![]() The radiation power factor of either kind of antenna is somewhat greater than (1/6π) (Ab/l2) in which Ab is the cylindrical volume occupied by the antenna, and l is the radianlength (defined as 1/2π wavelength) at the operating frequency. The practical efficiency relative to this ideal is limited by the "radiation power factor" of the antenna as compared with the power factor and bandwidth of the antenna tuning. A radiation shield, in the form of a conducting shell the size of the radiansphere, enables separate measurement of radiation resistance and loss resistance.Ī capacitor or inductor operating as a small antenna is theoretically capable of intercepting a certain amount of power, independent of its size, on the assumption of tuning without circuit loss. A fraction of this ratio is obtainable in various forms of small antennas (C or L) occupying a comparable amount of space. ![]() Its radiation power factor is equal to the ratio of its volume over that of the radiansphere. A small coil wound on a perfect spherical magnetic core is conceived as an ideal small antenna. Between two such dipoles, the far mutual impedance is that of mutual inductance, expressed in terms of space properties and the radiansphere. The power that theoretically can be intercepted by a hypothetical isotropic antenna is that which flows through the radiansphere or its cross section, the "radiancircle." From a small electric dipole, the far field of radiation is identified as a retarded magnetic field. A "small" antenna is one somewhat smaller than the radiansphere, but it has a "sphere of influence" occupying the radiansphere. Its radius is one radianlength (¿/2¿), at which distance the three terms of the field are equal in magnitude. The "radiansphere" is the boundary between the near field and the far field of a small antenna. These issues of measurement repeatability and accuracy were widely discussed during the COST 273 Action, with a focus on updating the standardised measurement techniques. However, both the position of the phantom hand, or in other words, the way a handset is held, and the position of the antenna handset relative to the head are also quite critical. One of the main factors influencing the measurement results is, of course, the electrical properties of the phantom material. In the first, which is based on the use of a reverberation chamber, multiple reflections on the walls reproduce ideal Rayleigh channels in the second, the transmitting antenna is placed in an anechoic chamber, allowing the modification of the antenna radiation pattern, due to a phantom that simulates the user's body, to be clearly demonstrated. ![]() Two of these techniques have been studied in depth. For this, measurement techniques must be reproducible. However, as outlined in Section 5.2, in order to characterise antenna efficiency precisely, it is first necessary to introduce and define such figures of merit as the mean effective gain and the mean effective radiated power, taking the multipath structure of the communication channel into account. Taken together, the total radiated power in the transmission mode and the total radiated sensitivity in the receiving mode provide a general idea of the handset terminal's behaviour in its environment. This is a critical point for handset antennas, for example, since the user's hand and body strongly affect the radiation efficiency. It is important to remember that antenna performances are strongly dependent on the electromagnetic properties of the medium in its immediate vicinity. This chapter provides new insights on antennas, including diversity schemes and UWB applications.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |