EMF measurement

EMF measurements are measurements of the electromagnetic fields with probes or sensors. Even if they are produced, having different characteristics, these probes can be generally regarded as a normal antenna. To obtain accurate and reliable measurements, the probes do not disturb the other hand, avoid couplings and reflections on the one hand, the electromagnetic field. The EMF measurements are becoming increasingly important and is used in countless contexts to the non- ionizing radiation, the people and the environment are exposed to determine exactly. According to a first classification can distinguish between two different types of EMF measurements:

• Broadband measurements, which are performed with a broadband sensor, which can identify the signals within a wide frequency band, and normally made of three detector diodes;
• Selective measurement frequency; in this case, the measurement system consists of an antenna and a selective receiver or a spectrum analyzer, with which you can control the desired frequency band exactly.

The probes also differ according to the type of measurement that you want to perform.

Ideal isotropic measurements

In this case, the EMF measurements are obtained by a sensor of the electric (E) or magnetic (M) field, the isotropic, mono - can be axial, passive or active. An omni-directional and mono- axial sensor is a device that the electric ( in that case it is a sensor which is equipped with a short dipole ) or collects linearly polarized magnetic field in a certain position.

Since a mono- axial probe is used three different measurements must be carried out; an orthogonal for each direction in which the axis of the sensor is provided.

For example, one may use a probe, in order to raise the electric field component which is parallel to the direction of its own axis of symmetry. When referring to the size of the incident electric field and the magnitude of the θ existing between the sensing axis and the direction of the electric field in these circumstances angle E, the signal type of the | D | cos θ (right). So one can express the total size of the field in the following form:

Or the magnetic field

Since the total size of the field may be charged by three measurements, a simplified three - axial probe measurement procedure, because the position of the sensor must not be changed in this case; since the geometry of the device from three independent broadband sensors, which are placed in orthogonal directions is, one does not have to change the position of the sensor. The output of each sensing element is in three consecutive periods calculated with the assumption that the field components do not change in time.

Active and passive sensors

Active sensors are devices that contain active components; normally allow the active sensors more precise measurements than the passive. The information received from a passive antenna or passive sensor energy is forwarded to an RF connector. This signal is then measured by the spectrum analyzer. It is to be noted however that the properties of the field could change, especially in a nearby field.

A more efficient solution, however, could be that you transfer the data collected by the probe components of the electric or magnetic field to an optical fiber. Which consists of an electro-optical system receiving antenna base component, the components of the electric or magnetic field, which are then detected as an electric signal to a converter optelektronischen available to be transferred to an optical fiber.

The optically -modulated transmission means is transmitted by a connection to an optical fiber to a converter which takes out the modulating signal and converts it into an electrical signal. It is transmitted cable with an ordinary 50 Ω RF.

Isotropic deviation

In the EMF measurements, the isotropic deviation describes the accuracy of the field intensities independent of the probe orientation. If the field may be added in an X, Y, Z orthogonal configuration in the following form:

A sufficient condition to make this statement true form for each triad of orthogonal coordinates ( X, Y, Z), is obtained when the probe radiation diagram nourishes as close as possible to the short dipole diagram (?). The diagram of this short dipole can be expressed in the following form:

Where A is a function of frequency. The difference between the ideal radiation diagram of the short dipole and the real of the probe is labeled with the term " isotropic deviation".