---
title: "Short note: Microwave radiation fields"
subject: "RF and Microwave Engineering"
chapter: 7
key: "semester_7/rf_and_microwave_engineering:microwave-radiation-fields-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwave-radiation-fields-note
site: BCT Engineering
---

# Short note: Microwave radiation fields

RF and Microwave Engineering · Chapter 7: Microwave Antennas and Propagation

- Asked in: 2071 Magh, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/microwave-radiation-fields-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-7

## Question

Write a short note on microwave radiation fields.

## Short answer

Space around an antenna is divided into reactive near field (r < 0.62 sqrt(D^3/lambda)), radiating near field (Fresnel) from that to 2D^2/lambda, and far field beyond 2D^2/lambda. Far field is a plane wave with E/H = 377 ohms and S = EIRP/4 pi r^2. Example D=2 m, lambda=0.333 m gives R1=3.04 m, R2=24 m.

## Answer

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

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

| Region | Range | Features |
|---|---|---|
| Reactive near field | r < 0.62√(D³/λ) | Stored energy, E and H not in phase, very high fields |
| Radiating near field (Fresnel) | 0.62√(D³/λ) to 2D²/λ | Pattern changes with distance, fields not simple |
| Far field (Fraunhofer) | r > 2D²/λ | Plane wave, E/H = 377 Ω, S = EIRP/4πr² |

- **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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the space around a radiating antenna divided?
   A. Reactive near field, radiating near (Fresnel) field, far (Fraunhofer) field
   B. Static, dynamic and quantum fields
   C. Only near field and far field
   D. Transmit, receive and reflect regions
   Answer: A. The regions depend on distance r, antenna size D and wavelength lambda. (Key point: Three regions)

2. What characterises the radiating near (Fresnel) field?
   A. Radiation dominates, but the pattern still depends on distance and power density varies irregularly
   B. Only stored energy with no radiation
   C. A fixed pattern and E/H = 377 ohm
   D. Zero fields
   Answer: A. Radiation dominates but the angular pattern has not yet become fixed. (Key point: Radiating near field)

3. In the far field, how are S and E related?
   A. S = PtGt/(4 pi r^2) = E^2/377
   B. S = E x 377
   C. S = E^2 x 377
   D. S = PtGt/r
   Answer: A. The wave is plane with E/H = 377 ohm, so S = E^2/377 and S falls as 1/r^2. (Key point: Far-field relations)

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