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Near and far field

Near and far field is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Near and far field rather than just read about it. In short: The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative near-field behaviors dominate close to the antenna or scatterer, while electromagnetic radiation far-field behaviors predominate at greater distances.

Near and far field — main illustration
Near and far field — illustration

Key takeaways

  • Near and far field belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Near and far field to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Near and far field from memory before moving on to harder problems.

Reference excerpt

The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative near-field behaviors dominate close to the antenna or scatterer, while electromagnetic radiation far-field behaviors predominate at greater distances. Far-field E (electric) and B (magnetic) radiation field strengths decrease as the distance from the source increases, resulting in an inverse-square law for the power intensity of electromagnetic radiation in the transmitted signal. By contrast, the near-field's E and B strengths decrease more rapidly with distance: The radiative field decreases by the inverse-distance squared, the reactive field by an inverse-cube law, resulting in a diminished power in the parts of the electric field by an inverse fourth-power and sixth-power, respectively. The rapid drop in power contained in the near-field ensures that effects due to the near-field essentially vanish a few wavelengths away from the radiating part of the antenna, and conversely ensure that at distances a small fraction of a wavelength from the antenna, the near-field effects overwhelm the radiating far-field.

Introduction

The distinction between near-field and radiated field can be illustrated by the Panofsky–Phillips equations, which give the general expression of the electric and magnetic fields produced by an arbitrary distribution of charges and currents. The terms of the Panofsky–Phillips equations can be separated into two categories: The "near field" terms have a 1 / r 2 {\displaystyle 1/r^{2}} dependence, and are significant only in the immediate vicinity of the sources. The "far field" terms have a 1 / r {\displaystyle 1/r} dependence. Due to their slower decrease with distance, they dominate as one moves away from the sources.

… excerpt ends here. Continue reading the full article.

Illustrations

Near and far field: Order of regions of an electromagnetic field emitted from an antenna: Inner, reactive near field, and outer, radiative near field (Fresnel diffraction), and lastly far field (Fraunhofer diffraction).
Order of regions of an electromagnetic field emitted from an antenna: Inner, reactive near field, and outer, radiative near field (Fresnel diffraction), and lastly far field (Fraunhofer diffraction).
Near and far field illustration
Near and far field: Near field: This dipole pattern shows a magnetic field B in red. The potential energy momentarily stored in this magnetic field is indicative of the reactive near field.
Near field: This dipole pattern shows a magnetic field B in red. The potential energy momentarily stored in this magnetic field is indicative of the reactive near field.
Near and far field: Far field: The radiation pattern can extend into the far field, where the reactive stored energy has no significant presence.
Far field: The radiation pattern can extend into the far field, where the reactive stored energy has no significant presence.
Near and far field: Field regions for antennas equal to, or shorter than, one-half wavelength of the radiation they emit, such as the whip antenna of a citizen's band radio, or an AM radio broadcast tower.
Field regions for antennas equal to, or shorter than, one-half wavelength of the radiation they emit, such as the whip antenna of a citizen's band radio, or an AM radio broadcast tower.

Worked examples

Example 1 — a first encounter with Near and far field

Start with the simplest possible case. Write down what Near and far field claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Near and far field before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Near and far field ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Near and far field

In research
Near and far field appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Near and far field in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Near and far field is common in secondary-school and first-year university syllabi. It links to neighbouring topics Antennas, Scattering, absorption and radiative transfer (optics), so understanding it makes those chapters shorter.
In everyday life
Look for Near and far field outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Near and far field in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Near and far field means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Near and far field out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Near and far field in simple terms?

The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative near-field behaviors dominate close to the antenna or scatterer, while electromagnetic radiation far-field…

Why does Near and far field matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Near and far field?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Near and far field.

Tags

  • Antennas
  • Scattering, absorption and radiative transfer (optics)

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