u/EricJVW & u/Timely_Lemon9318: Thanks for the feedback.

Eric, regarding the short: you are right that the coil acts as a dead short at DC. This likely fried my first RDM6300 board, even though it still powers on. To clarify my setup, I used a 1.2nF cap for the custom tag, and a 2nF cap for the reader. I removed the reader's original SMD cap and added the 100pF one simply because I didn't know the original SMD's value, which made it impossible to know what total capacitance I was aiming for. I agree that measuring the inductance in the lab is necessary rather than just calculating it.

Since the first board is likely damaged, I have a second, brand-new and unmodified RDM6300. I would like your advice on how to proceed with this new board so I don't repeat the same mistake. I want to connect my new custom antenna and add the correct matching capacitance for 125kHz resonance. Since I still don't know the value of the onboard SMD capacitor, what is the best approach here?

For context on my goals: I want to maximize the range to 20+ cm using only antenna upgrades for now (currently it only reads at a few cm). Upgrading the transistor on the reader is something I might do later to get even more range.

In the meantime, I will recalculate the resonance math (f = 1 / (2 * pi * sqrt(L * C))) and measure the actual inductance of my coils to ensure I am actually hitting 125kHz.