Arto Hujanen

Quality factor of an electrically small antenna radiating close to a conducting plane

Expressions are derived for the smallest achievable radiation quality factor (Q) 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 non-propagating energy in the near field is obtained explicitly using a spherical harmonics decomposition. The radiation Q 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 Q obtained from their fractional bandwidth are compared to the corresponding theoretical minima.

Bandwidth limitations of impedance matched ideal dipoles

Using circuit theory and Chus network representations of the wave impedance of the lowest spherical multipole field, the Ideal dipole, an explicit expression is derived for the upper limit for the impedance bandwidth obtainable for an ideal, lossless, linearly polarized antenna. The expression embodies the network character of an ideal antenna and its role in the guidance of energy between the feed point and the free-space interface, making it more appropriate as a reference than the theoretical radiation quality factor (Q). Moreover, practical antennas do not always manifest a clear single-pole behavior, undermining the basic assumption of a reciprocal relation between Q and the bandwidth.

Aspects on the Phase Delay and Phase Velocity in the Electromagnetic Near-Field

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.

Study of scattering by a perfectly conducting wedge with finite sized faces

In this work the scattering of plane waves from a finite sized perfectly conducting wedge as a function of its opening angle and the width of its faces is studied using the combination of physical optics (po) and the physical theory of diffraction (ptd). To find out under which circumstances the ptd contribution is significant compared to the PO, the ratio of the ptd field and the po field is evaluated as a maximum and a mean value over every direction of observation in the Keller cone, as well as in the special direction of backscattering. We employ the incremental length diffraction coefficients for a wedge with finite sized faces based on equivalent edge currents derived recently for truncated wedge strips. The numerical behaviour in the limiting cases of the diffraction coefficients are discussed extensively.RésuméDans ce travail nous étudions la diffraction ďondes planes par un dièdre conducteur parfait, dont les dimensions des faces sont limitées, par une combinaison de ľoptique physique (po) et de la théorie physique de la diffraction (ptd) en fonction de ľangle du dièdre et de la largeur des faces. Afin de déterminer les conditions ďutilisation efficace de la ptd, les valeurs maximales et les valeurs moyennes du rapport du champ de la ptd au champ de la po sont évaluées pour chaque direction ďobservation dans le cône de Keller et dans la direction spéciale de rétrodiffusion. Nous employons les facteurs de diffraction incrémentaux du dièdre, fondé sur des expressions des courants de bord équivalents ďun dièdre fini récemment présentées. Le comportement numérique des facteurs de diffraction dans les cas limites est examiné en détail.

On the effect of a spherical shield on the radiation Q and efficiency of ideal dipole antennas

A method is developed for calculating the radiation efficiency and quality factor, Q, for azimuthally symmetric electric (Tm) and magnetic (Te) multipole fields surrounded by a semi-transparent spherical shield of variable thickness, dielectric and magnetic constants involving losses. A matrix method is used to connect the transverse field components at adjacent interfaces. The Q and efficiency are determined by computing the energy stored in the near field of the multipole as well as the power radiated and dissipated in the shield. A numerical comparison of the performance of electric and magnetic dipole radiators is given as a function of frequency. An expression is derived for the permittivity of the shield which optimisés the Q.RésuméNous présentons une méthode developpée pour calculer le rendement du rayonnement et le facteur de qualité, Q, d’un multipôle électrique (champTm) ou magnétique (champTe) omnidirectionel, enfermé dans un boîtier semi-transparent sphérique dont l’épaisseur, la per-mittivité et la perméabilité, y compris les pertes, sont variables. Une méthode matricielle est utilisée afin de lier les champs électromagnétiques transversaux sur des interfaces adjacentes. Le facteur Q et le rendement sont déterminés en calculant l’énergie stockée dans le champ proche du multipôle ainsi que la puissance rayonnée et absorbée dans le boîtier. La caractéristique d’un dipôle électrique est comparée numériquement à celle d’un dipôle magnétique en fonction de la fréquence. Une expression algébrique est trouvée pour la per-mittivité relative du boîtier qui optimiserait le facteur de qualité.

CAST - RCS tool for modelling versatile size targets

Radar cross section (RCS) and inverse synthetic aperture radar (ISAR) are two important application areas for computation intensive RCS tools. Their objective is to control RCS signature in case of different types of mono-static and bistatic radars. To follow the quick development of radar technology, we need more flexible and more accurate RCS calculation tools. In this paper, we introduce CAST application and its techniques to compute RCS of different sized objects and their surroundings. For RCS and SAR analysis, we have added environment as a new spatiotemporal parameter in addition to traditional material and geometry design parameters.