Experiments with LC circuits   part 16

<< to part 15
Back to the index

In part 15 of this "experiments with LC circuit" series, I started experiments with LC circuits with ferrite rods in the coil.
On that page, a coil with 80 turns was used.
But after sticking a ferrite rod in that coil, the resonance frequency of the LC circuit became very low.
The lowest tuneable frequency was in the range of 200 kHz.
And the highest tuning frequency, didn't came further then the lower part of the medium wave band.

This time I made a new coil with only 30 turns, which hopefully will better cover the whole medium wave band.
At least I want to receive my local station "Groot Nieuws Radio" on 1008 kHz with it (100 kW 41 km distance).
This station is at the moment the only high power medium wave station left here in the Netherlands.

The coil has 30 turns of 0.8 mm solid enamelled wire.
It is wound on a 42 mm diameter toilet paper roll.

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

Measurement 111:  Q factor of 30 turns 42 mm coil without ferrite.

The tuning range is from 1200.6 to 3775 kHz, so without ferrite in the coil, I can't receive my local station on 1008 kHz.
On the lowest tuning frequency, the capacitance of the LC circuit is 526 pF.
And with the formula: L = (1 / (2. pi. f))² / C  we can calculate the inductance of the coil to be: 33.4 μH.

1 Ferrite rod 14 x 200 mm in the coil.
The first ferrite rod I put in the coil is a 14 x 200 mm rod.
The ferrite material is: 3C85 ( datasheet_3C85.pdf ).


Figure 1:  14 x 200 mm ferrite rod in the 30 turns coil.

The measured Q factors at the highest and lowest frequency, and two frequencies in between are:

Measurement 112:  Q factor of the 30 turns coil with one 14 x 200 mm ferrite rod in it.

The LC circuit can be tuned from 548.81 to 1704.20 kHz, this quite well covers the whole medium wave band.
The inductance of the coil is now: 160 μH.

Now the power can be measured which the coil picks up from the local station on 1008 kHz.
The voltages across the LC circuit are measured with several load resistor values.
And then the received power levels are calculated.
See part 12 for a description about the power measurement.

Measurement 113:  received power at 1008 kHz for the 30 turns coil with 1 ferrite rod 14 x 200 mm in it.

1 Ferrite rod 10 x 200 mm in the coil
Then a 10 x 200 mm ferrite rod was put in the coil.
The material of the 10 x 200 mm rods is 3B1 ( datasheet_3B1.pdf ).


Figure 2:   10 x 200 mm ferrite rod in the coil.

 

Measurement 114:  Q factor of the coil with 1 ferrite rod 10 x 200 mm in it.
The coil inductance is: 128 μH.

The received power with several load resistors is:

Measurement 115:  Received power at 1008 kHz for the 30 turns coil with a 10 x 200 mm ferrite rod in it.

2 Ferrite rods 10 x 200 mm parallel in the coil
Now 2 ferrite rods of 10 x 200 mm are placed parallel in the coil.


Figure 3:  two 10 x 200 mm ferrite rods parallel in the coil.

Measurement 116:  Q factor of the coil with two ferrite rods 10 x 200 mm in it.
The coil inductance is: 159 μH.

The received power is:

Measurement 117:  received power at 1008 kHz for the coil with 2 ferrite rods 10 x 200 mm parallel.

4 Ferrite rods 10 x 200 mm parallel in the coil.
The number of parallel ferrite rods is increased to 4.


Figure 4:  four ferrite rods 10 x 200 mm parallel in the coil.
 

Measurement 118:  Q factor of the coil with 4 ferrite rods 10 x 200 mm in it.
The coil inductance is: 192 μH.
 

Measurement 119: received power at 1008 kHz for the coil with 4 ferrite rods 10 x 200 mm in it.


8 Ferrite rods 10 x 200 mm in the coil.
The number of parallel ferrite rods is increased to 8.
The rods are mounted around a 16 mm plastic tube.

Figure 5.

The measured Q factors are:

Measurement 120:  Q factor of the coil with 8 ferrite rods 10 x 200 mm parallel.
The coil inductance is: 260 μH.
 

Measurement 121:  received power at 1008 kHz for the coil with 8 ferrite rods 10 x 200 mm parallel.
 

Figure 6:  measured Q factors and frequency tuning range with ferrite rods parallel in the coil.
For the coil without ferrite (the dotted line) only the lowest part of the tuning range is shown.

Placing ferrite bars parallel increases the Q factor.
I think because the magnetic flux in the coil then go through a larger area of ferrite.
As a result, the flux density in the ferrite gets lower.
Lower flux density decreases the core losses strongly.
And lower losses means: higher Q factor.

The Q factor with 8 ferrite rods in the coil, is however lower then with 4 rods.
This is probably caused by the fact  that with 8 rods, the ferrite comes very close to the coil windings.
This gives extra capacitance between coil and ferrite rods.
These ferrite rods are electrically conductive (about 40 kΩ between the ends ).
So when you place the rod very close to the coil winding, it is like placing a capacitor in series with a resistor across the coil, which will reduce the Q factor.

The 14 x 200 mm rod doesn't seem to work very good on medium wave frequencies, the Q factor is quite low at higher frequencies.



Figure 7:  received power with ferrite rods parallel in the coil.

Placing ferrite rods parallel in the coil, increases the received power.
With 8 rods parallel, the received power is the highest.

The 14 x 200 mm rod (material: 3C85) also here gives worse results then the 10 x 200 mm rod (material: 3B1).
 

Instead of placing the ferrite rods parallel, I will now place them in series.

 

Figure 8:  the ferrite rods in series are put in the tube which goes through the coil.

The tube is 1 metre long, and can contain up to 5 ferrite rods in series.
In this measurement I only use the 10 x 200 mm rods.
The ferrite rods are so positioned in the tube, that the coil is in the middle of the row of rods.

The situation with one rod in the coil was already measured in measurement 114 and 115.
 

2 Rods 10 x 200 mm in series.

2 Ferrite rods are put in the tube.

Measurement 122:  Q factor with 2 ferrite rods 10x200 mm in series.
The calculated inductance is 145 μH.
 

Measurement 123:  received power with 2 ferrite rods in series.


3 Rods 10 x 200 mm in series.

3 Ferrite rods are put in the tube.

Measurement 124:  Q factor with 3 ferrite rods 10 x 200 mm in series.
The calculated inductance is 179 μH.
 

Measurement 125:  received power with 3 ferrite rods in series.

4 Rods 10 x 200 mm in series.

4 Ferrite rods are put in the tube.

Measurement 126:  Q factor with 4 ferrite rods 10 x 200 mm in series.
The calculated inductance is 173 μH.
 

Measurement 127:  received power with 4 ferrite rods in series.

5 Rods 10 x 200 mm in series.

5 Ferrite rods are put in the tube.

Measurement 128:  Q factor with 5 ferrite rods 10 x 200 mm in series.
The calculated inductance is 188 μH.
 

Measurement 129:  received power with 5 ferrite rods in series.

Figure 9:  Q factors and frequency tuning range with ferrite rods in series in the coil.

Placing ferrite rods in series reduces the Q factor.

 

Figure 10:  received power at 1008 kHz with ferrite rods in series in the coil.

Placing ferrite rods in series in the coil, increases the received power.
 

2 x 8 Ferrite rods 10 x200 mm, coil at 50 % of the rod length.

In this measurement, I use 8 ferrite rods parallel, in series with another 8 rods.
So, a total of 16 ferrite rods.
The total length of ferrite is 400 mm, and the coil is in the middle of it.


Figure 11:  2x 8 ferrite rods, the coil is at 50% of the ferrite length.

 

Measurement 130:  Q factor with 2 x 8 ferrite rods 10 x 200 mm.
The coil is at 50% of the ferrite rod length.
The calculated inductance is 349 μH.
 

Measurement 131:  received power with 2 x 8 ferrite rods in the coil.
The coil is at 50 % of the length of the ferrite rod.
 

2 x 8 Ferrite rods 10 x 200mm, coil at 25 % of the rod length.

Now I use the same 2 x 8 ferrite rods, but the position of the coil is changed to 25 % of the ferrite length.

Figure 12:   2x 8 ferrite rods, the coil is at 25% of the ferrite length.
 

Measurement 132:  Q factor with 2 x 8 ferrite rods 10 x 200 mm.
The coil is at 25% of the ferrite rod length.
The calculated inductance is 322 μH.
 

Measurement 133:  received power with 2 x 8 ferrite rods in the coil.
The coil is at 25 % of the length of the ferrite rod.

Figure 13:  Q factor with 2 x 8 ferrite rods in the coil.
For comparison, also the result from measurement 120 ( 1 x 8 ferrite rods) is shown with the blue line in the diagram.

Figure 14:  received power with 2 x 8 ferrite rods in the coil.
For comparison, also the result from measurement 121 ( 1 x 8 ferrite rods) is shown with the blue line in the diagram.

We see the received power is the highest when the coil is in the middle of the 2x 8 ferrite rods.


<< to part 15
Back to the index