This paper explores the potential of metallic 3-D printing technology for millimeter-wave (mmWave) applications. It uses the selective laser melting (SLM) technology to melt the Cu-15Sn powder layer by layer to build up rectangular waveguides at E(60-90 GHz), D(110-170 GHz), and H-band (220-325 GHz). Different from nonmetallic 3-D printed waveguides, SLM Cu-15Sn waveguides are printed once in a whole piece. Post-electroplating and assembling are not necessary. Besides, the SLM waveguide also outperforms the nonmetallic 3-D printed waveguide in mechanical robustness. Compared with nonmetallic 3-D printed and commercial metallic waveguides, SLM waveguides demonstrate comparable performance at E-band with averaged dissipative attenuation 7.51 dB/m and 7.76 dB/m for 50and 100-mm waveguides, respectively. The attenuation of SLM waveguides is acceptable up to D-band. We prove the potential of the SLM technology for mmWave applications in both prototyping and mass production.

A compact circularly-polarized (CP) rectenna based on wide-slot antenna is designed at 5.8 GHz. The feed-back wide-slot antenna is used for its broadband, easy-to-integrate circuitry, simple structure, and low sidelobes. The designed receiving antenna has a bandwidth of 31.8%. In order to minimize the profile of the rectenna, the rectifier is integrated on the back side of the substrate with a matching circuit and a DC-pass filter. It features stable conversion efficiency on three azimuthal planes. The maximum efficiency of 66.2 % and an output DC voltage of 6.33 V have been measured with a 900 Ω load at the power density of 11.4mW/cm2. A 5 $\times$ 5 rectenna array is built for large physical aperture and increased output power. The maximum conversion efficiency of 62 % is observed with the array. Taking the advantage of immunity to misalignment of the CP antenna, the proposed rectenna and array are suitable to be used in mobile applications for wireless power transfer (WPT).

For most microwave power transmission (MPT) applications, the microwave power is received within the near field of a large-scale transmitting antenna from a limited distance. This letter explores the design of a near-field focused transmitting antenna to concentrate the microwave power on the receiving antenna's aperture, and thereby to enhance the transmission efficiency of the MPT. A near-field planar array with a size of 1 m × 1 m working at 5.8 GHz is manufactured. To focus the microwave power at a distance of 10 m, its 8 × 8 radiation elements are fed with an equal amplitude but different phase using coaxial cables of different lengths. An MPT experiment is conducted. The measurement shows that, compared with a traditional equal-phase array, the near-field focused array obtains a much more concentrated microwave beam. The transmission efficiency is significantly increased from 32.98% to 41.85%.

A compact circularly polarized (CP) rectenna with harmonic suppression for wireless power transmission at 5.8 GHz is proposed in this paper. The substrate integrated waveguide (SIW) structure is adopted for enhanced gain, unidirectional patterns and surface wave suppression. A rectifier, which consists of a matching network, a diode and a DC-pass filter, is integrated with the antenna at the back side. The proposed rectenna is fabricated and measured. It features 6.9 dB gain, 0.67dB axial ratio (AR) and 30 dB return loss. The conversion efficiency is 65.87 % with a 700 $\Omega$ load. A rectenna array is also build for increased aperture. It shows 61.49 % conversion efficiency with 650 $\Omega$ load in measurement. Both the rectenna and array are satisfactory choices for wireless power transfer (WPT) usage.

A substrate integrated waveguide multibeam antenna fed by a double-fold 7×8 Butler matrix (BM) beam-forming network (BFN) is presented. The proposed antenna is composed of a 7×8 BM feeding network and an 8×8 slot array. By merging the two central input ports of the BM into one port, the BM can provide output signals with equal phase, which can generate a boresight beam. In order to reduce the size of the antenna, the BM is folded twice into a three-layer structure. Measured results show that the proposed antenna has impedance bandwidth of 29.5 ∼ 30.5 GHz, whereas the gain lies in the range of 13.2–17.3 dBi. Seven beams, including a boresight beam, pointing from −58° to 58° are achieved. Compared with counterparts, the proposed antenna features boresight radiation and miniaturized size. It is a good candidate for the fifth-generation (5G) mobile network applications.

One novel single-feed single-layer wideband circularly polarized (CP) magneto-electric dipole antenna (MEDA) with enhanced gain property is presented in this communication. By loading co-planar spiral with the radiating patch, the unidirectional radiation gain of the antenna is effectively improved while the impedance bandwidth and axial ratio (AR) bandwidth unaffected. Surface current distributions with/without the spiral are compared to reveal the antenna’s mechanism. The measurement shows that the left hand circular polarization (LHCP) antenna features impedance bandwidth from 11.4 to 18.5GHz (or 52%), an AR bandwidth from 13.2 to 18GHz (or 32%), the maximum gain of 11.1 dBic at 16.9 GHz with 3-dB gain bandwidth from 12.5 to 18.6 GHz (or 36.1%). Owning to the low profile, broadband and high gain, the proposed antenna is a capable candidate for satellite communication.