On Mon, 22 Aug 2005 17:19:29 -0700, John Smith
wrote:

Fred:

As my last post gave away, my chokes are constructed on math and
observation in practical work... (well, prayer too grin)

I see roy is recommending 10x the impedance of the feedline (coax?) for
the choke/balun, minimum (I am guessing here a bit), he obviously knows
what he is speaking about, however, at 160m when you are struck with the
physical dimensions of the choke/balun needed--well, at those times I have
accepted a 5:1 ratio of choke/balun impedance to feedline impedance...

... but then, I am here to learn ...

John


On Mon, 22 Aug 2005 19:08:43 -0400, Fred W4JLE wrote:

How can I test how well a choke balun is performing? I know how to test a
transformer balun, but am cluless as how I can compare several different
choke balun approaches.

Fred, Roy's method is fine, but here's the second way I perform the
test with baluns constructed with ferrite beads surrounding a
transmission line. However, it does require two baluns.

To begin we connect the two baluns back-to-back, i.e. connect the
output of the first to the output of the second, and loading the
normally unbalanced input terminals of the second balun (now the
output) with a 50-ohm resistor.

Next, feed a signal into the input of the first balun and measure the
voltage between the outer surface of the line feeding the input of the
first balun and the outer surface of the unbalanced input terminals at
the output of the second balun. In the ideal balun the voltage between
these two points would be zero.

Next, disconnect the connections between the two baluns, and reconnect
them, reversing the connections. Now repeat the voltage measurement
between the same points as in the previous measurements.

In the ideal balun the voltage now measured will also be zero.

However, with a real balun, the voltage between the two points
specified will usually be zero when the connection between the two
baluns is such that the outer conductors and inner conductors of both
baluns are directly connected, respectively.

But the real test of the baluns comes when the outer conductor of the
first balun is connected to the inner conductor of the second balun,
and the outer conductor of the first balun is connected to the inner
conductor the second balun. The lower the voltage obtained in this
condition, the better the balun. With a perfect balun the voltage will
again be zero, but if the effect of the balun is zero the voltgage
measured across the specified point will equal the input voltage.

The ratio of the voltage at the input of the balun to the voltage
appearing at the points specified above is twice the ratio of the
forward voltage to the common mode voltage that would appear on the
outside surface of the feed line when that balun is used in practice.

Walt, W2DU