[gprutchick]

RodB

I was thinking of trying to use the 3M PET in lieu of mica for padders and trimmers.

I thought that I would ask ChatGPT for the dialectric constant of 3M PET transparency film,

For PET film, the dielectric constant (relative permittivity, εr) is typically:
εr ≈ 3.0 to 3.3 (at 1 kHz to 1 MHz)

For most RF and capacitor-type calculations, using:
εr = 3.1
is perfectly reasonable.

The dielectric constant varies slightly with: Frequency, Temperature
Exact formulation (some 3M films have coatings)

If you're working in the AM broadcast band (~500–1700 kHz), using 3.0–3.2 is absolutely fine for design calculations.

Dielectric strength for PET film is typically:
~200–300 V/mil
(very respectable for thin capacitor experiments)

For the mica used in trimmer and padder capacitors, the dielectric constant (relative permittivity, εr) is typically:
εr ≈ 5 to 7. Most commonly in radio service work, you’ll see: εr ≈ 6

I guess we can conclude that 2 layers of the 3M transparency would be equivalent to a single layer of mica if used in a padder or trimmer capacitor. However, the dielectric constant of the 3M material varies slightly with: Frequency, Temperature.

So to copare the temperature stability of mica with the 3M PET:

Mica (natural muscovite)
Dielectric constant (εr): ~5–7
Typical temp coefficient (TCC):

~ ±50 ppm/°C (often better in good capacitor-grade mica)

That’s very stable. Across a 30°C room change: 50 ppm/°C×30°?=1500 ppm=0.15%

PET (Polyethylene Terephthalate – 3M transparency film)

Dielectric constant (εr): ~3.0–3.3
Typical temp coefficient: ~ +400 to +700 ppm/°C

Let’s use 500 ppm/°C as a reasonable average.
Across 30°C: 500 × 30 = 15000 ppm = 1.5%

That’s 10× worse than mica.
In oscillator circuits, that’s noticeable.

Final Practical Verdict

For: RF padders, Oscillator trimmers, IF fixed caps, Anything affecting frequency accuracy Mica is vastly superior.

PET is fine for: Coupling caps, Bypass caps, Non-frequency-determining uses

But for frequency-setting components in your Philco-style circuits you want mica.

Learning something new every day!
gprutchick