Antenna Near Field & Far Field
When a radio antenna is excited with an electrical signal, there will be two distinct types of field generated: the near field and the far field.
Antenna Basics Includes:
Basic antenna theory Polarisation Antenna near & far fields Resonance & bandwidth Gain & directivity Feed impedance Antenna matching techniques
Signals that are applied to an antenna will generate field associated with the signal. These fields are electric fields associated with the potential on the antenna and magnetic associated with the current flowing in it.
However the characteristics of these fields change with the distance from the antenna and accordingly they are termed the near field and far field.
The near field and the far field have different characteristics, so when dealing with antennas and any measurements, it is necessary to understand the differences between them.
The two fields react differently, so it is essential to understand the difference between the near field and far field of an antenna and what this might mean in practice.
Understanding the way these fields react as well as their properties and their relationship to an antenna help significantly with an understanding of their operation for everything from reception to broadcasting, radio communications, mobile communications, EMC measurements, and much more.
Antennas and associated fields
When looking at the antenna near field and far field and their differences, it is first necessary to see how the different fields arise and what they are.
When a radio frequency signal is applied to any form of antenna, electric and magnetic fields are created.
The voltage element of the signal applied to an antenna gives rise to an electric field (often designated the E field). But current also flows in the antenna conductor and this current gives rise to a magnetic field (often designated the H field). This is just as one would expect from a basic understanding of the effects of a potential on a wire and a current flowing thought it.
However the electric and magnetic fields interact with each other, forming the electromagnetic field associated with the antenna.
Looking at these fields in a little more detail we find that the local E and H fields rise and flow with the frequency of the voltage and current in the antenna.
It is actually found that the near fields in the immediate vicinity of the antenna are 90 degrees out of phase with each other and as a result the net energy transfer as a result of these fields is zero.
Also these fields decay relatively rapidly with distance from the antenna.
However these local E and H fields become more important as the distance moves away from the antenna. These fields give rise to another form of field. They create new in-phase E and H fields which are the electromagnetic or in this case the radio wave that travels further out from the antenna.
As the electronic (E) field and magnetic (H) field for the electromagnetic waveform are in-phase, this waveform carries power with it. Also as the E and H fields are in phase and carry power, the electromagnetic wave propagates away from the antenna and decays far less rapidly than the original E and H fields that exist close to the antenna.
Near and far fields
It can be seen that the basic electric and magnetic fields exist close to the antenna, decaying rapidly with the distance away from the antenna. Also the electromagnetic wave travels much further away from the antenna, decaying more slowly.
Accordingly there are varying regions that are often mentioned where the different fields dominate, etc.
To complicate matters, the definitions of these regions sometimes vary according to the particular application and also antennas that could be termed 'short' antennas also behave slightly differently. As such different definitions and approaches may be used in some areas.
the aim here is to give an overview on which further research and understanding can be based.
Near field: As might be expected the antenna near field region is the region close to the antenna. It may also be referred to as the reactive near field region.
In this region, the electric and magnetic fields generated by the potential and current flow dominate. As mentioned, these fields are 90° out of phase with each other.
Transition region: This is the region between the near and far field regions where neither type of field dominates and there is a transition from one to the other. It may also be referred to as the radiative near field region.
Far field: As the name indicates, the far field region is the region beyond the transition region where the local electric and magnetic fields have decayed to a point where they are negligible and can be ignored and the electromagnetic wave dominates and is the only detectable form of field.
Antenna near field region
As the name suggests the near field region is closest to the antenna and it may be called the near field region or the reactive near-field region.
It is in this region that the electric and magnetic fields dominate and it is also characterised by the fact that the electric and magnetic fields are displaced by 90° from each other. This turns the field reactive as there is not resistance and no power transmission or loss.
The radiative property of the electromagnetic fields in the reactive near-field region is comparatively less, although it does exist.
The electric and magnetic fields are strongest in the reactive fields and separate measurements of the electric and magnetic fields separately is possible.
However, depending on the antenna type, one field dominates over the other in the near-field region. For example for a loop antenna, the magnetic field dominates in this region - a factor worth remembering when using a loop antenna.
There are several definitions of how far the reactive near field extends, dependent upon the application, antenna and the like.
Often the equation for the extent of the near field region is defined as:
It is worth remembering that within the near field region, coupling to conductive structures like power lines, internal plumbing, metal drain pipes and the like will be much greater than that resulting from the far field.
This can not only result in changes to the feed impedance and radiation pattern of the antenna, but it can also mean that significantly greater levels of interference may result. This is one of the reasons why it is always best to use external antennas, and ones that can be located remotely from conductive objects that may give rise to unwanted effects.
It can also help to understand this when making EMC measurements as the different fields react differently and will give rise to different forms of EMC issues.
Antenna transition zone
Moving out from the reactive near field zone, there is a transition from the dominance of the reactive nature of the separate electric and magnetic fields to the radiative nature of the far field.
Here there is a transition to the electromagnetic field where there is zero displacement between the electric and magnetic fields and hence it is able to carry power and travel further.
This transition region may also be called the radiative near field region or the Fresnel region.
The region can be mathematically designated as existing in the following region.
R is the region of the transition zone
L is the length of the antenna
Antenna far field region
The far field region is also called the Fraunhofer region and it is the region furthest from the antenna beyond the transition region.
This region is dominated by radiated electromagnetic fields, the electric and magnetic fields having fallen to negligible levels.
This inner border of this region can be mathematically expressed as an equation or formula as follows:
This may sometimes thought of as the antenna far field equation as it allows what might be thought of as the start of the region where it is safe to assume that only the far field effect fromt he antenna exists.
It is within the far field region where the radiation pattern of an antenna can be measured because it is largely independent of the distance from the antenna. The radiation follows the normal 1/d2 pattern, but over a sufficient distance any minor variations will not be significant as the antenna is rotated.
If the strength of the fields in this region require to be calculated, then a formula known as the Friis formula is used.
Understanding the importance of the near field and far field on the antenna operation gives a much greater understanding of the operation of antennas, especially when they are used for measurements, or when nearby objects may have affect their operation for reception, broadcast, two way radio communications, mobile communications, EMC measurements, etc. The near field and far field aspects of their operation can have a major impact on the way they work ad their effectiveness in a particular situation.
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