How match Molex antenna with NFC Tag ST25TA02KB-P

[ELECL.2]

Hi,

I would like to use an existing molex antenna(1462360131) with ferrite with a NFC Tag ST25TA02KB-P.

On datasheet all tests was made with a specific antenna :

External size : 75mmx48mm

Number of truns : 4

Width of conductor : 4

Space between two conductor : 0.5mm

Value of the tuning capacitor : 50pF

Value of the coil : 2.5uH

Tuning frequency : 14.2Mhz

Molex specification :

La : 3.5uH

Ra : 5.7ohms

Ca : 3.7pF

Fra : 45.8Mhz

Rp : 10.8kohmss

Q : 52.5

[Thomas Q.]

Hello,

The usage of your molex antenna design with a ST25TA02KB-P tag is not straightforward because of the big gap in tuning frequency (45.8MHz vs targeted 14MHz).

Please check document AN2866 for guidelines on NFC antenna design, and let us know if this helps in

customization of the molex antenna to match with the ST25TA02KB-P.

Best regards,

Thomas

[ELECL.2]

Thanks @Thomas Q.​ 

Fra is not the resonant frequency.

Attach to this reply, carateristics of this antenna :

[Henry Crane · Accepted solution]

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.