Low Scattering Photodiode-Modulated Probe for Microwave Near-Field Imaging

The space surrounding a radiating or scattering object is often divided into three regions, namely reactive near-field, near-field (NF) or Fresnel region and far-field (FF) or Fraunhoffer zones. In addition, the term “very-near-field" region is sometimes defined as very close to the antenna (e.g., antenna aperture). There are no abrupt boundaries between these three zones, however there are some commonly used definitions. For antennas with a size comparable to the wavelength (λ), the NF to FF boundary is calculated as r ≈ 2D2/λ , where D is the maximum dimension of the radiating device and r is the distance between the device and observation point. The most widespread use of near-field measurement is in antenna diagnostics. In this case, fields are sampled near the antenna, typically in the Fresnel region, and a NF-to-FF transformation is used to obtain the radiation patterns (Petre & Sarkar, 1992). Rather than extrapolating away from the antenna, another possible application consists of reconstructing the field and current on the radiating device. This may require sampling within the reactive near-field region, i.e., with r < λ. Such in-situ near-field diagnostics have been made on antennas (Laurin et al., 2001), microwave circuits (Bokhari et al., 1995) and device emissions (Dubois et al., 2008). They can also be used to measure the wave penetration into materials and their radio-frequency (RF) characterization purposes (Munoz et al., 2008). Dielectric properties reconstruction (Omrane et al., 2006) is another use of NF measurement. Measuring the coupling between components of microwave circuits (Baudry et al., 2007), calculating FF radiation pattern of large antennas (Yan et al., 1997), and testing for electromagnetic compatibility EMC (Baudry et al., 2007) and EMI (Quilez et al., 2008) are among the other uses of NF measurement.