[600MRG] Vacuum variable capacitor, low-pass/band-pass filters

[Joe]

On 6/21/2019 1:52 PM, Frank O'Donnell wrote:
> Hi all,
>
snip
> 1) Depending on how it's hung, the final circumference of the loop 
> could vary between about 100 and 130 feet. Using AA5TB's loop antenna 
> spreadsheet, it appears that I will need a range of about 1548 pF to 
> 2136 pF capacitance to tune the loop across the entire 630m band. I 
> have zero experience so far with vacuum variable capacitors, and 
> although I see them listed for sale on eBay and elsewhere, I'm 
> reluctant just to pick one and hope it's suitable. Does anyone have 
> suggestions for a source, possibly even for a specific cap?
>
> Frank K6FOD
Now this is something I have wondered about some, and why does it not 
happen?

Ok I know all the reasons for the vac caps.

But why can you not take a regular large bread slicer variable, and fill 
in the air gaps with a dialectic material. Gets two advantages

the arc over voltage point can be raised considerably!

and the amount of capacitance can be raised many many fold depending on 
the dialectic. some insulating plastics can be like 15 times!

Why is this not done ever that I have seen?

Joe WB9SBD