Fundamental Efficiency Limits for Small Metallic Antennas

Abstract

-Both the radiation efficiency and bandwidth of electrically small antennas are dramatically reduced as the size decreases. Fundamental limitations on the bandwidth of small antennas have been thoroughly treated in the past. However, upper bounds on radiation efficiency have not been established even though it is also of significant importance. Here, radiation from a thin metallic shell is rigorously analyzed to establish fundamental limits on the radiation efficiency of resonant, electrically small antennas in terms of the size and the metal conductivity. Metallic losses are systematically introduced into the circuit model proposed by Chu, and several resonant antennas with maximum radiation efficiencies are analyzed. Resonant electric and magnetic dipole antennas both have maximum radiation efficiencies near 100% until the size is reduced below a critical value, at which point the efficiency scales as electrical size to the fourth power ሺሺሻ ሻ. It is also shown that a helix antenna that resonantly couples the TM10 mode to the TE10 mode has a maximum radiation efficiency, and is about twice that of a resonant dipole or loop antenna. The closed form expressions reported here provide valuable insight into the design of small antennas with optimal efficiencies.

Dissipation Factors of Spherical Current Modes on Multiple Spherical Layers

“…A direct comparison with dissipation factors evaluated in [1] reveals that the dissipation factors evaluated above are approximately two times higher. The reason for this discrepancy is the assumption 1 made in [1] that the spherical shell is composed of an inner and outer surface, both exhibiting the same surface resistance R s .…”

“…The dissipation factor (12) of any resonant combination of two spherical modes on a single spherical layer can easily be evaluated by substituting (14)-(17) into (12). (14), (16) and (19) from [1] with corresponding results of this paper. A comparison of the asymptotic (solid line) and fullwave (solid line with marks) expressions derived in this paper is also shown.…”

“…The results correspond to a single spherical layer. The dissipation factors originating from [1] were multiplied by a factor of two, since [1] originally assumed two infinitesimally spaced resistive layers.…”

Selective Electron Beam Melting Manufacturing of Electrically Small Antennas

“…Generally, the radiation resistance decreases, while the reactive component of its impedance increases, leading to a poor match with the feed line or network. As such, there is a compromise between miniaturization and performance; small antenna theory dictates that a favourable compromise is reached when the antenna fully occupies a volume defined by the radius a [2][3][4][5][6][7][8]. Consequently, there is tremendous potential in the successful manufacture of novel, 3D antenna geometries -hitherto too complex to fabricate with traditional processes -using emergent technologies such as holographic photolithography, 3D printing, direct-write printing, direct transfer patterning, thermal transfer printing, and aerosol jet printing [9][10][11][12][13][14][15][16][17][18].…”

Quantum-Limited Atomic Receiver in the Electrically Small Regime

“…Here the quantum-limited capacity C SQL also reaches the optimum, that is C * SQL , and becomes flat. In the figure, C SQL is plotted for the experimentally observed effective atom number N eff = 63 (7) and effective observed signal size δω eff = 2π × 680(60) kHz for the 0.8 V/cm field. The experimentally measured channel capacity is shown in black points.…”