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HB9ABX > TECHNIK 02.07.20 16:42l 74 Lines 2941 Bytes #999 (999) @ WW
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Subj: measure current balun
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Sent: 200702/1522z @:DB0FHN.#BAY.DEU.EU [JN59NK Nuernberg] obcm1.07b12 LT:999
From: HB9ABX @ DB0FHN.#BAY.DEU.EU (Felix)
To: TECHNIK @ WW
Reply-To: HB9ABX @ HB9EAS.CHE.EU
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Measurements of Current Baluns (current chokes)
-----------------------------------------------
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.
73 de Felix, HB9ABX
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