NFC PCB Antenna Generator for KiCad

This Python script calculates and generates custom 13.56 MHz NFC/RFID planar antennas for Printed Circuit Boards (PCBs). It directly outputs a ready-to-use KiCad footprint (.kicad_mod) and calculates the specific 3-capacitor tuning network required for STMicroelectronics ST25 tags (like the ST25DV64KC).

Features

• Precision Calculation: Uses the Mohan equation for planar, circular spiral inductors to hit your target inductance (e.g., 4.83 µH).
• Hardware-Optimized Routing: Forces whole integer turns so the inner and outer connection pads perfectly align on the same axis. This prevents unwanted loop areas during PCB routing and improves EMI.
• Direct KiCad Export: Generates a .kicad_mod file that can be instantly imported into KiCad or Altium Designer.
• ST25DV Tuning Network: Automatically calculates the required external capacitance for a differential 3-capacitor adjustment network ($C_{p}$, $C_{s1}$, $C_{s2}$) based on the IC's internal tuning capacity ($C_{tune}$ = 28.5 pF).
• Input Validation: Catches invalid geometry parameters early with clear error messages.

Requirements

• Python 3.6 or later (no external dependencies — uses only the standard library)

Installation

git clone https://github.com/<your-user>/CoilDesigner.git
cd CoilDesigner

No additional packages need to be installed.

Usage

Run the script with default parameters (4.83 µH target, 50 mm diameter, 0.3 mm track):

python3 nfc_antenna.py

Or customise any parameter via command-line flags:

python3 nfc_antenna.py --target-l 2.5 --d-out 35.0 --width 0.2 --spacing 0.2 --thickness 18.0

To write output files into a specific directory:

python3 nfc_antenna.py --out-dir build/

Parameters

Flag	Type	Default	Description
--target-l	float	4.83	Target inductance in µH
--d-out	float	50.0	Outer coil diameter in mm
--width	float	0.3	PCB track width in mm
--spacing	float	0.3	Spacing between tracks in mm
--thickness	float	35.0	Copper layer thickness in µm (35 µm = 1 oz)
--out-dir	string	.	Output directory for generated files

Run python3 nfc_antenna.py --help to see all options.

Output Files

File	Description
NFC_Tag_Antenna_<d_out>mm_Integer.kicad_mod	KiCad footprint — import directly into your PCB project
ST25DV_Custom_Matching.txt	ASCII schematic with calculated capacitor tuning values for the ST25DV64KC

Using as a Library

The module can also be imported without side effects:

from nfc_antenna import calculate_inductance, find_best_integer_turns

turns, L = find_best_integer_turns(4.83, 0.05, 0.0003, 0.0003)
print(f"{turns} turns → {L:.3f} µH")

Network Topology

The script calculates values for the following differential tuning topology commonly used with ST25 dynamic tags:

 Tag AC0 ───────||─────────┬──────────────┐
              [ C_s1 ]     │              │
                          _│_             │
                          ___ C_p         § L_ant
                           │              § 
 Tag AC1 ───────||─────────┴──────────────┘
              [ C_s2 ]