Hello,
the resonant frequency of a tag antenna is given by Fr=1/(2*pi*sqrt(La_eq*Cs) where La_eq is the series equivalent inductance od the antenna at the desired tuning frequency, and Cs the series equivalent capacitance of the Tag IC.
Our ST25TA02K tag IC has a sereis equivalent Cs= 50pF. Recommended tuning frequency for an ISO 14443-A tag is around 14MHz. The corresponding antenna inductance is La_eq_14MHz= 1/(Cs.(2.pi.13.56e6))²= 2.75µH.
Molex antenna impedance model is Ca // (Ra + jLa.2.pi.f), where // means "in parallel to".
The 1462360131 total impedance is 7+342.1j => 7Ohm in series with La_eq=3.89µH at 14MHz which too high, based on explanation above. The 14MHz tuning frequency can be however reached by inserting two equal series caps ( to balance the impedance on both sides of the antenna) as described below:
Ct1a and Ct1b are equal. CT1a=CT1b=2xCT1 where CT1 is the theoretical unbalanced capacitor is series between the antenna and the ST25TA to adjust resonant frequency:
The CT1 capacitor comes in series with the ST25TA02K series tuning capacitance Cs:
The new series equivalent capaciatnce seen by antenna is Ctot= (Cs.Ct1)/(Cs+Ct1). Fr=14MHz with La_eq=3.85µH means CTtot=1/(La_eq.(2.pi.14e6)²)= 34.41 pF. The corresponding CT1 value is CT=110pF. Finally, Ct1a=Ct1b=220pF.
Another solution could be to move to the 1462360151 antenna using a parallel capacitance to increase the ST25TA02K capaciatnce wich is too llw in this case.
feel to ask for more if necessary
Henry Crane
RFID/NFC Application engineer.
