Experiments with LC circuits  part 9

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In this measurement, the Q is measured with different tuning capacitors.

Tuning capacitor C1c

500 pF
With polyethene insulators
and copper plates.

 

Tuning capacitor C4 

2x 365 pF
Brand: Hopt.
With ceramic insulators and aluminium plates.

 

Tuning capacitor C5 

2x 500 pF
From a old tube radio.
With ceramic insulators and copper plates.

The used coil (L12a) is coil L12 with the last winding removed.
The induction of L12a is about 169 uH.
Coil L12a is mounted in my "antenna unit 1" (description comes later), in the antenna unit 1 there are two tuning capacitors placed near the coil, this can give some reduction in Q compared to measurement 74.

In measurement 84, I placed back two sets of old insulation rings on tuning capacitor C1c (these are the rings removed in measurement 5 ).
Original there were 4 sets of insulation rings on C1, but I couldn't replace the other two set's without dismantling the whole antenna unit 1, and this was too much work.

With tuning capacitor C5, one measurement is made with one 500 pf section, and one with the two 500 pF section parallel.

Measurement
 number
LC combination F min
kHz
F max
kHz
Q 600 kHz  Q900 kHz Q 1200 kHz Q 1500 khz
83 L12a C1c 550 2184 1111 1084 961 766
84 L12a C1c
with old insulation rings
546 2005 731 523 339 252
85 L12a C4 (2x365 pF) 451 2312 1111 1046 923 750
86 L12a C5
one 500 pF section
547 2670 638 725 685 604
87 L12a C5
2x 500 pF parallel
390 2160 674 725 655 595

Conclusions: 
Tuning capacitor C1c and tuning capacitor C4 almost give the same circuit Q.

Placing back the old insulation rings on tuning capacitor C1c gives a large reduction in Q.
Here we see the importance of good quality insulators in tuning capacitors

The old tuning capacitor C5 gives a lower Q than C1c and C4, maybe this is caused by:
- less quality of the ceramic insulators
- contact resistance between rotor and frame of the capacitor
- oxidation of the plates.

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