Expressions are derived for the smallest achievable radiation quality factor () of an electrically small antenna in front of a conducting plane. Applying the low-frequency approximation to the source region involving an electric or a magnetic point dipole plus their images behind the plane, an expression is formed for the field in the radiation zone. The contribution of nonpropagating energy in the near field is obtained explicitly using a spherical harmonics decomposition. The radiation is found to depend on the radius (relative to wavelength) of the smallest sphere that encloses the antenna and its image, the ratio of the vertical and horizontal dipole moments, as well as the positions of the dipoles relative to each other and to the plane. A number of simple wire structures are analysed with NEC (based on the Method of Moments), and the approximate obtained from their fractional bandwidth are compared to the corresponding theoretical minima.
Abstract-The phase of a complex field and its speed of propagation are fundamental concepts of electromagnetic wave motion. Although it seems to be well-known that faster than light propagation of the phase may occur in, e.g., waveguides and certain dispersive media, it is often ignored that a similar phenomenon, in fact a very marked one, presents itself in the near-field of an arbitrary oscillating current in vacuum. Connected herewith is the observation that the phases of the transverse field components of a dipole approach kr − π/2, and not kr, in the radiation zone. This article illustrates these phenomena by theoretical and numerical examples as well as indicates their consequences for broad-band wireless communication over short distances.
Three methods suitable for on-site window and wall penetration loss measurements are presented and compared. The methods are (i) outdoor-to-indoor channel measurement, (ii) far-field penetration loss measurement, and (iii) near-field penetration loss measurement. It is shown that the different methods give similar penetration loss results. The measured window exhibits bandstop characteristics due to the internal reflections in the periodic structure of the triple-layer window. The brick wall has penetration loss that increases as a function of the frequency.
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