Commercial wireless communications systems are moving higher in frequency as the demands continue to grow for faster exchanges of voice, video, and data. Available bandwidth at 60 GHz makes those millimeter-wave frequencies attractive for short-range, high-data-rate backhaul links in emerging 5G wireless communications networks, notably in personal wireless devices (PWDs). Of course, signal energy at those frequencies is not always easy to come by, making antennas critical components in any 60-GHz system.
Ideally, antennas can provide high gain at 60 GHz without adding too much size, weight, or cost to a PWD. Traditional 60-GHz millimeter-wave antenna types, such as reflector, lens, and horn antennas, may not be the most promising candidates for commercial 60-GHz PWDs. Since the antennas for these PWDs will often be operating at close distances to a user’s head, the specific absorption rate (SAR) of the head and tissues around the antenna of a 60-GHz PWD should be known in order to determine a safe antenna design for future millimeter-wave PWDs.
Researchers at the Tshwane University of Technology (Pretoria, South Africa) and Sri Ramaswami Memorial University (Chennai, India) explored the possibilities of using antipodal linear tapered slot antennas (ALTSAs) in 60-GHz PWDs. The antenna has a planar geometry, with corrugated structure and substrate integrated waveguide (SIW). The experimenters first compared their ALTSA design, with 16.5-dB gain and 34.6-deg. beamwidth at 60 GHz, with various other compact antenna configurations with similar gain levels and beamwidths, at 28 GHz, 41 to 61 GHz, 35 GHz, and 50 to 70 GHz.
The researchers also detailed the dielectric properties of the human head and the tissues around it, as well as the typical thicknesses of those tissues, so that the SARs of these tissues could be understood with respect to various radiation levels. They investigated the radiation risks from such antennas in close vicinity to a user’s head by performing SAR distribution analysis of an ALTSA to better understand the interactions between the antenna and the human body of the user—especially when the antenna would be used in wireless electronic devices, such as audio headphones and laptop computers that would be close to a user’s body.
Due to the planar geometry and the corrugated structure of the ALTSA, it exhibited very uniform SAR distributions across its aperture. The SAR values for the antenna at the head and anterior thighs of a test subject were below safety levels at 60 GHz, making the ALTSA a dependable candidate for millimeter-wave PWDs, expected to become commercially widespread as the wide bandwidths available at millimeter-wave frequencies encourage the use of signal frequencies at 60 GHz and higher.
See “Specific Absorption Rate Distributions of a Tapered Slot Antenna at 60 GHz in Personal Wireless Devices,” IEEE Antennas & Propagation Magazine, December 2017, p. 140.