Technical abt. Current Baluns
10th. Aug. 2005
Felix
Meyer, HB9ABX
Measurements
of Current Baluns (current chokes)
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The
efficiency or the quality of a current balun
(current choke, surface current
filter) can be measured in the following way:
This applies for 50 ohm 1:1coax
current baluns:
A current balun has the function of preventing surface
currents
on the braid of a coax cable, while the attenuation of
the
passing RF shall be kept minimal.
I recommend the following
measurements:
1: Measuring of isolation
resistance.
The filter has to block surface current (surface
waves)
with the isolation resistance on the braid.
For that we measure the
RF resistance using e.g. an antenna analyzer
between the ground input and
ground output of the filter.
Measurement has to be performed over the
interested frequency
range of use.
The attenuation is good, if this
resistance is 10 times the cable
impedance, or more. That is, 500 Ohm or
more.
Higher resistance is better.
2. Measuring of power
loss.
To measure the loss of the filter, one connects to identical
filters
in series.
The input is fed with a transmitter with maximum power
of the filter
and the output is terminated wit a dummy load of
appropriate
power rating.
In order to assure that the filter operates as
isolating balun,
the ground connection between the two filters shall
be
isolated from ground (= floating) and at the same point,
the center of
the coax shall be connected to ground!
The ground connection of the output
shall be connected
to the ground connection of the input.
A power meter
inserted at the input and at the output is
used to measure the power
loss.
The power loss is the difference between input power
and output
power.
This measurement shall be performed in the interested
frequency
range of operation.
A good filter has 5% or less power attenuation.
5
% power loss corresponds to 0.223 dB attenuation,
what in practical operation
is not noticed.
A word of
caution: Ferrite
Toroids
--------------------------------------------
Recently I
purchased toroids in a HAM fair, in order to
build current
baluns.
Unfortunately I received unusable material.
It concerns
material of the type FT-140-43.
Before the use of toroids, I
recommend to measure them,
in order to prevent unexpected
results!
1.
Measurement:
Wind 10 turns of isolated copper wire around the toroid
of
the FT-140-43 and measure the inductivity.
You should read between 90
and 100 Micro Henrys.
2.
Measurement:
Measure the resistance of 10 turns wound on FT-140-43
using
e.g. MFJ-269 or MFJ-259:
Indicated R : above 1.7 MHz : higher than
500 Ohm, rising with
the frequency to > 1500.
Above 25 MHz, R is
lowering towards 500 Ohm, while Xs remains at 0.
If the measured values
differ much from the indicated values,
you have some different marerial.