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.

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.

One possible method comes to mind... use an RF current probe to see
how much RF is actually flowing on the outside of your feedline. I've
seen plans for such probes, made from a clamp-on ferrite choke core
and winding (making a simple RF transformer) plus a diode, cap, and
meter.

In order to distinguish conducted RF (which the balun would be
expected to be able to stop) from induced RF from the antenna itself
(which would probably go right around the balun) you might want to
try disconnecting the antenna itself from the output of the balun, and
substituting an equivalent resistance/reactance at the frequency of
interest. Push a few watts into the "dummy load", run the current
probe down the feedline from the balun to your rig, and see what the
peak current is. Repeat at multiple frequencies, and (if practical)
with different feedline lengths. Using a half-wavelength-long coaxial
feedline would probably maximize the sensitivity of the test, as it
would present a low RF impedance on the outside of the coax at the
"bottom" of the current balun.

Repeat with other current baluns and compare.

To be complete, I suppose that you'd also want to measure the
temperature of each balun before and after a high-power test, to try
to get some feeling for the amount of loss in the balun itself.

--
Dave Platt AE6EO
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"Fred W4JLE" wrote in message
...
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

I dont have the skills nor knowledge to tell you how to test the choke
balun. I do have some experience with testing of my home made choke
baluns.
I made connection to the shield of the coax that is surrounded by the
"choke balun", and measured the impedance across it. My reasoning was that
the major objective of the balun was to present a high impedance to current
along the outside of the coax.
It occurres to me that my reasoning might be invalid, since it seems too
obvious, and you already know the "impedance" test isnt adequet.

Jerry

Fred:

With a toroid core, using the manufacturers data on uh/mh per number of
turns, al factor and such, should allow you to compute the inductance and
come to a value of the impedance that inductance poses at the operating
freq(s) in question...

Major problem I see is a resonance(s) occurring with feedline length and
the inductance chosen, but even that could be designed out with a pencil
and paper... the physical testing you are inquiring about has my interest
also...

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 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.


You can measure the impedance by shorting the input conductors together
and the output conductors together. Then connect the input conductors to
one terminal of an impedance measuring device such as an antenna
analyzer or impedance bridge, and the output conductors to the other
terminal. An impedance of around 500 - 1000 ohms is generally adequate.

If you want to measure its effectiveness in place, you can construct a
current probe like the one shown in Fig. 12 of
http://www.eznec.com/Amateur/Articles/Baluns.pdf. It'll give you a
quantitative measure of the feedline current. You can use a snap-on
split core for convenience. The value at the choke should be very low,
but it might increase considerably about a quarter wavelength down the
line due to induced common mode current. The solution to that is to
insert another choke balun at that point.

Roy Lewallen, W7EL

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.

I have a toroidal pick-up coil slipped over my coax in the
shack to monitor the magnitude of common-mode currents.
That tells me how chokes, etc., are performing.
--
73, Cecil http://www.qsl.net/w5dxp

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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.

Hi Fred, If you have an MFJ antenna analyzer it is easy. You have
to measure the choking Impedance at the frequency of interest. Rule of
thumb is 10X the transmission line or Antenna Impedance, depending on
where the the balun is located. 50 ohm impedance, 500 ohm choke
impedance.
The 259 manual has a section on testing baluns. You can download the
manual from MFJ.
Gary N4AST

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.

Thanks to all for the information. Roy I particularly wanted to thank you
for the copy of your article from the antenna compendium.

I appreciate all the input from others as well. I was planning to do a
number of tests with ferrite beads and air coil baluns.

The data accumulated will be presented here for all to peruse. Thanks again
guys!

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