
you get to many Watts. A string of 20 beads can present upwards to
1000 Ohms resistance, but I wouldn't try to dissipate more than 20W
throughout.

Now, as to efficiency. I presume the load is a nominal 50 Ohms, and
that it is paralleled by that choke's 1000 Ohms. If we were in a
condition where that same 1000 Ohms were dissipating 20W then it
follows that the load is dissipating 400W. This is about the time
when some Hams begin to tear their hair and fulminate at the BalUn
(which is doing its job faithfully) rather than asking themselves the
question:
"why is my antenna system trying to pump
that current down the transmission line?"

If they solved that question, then they would have more power
available for their intended load. Others who would have reflected on
that question naturally, they would have shrugged their shoulders and
figure chasing that last 5% efficiency exceeded the small price they
paid for the BalUn - which was working quite well.

Now, consider if that BalUn were to suddenly vanish. The same current
which formerly saw 1000 Ohms, would see none. We can all work out the
equivalent resistance of 0 Ohms paralleled to the intended load of 50
Ohms to observe where the power would eventually go. This is so
extreme as to be absurd (or at least rare). Such a radical
possibility necessarily presumes a rather disastrous implication in
the state of the antenna's design - or damage.

However, none of this actually answers your question about "wrong
impedances," but it does respond to the worst efficiency you could
expect. By and large, when you are in the middle-to-near fringes of
the BalUn's operational range, efficiencies are exceptionally high
(95% to 99+%). Whatever power that is lost to improper matching
occurs in far greater abundance elsewhere.

73's
Richard Clark, KB7QHC