[Milsurplus] [Boatanchors] Mica compression trimmers?

[Gordon Smith]

Hi All,
Per the beginning of an ARCO Mica compression trimmer data sheet 
(edited slightly to help readability and available here: 
<https://datasheet.datasheetarchive.com/originals/scans/Scans-058/DSA2IH0026119.pdf>DSA2IH0026119.pdf 
): "variable compression mica trimming capacitors are produced by 
stacking mica dielectric capacitance units. A capacitor section 
consists of a thin film of mica between two spring loaded nonferrous 
metal conducting plates........A panhead adjusting screw.....is 
inserted through the center holes of the plates, the mica films and 
(a) threaded bushing. This screw provides variable compression on the 
formed metal plates, varying the plate separation capacitance."

Because of this style of construction technique, large capacitance 
trimmer units appear to be able to be modifiable to lower capacitance 
levels  by the removal of plates/mica sheets for lower capacitance 
levels. One ebay seller notes this in his listing : 
https://www.ebay.com/itm/315-Variable-Compression-Mica-Trimmer-Capacitor-Please-See-My-Note-/264803509560 


Hope this helps,

73, Gordon KJ6IKT

At 12:19 PM 11/8/2020, KD7JYK DM09 wrote:
>>I believe your observation is wrong. Mica compression trimmers 
>>indeed do have mica between the plates, so the compression is on 
>>the mica and not air. Consequently you can obtain a higher 
>>capacitance for the same size as the dielectric constant for air is 
>>1 and for mica it is about 3 to 6.
>
>And here I though rock didn't compress, and only the air-gap between 
>the plates was changing relatively electrically, until the plates 
>stopped. I do however, recall a saying of squeezing blood from a 
>stone, so maybe that's how they work, but I can't imagine it 
>designed to shatter eisenglass, bend plates, strip screws, and crack 
>the body, for that last little uuF.  I figured the mica was the 
>dielectric to increase the operating voltage...
>
>Kurt
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