Dielectric lens antenna with enhanced aperture efficiency Quantity Typical Minimum Maximum Polarisation Linear - - Radiation pattern Single broadside lobe - - Gain 25 dBi 15 dBi 30 dBi Performance bandwidth 25 % - - Complexity Medium - - Impedance - - - Balun None required - - Background Dielectric lens antennas have grown in popularity for use in millimeter and sub-millimeter applications. They provide good efficiency, especially in the case of the elliptical dielectric lens, which has high focusing properties. [Neto et al.] By relating the eccentricity of the lens to the dielectric constant, an in-phase ray-field contribution may be achieved, thus resulting in a high directivity. [Neto et al.] In the case of the elliptical lens, the high aperture efficiency ensures an antenna with a reduced diameter when compared to a standard equal-gain conical horn antenna. This reduced size was the driving factor for its use as the antenna in an industrial radar application, a so called tank level probing radar (TLPR). [Pohl] Antenna Magus 2024.1: 07-10-2024, Content Copyright Magus Pty (Ltd) Quick Summary Dielectric lens antenna with enhanced aperture efficiency 2 Physical Description The antenna comprises of a circular waveguide, which feeds the elliptical lens. A dielectric stepped impedance transformer section is added at the waveguide-lens junction in order to improve the reflection performance of the structure. Feed Method The axial symmetry of the antenna makes it suitable for any polarisation. Common polarisations include linear, dual-linear and circular. Linear polarization may be achieved by feeding the antenna with a single coaxial-probe or waveguide feed. This particular antenna is fed using a waveguide port. Operation Mechanism By properly relating the eccentricity of the lens to the dielectric constant, and feeding the lens at one of its focal points, the majority of the rays are refracted in the ‘forward’ direction, thereby constructing a plane phase front. On the other hand the lens interface gives rise to reflections inside the lens, which may significantly affect the input impedance and the radiation properties. As expected, the PO contribution of the doubly reflected ray currents is dominant in describing the input impedance, according to the geometrical evidence by which all rays emanating from a focal source exactly return to the starting point after a double reflection at the lens surface. [Neto et al.] By using a lens material with a low relative permittivity (smaller than 3), the dielectric contrast with free-space is usually weak enough to neglect the effects of multiple internal reflections [van der Vorst et al.]. While additional matching layers may be used to reduce reflections for materials with higher dielectric constants, they come at the expense of increased cost and production efforts [Pohl]. When using dense materials without such matching layers, internal resonances are excited within the lens, which may substantially affect not only the input impedance, but also the radiation properties in Performance The antenna may be designed for a centre frequency, lens relative permittivity and a gain between 15 and 30 dBi. Results shown below are for an antenna designed for a relative permittivity of 2.08 and a gain of 25 dBi. Impedance Characteristics The reflection performance is mainly controlled by the internal reflections present in the lens structure. A basic stepped impedance transformer improves the S11 performance. Antenna Magus 2024.1: 07-10-2024, Content Copyright Magus Pty (Ltd) terms of gain reduction, beam distortions and increase of side lobes [Nguyen et al.]. Dielectric lens antenna with enhanced aperture efficiency 3 Typical reflection coefficient vs frequency Radiation Characteristics Antenna Magus 2024.1: 07-10-2024, Content Copyright Magus Pty (Ltd) Typical total gain pattern at the centre frequency Dielectric lens antenna with enhanced aperture efficiency 4 Typical normalised gain patterns at the centre frequency References A. Neto, S. Maci, P.J.I. de Maagt, "Reflections inside an elliptical dielectric lens antenna," Microwaves, Antennas and Propagation, IEE Proceedings, vol.145, no.3, pp.243-247, Jun 1998 N. Pohl "A dielectric lens antenna with enhanced aperture efficiency for industrial radar applications." Antennas and Propagation (MECAP), 2010 IEEE Middle East Conference on. IEEE, 2010. M.J.M. van der Vorst, P.J.L. de Maagt, M.H.A.J. Herben, "Effect of internal reflections on the radiation properties and input admittance of integrated Ngoc Tinh Nguyen; R. Sauleau, C.J.M. Perez, "Very Broadband Extended Hemispherical Lenses: Role of Matching Layers for Bandwidth Enlargement," Antennas and Propagation, IEEE Transactions, vol.57, no.7, pp.1907-1913, July 2009. Model Information (FEKO) Model 1 Model using the MoM/SEP solution. The model uses a waveguide feed. Antenna Magus 2024.1: 07-10-2024, Content Copyright Magus Pty (Ltd) lens antennas," Microwave Theory and Techniques, IEEE Transactions, vol.47, no.9, pp.1696-1704, Sep 1999. Dielectric lens antenna with enhanced aperture efficiency 5 The model uses a circular waveguide port to feed the antenna, as well as two planes of symmetry to reduce simulation time. Model Information (CST Studio Suite) Model 1 Model using the T-solver. The model uses a waveguide feed. The model uses a waveguide port to feed the antenna, as well as two planes of symmetry to reduce simulation time. Model Validation The performance of the antenna has been validated against simulations / measurements from Pohl. Each export model has been validated to give the expected results for several parameter variations in the design space. Magus Analysis The internal performance estimation is expected to be similar to a full 3D-EM analysis. Expect: - Small frequency offsets (-3% to +3%) - Possibly inaccurate reflection coefficients below -15 dB The Antenna Magus design allows a relative permittivity between 1 and 10, and a gain between 15 and 30 dBi, to be specified. Approximate design guidelines are outlined below. - The input resistance may be improved by optimizing the stepped impedance transformer diameter and length. - The gain may be increased (decreased) by increasing (decreasing) the dielectric diameter of the ellipsoid. - The frequency may be increased (decreased) by decreasing (increasing) all the length parameters of the model. Antenna Magus 2024.1: 07-10-2024, Content Copyright Magus Pty (Ltd) Design Guidelines
