Conformal coating of UHF (radio frequency) circuitry 400 - 1000 MHz

Question

Do you have experience applying conformal coating on UHF circuitry (ISM band)?

Are there known issues (e.g. degradation of UHF performance) which arise when coating UHF components like filters and matching networks or even Chip-Antennas?

Would be glad to hear about your experience.

edit: I wanted to provide some results from my investigation. The Smith-Chart shows the impedance looking into the matching network (from transceivers point of view), before and after coating was applied. In particular both smith charts are overlapped. The orange arrow shows how marker 1, the center frequency is beeing affected.

Question: does that mean, that coating acts as a series inductor? Would I need to counteract that with an equvalent series capacitor or by choosing a lower value for the existing series inductor?

Answer 1

The short answer is yes, the conformal coating, or, for example, solder mask, does change things at high frequencies. For example, the plastic in the coating likely has a dielectric constant of about 4, so the capacitance between traces is increased. The characteristic impedance of transmission lines will be lower. Resonant antennas will be de-tuned.

You can use CAD programs to look at this, or estimate its effects using microwave formulas, or just build your circuit and measure it to see if the changes are significant in your particular case.