astra/iphone-encoder-counter

ANTENNA.md

# Antenna audit — prototype, not RF-qualified

The current design cannot yet be signed off as working on the back of an iPhone. Digital clearance checks and an inductance estimate do not establish RF compliance or successful same-phone communication.

## Design basis and changes

ST25DV04KC uses 28.5 pF nominal internal tuning capacitance (26.5–30.5 pF). With a nominal 3 µH coil, resonance at 13.56 MHz requires about 45.9 pF total effective capacitance. The earlier 10 pF C9 depended on approximately 7.4 pF of additional capacitance. That value was not justified by measurement.

C9 is now 15 pF ±1%, C0G, Murata GJM1555C1H150FB01D / JLC C441742. This is a prototype starting value: it allows about 2.4 pF of additional capacitance at a nominal 3 µH. The layout brings the NFC chip and tuning capacitor closer to the antenna terminals. The outer-terminal path is shortened from about 18.0 mm to 8.9 mm; the inner-terminal path is about 19.1 mm versus 21.9 mm before. The longest individual branch is still 13.34 mm, so the layout is not yet an ideal few-millimeter feed as recommended by ST. Those feeds must be included in the measured RF model. Exact branch lengths are in validation/antenna-audit.json.

The printed spiral remains seven turns with 0.25 mm trace and spacing. A straight-segment partial-inductance screening model estimates about 3.0 µH for the spiral alone. That model excludes feed inductance, dielectric parasitics, RF losses, phone metal, magnets, ferrite and reader coupling; it is not a calibrated electromagnetic simulation. For 3 pF assumed parasitic capacitance, the nominal resonance is about 13.5 MHz. Neither that capacitance nor the assembled resonance has been measured. No ground pour lies beneath or around the spiral.

## Required physical acceptance tests

1. Assemble a first article with the intended 1.6 mm FR-4 / 35 µm copper stack. Verify actual trace dimensions and continuity.
2. Install it in the exact case with the intended phone, rotor and attachment magnets. Select any NFC-grade ferrite backing as part of that stack; do not assume an unspecified ferrite sheet solves metal loss.
3. Measure resonance contactlessly with a weakly coupled loop/VNA, or measure antenna impedance using the manufacturer procedure. Record the complete fixture and case stack. Avoid probe capacitance dominating the small tag capacitance.
4. Adjust C9 based on the measured resonant frequency. Aim near 13.56 MHz and optimize for reliable read/write with the actual phone. This target is a design objective, not a claim of NFC certification.
5. Exercise live NFC request/snapshot transactions while attached, at battery voltage extremes and with the rotor throughout a full revolution. Verify repeatable reads and writes, not merely detection of an NDEF tag.

Phone metal changes inductance and losses, so free-space tuning alone does not satisfy this application. Antenna tuning remains a fabrication-release hold until these measurements pass. Firmware/app integration is also required for live counts.

References: [ST AN2972, Rev 10](https://www.st.com/resource/en/application_note/an2972-how-to-design-an-antenna-for-dynamic-nfc-tags-stmicroelectronics.pdf), [ST25DVxxKC datasheet](https://www.st.com/resource/en/datasheet/st25dv04kc.pdf). The ST online calculator could not be edited without account access; no result from it is claimed.