Hello Dan,

About your matching network for loop antennas:

Breakdown will not occur through the material in your case. Major issue
is dielectric losses.

As you know the antenna impedances, you are able to calculate the
apparent power for the capacitor (just Ic*Uc [rms values]). Both
Epsilon and loss factor for FR4 varies (depending on the manufacturer).
You can use 0.01 as a rule off thumb. So if your capacitor "consumes"
an apparent power of 500W (at 100W input), the loss inside the
capacitor is about 500*0.01 = 5W.

I hope this will help you.

Wim
PA3DJS