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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 26th May 2014, 8:53 pm   #1
threeseven
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Default Air spaced tuning caps, possibly a silly question!!

Was thinking, as you do, about air dielectric tuning caps, the aluminium vane type seen in most old radios.
As air is the dielectric, and air is prone to changes in density and humidity, this must change the leakage between the vanes. So therefore capacitance will vary accordingly at any given setting. This would therefore change tuning. Which would be one of the reasons for slight tuning drift on older sets I suspect? Particularly as the set warms up. Or am I totally wrong and missing something?
As I said, possibly a silly question!
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Old 26th May 2014, 9:48 pm   #2
GrimJosef
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Default Re: Air spaced tuning caps, possibly a silly question!!

The capacitance and the leakage are different things. As far as the capacitance goes this depends directly on the permittivity of the dielectric. The permittivity of air is 1.00059. The reason we don't worry about atmospheric changes is that these only affect the 0.00059 part, mostly through the density of the air. And the density of air doesn't vary very much - 950 mbar is an unusually low pressure and 1050 mbar an unusually high one. So even over this +/-5% range the permittivity of the air would still only vary from about 1.00056 to 1.00062 which is a 0.006% variation. I suspect that other factors (maybe simple thermal contraction and expansion ?) would lead to bigger changes in the tuning than this.

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GJ
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Old 27th May 2014, 11:02 am   #3
G8HQP Dave
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Default Re: Air spaced tuning caps, possibly a silly question!!

The main reason for tuning drift is the inductor warming up.
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