Limitations of Incident Power Density as a Proxy for Induced Electromagnetic Fields

2020 · Mixed · Insufficient evidence

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MixedInsufficient evidencePaper 1520

Study snapshot

Publicationmanual · 2020

Study typeengineering

Reported directionMixed

Evidence ratingInsufficient

Abstract

Limitations of Incident Power Density as a Proxy for Induced Electromagnetic Fields Christ A, Samaras T, Neufeld E, Kuster N. Limitations of Incident Power Density as a Proxy for Induced Electromagnetic Fields. Bioelectromagnetics. 2020 May 14. doi: 10.1002/bem.22268. Abstract The most recent safety guidelines define basic restrictions for electromagnetic field exposure at frequencies more than 6 GHz in terms of spatial- and time-averaged transmitted power density inside the body. To enable easy-to-perform evaluations in situ, the reference levels for the incident power density were derived. In this study, we examined whether compliance with the reference levels always ensures compliance with basic restrictions. This was evaluated at several distances from different antennas (dipole, loop, slot, patch, and helix). Three power density definitions based on integration of the perpendicular real part of the Poynting vector, the real part of its three vector components, and its modulus were compared for averaging areas of λ2 /16, 4 cm2 (below 30 GHz) and 1 cm2 (30 GHz). In the reactive near-field (d < λ/(2π)), the transmitted power density can be underestimated if an antenna operates at the free space exposure limit. This underestimation may exceed 6 dB (4.0 times) and depends on the field source due to different coupling mechanisms. It is frequency-dependent for fixed-size averaging areas (4 and 1 cm2 ). At larger distances, transmission can be larger than the theoretical plane-wave transmission coefficient due to backscattering between the body and field source. Using the modulus of the incident Poynting vector yields the smallest underestimation. ncbi.nlm.nih.gov Conclusions The ratio of the transmitted to incident averaged PD was quantified considering near- and far-field effects with the objective of establishing a correlation that can be used for the assessment of the compliance of wireless devices operating above 6 GHz based on free space measurements. The following additional aspects need be taken into consideration for the quantification of the average transmitted PD: * Increased coupling in the near-field, which has already been described by Kuster and Balzano [1992], can lead to an underestimation of the average transmitted PD by more than 6 dB (4.0 times) or a factor of 4 for d < λ/2 if the real part of the Poynting vector that is perpendicular to the phantom (Equation (1)) is applied to calculate the averaged PD as the exposure metric. * The underestimation is frequency-independent, if the averaging area is normalized to (λ/4)2, instead of being kept fixed at 4 or 1 cm2. If the averaging area is normalized to the wavelength, frequency independence for array antennas can be assumed provided that the array topology (e.g. number and spatial arrangement of array elements) is kept constant. * Calculating the averaged PD using the modulus of the Poynting vector (Equation (3)) yields a better proxy for the transmitted power in the near-field, i.e. for d < λ/(2π), because it takes into account losses that are induced by the reactive field components, which do not contribute to the real part of the Poynting vector. * At distances d > λ/(2π), at which the Poynting vector approaches the PD of TEM wave propagation, the transmission coefficient may deviate by more than ±2 dB (1.6 times) from the plane‐wave equivalent transmission due to backscattering and constructive or destructive interference. Within the considered distance range of up to 2λ, the impact of backscattering cannot be neglected. These results should be considered in the assessment of compliance of wireless devices with the basic restrictions. The incident PD calculated from the real part of the Poynting vector that is perpendicular to the phantom (Equation (1)) is a poor proxy for any EM source in the reactive near-field, i.e. at d < λ/(2π) or at a distance of 2 mm below 24 GHz. According to the results of the evaluated EM sources in this study, the modulus of the Poynting vector (Equation (3)) approximates the exposure much more reliably. The most reliable method of determining the exposures at 2 mm below 24 GHz, however, is the direct assessment of the transmitted PD in a skin simulating phantom instead of the incident PD, as it also takes into account the coupling and backscattering effects. Above 24 GHz, current methods have proven to be sufficiently accurate and reliable.

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AI evidence extraction

Structured fields below were generated from the paper’s abstract and metadata to make the evidence easier to inspect. Automated extraction may be incomplete or incorrect; verify important details against the original paper.

Study typeengineering

Effect directionmixed

PopulationNot supplied

Sample sizeNot supplied

ExposuremmWave

Evidence strengthInsufficientConfidence: 74% · Peer reviewed: yes

Main findings

The study evaluated whether compliance with incident power density reference levels ensures compliance with basic restrictions for >6 GHz exposures. In the reactive near-field, incident PD based on the perpendicular real part of the Poynting vector can underestimate transmitted PD by more than 6 dB (4×), with dependence on antenna type and (for fixed averaging areas) frequency; using the modulus of the Poynting vector produced the smallest underestimation. At distances beyond the reactive near-field, the transmission coefficient could deviate by more than ±2 dB from plane-wave equivalents due to backscattering and interference; direct assessment of transmitted PD in a skin-simulating phantom was described as most reliable below 24 GHz at very small distances, while above 24 GHz current methods were reported as sufficiently accurate.

Outcomes measured

Accuracy/validity of incident power density (PD) reference levels as a proxy for transmitted/induced fields inside the body above 6 GHz
Comparison of incident PD definitions based on Poynting vector (perpendicular real part; real part of 3 components; modulus)
Near-field and far-field effects on transmitted-to-incident averaged PD ratio (coupling, backscattering/interference)

View raw extracted JSON

{
  "study_type": "engineering",
  "exposure": {
    "band": "mmWave",
    "source": "wireless devices/antennas (dipole, loop, slot, patch, helix)",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": null,
  "sample_size": null,
  "outcomes": [
    "Accuracy/validity of incident power density (PD) reference levels as a proxy for transmitted/induced fields inside the body above 6 GHz",
    "Comparison of incident PD definitions based on Poynting vector (perpendicular real part; real part of 3 components; modulus)",
    "Near-field and far-field effects on transmitted-to-incident averaged PD ratio (coupling, backscattering/interference)"
  ],
  "main_findings": "The study evaluated whether compliance with incident power density reference levels ensures compliance with basic restrictions for >6 GHz exposures. In the reactive near-field, incident PD based on the perpendicular real part of the Poynting vector can underestimate transmitted PD by more than 6 dB (4×), with dependence on antenna type and (for fixed averaging areas) frequency; using the modulus of the Poynting vector produced the smallest underestimation. At distances beyond the reactive near-field, the transmission coefficient could deviate by more than ±2 dB from plane-wave equivalents due to backscattering and interference; direct assessment of transmitted PD in a skin-simulating phantom was described as most reliable below 24 GHz at very small distances, while above 24 GHz current methods were reported as sufficiently accurate.",
  "effect_direction": "mixed",
  "limitations": [],
  "evidence_strength": "insufficient",
  "confidence": 0.74,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "power density",
    "Poynting vector",
    "incident power density",
    "transmitted power density",
    "basic restrictions",
    "reference levels",
    "near-field",
    "reactive near-field",
    "backscattering",
    "antenna",
    "mmWave",
    ">6 GHz"
  ],
  "suggested_hubs": [
    {
      "slug": "who-icnirp",
      "weight": 0.6,
      "reason": "Discusses safety guidelines/basic restrictions and reference levels for >6 GHz exposure assessment."
    },
    {
      "slug": "5g-policy",
      "weight": 0.55,
      "reason": "Focuses on compliance assessment methods for wireless devices operating above 6 GHz (mmWave-relevant)."
    }
  ]
}

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AI-extracted fields are generated from the abstract and metadata and may be incomplete or incorrect. This material is for informational purposes and is not medical advice.

Identifiers and source

DOI: 10.1002/bem.22268
PubMed: Not supplied

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