We design and fabricate a double-layer hollow-waveguide slot array antenna with wide bandwidth and high antenna efficiency for the 120 GHz band. The antenna is fabricated by diffusion bonding of laminated thin metal plates for high precision and perfect electrical contact. The 1616-element antenna shows more than 70% antenna efficiency over a 13 GHz bandwidth. Furthermore, it realizes error-free data transmission in 2.5 m distance at up to 10 Gbit/s. To our knowledge, this is the first report of the design and fabrication of a high-efficiency wideband planar antenna for the 120 GHz band.

This paper presents non-destructive millimeter-wave (MMW) imaging of sub-millimeter-wide cracks on a concrete surface covered with paper. We measured the near-field scattering of 76.5 GHz-MMW signals at concrete surface cracks for detection of the sub-millimeter-wide cracks. A decrease in the received signal magnitude by near-field scattering at the fine concrete surface crack was slight, which yielded an unclear MMW image contrast of fine cracks at the concrete surface. We have found that the received signal magnitude at concrete surface crack is larger than that at the surface without a crack, when the paper thickness is almost equal to n/4 of the effective wavelength of the MMW signal in the paper (n=1, 3, 5 ...), thus, making MMW image contrast at the surface crack reversed. By calculating the difference of two MMW images obtained from different paper thickness, we were able to improve the MMW image contrast at the surface crack by up to 3.3 dB.

10-Gbit/s wireless data transmission using 120-GHz millimeter-wave (MMW) photonic technologies is presented. For such high-data-rate transmission, we have newly developed a planar broadband receiver that employs a > 200-GHz Schottky diode and a slot-ring antenna with a 10-dB bandwidth of 30 GHz. The receiver achieves a high video sensitivity of 190 mV/mW at 120 GHz due to its optimized data output circuit. The MMW wireless link using the receiver and photonic transmitter has data transmission bandwidth of 8.5 GHz, and succeeded in 10-Gbit/s data transmission, which is the fastest ever achieved through a MMW wireless link.

A compact 120-GHz-band finline orthomode transducer (OMT) with high isolation between orthogonal ports (Iop) was designed and fabricated for bidirectional wireless data transmission with polarization multiplexing. To achieve high Iop, finline OMTs normally use a resistive card to decrease unwanted resonance, that occurs on the finline, but adding a resistive card complicates the fabrication process and raises the cost of fabrication. Our proposed finline OMT uses an improved finline design in which the resonance frequency is controlled in order to expel unwanted resonance from the operation bandwidth of the 120-GHz-band wireless link. The proposed finline design enables high Iop without using a resistive card, which simplifies the fabrication process and lowers the cost of fabrication. A square horn antenna, which is attached to the finline OMT, is also designed to suppress unwanted polarization rotation of reflected waves, which further improves Iop. The proposed finline OMT has a transmission loss of less than 1.2dB, return loss of more than 12dB, cross polarization discrimination of more than 30dB, and Iop of more than 50dB across the entire occupied bandwidth of the 120-GHz-band wireless link. These characteristics are sufficient not only for 10-Gbit/s bidirectional data transmission but also for 20-Gbit/s unidirectional 2-ch data transmission by polarization-multiplexing.

This paper proposes the absorber integrated planar array antenna for a 120-GHz-band close proximity wireless system. It consists of split-ring resonators (SRRs) patterned on a quartz substrate and a plate-laminated-waveguide planar slot array antenna. Precise alignment and multiple reflection between Tx-Rx antenna become severe problem as the carrier frequency increases, such as >100GHz. The absorber integrated planar slot array antenna solves these problems. We designed a SRR unit cell that acts as a millimeter-wave (MMW) absorber, and the simulated S11 of the SRR absorber at 125GHz is -37dB. The use of the SRR absorber on the planar slot antenna suppresses the multiple reflection between Tx and Rx antennas, however the transmission loss between Tx and Rx antennas increases. We changed the conductivity and cell size of 2×3 element SRR unit cells directly above the waveguide slots in order to make them act as an SRR director, and the use of the SRR director improved the transmission loss by 2.7dB. We simulated the transmission characteristics of a close-proximity wireless system using the SRR absorber integrated planar slot antennas. The simulated fluctuation of S21 in the 120-130GHz band is below 2.6dB, and the delayed waves that come from the multiple reflection between Tx and Rx antennas were suppressed.

This paper presents a 120-GHz-band split ring resonator (SRR) bandstop filter whose insertion loss can be controlled by coupling another lattice pattern substrate. The SRR bandstop filter and lattice pattern substrate is composed of 200-µm-thick quartz substrate and 5-µm-thick gold patterns. S21 of the SRR bandstop filter is -37.8 dB, and its -10-dB bandwidth is 115-130 GHz. S21 of the SRR bandstop filter changes to -4.1 dB at 125 GHz by arranging the lattice pattern substrate in close proximity to the SRR stopband filter, because coupling between the SRR and the lattice pattern occurs when the SRR and lattice pattern are opposed in close proximity. It was found that 10 Gbit/s data transmission can be achieved by setting the lattice pattern substrate just above the SRR bandstop filter with a spacer thickness of 50 µm, even though data transmission is impossible when only the SRR bandstop filter is inserted between the transmitter and the receiver.

In this paper, we present the transmission characteristics control of a 125 GHz-band split-ring resonator (SRR) bandstop filter by coupling an alignment-free lattice pattern. We demonstrate that the transmission characteristics of the SRR filter can be controlled by coupling the lattice pattern; however, the required accuracy of alignment between the SRR filter and lattice pattern was below 200 µm. Therefore, we designed an alignment-free lattice pattern whose unit cell size is different from that of the SRR unit cell. S21 of the SRR bandstop filter changes from -38.7 to -4.0 dB at 125 GHz by arranging the alignment-free lattice pattern in close proximity to the SRR stopband filter without alignment. A 10 Gbit/s data transmission can be achieved over a 125 GHz-band wireless link by setting the alignment-free lattice pattern substrate just above the SRR bandstop filter.

This paper presents 10-Gbit/s bidirectional and 20-Gbit/s unidirectional wireless data transmission systems using 120-GHz-band finline orthomode transducers (OMTs). A new finline OMT was fabricated with two improved designs, to adapt it to the data transmission characteristics of the 120-GHz-band wireless link. One improvement is higher isolation between orthogonal ports and the other is lower group delay variation. The measured isolation is more than 59dB at the carrier frequency of the 120-GHz-band wireless link, and the measured group delay variation is 43ps. Using the finline OMT, we developed 10-Gbit/s bidirectional and 20-Gbit/s unidirectional wireless equipment that can transmit two channels of 10-Gbit/s data using polarization multiplexing. With this wireless equipment, we succeeded in 10-Gbit/s bidirectional and 20-Gbit/s unidirectional wireless data transmission, which leads to successful seamless connection to 10 Gigabit Ethernet and 12-ch high definition television signal transmission.

This paper investigates space and polarization multiplexing for multichannel transmission in a 120-GHz band wireless link system. The 120-GHz-band wireless equipment employs Cassegrain antennas with a gain of about 49dBi and cross-polar discrimination of 23dB. When each of two 120-GHz wireless links transmits a 10-Gbit/s data signal in the same direction over a distance of 800m, a bit error rate (BER) of below 10-12 is obtained when the receivers are set 30m apart. When forward error correction and polarization multiplexing are used for each wireless link, we can set two wireless links within 1m of each other and obtain a BER below 10-12. Moreover, we have experimentally shown that the rain attenuation of V- and H-polarization 120-GHz-band signal is almost the same.

An alignment control system using beam-tilting 1-D arrays for a 120-GHz-band corporate-feed 2-D waveguide-slot array antenna is presented. The 2-D waveguide-slot array antenna transmits data, and the 1-D arrays are used to determine array alignment. We design two types of 1-D array antenna and fabricate a corporate-feed 2-D waveguide-slot array antenna surrounded by four beam-tilting 1-D arrays. We then construct an alignment control system and evaluate the performance of the control. We find that the angular accuracy of the antenna alignment control was within ±1deg.

We present a low-phase-noise and frequency-tunable photonic millimeter-wave (MMW) generator based on two-mode beating. The generator consists of a single-mode laser, an external optical intensity modulator, and a planar lightwave circuit (PLC) on which an arrayed-waveguide grating (AWG) and 3-dB optical combiners are integrated. Because the AWG and the optical combiners are connected with optical waveguides and the optical path length difference between the two modes filtered by the AWG is kept constant, the phase fluctuation of the generated MMW signal is suppressed. The generator can generate MMWs with a phase noise of less than -75 dBc/Hz at 100 Hz and has a frequency tunability in a range of 90 to 125 GHz. The generator can be applied for the local oscillator (LO) in 10-Gbit/s wireless links that use heterodyne detection.

Wireless data transmission at 3.0 Gbit/s was achieved by using millimeter-wave photonic techniques, such as optical 120-GHz subcarrier generation, optical modulation, and high-power photonic millimeter-wave emission. We have successfully demonstrated the transmission of optical Gigabit Ethernet signals over this link.

We experimentally evaluated transmission characteristics of 120-GHz-band close-proximity wireless link that employs a split-ring resonator (SRR) millimeter-wave (MMW) absorber integrated on planar slot antennas in 120-GHz-band close-proximity wireless links. We fabricated the SRR MMW absorber made of a 0.28-μm-thick TaN film on a quartz substrate, and integrated it on planar single slot antennas. When the TaN SRRs are not integrated on the planar slot antennas, multiple reflections between the two antennas occur, and a >10-dB fluctuation of S21 at 100-140GHz is observed. When the TaN SRRs are integrated on the planar antennas, the fluctuation of S21 is suppressed to be 3.5dB at 100-140GHz. However, the transmittance of the close-proximity wireless link decreases by integrating TaN SRRs on the planar slot antenna because of reflection at the quartz substrate surface. The integration of the radiator that is composed of single SRR with two capacitors just above the slot antenna increased S21 by 3.5dB at 125GHz. We conducted a data transmission experiment over a close-proximity wireless link that employs radiator-and-TaN-SRR-integrated slot antennas for Tx and Rx, and succeeded to transmit 10-Gbit/s data over the close-proximity wireless link for the first time.

This study investigates the improvement of the channel capacity of 5-GHz-band multiple-input multiple-output (MIMO) communication using microwave-guided modes propagating along a polyvinyl chloride (PVC) pipe wall for a buried pipe inspection robot. We design a planar Yagi-Uda antenna to reduce transmission losses in communication with PVC pipe walls as propagation paths. Coupling efficiency between the antenna and a PVC pipe is improved by attaching a PVC adapter with the same curvature as the PVC pipe's inner wall to the Yagi-Uda antenna to eliminate any gap between the antenna and the inner wall of the PVC pipe. The use of a planar Yagi-Uda antenna with a PVC adaptor decreases the transmission loss of a 5-GHz-band microwave signal propagating along a 1-m-lomg straight PVC pipe wall by 7dB compared to a dipole antenna. The channel capacity of a 2×2 MIMO system using planar Yagi-Uda antennas is more than twice that of the system using dipole antennas.

This paper presents a 120-GHz-band amplifier module with a hermetic sealing structure for a broadband wireless system. The sealing structure for F-band waveguides is a laminate composed of two sealing plates and a spacer. Each sealing plate has a fused glass window and separates inside air from the ambient atmosphere. The design process of the sealing structure is simple and has good simulation fidelity. The hermetic sealing structure designed for an amplifier in a 120-GHz-band wireless link has an insertion loss of less than 1dB and a return loss of more than 15dB in the operating band. We made three kinds of sealed modules to evaluate the sealing function. The modules sealed with this technique meet the hermetic-seal standard in MIL-STD-883F. We then verified that the sealing structure on the sealed modules has a small enough effect for the transmittance of the intrinsic characteristics. In addition, we performed 10-Gbit/s data transmission using a sealed amplifier module with the bit error rate of less than 10-10.

We present low-cost millimeter-wave (MMW) photonic techniques for implementing gigabit/s wireless links. A passive mode-locked laser consisting of a Fabry-Perot laser and a single-mode fiber is used to generate 120-GHz optical MMW signals. We modulated these MMW signals by controlling the bias voltage of the photodiode. The MMW generation and modulation methods do not need expensive photonic components or high-power drivers. A link employing these low-cost photonic techniques achieved 1.25-Gbit/s wireless data transmission.

For 6G mobile communications, it is important to realize a 300 GHz band bandpass filter that fits the occupied bandwidth of wireless communication system to prevent inter-system interference. This paper presents the design of a 300-GHz-band dual-band bandstop filter composed of two types of different sized split ring resonator (SRR) unit cells. The SRR unit cells are formed by a 5-μm-thick gold pattern on a 200-μm-thick quartz substrate. When two different-sized SRR unit cells are placed alternately on the same quartz substrate and the SRR unit cell size is over 260 μm, the stopbands of the dual-band bandstop filter are almost the same as those of the bandstop filter, which is composed of a single SRR unit cell. The insertion loss of the dual-band bandstop filter at 297.4 GHz is 1.8 dB and the 3-dB passband becomes 16.0 GHz (290.4-306.4 GHz). The attenuation in the two stopbands is greater than 20 dB. Six types of dual-band bandstop filters with different arrangement and different distance between SRR unit cells are prototyped, and the effect of the distance and arrangement between different sized SRR unit cells on the transmission characteristics of dual-band bandstop filters were clarified.