Experiments with LC circuits   part 15

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On this page, we do some experiments with ferrite rods.
The ferrite rods are put inside a coil, and the effect on the coil properties is measured.

On the next picture you see the test setup.


Figure 1: overview of the test setup.

With the amplifier , the diode detector and the voltmeter, the amplitude of the signal across the LC circuit is measured.
Via the headphone, the demodulated signal can be heard.

For determining the received power, the LC circuit is tuned to a local station.
And the received signal amplitude is measured for several values of load resistors across the LC circuit.
The method is described in part 12
For determining the received power, the signal generator is not used.

With this test setup, also the Q factor of the LC circuit can be measured at several frequencies.
For this we need the signal generator, and we need to remove the load resistor.
The LC circuit is tuned to the frequency of the signal generator.
The method for determining the circuit Q can be found on this page

The Q factor measurements are done with 200 mV (peak) across the LC circuit at resonance.
And 141 mV (peak) at the -3 dB points.
In this voltage range my diode detector  works very accurate, it is really a great tool for Q factor measurements.

For ferrite cores, the Q factor might change with signal amplitude, so every time I used the same amplitude to get a good comparison.

Figure 2: detail of the tuning capacitor with the loading resistor attached to it via two crocodile clips.
 

Measurement with no ferrite in the coil.


Figure 3.

In this picture you see the coil of the LC circuit.
It has 80 turns of 0.8 mm solid enamelled copper wire.
The coil is wound on a 42 mm diameter toilet paper roll.

First the Q factor of the LC circuit is measured without a ferrite rod in the coil.

Measurement 100:  the Q factor of the LC circuit without ferrite rod.
The minimum tuning frequency is 635 kHz.
And the maximum tuning frequency is 1987 kHz.

The maximum capacitance of the tuning capacitor, inclusive the input capacitance of the amplifier is 526 pF.
When the minimum tuning frequency is 635 kHz, the inductance of the coil can be calculated to be: 119 μH.
This is calculated with the formula:  L = (1 / (2. pi. f))² / C.



Figure 4: results of measurement 100, the Q factor without ferrite in the coil.

Without ferrite in the coil, the LC circuit can be tuned across almost the whole medium wave band, except the lowest frequencies.
Therefore it was possible to measure the received power at my local station on 1008 kHz (100 kW 41 km distance).
 

Measurement 101: power received by the LC circuit without ferrite in the coil at 1008 kHz.

The coil has only a small area, so it picks up little voltage and power.
The received voltages are in fact so low, that it is near the limit of what the diode detector can measure.
I didn't want to make the load resistor value lower then 220 kΩ, because that would make the received voltage even lower, and introduce too much error.
But anyway, we see some values for received power in the range of 0.18 nano Watt.

1 Ferrite rod 14 x 200 mm in the coil
Next, a ferrite rod is placed in the coil, and the Q factor is measured.
This ferrite rod is 14 mm thick and 200 mm long.
The coil is placed over the middle of the rod.
The material of this ferrite rod is: 3C85 ( datasheet_3C85.pdf ).



Figure 5: One 14 x 200 mm ferrite rod inside the coil.

The Q factor at the maximum and minimum frequency, and two frequencies in between are:

Measurement 102: Q factor with one 14 x 200 mm ferrite rod in the coil.

The minimum frequency of the LC circuit is now, 230.18 kHz.
This means the inductance of the coil is now 909 μH, which is much more then the 119 μH it was without ferrite in the coil.

The highest tuning frequency is 717 kHz, which is in the lower part of the medium wave band.
The circuit cannot be tuned to my local station on 1008 kHz anymore, so no receiving power measurement can be done.



2 Ferrite rods 14 x 200 mm in series
In the next measurement, two ferrite rods are placed in series, like the next picture shows.


Figure 6:  two 14 x 200 mm ferrite rods placed in series.

The coil is in the centre of the two rods.
And the rods goes through the centre of the coil.

The Q factor at the maximum and minimum frequency, and two frequencies in between are:

Measurement 103:  Q factor with two 14 x 200 mm ferrite rods in series in the coil.
The tuning range is now 198.005 to 615.26 kHz.
The coil inductance is about 1228 μH.
 

2 Ferrite rods 14 x 200 mm parallel
I have only two of these 14 x 200 mm ferrite rods.
So the inevitable next measurement is to place these two rods parallel in the coil.


Figure 7:  two 14 x 200 mm ferrite rods parallel in the coil.

In this picture you can clearly see the grooves in these ferrite rods.
I thought these grooves are for increasing the Q factor (but I don't know for sure about that).

Measurement 104:  Q factor with two 14 x 200 mm ferrite rods parallel in the coil.

The coil inductance is now: 1140 μH.


The next diagram shows the results of measurement 102, 103 and 104.


Figure 8:   Q factor with 14 x 200 mm ferrite rods in the coil.
 

1 Ferrite rod 10 x 200 mm in the coil.
Now we are going to use another type of ferrite rod.
This one is 10 mm in diameter, and 200 mm long.
The material is: 3B1 ( datasheet_3B1.pdf ).
The coil is the same as before, so 80 turns solid wire on a 42 mm coil former.


Figure 9:  one 10 x 200 mm ferrite rod in the coil.

The measured Q factors are:

Measurement 105:  Q factor with one 10 x 200 mm ferrite rod in the coil.
The calculated inductance of the coil is: 711 μH.


3 Ferrite rods 10 x 200 mm in series
In this measurement three 10 x 200 mm ferrite rods are placed in series.
The rods are put inside a PVC tube to easily keep them in the right position.

Figure 10:  three ferrite rods 10 x 200 mm in series.

The measured Q factors are:

Measurement 106: Q factor with three 10 x 200 mm ferrite rods in the coil.
The calculated coil inductance is 1094 μH.


8 Ferrite rods 10 x 200 mm parallel.
And then, 8 rods where put parallel, and placed in the coil.

Figure 11:  eight 10 x 200 mm ferrite rods parallel in the coil.

The measured Q factors are:

Measurement 107:  Q factor with eight 10 x 200 mm rods parallel in the coil.
The calculated inductance is: 1485 μH.

It was fun hearing several European long-wave (153-279 kHz) broadcast stations in the headphone while turning the tuning capacitor.


The results of measurement 105, 106 and 107 are shown in the next diagram.

Figure 12:  Q factor with 10 x 200 mm ferrite rods in the coil.

A coil with 16 mm diameter.

Then I made another coil, again with 80 turns.
But this times, the coil is wound on a 16 mm PVC tube.
So, the coil diameter is reduced from 42 mm to16 mm.


Figure 13:  a coil with 80 turns wound on a 16 mm tube.
In this picture there are 3 ferrite rods in the tube, partly visible at the ends of the tube.


Without ferrite in the coil.
First the Q factor is measured without ferrite in the coil

Measurement 108:  Q factor of 16 mm 80 turns coil without ferrite core.
The coil inductance is 21.6 μH

1 Ferrite rod 10 x 200 mm in the coil.
Then one 10 x 200 mm ferrite rod was shifted in the tube.
The centre of the ferrite rod was at the centre of the coil.
The Q factor was:

Measurement 109:  Q factor with 1 ferrite rod 10 x 200 mm in the coil.
The coil inductance is: 713 μH

3 Ferrite rods 10 x 200 mm in series in the coil.
The number of ferrite rods in the tube was increased to 3

Measurement 110:  Q factor with 3 ferrite rods 10 x 200 mm in the coil.
The coil inductance is: 1061 μH.



Figure 14:  Q factor of the 16 mm coil, with 1 and 3 ferrite rods in it.

 

Without ferrite in the coil, there is a large difference in inductance between the two coils.
After putting a ferrite rod in the coil, the inductance greatly increases, and then both coils have almost the same inductance.

The Q factor of the 42 mm coil is however higher then the Q factor of the 16 mm coil.
You can see that when you compare figure 12 with figure 14.

The ferrite rods so much reduce the tuning frequency of the LC circuit, that I can't receive my local station on 1008 kHz anymore with these coils.
And because I want to measure the received power at 1008 kHz with ferrite rods in the coil, there is a need to reduce the number of turns.
But more about that in part 16 of this story.

<< to part 14   to part 16>>
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