Sungtek Kahng

As the physical size of a communication system for satellites or unmanned aerial vehicles demands to be reduced, a compact antenna with high directivity is proposed as a core element essential to the wireless device. Instead of using a horn or an array antenna, a unit planar antenna is combined with a surface-modulated lens to convert a low antenna gain to a high antenna gain. The lens is not a metal-patterned PCB but is dielectric, which is neither curved nor very wide. This palm-sized lens comprises pixels with different heights from the backside of PolyPhenylene Sulfide (PPS) as the dielectric base. The antenna gain from the unit antenna of 4.5 cm × 4.5 cm is enhanced by 10 dB with the help of a compact dielectric lens of 7.5 cm × 7.5 cm at 24.5 GHz as the frequency of interest. The antenna design is verified by far-field measurement as well as near-field observation, including sensing a metal object behind a blocking wall by using an RF test bench. Moreover, antenna performance is understood by making a comparison with conventional designs of antennas in terms of directivity and physical sizes.

The recent confluence of major thrusts, such as 5G/6G, satellite communications, automated driving, human behavior recognition, and space applications, has triggered widespread research and development activities at the millimeter-wave (mm-wave) and terahertz (THz) frequency spectrums. Many of these applications require new approaches in the wireless architecture, oftentimes requiring new strategies and topologies for corresponding antennas. Naturally, the current landscape has increasingly rendered conventional antenna measurement techniques and infrastructures insufficient for addressing future mm-wave and THz antenna measurement objectives. The aim of this special issue is to solicit and assemble the latest studies that highlight current research involving mm-wave and THz antenna measurements as well as provide discussion about measurement challenges.

An array antenna for millimeter-wave communication and an array antenna for millimeter-wave sensing are designed and put together into one structure. Because millimeter-wave signals become weaker fast with the increasing distance and any kind of error in the required functions of the antenna has to be minimized, pointing error from the target direction should be prevented. The device is a millimeter-wave sensing antenna with high directivity to check the straight link between the TX and RX sides of wireless communication. A 24 GHz 8-by-16 array antenna which generates stronger signals for sensing resolves the drawback of a 28 GHz 1-by-4 array antenna that is commonly seen in 5G wireless terminals. The sensing and communication antennas are integrated as a planar structure mountable on platforms, which is investigated with regard to forming wireless links over a distance of several meters with an input power of less than 0 dBm. Additionally, in the event of a reflecting surface disturbing the straight path and worsening the pointing error in RF signal transfer, the dual-capability of the combination is presented on the basis of intuitive electromagnetic experiments.

Subterahertz (sub-THz) wireless links are investigated on small and thin metamaterial structures as part of an antenna measurement system. In order to realize the device under test (DUT) at 330 GHz beyond the millimeter-wave bands, Schott glass as ultrathin glass (UTG) is chosen as the substrate more than Teflon and laminates, and it goes through the sputter deposition of a metallic thin-film process to form a pattern on its surface as the metamaterial. The signal strength at the receiver (RX) antenna for the line-of-sight (LoS) becomes weaker when the glass analogous to the phone display or room window is in between the transmitter (TX) and the RX. This problem is circumvented by placing the metallic patterned glass metamaterial in the signal path from the TX to the RX. The wireless links for cases of the LoS, the glass plane inserted, and the metasurface inserted in the propagation axis are constructed, and their electromagnetic (EM) functions are experimentally characterized. The degradation in S ₂₁ is increased by more than 3 dB, both in simulation and measurement. In conjunction, the 64-QAM constellation as a communication standpoint of indicator of 6G mobile is evaluated in the three cases, revealing positive effects of the sub-THz metamaterial surface.

Radar systems are a type of sensor that detects radio signals reflected from objects located a long distance from transmitters. For covering a longer range and a higher resolution in the operation of a radar, a high-frequency band and an array antenna are measures to take. Given a limited size to the antenna aperture in the front end of the radar, the choice of a millimeter-wave band leads to a denser layout for the array antenna and a higher antenna gain. Millimeter-wave signals tend to become attenuated faster by a larger loss of the covering material like the radome, implying this disadvantage offsets the advantage of high antenna directivity, compared to the C-band and X-band ones. As the radome is essential to the radar system to protect the array antenna from rain and dust, a metamaterial surface in the layer is suggested to meet multiple objectives. Firstly, the proposed electromagnetic structure is the protection layer for the source of radiation. Secondly, the metasurface does not disturb the millimeter-wave signal and makes its way through the cover layer to the air. This electromagnetically transparent surface transforms the phase distribution of the incident wave into the equal phase in the transmitted wave, resulting in an increased antenna gain. This is fabricated and assembled with the array antenna held in a 3D-printed jig with harnessing accessories. It is examined in view of S₂₁ as the transfer coefficient between two ports of the VNA, having the antenna alone and with the metasurface. Additionally, the far-field test comes next to check the validity of the suggested structure and design. The bench test shows around a 7 dB increase in the transfer coefficient, and the anechoic chamber field test gives about a 5 dB improvement in antenna gain for a 24-band GHz array antenna.

This paper presents an effective method to boost the RSSI, the received signal strength on a moving path of LoRa wireless networking without change in digital circuitry or power amplifiers. Replacing the wire antenna as the conventionally used sensor by a printed Yagi-Uda antenna as a novel one designed to have a higher gain signifies increments of the RSSI at distant positions in the LoRa system with the same battery. The electromagnetic properties of the novel antenna are given by observing its beam-patterns in the anechoic chamber following the design procedures and fabrication. And the field-test for the radio link is conducted to see the proposed antenna enhance the RSSI at positions along the street of a wide-open area for LoRa communication. The RSSI has risen by more than 10 dB at distances from 200m to 700m with DLP-RFS1280 as the LoRa device in comparison to the conventional configuration in the device.

In this paper, a novel method is proposed to effectively reduce the size of a waveguide bandpass filter (BPF). Because the metallic cavities make the conventional waveguide end up with a large geometry, especially for high-order BPFs, very compact waveguide-type resonators having metamaterial zeroth-order resonance (WG ZOR) are designed on the cross section of the waveguide and substituted for the cavities. While the cavities are half-wavelength resonators, the WG ZOR is shorter than one-eighth of a wavelength. A substantial reduction in the size and weight of the waveguide filter is observed as the resonators are cascaded in series through coupling elements in the X-band that is much longer than that in K- or Ka-bands. The proposed metamaterial filter is realized as a 3D-printed structure to be lighter and thus more suitable for low earth orbit (LEO) satellites. An X-band of 7.25–7.75 GHz is chosen to verify the method as the passband with an attenuation of 40 dB at 7.00 GHz and 8.00 GHz as the roll-off in the stopband. The BPF is manufactured in two ways, namely the CNC-milling technique and metal coating–added 3D printing. The design is carried out with a geometrical parameter of not 10⁻² mm but rather 10^-1 mm, which is good for manufacturers but challenging for component designers. The measurement of the manufactured metal waveguide filters reveals that the passband has about ≤1 dB and ≤−15 dB as the insertion loss and the reflection coefficient, respectively, and the stopband has an attenuation of ≤−40 dB, which are in good agreement with the results of the circuit and the simulation. The proposed filter has a length of 14 cm as the eighth-order BPF, but the conventional waveguide is 20 cm as the seventh-order BPF for the same area of the cross section.

In this paper, an intuitive approach to assessing advantages of beamforming in 5G wireless communication is proposed as a novel try and practical demonstration of importance of alignment between the transmitter's and receiver's beams working in millimeter-wave frequency bands. The effects of the misalignment and alignment between beams need to be checked for, which was conducted with a horn antenna and the 4 × 4 Butler matrix RF-to-RF wireless connectivity between the horn and the microstrip line beamformer, concerning the changing angle of the beam from the Butler matrix, was tested, showing over 12 dB enhancement in received power. This direct electromagnetic link test was accompanied by examining 64-QAM constellations for beam-angle change.

Satellite communication has begun being taken to the expanded scope of mobile services in the upcoming future such as 6G or beyond, attention has been paid to methodologies to combine the on-the-ground with off-the-ground wireless link. Along with advancing the modulation schemes, protocols and signal processing, it is a must-thing to secure physical devices working for them. As all the wireless systems need RF components and modules as in beamforming for 5G, the antenna compatible to the demands of accomplishing the NTN should be devised. In this paper, for the purpose of long-range signal transmission and mountability on the payload of an LEO satellite, we show a novel antenna system of high directivity as a smaller structure. A metamaterial surface is designed to reduce the volume of the classic satellite antenna system, because this is a flat geometry instead of a parabolic one. The high directivity of the antenna is shown increasing the gain of the primary antenna by over 5 dB as the frequency band is kept with no degradation in signals.