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Abstract Wireless radio communications provide a backbone to our technological civilization. However, radio communications are widely believed to be impossible in many situations where radios are surrounded by conductive media, such as underwater or underground, thus making ocean exploration difficult and creating well-known mine safety problems. In addition, since most imaging techniques rely on electromagnetic waves, the difficulty of electromagnetic wave propagation through biological tissues, which are mostly made of water, also severely limits bioimaging. Here we show that contrary to common beliefs, radio signals may be efficiently propagated through water over useful distances. Both radio communication and radio imaging through water may be enabled by superlensing of surface electromagnetic waves propagating along the water surface. We have demonstrated underwater radio communication over distances of several hundred skin depth in the MHz frequency range, which would require sensitivity below 10−100 W in a conventional radio communication channel. We also demonstrated subwavelength super-resolution radio imaging in the GHz range by using water surface as a superlens. Our results indicate new ways to perform bioimaging, as well as marine life safe techniques of wireless radio communication and imaging underwater, which are essential for ocean and seafloor exploration. We also anticipate that the developed techniques will provide invaluable means of studying the extraterrestrial water worlds, such as potentially inhabitable Jovian moons.

Underwater radio communication using surface electromagnetic waves propagating along the seawater-air and seawater-sea floor interfaces may provide a promising alternative to commonly used acoustic communication underwater. Here we present a detailed analytical and numerical consideration of TE and TM polarized surface electromagnetic wave properties. The resulting surface waves have propagation constants that permit communication ranges far longer than those of bulk propagation in the lossy media. We demonstrate theoretically and experimentally their ability to carry broadband radio signals over practical communication distances underwater. A 50 MHz radio signal was successfully carried along the sandy seabed over 7 m distance.

While evaluating the near field of dipole antennas, it is noted that different software suites yield values of the electric and the magnetic fields, at the surface of antennas, that can be substantially different, especially at the tips and at the feed gap. The close near field of dipoles has not been yet analysed in detail. An asymptotic expansion method for the near fields at the surface of these antennas has been developed and compared to the computed fields. The results show that the software of the computed field should not use a uniform mesh. The mesh should be much tighter at the metal extremities than in the dipole body. The proposed technique can be used to check complex field computations, producing diverging values, with simple analytical equations.

This study examines thermal responses of skin to pulsed millimeter wave (mm-wave) and radiofrequency (RF) radiation. We review limits for pulse fluence in the IEEE Std. C95.1-2019 and the 2020 guideline of the International Commission on Nonionizing Radiation Protection (ICNIRP), as well as the recently re-affirmed guidelines of the U.S. Federal Communications Commission (FCC). The focus of the study is on millimeter-wave frequencies (30-300 GHz) where energy is absorbed close to the body surface and intense pulses could potentially cause high temperature gradients at the skin, but the model is extended to lower frequencies as well. The study employs a simple one-dimensional baseline thermal model for skin and Pennes' bioheat equation (BHTE), together with a baseline model for thermal damage to skin based on a standard model. The predicted temperature increases produced by 3-sec pulses at 94 GHz are consistent with previous experimental results with no adjustable parameters in the model. The few reported data on thermal damage to the skin from pulsed 94 GHz energy are insufficient to enable a conventional analysis of damage thresholds and the data may be affected by errors in dosimetry. The baseline model suggests that the implicit limits on pulse fluence in the present FCC guidelines might allow, in extreme (but in practice unrealistic) cases, transient increases in skin temperature that approach thresholds for thermal pain but which remain well below levels anticipated to cause thermal damage. Limits on pulse fluence in the current IEEE and ICNIRP exposure guidelines would preclude such effects. Such extreme pulses are far above those that are emitted by wireless and other technologies but may be emitted by some nonlethal weapons systems. FCC's proposed “device-based time averaging” rules will restrict thermal transients in skin from mm-wave transmitters to levels that are roughly an order of magnitude below the slower temperature increases produced by the low-frequency components of the modulation waveform and appear to be excessively conservative. An appendix discusses the applicability of two approximations to the analytical solutions to the bioheat equation that can be used to estimate temperature increases in skin from exposure to mm-waves.

This paper considers the problem of choosing an appropriate “averaging time” in radiofrequency (RF) exposure limits to protect against thermal hazards, focusing on the RF frequency range above 3-10 GHz. Analysis is based on examination of the dynamic properties of thermal models for tissue using Pennes' bioheat equation. Three models are considered: a baseline model consisting of a uniform half space with dielectric and thermal properties similar to those of human skin with adiabatic boundary conditions; a layered 1D model with dielectric and thermal properties similar to those of skin, fat, and underlying muscle, with convective boundary conditions appropriate for room environments; and exposures to the head of an anatomically detailed image-based model (“Taro”). RF exposure consisted of plane wave radiation incident on the two planar models, and radiation from resonant dipoles located 1.5 cm from the head model, at frequencies ranging from 1 to 300 GHz. The dynamic properties of the models were explored by analytic solution of the baseline model, and from numerical solutions of the thermal responses of the layered and head models. From the step responses of the models (increases in surface temperature to a suddenly imposed exposure), the impulse and frequency responses of the models were obtained. In the frequency domain, the thermal models exhibit extreme lowpass characteristics with cutoff (-3 dB response) frequencies below 1 mHz. The impulse response to millimeter wave radiation (30-300 GHz) shows a sharp peak at zero time, due to short term accumulation of heat near the surface, which dissipates quickly as heat is conducted into deeper layers of tissue. Simple analytical results of a further simplified model assuming purely surface heating agree well with results of a more detailed assessment for millimeter waves. Response of the model to pulse trains and to single maximum fluence “big bang”pulses in which all allowable energy over a 6-min averaging time is delivered in one short pulse raises the possibility of excessive transient temperature increases at the tissue surface from exposure to short high-fluence pulses at mm-wave frequencies. Such exposures are not produced by current technologies apart from certain military weapons systems but may occur from future high-power mm-wave technology. By contrast, simulations of exposure from a communications waveform at 1.9 GHz show extremely tiny transient temperature fluctuations. The results generally confirm the present choice of 6 min for an averaging time in the current generation of RF exposure limits but suggest the need for additional limits on fluence for brief high-fluence pulses at mm-wave frequencies. This paper addresses thermal hazards only, and a larger range of evidence would need to be evaluated as well in revising exposure limits.

In this paper, mechanisms of RF energy absorption by body tissue in close proximity to wireless equipment, are studied using numerical simulations at frequencies above 24 GHz. It is shown that at millimeter-wave (mmW) frequencies, of relevance for 5G mobile communications, and for realistic source to body separation distances, the contribution from the reactive near-field to the energy deposition in the tissue is small. Furthermore, the interaction between the source and the exposed body is modest. The results suggest that the effects of the near-field body interactions are small when evaluating electromagnetic field compliance at mmW frequencies.

This paper presents several novel designs of small form factor underwater radio antennas operating in the 2 MHz, 50 MHz and 2.4 GHz bands. These antennas efficiently excite surface electromagnetic waves (SEW) which propagate along the surface of seawater. The antenna operation is made possible due to implementation of an impedance matching enclosure, which is filled with de-ionized water. Enhanced coupling to surface electromagnetic waves is enabled by the enhancement of the electromagnetic field at the antenna apex. These features allow us to make antenna dimensions considerably smaller compared to typical free space designs. They also considerably improve coupling of electromagnetic energy to the surrounding seawater. Since SEW propagation length is considerably larger than the skin depth in seawater, this technique is useful for underwater broadband wireless communication. We conclude that the developed broadband underwater radio communication technique will be useful in networking of unmanned underwater vehicles.

Biological experiments that expose living cells or tissues to RF energy must have an aqueous medium to provide essential water, ions, nutrients, and growth factors. However, as we show here, the medium inherently functions as a receiving antenna that conveys RF energy to the biological entity in a manner entirely determined by exposure vessel geometry, orientation to the incident RF flux, frequency, and dielectric properties of the medium. We show for two common experimental arrangements that basic antenna theory can predict electromagnetic energy patterns that agree well with those otherwise obtained by computationally intensive methods that require specialized resources.

This paper considers the specific absorption rate (SAR) in tissue of radiofrequency (RF) energy and temperature increases produced by RF currents on short conductors (0.03–0.1λ). We consider a cylindrical model in which a center-feeds, insulated antenna is embedded in tissue. We introduce a new method for the analytic evaluation of the fields in the cylindrical phantom taking advantage of the axial symmetry of the antenna and the tissue. Results of the analytical model are compared to results of numerical (finite difference time domain) simulations; in addition, the thermal response of the exposed material is calculated by finite element solution of the heat conduction equation. For model antennas of 1 to 3 cm total length with a feedpoint current of 10mA RMS at 900MHz, the maximum SAR (in tissue next to the antenna) is less than ∼2.5W/kg. SAR decays rapidly with radial distance from the antenna (∼r−4 for the 1cm antenna) and creates a steady-state temperature rise less than 0.05K at the location of SARmax. Heat conduction causes the temperature to decline steeply with radius (depth into tissue).

The Neotropical region hosts 4225 freshwater fish species, ranking first among the world's most diverse regions for freshwater fishes. Our NEOTROPICAL FRESHWATER FISHES data set is the first to produce a large-scale Neotropical freshwater fish inventory, covering the entire Neotropical region from Mexico and the Caribbean in the north to the southern limits in Argentina, Paraguay, Chile, and Uruguay. We compiled 185,787 distribution records, with unique georeferenced coordinates, for the 4225 species, represented by occurrence and abundance data. The number of species for the most numerous orders are as follows: Characiformes (1289), Siluriformes (1384), Cichliformes (354), Cyprinodontiformes (245), and Gymnotiformes (135). The most recorded species was the characid Astyanax fasciatus (4696 records). We registered 116,802 distribution records for native species, compared to 1802 distribution records for nonnative species. The main aim of the NEOTROPICAL FRESHWATER FISHES data set was to make these occurrence and abundance data accessible for international researchers to develop ecological and macroecological studies, from local to regional scales, with focal fish species, families, or orders. We anticipate that the NEOTROPICAL FRESHWATER FISHES data set will be valuable for studies on a wide range of ecological processes, such as trophic cascades, fishery pressure, the effects of habitat loss and fragmentation, and the impacts of species invasion and climate change. There are no copyright restrictions on the data, and please cite this data paper when using the data in publications., (© 2022 The Ecological Society of America.)

The spatial gradients created after the installation of large reservoirs produce a great diversity of biotopes, which affects the distribution and structuring of aquatic communities, mainly fish. Thus, we hypothesized that the fishes living in the lotic stretch (river zone, closest to the natural environment) of the reservoir would have lower overlap and higher niche breadth than fishes living in the lentic stretch. Samples were collected in six sections (lentic and lotic) of the Chavantes Reservoir, middle Paranapanema River. A total of 1478 individuals distributed in 13 species were sampled in both stretches. Many resources were acquired by species, and we observed that nine species present significant differences when comparing the two stretches. Furthermore, only \emph{Schizodon nasutus.