Experiments with LC circuits   part 6

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From polypropylene sheet, I made a small table with a height of 25 cm.
On it are placed; the coil, tuning capacitor and the amplifier.
The coil is placed on a 12 cm height support made of polypropylene.
The amplifier box is covered with a small aluminium plate.
Under the test setup there is a larger aluminium sheet which is connected to the test setup ground.
Also the signal generator ground is connected to the rest of the test setup ground.
The coupling coil is placed on the same height near the coil of the LC circuit.
On this picture you see the complete test setup.

Because the LC circuit is now at some height above the wooden table, I hope to get a higher circuit Q.

In the following measurement, the effect is measured of the aluminium under the test setup and the aluminium on the amplifier box.
There is only measured at 1500 kHz.

Measurement number  LC combination conditions Q at 1500 kHz
61 L9 C2b Without aluminium under test setup
Without aluminium on amplifier
62 L9 C2b Without aluminium under test setup
With aluminium on amplifier
63 L9 C2b With aluminium under test setup
With aluminium on amplifier


Now the distance between the coupling coil and the coil of the LC circuit is varied, and the Q's measured.
The distance is measured between the outside windings of the coils, so with a distance of 0 cm the windings are touching each other.

Measurement number  distance coil - coupling coil (cm)  Q at 1500 khz 
64 40 891
65 35 881
66 30 881
67 25 881
68 20 881
69 15 876
70 10 897
71 5 907
72 0 839

The influence is less then I expected.
There is a small increase in Q at 5 and 10 cm.
Maybe this is caused by the fact that at such a small distance, there is also capacitive coupling between coils, which can add or subtract from the inductive coupling.
To reduce the capacitive coupling, I always connect the outside winding of the coils to ground.

In the following measurements the coils have a distance of 20 cm.

Spiderwebcoils with polypropylene former

Now I made some polypropylene coilformers, because this material gives less losses then the first used foam PVC.

Coil L12

Wire length on the coil: 14.5 meters and two leads of 0.25 m.
Total wire length: 15 meters.
Inside coil diameter: 6 cm
Outside coil diameter: 16 cm
Number of windings: 39.5
Wire: litzwire 660x0.04 mm
Coilformer material: 3 mm polypropylene.
Induction: about 175 uH

Coil L12 ia about the same as coil L9, only the former is not made of foam PVC but polypropylene.
I unwounded coil L9 and used the wire for L12, this because of the price of the 660x0.04 litzwire.


Coil L13 

Wire length on the coil: 17 meters and two leads of 0.25m.
Total wire length: 17.5 m
Inside coil diameter: 6 cm
Outside coil diameter: 17.5 cm
Number of windings: 44
Wire: litzwire 660x0.04 mm
Coilformer material: 3mm polypropylene
Induction: about 230 uH

L13 has a longer wire length than L12, and so more induction.
With L13 the whole MW band can be tuned, with the use of a 390 pF tuning capacitor.

In the following measurement, the test setup described at the top of this page is used.


LC combination 

F min 

F max 

600 kHz 

900 kHz 

1200 kHz 

1500 kHz 


L9 C2b 








L12 C2b








L13 C2b 







We see the polypropylene formers gives a higher Q, compared to the foam PVC former used in measurement 73.

When measuring the Q at 900 kHz, something interesting happens.
When the circuit is tuned to 900 kHz, and the DDS signalgenerator is also tuned to 900 kHz, I measure for instance 3 Volt peak-peak on my oscilloscope.
But then I see the amplitude vary a little bit (about 0.1 to 0.2 Volt).
The variation has a frequency of 3 Hz.
If I tune the DDS generator 3 Hz higher, the frequency of the variation reduces to 0 Hz.
There must be a station on 900 kHz interfering with my DDS generator signal.
Probably this station is: RAI 1 from Italy on 820 km distance (600 kW power).
This is amazing, because I don't have a antenna connected to the LC circuit.
We can also conclude that my DDS generator has a frequency error of 3 Hz (at 900 kHz).

This variation in amplitude only happens when the sun is down, so when there is a good reception of foreign stations.
On 600 kHz, 1200 kHz, and 1500 kHz this don't happen, because these frequencies are not on the 9 kHz frequency raster which is used here in Europe.

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