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EMF measurements are measurements of ambient (surrounding) electromagnetic fields that are performed using particular sensors or probes, such as EMF meters. 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 as possible in order to obtain precise results. There are two main types of EMF measurements: *''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 detectors; *''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. EMF probes may respond to fields only on one axis, or may be tri-axial, showing components of the field in three directions at once. Amplified, active, probes can improve measurement precision and sensitivity but their active components may limit their speed of response. ==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 the 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 . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「EMF measurement」の詳細全文を読む スポンサード リンク
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