Microwave Antennas and Propagation

RF/microwave radiation is non-ionising (photon energy far below that needed to break chemical bonds), but it is absorbed by the body and converted to heat. Hazards arise when the power density or SAR is above safe limits.

RF/MW radiation hazards

Thermal effects (main, well-established):

• Water molecules in tissue rotate with the field; the energy becomes heat (dielectric heating).
• Organs with poor blood flow cannot remove heat: eyes (cataract) and testes (temporary sterility) are most sensitive.
• Whole-body heating raises body temperature, causing heat stress, fatigue and burns at high levels.
• Penetration depth falls with frequency: at 2.45 GHz a few cm, above 10 GHz mostly skin heating.
• Human body resonance (about 70–100 MHz for an adult) gives the highest whole-body absorption.

Non-thermal effects (studied, less certain): headaches, sleep disturbance, changes in nervous and cardiovascular response, and possible genetic effects. IARC (WHO) classes RF fields as "possibly carcinogenic to humans" (Group 2B).

Other hazards: sparks and ignition of fuel (HERF), triggering of electro-explosive devices (HERO), and interference with pacemakers and medical implants.

The absorbed power is measured as SAR = σ|E|²/ρ (W/kg).

Safety practices

1. Follow exposure limits: ICNIRP / IEEE C95.1: whole-body average SAR 0.08 W/kg (public) and 0.4 W/kg (occupational); localised (head/trunk) 2 W/kg and 10 W/kg over 10 g.
2. Distance: power density falls as 1/r², so keep away from antennas, especially in the main beam; mark exclusion zones around BTS and radar antennas.
3. Time: limit exposure time (limits are averaged over 6–30 minutes).
4. Shielding: metal enclosures, mesh, RF-absorbing material; microwave ovens use door mesh and chokes.
5. Switch off / lock-out transmitters before working on antennas or waveguides; never look into an open waveguide or horn.
6. Monitoring: survey sites with calibrated field meters; workers can wear personal RF monitors.
7. Signs and training: warning signs, fencing, and training of technicians.
8. Equipment maintenance: check oven door seals and waveguide flanges for leakage.

Microwave radiation (300 MHz–300 GHz) is non-ionising, but it becomes hazardous when the body absorbs more energy than it can remove as heat.

How it becomes hazardous:

1. Microwaves penetrate tissue and make polar water molecules rotate and ions move; the energy turns into heat (dielectric heating).
2. The rate of absorption is the SAR = σ|E|²/ρ (W/kg). Above about 4 W/kg whole body, core temperature rises by about 1 °C.
3. Tissues with poor blood circulation cannot remove heat: the eye lens (cataract) and testes (reduced sperm count) are most at risk. Burns may occur at high power density.
4. Penetration depends on frequency: lower microwave frequencies heat deep tissues (no warning from skin sensors), higher frequencies heat the skin.
5. Possible non-thermal effects: headache, fatigue, sleep and nervous system effects (not fully established).
6. Indirect hazards: interference with pacemakers, ignition of fuel and explosives.

Limits: ICNIRP/IEEE limit whole-body SAR to 0.08 W/kg for the public and 0.4 W/kg for workers; microwave ovens must leak less than 5 mW/cm² at 5 cm (US FDA).

SAR (Specific Absorption Rate) is the rate at which RF/microwave energy is absorbed per unit mass of body tissue. Unit: W/kg.

SAR = σ|E|²/ρ      (W/kg)
SAR = c·(dT/dt)    (from temperature rise)
σ = tissue conductivity (S/m)
E = rms electric field in tissue (V/m)
ρ = tissue density (kg/m³)
c = specific heat (J/kg·°C)

SAR is averaged over the whole body or over a small mass (1 g or 10 g) for local exposure, and over time (6 min or 30 min).

Hazards based on SAR

• Thermal effects: energy is converted to heat. Whole-body heating causes heat stress; localised heating hurts organs with poor blood flow, such as the eyes (cataract) and testes. The safety limits are set at 1/10 (workers) and 1/50 (public) of the 4 W/kg threshold.
• Frequency dependence: the whole body absorbs most near its resonance (about 70–100 MHz for an adult); at microwave frequencies absorption is concentrated near the surface, so local SAR matters more.
• Local exposure: mobile phones close to the head produce high local SAR. Limits: 2 W/kg over 10 g (ICNIRP/IEEE, head and trunk); 1.6 W/kg over 1 g (FCC, USA).
• Non-thermal effects: possible effects on the nervous system, sleep and cell function at low SAR are still being studied; IARC classes RF as Group 2B (possibly carcinogenic).

Standard SAR limits (ICNIRP 2020 / IEEE C95.1-2019)

SAR is measured with a phantom head/body filled with tissue-simulating liquid and an isotropic E-field probe, and it is the basis for certifying mobile phones.

Microwave radiation fields (public exposure)

The space around an antenna is divided into three regions (D = largest antenna dimension, λ = wavelength):

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ

• In the reactive near field, E and H must be measured separately and exposure can be very high; only trained staff should enter.
• In the radiating near field, power density varies irregularly with distance.
• In the far field, power density S = PtGt/(4πr²) and E = √(377·S), so exposure can be calculated. Public areas are normally in this region.

International EMR safety standards

1. ICNIRP (1998, updated 2020): International Commission on Non-Ionizing Radiation Protection; adopted by WHO and many countries. Basic restrictions on SAR: 0.08 W/kg whole body (public), 0.4 W/kg (occupational); 2 W/kg local (public). Reference levels for public power density (1998): 2 W/m² at 10–400 MHz, f/200 W/m² at 400–2000 MHz (4.5 W/m² at 900 MHz, 9 W/m² at 1800 MHz), 10 W/m² at 2–300 GHz.
2. IEEE C95.1 (2019): similar SAR limits for "unrestricted" (public) and "restricted" (occupational) environments.
3. FCC (USA, OET Bulletin 65): public limit 1.6 W/kg over 1 g for phones; MPE 1 mW/cm² above 1.5 GHz for the public.
4. IEC 62209 / IEC 62232: measurement methods for phone SAR and for BTS RF fields.
5. ITU-T K.52 / K.70: guidance for telecom operators on compliance with exposure limits.

Recommended practices

• Keep public areas outside the compliance distance; fence and sign antenna sites.
• Mount BTS antennas high, with the main beam above people and buildings.
• Use field surveys to verify compliance after installation.
• Workers switch off or reduce power before climbing towers; use personal monitors.
• Limit exposure time; use shielding; follow manufacturer guidelines for ovens and phones (hands-free use).

How microwaves are hazardous to humans

Thermal effects (main, well-established):

• Water molecules in tissue rotate with the field; the energy becomes heat (dielectric heating).
• Organs with poor blood flow cannot remove heat: eyes (cataract) and testes (temporary sterility) are most sensitive.
• Whole-body heating raises body temperature, causing heat stress, fatigue and burns at high levels.
• Penetration depth falls with frequency: at 2.45 GHz a few cm, above 10 GHz mostly skin heating.
• Human body resonance (about 70–100 MHz for an adult) gives the highest whole-body absorption.

Non-thermal effects (studied, less certain): headaches, sleep disturbance, changes in nervous and cardiovascular response, and possible genetic effects. IARC (WHO) classes RF fields as "possibly carcinogenic to humans" (Group 2B).

Other hazards: sparks and ignition of fuel (HERF), triggering of electro-explosive devices (HERO), and interference with pacemakers and medical implants.

Radiation zones of a microwave oven

A domestic oven works at 2.45 GHz (λ = c/f = 3×10⁸/2.45×10⁹ = 12.24 cm). The magnetron energy is confined in a metal cavity; only small leakage escapes through the door gaps and mesh. Around the oven three zones are defined:

 [OVEN]|<-- ~2 cm -->|<-- few λ -->|
 cavity| reactive    | radiating   | far field
       | near field  | near field  | (> ~0.5 m)
       | (door/mesh) |             |

1. Inside the cavity: very high fields (standing waves, kW level). Hazardous; interlocks switch off the magnetron when the door opens.
2. Reactive near field (within about λ/2π ≈ 2 cm of the door): leakage fields are stored around gaps; E and H are measured separately. Leakage is measured at 5 cm from the surface.
3. Radiating near field (few cm to several λ): leakage power spreads and varies with position.
4. Far field (beyond about 0.5–1 m): power density falls as 1/r² and is far below limits.

Safety limit (US FDA 21 CFR 1030.10): leakage must be under 1 mW/cm² at 5 cm for a new oven and 5 mW/cm² during its life. Damaged doors, hinges or seals can increase leakage, so they must be repaired.

RF radiation fields around an antenna are classified by distance r from the antenna (D = largest dimension, λ = wavelength):

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ

Comparison

How they are hazardous

• Reactive near field: strong E and H fields close to the antenna can deliver very high local SAR, causing burns and RF shock; workers on a live tower are at risk. Even a small antenna (phone) gives high local SAR to the head.
• Radiating near field: in the main beam of a BTS or radar dish, power density can exceed the limit; tissues heat, the eye lens and testes are most at risk (cataract, sterility).
• Far field: power density S = PtGt/(4πr²) falls with distance; for the public it is usually below ICNIRP reference levels, but long exposure to high-power radars or broadcast stations can still cause heating.
• In all zones, the body absorbs energy as heat (SAR = σE²/ρ). Above about 4 W/kg whole-body SAR, core temperature rises about 1 °C; limits are 0.4 W/kg (workers) and 0.08 W/kg (public).

Hence exclusion zones are set within the near field and the main beam, and exposure in the far field is checked against standards.

How microwaves are hazardous to humans

Thermal effects (main, well-established):

• Water molecules in tissue rotate with the field; the energy becomes heat (dielectric heating).
• Organs with poor blood flow cannot remove heat: eyes (cataract) and testes (temporary sterility) are most sensitive.
• Whole-body heating raises body temperature, causing heat stress, fatigue and burns at high levels.
• Penetration depth falls with frequency: at 2.45 GHz a few cm, above 10 GHz mostly skin heating.
• Human body resonance (about 70–100 MHz for an adult) gives the highest whole-body absorption.

Non-thermal effects (studied, less certain): headaches, sleep disturbance, changes in nervous and cardiovascular response, and possible genetic effects. IARC (WHO) classes RF fields as "possibly carcinogenic to humans" (Group 2B).

Other hazards: sparks and ignition of fuel (HERF), triggering of electro-explosive devices (HERO), and interference with pacemakers and medical implants.

Device to map radiation zones around a GSM BTS

The usual instrument is a broadband isotropic RF field meter (EMF/radiation hazard meter) with a probe; for frequency-selective mapping a calibrated antenna + spectrum analyzer is used.

Broadband isotropic probe meter: working principle

  3 orthogonal      diode/thermocouple    meter unit
  dipoles (x,y,z)   detectors + filter    (√sum, display)
     |                  |                     |
  [ Ex Ey Ez ] ---> [ DC ∝ E² ] ---> [ E = √(Ex²+Ey²+Ez²) ]
                                       → V/m, W/m², % limit

1. The probe has three mutually perpendicular small dipoles (for E) or loops (for H), so the reading does not depend on polarisation or direction (isotropic).
2. Each element feeds a diode detector (fast, sensitive) or thermocouple (true rms). Resistive leads carry the DC to the meter without picking up RF.
3. The meter adds the three squared components: E² = Ex² + Ey² + Ez², and shows E (V/m), power density S = E²/377 (W/m²), or % of the ICNIRP limit. Readings are averaged over 6 minutes.

Frequency-selective method: a calibrated log-periodic/biconical antenna and spectrum analyzer measure GSM 900 and 1800 carriers separately; E = received level + antenna factor + cable loss.

Mapping procedure:

1. Measure on a grid of points around the tower at several heights (e.g. 1.1, 1.5, 1.7 m) and in the main beam direction.
2. Record the BTS power and traffic; scale to maximum power.
3. Compare each reading with the limits (public at 900 MHz: 4.5 W/m², 41 V/m; at 1800 MHz: 9 W/m², 58 V/m; ICNIRP 1998).
4. Plot contours: exclusion (occupational) zone, public compliance boundary, and safe zone.

GSM 900: λ = 33.3 cm; GSM 1800: λ = 16.7 cm. For a 2 m panel antenna at 900 MHz, the far field begins about 2D²/λ = 24 m away.

Radiation fields for λ/4 antennas

λ = c/f
900 MHz:  λ = 3×10⁸/9×10⁸  = 33.33 cm,  D = λ/4 = 8.33 cm
1800 MHz: λ = 3×10⁸/1.8×10⁹ = 16.67 cm, D = λ/4 = 4.17 cm

For a small antenna (D < λ), the formula 2D²/λ gives a distance smaller than λ/2π, so the reactive near field ends at about λ/2π and the far field starts beyond about one wavelength (~33 cm at 900 MHz, ~17 cm at 1800 MHz).

Comparison:

• All field regions at 1800 MHz are half as large as at 900 MHz, because λ and D are both halved.
• At 1800 MHz the energy is absorbed nearer the skin; at 900 MHz it penetrates deeper.
• Real BTS panel antennas are much larger (≈ 1–2 m), so their far field begins tens of metres away.

How microwave EMR becomes hazardous

The body absorbs RF energy, which turns into heat (SAR = σE²/ρ). Above about 4 W/kg, core temperature rises by about 1 °C. Organs with low blood flow, such as the eyes and testes, are damaged first (cataract, sterility); near-field exposure can cause burns. Possible non-thermal effects (headache, sleep disturbance) are still being studied.

IEEE safety standard

IEEE C95.1-2019 (limits for human exposure, 0 Hz–300 GHz):

• Whole-body SAR: 0.08 W/kg (unrestricted/public), 0.4 W/kg (restricted/occupational).
• Localised SAR (10 g): 2 W/kg (public), 10 W/kg (occupational).
• Public power density at 900 MHz ≈ f/200 = 4.5 W/m², at 1800 MHz ≈ 9 W/m².

Hazardous and safe zones at 2600 MHz, D = 0.5 m

λ  = c/f = 3×10⁸ / 2.6×10⁹ = 0.1154 m
R1 = 0.62√(D³/λ) = 0.62√(0.125/0.1154)
   = 0.62 × 1.0408 = 0.645 m
R2 = 2D²/λ = 2×0.25/0.1154 = 4.33 m

 [ant]|--0.65 m--|-------4.33 m-------|----->
      | reactive |  radiating near    | far field
      | DANGER   |  CAUTION (beam)    | SAFE (public)

So the hazardous zone is within about 4.3 m of the antenna (mainly in front of it), and the public safe zone is beyond about 4.3 m, provided the far-field power density there is below the ICNIRP public limit of 10 W/m² (2–300 GHz). The exact compliance distance depends on EIRP; for example, an EIRP of 1 kW gives S = 1000/(4π × 4.33²) = 4.2 W/m² at 4.33 m, which is already below 10 W/m².

How non-ionising radiation is hazardous

(The "non-radiating radiation" in the question is read as non-ionising radiation.)

• RF photons cannot ionise atoms, but the fields make polar molecules (water) rotate and ions move; this energy turns into heat.
• The absorption is the SAR = σE²/ρ (W/kg). Above about 4 W/kg the core temperature rises by about 1 °C, giving heat stress.
• At 2.6 GHz penetration is a few centimetres, so the skin, eyes and testes are heated most. The eye lens has poor blood flow and can develop cataract; burns occur at high power density.
• Long-term, possible non-thermal effects (headache, sleep disturbance, IARC Group 2B "possibly carcinogenic") are still being studied.

Propagation fields of a GSM BTS

A GSM BTS antenna (900/1800 MHz panel, length D ≈ 1–2 m) produces three field regions:

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ

Example: 900 MHz (λ = 0.333 m), D = 2 m → R1 = 0.62√(8/0.333) = 3.04 m, R2 = 2×4/0.333 = 24 m.

Radiation parameters used to describe BTS exposure:

• EIRP = Pt·Gt (W or dBm): effective radiated power in the main beam.
• Power density S = EIRP/(4πr²) (W/m²) in the far field.
• Electric field E = √(377·S) (V/m), magnetic field H = E/377 (A/m).
• Antenna gain, beamwidth and tilt decide where the main beam reaches the ground.
• SAR (W/kg) for absorbed power in the body.

Non-ionising SAR and its hazard

Microwave radiation is non-ionising: each photon carries too little energy to break chemical bonds. Its effect on the body is measured by the Specific Absorption Rate:

SAR = σ|E|²/ρ   (W/kg)

where σ is tissue conductivity, E the internal rms field and ρ the tissue density.

How it becomes hazardous:

1. Absorbed power becomes heat. Whole-body SAR above about 4 W/kg raises core temperature by about 1 °C, causing heat stress.
2. High local SAR heats organs with poor blood flow: eye lens (cataract), testes (temporary sterility); very high levels cause burns.
3. Limits: ICNIRP/IEEE whole-body 0.08 W/kg (public), 0.4 W/kg (occupational); local 2 W/kg (public, 10 g).
4. Near a BTS, workers in the near field or main beam can exceed limits; the public at ground level is usually far below them.

HERP (Hazards of Electromagnetic Radiation to Personnel) is the term (used originally by the US Navy and defence standards) for the danger that high-power RF/microwave fields pose to people. It is one of three EMR hazard classes:

• HERP – hazard to personnel (people).
• HERO – hazard to ordnance (electro-explosive devices may be triggered).
• HERF – hazard to fuel (sparks may ignite fuel vapour).

Main points of HERP:

1. The cause is thermal heating: absorbed power (SAR = σE²/ρ) raises tissue temperature. Eyes (cataract) and testes are most sensitive; burns and RF shock occur near high-power antennas.
2. HERP is controlled by keeping exposure below permissible exposure limits (IEEE C95.1, ICNIRP): e.g. whole-body SAR 0.4 W/kg for workers and 0.08 W/kg for the public.
3. HERP distance/zone: around radars, transmitters and BTS antennas, a calculated distance (from S = PtG/4πr²) inside which exposure exceeds the limit is marked and access is controlled.
4. Controls: warning signs, interlocks, switching off before work, time limits, shielding and RF monitoring.

Five major RF safety practices

1. Observe exposure limits: keep exposure below ICNIRP/IEEE C95.1 limits (whole-body SAR 0.08 W/kg public, 0.4 W/kg workers; local 2 W/kg public). Calculate compliance distances for every antenna site.
2. Distance and exclusion zones: power density falls as 1/r²; fence, mark and restrict areas in front of antennas; mount antennas high with the beam above people.
3. Power-down and lock-out: switch off or reduce transmitter power before working on towers, antennas or waveguides; never look into a live waveguide or horn.
4. Shielding and equipment integrity: use metal enclosures, RF absorbers and screened rooms; check oven seals and waveguide flanges for leakage.
5. Monitoring, signage and training: survey sites with calibrated field meters, use personal RF monitors, display warning signs and train workers; limit exposure time.

Reactive near, radiating near and far fields

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ

Example: a 0.5 m antenna at 2.6 GHz (λ = 0.115 m) has R1 = 0.65 m and R2 = 4.33 m.

The space around a radiating antenna is divided into three field regions, depending on distance r, antenna size D and wavelength λ:

 Antenna  |<-- R1 -->|<------- R2 ------->|  far field
   [D]    | reactive |  radiating near    |  (Fraunhofer)
          | near     |  (Fresnel)         |
R1 = 0.62√(D³/λ)       R2 = 2D²/λ

• Reactive near field: closest to the antenna; energy is stored and exchanged between antenna and space. E and H are not in phase, so both must be measured. Fields can be very strong, giving high local SAR.
• Radiating near field (Fresnel region): radiation dominates, but the angular pattern still depends on distance; power density varies irregularly.
• Far field (Fraunhofer region): the wave is nearly plane, E ⊥ H, E/H = 377 Ω, pattern is fixed, and S = PtGt/(4πr²) = E²/377.

Example: GSM 900 MHz panel, D = 2 m, λ = 0.333 m: R1 = 3.04 m, R2 = 24 m.

For safety, exposure limits are checked with power density in the far field, while near-field zones are usually made exclusion zones for workers only.