Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 116

The radar cross-section σ (RCS, scatter aperture) is a measure of how well an
object reflects electromagnetic waves. The radar cross-section depends upon a range of
parameters, such as object size, shape, material, surface structure, but also wavelength
and polarisation.
The radar cross-section can only be calculated precisely for simple surfaces such
as spheres, flat surfaces and the like (for example see Baur, 1985). The material also
has a significant influence. For example, metal surfaces reflect much better than plas-
tic or composite materials. Because the dependence of the radar cross-section σ on
wavelength plays such an important role, objects are divided into three categories:

• Rayleigh range: the wavelength is large compared with the object dimensions. For

objects smaller than around half the wavelength, σ exhibits a λ − 4 dependency
and so the reflective properties of objects smaller than 0.1 λ can be completely
disregarded in practice.

• Resonance range: the wavelength is comparable with the object dimensions. Vary-

ing the wavelength causes σ to fluctuate by a few decibels around the geometric
value. Objects with sharp resonance, such as sharp edges, slits and points may, at
certain wavelengths, exhibit resonance step-up of σ . Under certain circumstances
this is particularly true for antennas that are being irradiated at their resonant
wavelengths (resonant frequency).

• Optical range: the wavelength is small compared to the object dimensions. In this

case, only the geometry and position (angle of incidence of the electromagnetic
wave) of the object influence the radar cross-section.

Backscatter RFID systems employ antennas with different construction formats as
reflection areas. Reflections at transponders therefore occur exclusively in the resonance
range. In order to understand and make calculations about these systems we need to
know the radar cross-section σ of a resonant antenna. A detailed introduction to the
calculation of the radar cross-section can therefore be found in the following sections.
It also follows from equation (4.67) that the power reflected back from the
transponder is proportional to the fourth root of the power transmitted by the reader
(Figure 4.61). In other words: if we wish to double the power density S of the reflected

Reflected
wave

R

Reader Transponder

Figure 4.61 Propagation of waves emitted and reflected at the transponder