10-03-2018, 01:28 PM
So, that's different to this then in Post#3:
""I've just done some sums, and come up with a typical vane/stator spacing of about 0.3mm, which is about right for a 10 vane 500pf tuning capacitor. Now suppose that there is 0.005mm of corrosion (ie 5 microns), so the total on both sides of the gap will be 0.01mm. The corrosion will be aluminium oxide typically, which has a relative permittivity of 10. This increases the effective gap by 0.01 x 10 = 0.1mm. So the effective gap is now 0.3mm+ 0.1mm = 0.4mm. So the capacitance will go up from 500pF to 750pF, and the tuning frequency by the root of that - a shift to longer wavelengths of ~20%.
Now that is an over-estimate in capacitance shift. The oxide layer could be thinner, which would change the values, and of course the aluminium is a tiny bit thinner because some of it has been converted to oxide.
But the principle remains - could a visible oxide layer change the value of a tuning capacitor? Yes.""
Lawrence.
""I've just done some sums, and come up with a typical vane/stator spacing of about 0.3mm, which is about right for a 10 vane 500pf tuning capacitor. Now suppose that there is 0.005mm of corrosion (ie 5 microns), so the total on both sides of the gap will be 0.01mm. The corrosion will be aluminium oxide typically, which has a relative permittivity of 10. This increases the effective gap by 0.01 x 10 = 0.1mm. So the effective gap is now 0.3mm+ 0.1mm = 0.4mm. So the capacitance will go up from 500pF to 750pF, and the tuning frequency by the root of that - a shift to longer wavelengths of ~20%.
Now that is an over-estimate in capacitance shift. The oxide layer could be thinner, which would change the values, and of course the aluminium is a tiny bit thinner because some of it has been converted to oxide.
But the principle remains - could a visible oxide layer change the value of a tuning capacitor? Yes.""
Lawrence.
