68 research outputs found
Maximum Gain, Effective Area, and Directivity
Fundamental bounds on antenna gain are found via convex optimization of the
current density in a prescribed region. Various constraints are considered,
including self-resonance and only partial control of the current distribution.
Derived formulas are valid for arbitrarily shaped radiators of a given
conductivity. All the optimization tasks are reduced to eigenvalue problems,
which are solved efficiently. The second part of the paper deals with
superdirectivity and its associated minimal costs in efficiency and Q-factor.
The paper is accompanied with a series of examples practically demonstrating
the relevance of the theoretical framework and entirely spanning wide range of
material parameters and electrical sizes used in antenna technology. Presented
results are analyzed from a perspective of effectively radiating modes. In
contrast to a common approach utilizing spherical modes, the radiating modes of
a given body are directly evaluated and analyzed here. All crucial mathematical
steps are reviewed in the appendices, including a series of important
subroutines to be considered making it possible to reduce the computational
burden associated with the evaluation of electrically large structures and
structures of high conductivity.Comment: 12 pages, 15 figures, submitted to TA
Bandwidth-Constrained Capacity Bounds and Degrees of Freedom for MIMO Antennas
The optimal spectral efficiency and number of independent channels for MIMO
antennas in isotropic multipath channels are investigated when bandwidth
requirements are placed on the antenna. By posing the problem as a convex
optimization problem restricted by the port Q-factor a semi-analytical
expression is formed for its solution. The antennas are simulated by method of
moments and the solution is formulated both for structures fed by discrete
ports, as well as for design regions characterized by an equivalent current. It
is shown that the solution is solely dependent on the eigenvalues of the
so-called energy modes of the antenna. The magnitude of these eigenvalues is
analyzed for a linear dipole array as well as a plate with embedded antenna
regions. The energy modes are also compared to the characteristic modes to
validate characteristic modes as a design strategy for MIMO antennas. The
antenna performance is illustrated through spectral efficiency over the
Q-factor, a quantity that is connected to the capacity. It is proposed that the
number of energy modes below a given Q-factor can be used to estimate the
degrees of freedom for that Q-factor.Comment: 13 pages, 17 figure
Dissipation Factors of Spherical Current Modes on Multiple Spherical Layers
Radiation efficiencies of modal current densities distributed on a spherical
shell are evaluated in terms of dissipation factor. The presented approach is
rigorous, yet simple and straightforward, leading to closed-form expressions.
The same approach is utilized for a two-layered shell and the results are
compared with other models existing in the literature. Discrepancies in this
comparison are reported and reasons are analyzed. Finally, it is demonstrated
that radiation efficiency potentially benefits from the use of internal volume
which contrasts with the case of the radiation Q-factor.Comment: 5 pages, 5 figure
Physical Bounds on Antennas with Feed Constraints
Antenna current optimization has been used to derive physical bounds on antenna parameters such as Q-factor, radiation efficiency, gain, directivity, capacity, and radiation patterns. The success of the methodology is partly due to the assumption of absolute control of the antenna current in the antenna region which in practice producing an array antenna with multiple feeds. Details of the feed such as input impedance and placement are however essential in antenna synthesis and there has so far been no successful approach to include these types of constraints. In this presentation, we illustrate how feed constraints can be included in current optimization and discuss their associated challenges
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