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Register a new userMechanical properties of the radio frequency field emitted by an antenna array
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Angular momentum densities of electromagnetic beams are connected to helicity (circular polarization) and topological charge (azimuthal phase shift and vorticity). Computing the electromagnetic fields emitted by a circular antenna array, analytic expressions are found for the densities of energy, linear and angular momentum in terms of helicity and vorticity. It is found that the angular momentum density can be separated into spin and orbital parts, a result that is known to be true in a beam geometry. The results are of importance for information-rich radio astronomy and space physics as well as novel radio, radar, and wireless communication concepts.
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Low frequency communication systems offer significant potential in portable electronics and internet of things (IoT) applications due to the low propagation loss and long transmission range. However, because the dimension of electrical antenna is comparable to one quarter wavelength of the electromagnetic wave that it transmits and receives, currently a gap exists between LF communication systems and IoT, as IoT generally implement miniaturized electrical antennas with a small wavelength at a high frequency while traditional LF electrical antennas are too bulky. In this paper, we present an LF magnetoelectric mechanical transmitting and receiving antenna pair to significantly miniaturize the dimension of LF antennas. As the operation principle of the magnetoelectric mechanical antenna pair is based on mechanical resonance, its dimension is reduced by four orders of magnitude compared with an electrical antenna counterpart. The transmitting and receiving antennas are the same in structure and dimensions: they are both composed of magnetostrictive Terfenol-D and piezoelectric PZT laminate, and their dimensions are both 38x12x8.2mm3. Both the transmitting and receiving antennas are integrated with DC magnetic bias to improve its performance. Antenna pair performance measurement demonstrates that a maximum operation distance of 9m is demonstrated with DC magnetic bias, while maximum operation distance without DC magnetic bias is 4m, which is reduced by 56%. The LF magnetoelectric mechanical antenna pair may be a promising candidate for the portable electronics and IoT applications.
We apply the transformation-optics approach to the design of a metamaterial radome that can extend the scanning angle of a phased-array antenna. For moderate enhancement of the scanning angle, via suitable parameterization and optimization of the coordinate transformation, we obtain a design that admits a technologically viable, robust and potentially broadband implementation in terms of thin-metallic-plate inclusions. Our results, validated via finite-element-based numerical simulations, indicate an alternative route to the design of metamaterial radomes which does not require negative-valued and/or extreme constitutive parameters.
An analysis of the influences of a high frequency (30 kHz) alternating current on the uniformity of the magnetic field (B) in an electromagnetic casting (EMC) mould is investigated by means of parametric numerical simulations where the induction current (Js) varies in the range of [1 to 10000 A]. The results show that values of the magnetic flux density along the casting direction (Bz) near the square mould corners are small, compared to those at the other locations where Js < 10000 A, and that the magnitude of Bz increases with an increased induction current (Js). However, it is shown that, for the EMC mould structure investigated in this paper, the variations of Js have no significant influences on the uniformity of the magnetic field, especially for the regions near molten steel level. Moreover, the effective acting region (Rbz) for the critical magnetic field (Bzc) is first introduced in this paper, which opens an interesting topic for future research.
The boundary-value problem for the perturbation of an electric potential by a homogeneous anisotropic dielectric sphere in vacuum was formulated. The total potential in the exterior region was expanded in series of radial polynomials and tesseral harmonics, as is standard for the Laplace equation. A bijective transformation of space was carried out to formulate a series representation of the potential in the interior region. Boundary conditions on the spherical surface were enforced to derive a transition matrix that relates the expansion coefficients of the perturbation potential in the exterior region to those of the source potential. Far from the sphere, the perturbation potential decays as the inverse of the distance squared from the center of the sphere, as confirmed numerically.
To better understand the radio signal emitted by extensive air-showers and to further develop the radio detection technique of high-energy cosmic rays, the LOPES experiment was reconfigured to LOPES-3D. LOPES-3D is able to measure all three vectorial components of the electric field of radio emission from cosmic ray air showers. The additional measurement of the vertical component ought to increase the reconstruction accuracy of primary cosmic ray parameters like direction and energy, provides an improved sensitivity to inclined showers, and will help to validate simulation of the emission mechanisms in the atmosphere. LOPES-3D will evaluate the feasibility of vectorial measurements for large scale applications. In order to measure all three electric field components directly, a tailor-made antenna type (tripoles) was deployed. The change of the antenna type necessitated new pre-amplifiers and an overall recalibration. The reconfiguration and the recalibration procedure are presented and the operationality of LOPES-3D is demonstrated.
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