Important definitions to know and understand when looking at the Near Field/Far Field discussion. I appreciate there will be a tendency for the eyes to glaze over with these definitions. The definitions are not a big challenge to follow if one looks at them one piece at a time and retains this substack for a reference .
Unless noted otherwise this is based directly on Innovation, Science and Economic Development (ISED) TN 261 Safety Code 6 (SC6) Radio Frequency Exposure Compliance Evaluation Template (Uncontrolled Environment Exposure Limits)
D - the maximum size of the antenna whether length or diameter for a dish antenna.
Electrically Small Antennas - Where the maximum size of the antenna whether length or diameter for a dish antenna is (D) is smaller than the wave length (lamba)λ ie D<λ - Typically most cell phones.
Electrically Large Antennas - Where the maximum size of the antenna whether length or diameter for a dish antenna is (D) is greater than the wave length λ ie D>λ Typically some routers and most other devices including Cell Towers.
Electric Field - (E) measured in volts/metre.
Energy - the capacity for doing work.
Far-field region - (Fraunhofer region) – This region is also referred to as the Fraunhofer region. This region is sufficiently far from the source that the phase and amplitude relationships of the waves stemming from different areas of the antenna do not change appreciably with distance. The antenna gain and angular pattern are independent of distance, and the power density is inversely proportional to the square of the distance from the source. Although the transition from the reactive near-field region is a gradual one, in antenna design and engineering, the far-field region is commonly assumed to begin at a distance of approximately 2D2/λ for electrically large antennas and extends to infinity (“D” being the largest linear aperture dimension and λ the wavelength at the frequency of interest). The E and H fields are orthogonal and Z=E/H=377 ohms. This region extends from 2D2/λ to infinity, but the SC6 guideline recommends that it generally be taken as 1⁄2D2/λ to infinity because it considers the transition region and the far-field region to be one. (This definition according to Health Canada is at odds with most other authorities and Health Canada offers no science to support their contrary definition. Most other authorities on the subject consider the transitional zone as a relevant 3 rd zone where the peaks and valleys of radiation while less in difference than the near field (<1⁄2D2/λ) still have significant variations.)
Impedance in Free Space - (Z) (Ώ) (assume free space for most situations) measured in Ohms is 377 Ohms.
Magnetic Field - (H) measured in amps/metre – few if any consumer class meters measure H, those that do measure the E value and calculate the H assuming the reading is in the far field where the formula H = E/377 where the impedance in free space is 377 ohms ( Ώ)
Orthogonal – Just the $10 word for right angles.
Power - in Radio Frequency/Electromagnetic Radiation terms is calculated as Watts or Volt x Amps and is the amount of energy converted over a distance i.e. work done
Power Density - (PD) measured in Watts/metre2 :Note this is the product of E x H or (volts/metre) x (amps/metre) Consumer class meters calculate the power density assuming the measurement is in the far field using PD= E2/377.
Reactive near-field region (evanescent ‘fading quickly’ region) – This is sometimes referred to as the evanescent (fading quickly) region and is the space immediately surrounding the antenna or leakage source, where the reactive (stored energy) components predominate and energy is stored in the field. In this region, the E and H fields are not orthogonal. Therefore, the impedance (Z) is not 377 ohms, but rather a complex impedance.
However, the mathematical relationship of Z=E/H still applies. Z could be a small fraction of 377 ohms for a predominately magnetic field or many times 377 ohms for a predominately electric field. The region extends up to a distance of λ/2π or 0.159 λ. For electrically small antennas (where, D < λ), the end of the reactive near-field region is also the boundary where the far-field region begins. Ιn distance terms, it is: 1.6 m at 30 MHz, 32 cm at 150 MHz, 11 cm at 450 MHz, 5 cm at 875 MHz, and 2.5 cm at 1950 MHz. ( I assume this is the reason why cell phone safety manuals use a distance of 1.9 cm as the recommended distance to hold a phone a way from the head and body and some recommend to hold it as much as 2.5 cm this distance appears to be intended to keep one out of the reactive near field zone - not that anyone reads their phones safety manual)
Radiating near-field region (Fresnel region)– This is sometimes referred to as the Fresnel region. In this region, which starts at a distance from the antenna where the reactive near-field energy has diminished to an insignificant amount, the antenna gain and the angular distribution of the radiated field vary proportionally with the distance from the antenna. This is because the phase and amplitude relationships of the various waves arriving at the observation point from different areas of the antenna change with distance. For electrically large antennas (D > λ), this region extends from λ/2π to 1⁄2D2/λ.
Transition zone - (intermediate-field region) – For an antenna that is electrically small compared to the wavelength in question, the transition zone is considered to exist at distances anywhere between 0.1 wavelength and 1.0 wavelength from the antenna, essentially between the radiating near-field and the far-field regions. This region is comprised of a combination of the characteristics found in both the near-field and the far-field regions, but the far-field characteristics are becoming more evident moving outwards. The E and H fields are almost orthogonal (Z is almost 377 ohms). This region extends from 1⁄2D2/λ to 2D2/λ and, for the purpose of SC6, is assumed to be the region in which the far-field starts.
Wavelength - (λ) the distance between one wave and the same position on the next or previous wave.
c – Speed of light = 3 x 10 8 metres/second or more precisely 299,792,458 metres/second (for most calculations 300,000,000) works well enough.
f - cycles per second usually referred to as Hertz.
For electrically small antennas, the near-field/far-field boundary is given as: λ/2π
Frequency = c/λ
For electrically large antennas, the near-field/far-field boundary is given as:
Near-Field (Reactive)/Near-Field (Radiating) = λ/2π
Near-Field (Radiating) =Far-Field = 0.05 D2/λ
There are 10,000 centimetres 2 in a metre 2
1 Unit ie Watt = 1,000 milliwatts (mwatts) =1,000,000 microwatts (watts)
For simplicity I will Endeavour to keep consistent in terms of units using watts/meter 2.
There is one more complicated formula one may need to use – to calculate the assumed power density based on the antenna gain distance from the antenna and input into the antenna
PD = power density in watts/m2
Pout= output power from the antenna in watts (W)
Gtx= gain of the antenna expressed as db gain where every 3db gain increase represents a doubling of the concentration or focusing of the antenna. Care must be taken with this formula to get the correct Pout as according to ISED and others the power could be 4 times the assumed power and according to Dr Robert Kane in Cellular Telephone Russian Roulette. The stored energy is 10 to 100 times greater than the radiated energy. It depends to a great extent on the configuration of the antenna. (page 166 )
D= distance from the antenna in meters (m)
PD=PoutGtx4∗π∗D2
For this formula I admit I cheat and use an online calculator such as:
https://www.everythingrf.com/rf-calculators/rf-power-density
No there will not be a test on these definitions, but I do recommend keeping them as a reference.
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