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Fifth generation (5G) wireless communication systems demand compact and low-profile high gain wideband antenna. Planar log-periodic array antenna (pLPA) has such potential qualities for 5G base-station and user-devices. However, many important features and RF analysis are rarely elaborated for pLPA which include equivalent circuit modeling. In this article, authors present the systematic RF analysis, fabrication, and equivalent circuit modeling for sub-6GHz 5G application using theory of characteristic modes (TCM). The fabricated pLPA is designed to cover the 5G bands with a high gain of >5 dBi and a fractional bandwidth of >1. Using simulation tool based on method of moments, radiating and non-radiating modes are tracked and studied with detailed physics. These modes are modeled using analytically derived equivalent circuit and assembled in an RF/microwave circuit simulator. The characteristic mode analysis and RF circuit modeling of pLPA help in the advancement of future 5G RF front-end modules.

We show theoretically that the characteristic modes of dielectric resonator antennas (DRAs) must be capacitive in the low frequency limit and that as a consequence of this constraint and the Poincaré Separation Theorem, the modes of any DRA consisting of partial elements of an encompassing super-structure with the same spatial material properties cannot resonate at a lower frequency than the encompassing structure. Thus, design techniques relying on complex sub-structures to miniaturize the antenna, including topology optimization and meandered windings, cannot apply to DRAs. Due to the capacitive nature of the DRA modes, it is also shown that the Q factor of any DRA sub-structure will be bounded from below by that of the super-structure at frequencies below the first self-resonance of the super-structure. We demonstrate these bounding relations with numerical examples.

A novel Green’s function-free characteristic modes formulation is introduced in this work. The desired impedance or admittance matrix is obtained utilizing and appropriately modifying the versatile finite element method. For this purpose, the generalized eigenvalue problem of the electric or magnetic field vector wave equation is formulated. In the case of the electric field wave equation, using the Schur complement, the system is reformulated and expressed only in terms of the tangential electric field over the radiating apertures, retaining the equivalent magnetic currents. Similarly, in the case of the magnetic field wave equation, the electric current density on radiating metallic surfaces is isolated using the Schur complement. In both cases, the obtained matrix is split into its real and imaginary part to yield the characteristic modes eigenvalue problem. Key advantage of the proposed formulation is that it does not require the evaluation of Green’s function, thereby the study of any arbitrarily shaped, multilayered geometry loaded with anisotropic and inhomogeneous materials is feasible. To prove the validity of the proposed methodology various classical structures, with both homogeneous, and inhomogeneous and anisotropic materials, published in the bibliography are studied. Both the eigenvalues and eigenvectors compared with the published results show good agreement.

A co-designed (3+3)-port antenna for dual-band operation that requires no feeding or decoupling network is proposed and verified. The antenna consists of six ports which are divided into two groups that resonate in two different frequency ranges. The basic radiating element is a tri-modal patch in a folded snowflake-shaped structure with the largest antenna dimension of $0.48\mathit {\lambda _{1}}$ or $0.68\mathit {\lambda _{2}}$ , where $\mathit {\lambda _{1}}$ and $\mathit {\lambda _{2}}$ are the wavelengths in air at the center frequencies of the lower and upper operating bands, respectively. Measurement results show that the 10 dB impedance bandwidths of the two respective bands are 19.7% and 14.4%. The proposed antenna exhibits compact, multiport, multiband and broadside radiation characteristics which are not only suitable for dual-band MIMO applications, but also for energy harvesting systems with spatial and frequency diversities, or dual-function wireless systems with simultaneous information and power transfer.

Characteristic mode analysis (CMA) is based on time-harmonic electromagnetics and applied mostly in frequency domain. In this paper, CMA is extended to time domain (TD) for transmission problems. Based on the modal self- and mutual-admittance of two-port systems, the modal TD response is obtained using inverse fast Fourier transform, extrapolation and Hilbert transform. TD-CMA represents modal impulse response on specified ports, and impulse response of the system is the linear superposition of the modal ones. Numerical examples are provided to explain this new concept. For a pair of parallel wires, the calculated TD-CMA is alike the common and differential modes. For a pair of dipoles, TD-CMA can be divided into strong and weak coupling modes according to their magnitudes. The strong coupling modes are due to the pulse radiation and oscillations at dipole tips and feedings. Time delay paths are clearly observed in the near field but are not discernable in the intermediate field. This shows different coupling mechanisms for the two cases. TD-CMA expands research field of CMA and may find applications for analyzing transient behaviors of microwave systems.

Multiple-input multiple-output (MIMO) is a key enabler for high data rates in mobile communications. However, it is challenging to design MIMO terminal antennas for LTE bands below 1 GHz, due to the conventional chassis offering only one resonant characteristic mode (CM). Recently, it was shown that minor structural changes can yield up to two additional resonant modes for designing two-port MIMO antennas. Nonetheless, the resulting bandwidth for the second port is relatively small. To simultaneously meet bandwidth and other practical requirements (including low profile and no off-ground clearance), a step-by-step approach for structural changes and feed design is applied in this work to exemplify the use of physical insights from CM analysis to achieve a competitive wideband two-port solution. The main novelty is that an entirely new mode is identified and appropriately tuned by structural modification for creating an additional resonance below 1 GHz. Moreover, two simple probe-feed ports are designed to jointly excite different subsets of four modes over frequency. In addition, far-field pattern orthogonality is guaranteed by the different phase shifts of the characteristic electric fields at the port locations. Furthermore, bulkier self-resonant antenna elements are avoided. To show design flexibility, a three-port version is also demonstrated.

A technique for enhancing the bandwidth of a broadside tri-modal patch antenna is described. The key idea of the technique is to incorporate a dual-resonance structure into the broadside tri-modal patch geometry. By increasing one edge of the tri-modal patch while decreasing its size at the opposite edge, the resulting structure can be viewed as two superimposed Y-shaped structures of different resonant frequencies. This intuition is confirmed using characteristic mode analysis (CMA). Furthermore, guided by CMA, further modifications enable two sets of resonant modes to be tuned for increasing the bandwidth of the tri-modal patch antenna. Importantly, the proposed bandwidth enhancement technique does not affect the desired broadside radiation patterns significantly. Therefore, it can be utilized to modify the tri-modal patch antenna without degrading its potential for massive MIMO array application. Measurement results show that the technique enhances the 10 dB impedance bandwidth from 4.3% to 19.7% with the largest antenna dimension of $0.48{\lambda _{c}}$ , where ${\lambda _{c}}$ is the wavelength in air at the center frequency. The design example of the proposed technique is able to cover widely used 3 GHz bands in 5G communication systems and its potential usage in massive MIMO arrays is demonstrated.