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 , 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.

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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