Physical Limitations of Omni-Directional Antennas

Abstract

The physical limitations of omnidirectional antennas are considered. With the use of the spherical wave functions to describe the field, the directivity gain G and the Q of an unspecified antenna are calculated under idealized conditions. To obtain the optimum performance, three criteria are used: (1) maximum gain for a given complexity of the antenna structure, (2) minimum Q, (3) maximum ratio of G/Q. It is found that an antenna of which the maximum dimension is 2a has the potentiality of a broad bandwidth provided that the gain is equal to or less than 4a/. To obtain a gain higher than this value, the Q of the antenna increases at an astronomical rate. The antenna which has potentially the broadest bandwidth of all omnidirectional antennas is one which has a radiation pattern corresponding to that of an infinitesimally small dipole. PHYSICAL LIMITATIONS OF OMNIDIRECTIONAL ANTENNAS

On Some Optimal Mimo Antenna Coefficients in Multipath Channels

“…Furthermore, if the angle spread is large at both the transmitter and the receiver the optimized antennas would provide a diagonal correlation matrix with elements of equal magnitudes, i.e., links with identical power. As predicted in Chu's seminal work [30], the radiation properties of an antenna are related to its size. Moreover, the size of the antenna poses limits to the magnitude of svw modes.…”

“…The applicability of model (30) to real propagation channels has been proved limited [25]. However, under some special conditions it can still be a good working model, e.g., when the correlation statistics at one end of the link is determined by a local scattering process and is less sensitive to changes at the other end of the link [26,27].…”

Synthesis of SEM-Derived Equivalent Circuits for Energy-Collecting Structures.

“…For the spherical antenna [5tratton and Chu, 1941] the n term involves a ratio of a spherical Hankel function of order n and its derivative. As shown by Chu (1948], this ratio can be expanded in a 4 finite continued fraction with positive real coefficients, which can be physically realized by an LC ladder network terminated into a single resistance. A complete equivalent circuit consists of a parallel connection of an infinite number of such "eigennetworks".…”

Planar Monopole Uwb Antenna for Usb Dongle Application

“…The modal analysis has been long used in electro-magnetics for the analysis of close structures such as wave-guides and cavities but the calculation of modes for real world classical problems that involve open radiating structures such as antennas and scatterers was time consuming and probably is one of the reason that the modal analysis is not used for antenna design in mid-90s. The fundamental limitations of modal analysis in antenna design was investigated by Chu in 1948 and subsequently by Harrington in 1960 while, Chu [5] postulated that, since any radiating field can be written as sum of spherical vector waves, the antenna can be enclosed in a sphere and the radiated power of the antenna is calculated from the propagating modes within the sphere and thus antenna can be considered as closed structure. A popular notion called the Characteristic Modes proposed by Garbacz corresponds with eigenvectors of a weighted eigenvalue equation and is the most common used techniques in modal analysis.…”