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The influence of temperature changes in Common Mode Chokes.
pa0nhc 20200602 / 20220917

FIRST press F5.

Mix #31 material :
Variation of Ui with temperature.
30C : Ui ~ 1700.  90C : Ui ~ 3600



Top :
Impedance measurment of :
three CMCs in series.
Each CMC :
2 cores stacked,
#31 mix ferrite 29x19x14mm ,
13 turns 1mm thin test wire.

Bottom :
 1 core #31 mix ferrite 29x19x14mm ,
13 turns thin 2.5mm RG316 PTFE coax demonstrates possible winding with thin coax.


NanoVNA impedance measurement
on the three CMCs connected in series at the picture on the left.
Z3.7MHz = 22 kOhm.
Each CMC consists of 2 stacked cores.
Each core : #31 mix ferrite 29x19x14mm.

Totally 3 pairs of 2 cores = 6 cores. 
Each cora pair wound with
13 turns 1mm thin test wire.
Winding thin RG316 coax will give the same reults.

In the right hand picture, the green line in the Smith Chart stays very close to the Xaxis. Which means that the impedance is dominantly resistive, and becomes purely resistive at the 3.7 MHz marker. 

When the temperature of a mix31 core rises, the initial Permeability (U's) rises too.
Then causing a CMC inductance to rise, and the CMC resonance frequency to shift down.
As the core losses are not only temperature dependant, but also frequency dependant, this should be given in a bundle of characteristics. 
The FairRite catalog gives no such data.


1 core with 13 turns RG316 PTFE coax. @30C
1 core with 13 turns RG316 PTFE coax. @90C

Frequency characteristics of a CMC, made from one 29x19x14mm #31 mix ferrite core, with 13 turns RG316 coax.
Temperature rise has nearly no practical influence on these coil its properties.

To get an indication of the impedance behavior when the core temperature changes, i tested a CMC by heathing it with a hairdryer, and measuring its temperature with an infrared thermometer.

When the CMC  temperature rises from 30C to 90C, the frequency of maximal impedance shifts down from ca.10 MHz to ca.7.2 MHz, or by a factor 1.4. But as the impedance characteristic is flat, the attenuation changes very little between 1.8 MHz and 7.2 MHz when shifting to the left.

Conclusion :
Used in antenna systems for 1.8MHz to 28MHz, the CMC temperature change will have very little effect on antenna properties like VSWR.

 

Application in the pa0nhc Cobra Antenna :
Seven of these little CMCs with distances > 1cm connected in series, should work well as a Cobra antenna CMC.
With air cooling, the dissipation per choke core  at 100W power is estimated as maximal 0.4W.