Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 61

Table 4.2 Units and abbreviations used

Variable Symbol Unit Abbreviation

Magnetic field strength H Ampere per meter A/m

Magnetic flux ( n = number Volt seconds Vs

of windings)

= n

Vs/m 2 Magnetic inductance B Volt seconds per meter

squared

Inductance L Henry H

Mutual inductance M Henry H

Electric field strength E Volts per metre V/m

Electric current I Ampere A

Electric voltage U Vo l t V

Capacitance C Farad F

Frequency f Hertz Hz

Angular frequency ω = 2 π f 1/seconds 1/s

Length l Metre m

m 2

Area A Metre squared

Speed v Metres per second m/s

Impedance Z Ohm

Wavelength λ Metre m

Power P Watt W

W/m 2

Power density S Watts per metre squared

x

H

r

d

Figure 4.3 The path of the lines of magnetic flux around a short cylindrical coil, or conductor
loop, similar to those employed in the transmitter antennas of inductively coupled RFID systems

If the measuring point is moved away from the centre of the coil along the coil axis
( x axis), then the strength of the field H will decrease as the distance x is increased.
A more in-depth investigation shows that the field strength in relation to the radius
(or area) of the coil remains constant up to a certain distance and then falls rapidly
(see Figure 4.4). In free space, the decay of field strength is approximately 60 dB per