We are interested in near field, so consider magnetic field produced by a circular loop antenna:
Bz=μ0∗I∗N∗a2/2(a2+r2)3/2 ,
where I- current, r-distance from the center of wire, μ0 - permeability of free space and given as 4π∗107 (Henry/meter), a-radius of loop
The maximum magnetic flux that is passing through the tag coil is obtained when the two coils (reader coil and tag coil) are placed in parallel with respect to each other.
An optimum coil diameter that requires the minimum number of ampere-turns for a particular read range can be found from previous equation such as:
NI=K∗(a2+r2)3/2/a2 , K=2∗Bz/μ0
By taking derivative with respect to the radius a,
d(NI)/da=K∗3/2∗(a2+r2)1/2∗(2a3)−2a∗(a2+r2)3/2/a4=K(a2−2∗r2)(a2+r2)1/2/a3
the above equation becomes minimized when:
a=2–√∗r
The result indicates that the optimum loop radius, a, is 1.414 times the demanded read range r.
NFC就老老实实做好近距离的应用就可以了。