cap dialectric

[p.larner]

i am thinking of making some fixed caps for an atu,it will be copper fr4 sandwitched with 0.36mm nomex sheet as seperators,the fr4 boards are 6 inchx4 inch,will that work for high voltage?https://www.ebay.co.uk/itm/144648265472

[TimFox]

Why not use a well-characterized low-loss dielectric material such as polypropylene?
In any sandwich construction, be careful to avoid air gaps in the stack, where the adjacent dielectric will increase the E field (voltage gradient) in the air by it’s dielectric constant.

[p.larner]

simple realy as i have a sheet of the stuff,i brought it to use the insulate the windings from the stator on an ebike hubmotor rewind,so ive a lot left over+its tough.

[TimFox]

Nomex is an electrical insulator commonly used for fire protection, but I don’t know it’s dielectric properties, constant and loss. It is often a fibrous construction.
Did you look up its properties to design the capacitor?
Polypropylene has excellent voltage capability, so thin sheets can be used.
Polyester (Mylar^TM) is mediocre for loss.

[p.larner]

i found this

[TimFox]

The two Nomex dielectric loss data are mediocre, similar to polyester/Mylar at a few percent.
Note that the data are at sub-RF frequencies.
Polypropylene should be about 0.1%.
Depends on how good a capacitor you want, which usually depends on the dielectric.

[MathWizard]

What about something like hot glue ? If you glued together some metal sheets. I have an LCR meter and calculator, I need to try some of this. Like making variable caps out of tin can's.

What about if you wind some inductor and coat it in hotglue ? If the coil radius is large enough, wouldn't still be an air core inductor. I need to play around with inductors more, like putting different materials inside them.

[TimFox]

Back in the day, we used “Q dope”, which was polystyrene dissolved in a solvent (acetone?) to exploit the low dielectric loss of polystyrene.

[p.larner]

would 2 sheets of fr4 copper clad  (1.2mm thickness? glued back to back work as a cap at hf? then?.i have about 150 5"x4" sheets f it,its  to make padding caps for an atu.

[TimFox]

With the data sheet values for dielectric constant and loss, and your plate area and dielectric thickness, what capacitance and Q do you calculate?

[p.larner]

i have no datasheet!.just wanted to know if it would work.

[Andy Chee]

Put a sheet of Nomex in your microwave oven for 30 seconds.  Make sure you have a cup of water in the microwave alongside your sample Nomex.

If the sample gets warm, it's no good for your RF antenna tuner application.

[TimFox]

i have no datasheet!.just wanted to know if it would work.

I’m sure you can locate dielectric data for FR4 through Google.
How can anyone else answer the question if it would work without a clue about the required capacitance and Q?