Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements

Abstract

Abstract-The theory as well as numerical and experimental results are presented for a superdirective array composed of closely spaced electrically small resonant magnetic dipole elements. The array operates on a metal ground plane and can exhibit a maximum directivity of 11.5 dBi, 15.2 dBi, and 17.8 dBi (including 3 dB due to the ground plane), for 2, 3, and 4 magnetic dipoles, respectively. The array is self-resonant and is directly excited by a 50-ohm coaxial cable through the ground plane. The array radiates essentially the |µ| = 1 spherical modes, which, despite a narrow bandwidth, makes it an excellent firstorder probe for spherical near-field antenna measurements at low frequencies.Index Terms-Electrically small antennas, supergain array, superdirectivity, magnetic dipole, spherical near-field antenna measurements, first-order probe

Physical Bounds of Antennas

“…Antennas with a higher directivity than typical antennas of the same size are often referred to as superdirective antennas [8,56,79,84]. The increase of the Q-factor for small antennas with high directivity is analyzed by adding the constraint D ≥ D 0 to (6.4).…”

Compact First-Order Probe for Spherical Near-Field Antenna Measurements at Low Frequencies

“…Despite being approximate, expression (1) immediately suggests how the bandwidth of a superdirective array can be improved -by extending the bandwidth of individual array elements (reducing Q e ) and by manipulating the geometry of the array to reduce Q g . In our case, individual array elements are in fact electrically small self-resonant magnetic dipole antennas [2], and thus the Q theory of electrically small antennas applies here. In particular, the Chu lower bound on Q [7] is inversely proportional to the cube of the electrical size of an antenna…”

“…Further bandwidth improvement can be gained by efficiently utilizing the volume surrounding an electrically small antenna. For example, being effectively thin disks in shape, the capacitively loaded loops (CLL) used as array elements in [2] are inherently inferior in terms of Q to volumetric magnetic dipole antennas of the same size. This comes from the fact that the lower bound on Q for a thin disk is about 2.2 times higher than that for a spherical magnetic dipole of the same radius in free space (6.7Q Chu and 3.0Q Chu , respectively [8]; both values are for ka = 0.1); the potential bandwidth is therefore 2.2 times narrower.…”

“…A lightweight and compact alternative to legacy probes was suggested in [2], where a superdirective linear array of two electrically small magnetic dipole elements was shown to be a viable solution for SNF measurements at low frequencies. The new probe exploits the cut-off property of spherical waves [1] that limits the mode spectrum of an electrically small source to dipole modes -the lowest-order µ = ±1 modes.…”

“…In this paper, guidelines for designing superdirective probes with excellent first-order characteristics and improved impedance bandwidth are explained theoretically, illustrated numerically and validated experimentally. They have been implemented in a new compact first-order probe design, which has been fabricated and measured, providing a more than tenfold improvement in the bandwidth over the first prototype presented in [2]. While both probes represent a new class of probes for SNF antenna measurements -superdirective first-order probes -the one in [2] was built for a very specific task and was used only once, while the new probe was designed for regular use at the DTU-ESA SNF Antenna Test Facility [4] extending its operational frequency range covered by the facility's collection of first-order probes (conventional conical horns and open-ended cylindrical waveguides) to the lower UHF-band.…”