EMF measurements
Encyclopedia
EMF measurements are measurements of ambient (surrounding) electromagnetic fields that are taken with particular sensors or probes, such as EMF meter
s. These probes can be generally considered as antennas
although with different characteristics. In fact probes should not perturb the electromagnetic field and must prevent coupling and reflection as much possible in order to obtain a precise measure. EMF measurements are nowadays becoming important and widespread in different sectors to assess environmental and human exposure to non-ionizing radiation
in many contexts.
There are two main EMF measurements types:
Moreover there are different types of probes in relation to what is measured and to what the probe is made by.
A mono-axial, omnidirectional probe is a device which senses the Electric (short dipole) or Magnetic field linearly polarized in a given direction.
Using a mono-axial probe implies the need for three measurements taken with the sensor axis set up along three mutually orthogonal directions, in a X, Y, Z configuration.
As an example, it can be used a probe which senses the Electric field component parallel to the direction of its axis of symmetry. In these conditions, where E is the amplitude of incident electric field, and θ is the amplitude of the angle between sensor axis and direction of electric field E, the signal detected is proportional to |E|cos θ (right). This allows to obtain the correct total amplitude of the field in the form of
or, in case of Magnetic field
An isotropic
(tri-axial) probe simplifies the measurement procedure because the total field value is determined with three measures taken without changing sensor position: this results from the geometry of the device which is made by three independent broadband sensing elements placed orthogonal to each other. In practice, each element’s output is measured in three consecutive time intervals supposing field components being time stationary .
In fact, a passive receiving antenna collects energy from the electromagnetic field being measured and makes it available at a RF cable connector. This signal then goes to the spectrum analyzer but the field characteristics can be someway modified by the presence of the cable, especially in near-field
conditions.
On the other hand an effective solution is to transfer on an optical carrier, the electric (or magnetic) field component sensed with an active probe.
The basic components of the system are a receiving electro-optical antenna which is able to transfer, on an optical carrier, the individual electric (or magnetic) field component picked up and to return it in the form of an electrical signal at the output port of an opto-electric converter.
The modulated optical carrier is transferred by means of a fiber-optic link to a converter which extracts the modulating signal and converts it back to an electrical signal.
The electrical signal thus obtained can be then sent to a spectrum analyzer with a 50 Ω common RF cable.
If the field is obtained by three measurements in an orthogonal X, Y, Z configuration in the form:
a sufficient condition for the expression to be true for every three orthogonal coordinates (X,Y,Z) is for the probe radiation pattern
to be as close as possible to ideal short dipole pattern, called sin θ:
,
where A is function of frequency.
The difference between ideal dipole radiation pattern and real probe pattern is called isotropic deviation.
EMF Meter
An EMF meter is a scientific instrument for measuring electromagnetic fields . Most meters measure the electromagnetic radiation flux density or the change in an electromagnetic field over time , essentially the same as a radio antenna, but with quite different detection...
s. These probes can be generally considered as antennas
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
although with different characteristics. In fact probes should not perturb the electromagnetic field and must prevent coupling and reflection as much possible in order to obtain a precise measure. EMF measurements are nowadays becoming important and widespread in different sectors to assess environmental and human exposure to non-ionizing radiation
Non-ionizing radiation
Non-ionizing radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule...
in many contexts.
There are two main EMF measurements types:
- broadband measurements performed using a broadband probe, that is a device which senses any signal across a wide range of frequencies and is usually made with three independent diode detectorsDetector (radio)A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
; - frequency selective measurements in which the measurement system consists of a field antenna and a frequency selective receiver or spectrum analyzer allowing to monitor the frequency range of interest.
Moreover there are different types of probes in relation to what is measured and to what the probe is made by.
Ideal isotropic measurements
Measurements of the EMF are obtained using an E-field sensor or H-field sensor which can be isotropic or mono-axial, active or passive.A mono-axial, omnidirectional probe is a device which senses the Electric (short dipole) or Magnetic field linearly polarized in a given direction.
Using a mono-axial probe implies the need for three measurements taken with the sensor axis set up along three mutually orthogonal directions, in a X, Y, Z configuration.
As an example, it can be used a probe which senses the Electric field component parallel to the direction of its axis of symmetry. In these conditions, where E is the amplitude of incident electric field, and θ is the amplitude of the angle between sensor axis and direction of electric field E, the signal detected is proportional to |E|cos θ (right). This allows to obtain the correct total amplitude of the field in the form of
or, in case of Magnetic field
An isotropic
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...
(tri-axial) probe simplifies the measurement procedure because the total field value is determined with three measures taken without changing sensor position: this results from the geometry of the device which is made by three independent broadband sensing elements placed orthogonal to each other. In practice, each element’s output is measured in three consecutive time intervals supposing field components being time stationary .
Active and passive sensors
Active sensors are sensing devices which contain active components; usually this solution allows for a more precise measurement with respect to passive components.In fact, a passive receiving antenna collects energy from the electromagnetic field being measured and makes it available at a RF cable connector. This signal then goes to the spectrum analyzer but the field characteristics can be someway modified by the presence of the cable, especially in near-field
Near and far field
The near field and far field and the transition zone are regions of the electromagnetic radiation field that emanates from a transmitting antenna, or as a result of radiation scattering off an object...
conditions.
On the other hand an effective solution is to transfer on an optical carrier, the electric (or magnetic) field component sensed with an active probe.
The basic components of the system are a receiving electro-optical antenna which is able to transfer, on an optical carrier, the individual electric (or magnetic) field component picked up and to return it in the form of an electrical signal at the output port of an opto-electric converter.
The modulated optical carrier is transferred by means of a fiber-optic link to a converter which extracts the modulating signal and converts it back to an electrical signal.
The electrical signal thus obtained can be then sent to a spectrum analyzer with a 50 Ω common RF cable.
Isotropic deviation
Isotropic deviation, in EMF measurements, is a parameter that describes the accuracy in measuring field intensities irrespective of the probe’s orientation.If the field is obtained by three measurements in an orthogonal X, Y, Z configuration in the form:
a sufficient condition for the expression to be true for every three orthogonal coordinates (X,Y,Z) is for the probe radiation pattern
Radiation pattern
In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....
to be as close as possible to ideal short dipole pattern, called sin θ:
,where A is function of frequency.
The difference between ideal dipole radiation pattern and real probe pattern is called isotropic deviation.
External links
- Clampco Sistemi Isotropic antenna AT3000
- Clampco Sistemi Electric field probe FP2000
- Clampco Sistemi EMF measurement antennas
- EMC Zurich