Standard designation: National Standard of electromagnetic field strength
Internal identifier: ECM 240-5/03-024
Year of approval: 2003
Department: Primary metrology of RF electrical quantities 1013, Regional branch Prague
Person in charge: Ing. Karel Dražil, Ing. František Hejsek (2001 – 2008)
Number of CMC rows: 3
Basic metrological parameters of the standard:
Th electromagnetic field with calculable intensity is generated by means of:
- rectangular TEM cell, frequency range up to 250 MHz
- distance of the wall from the septum 250 mm
- field strength up to 140 V/m
- field probe calibration uncertainty (0.5 – 1) dB (k = 2)
- rectangular TEM cell, frequency range up to 1 GHz
- distance of the wall from the septum 35 mm
- field strength up to 60 V/m
- miniature field probe calibration uncertainty 0.5 dB (k = 2)
- pyramidal TEM cell, frequency range 300 MHz to 3 GHz
- distance of the wall from the septum approx. 200 mm
- field strength up to 50 V/m
- field probe calibration uncertainty 1 dB (k = 2)
- rectangular waveguides R14 and R22, frequency range 1 GHz to 2.5 GHz
- field strength up to 200 V/m
- field probe calibration uncertainty 0.4 dB (k = 2)
- anechoic chamber, frequency range (1 – 18) GHz
- field strength up to 100 V/m
- field probe calibration uncertainty 0.8 dB (k = 2)
Precise knowledge of electromagnetic field strength plays an important role in electromagnetic compatibility (EMC) measurements, telecommunications and health protection. General principle of a traceable field strength measurement is the use of calculable structures, i.e., the field strength can be calculated in dependence on geometrical dimensions, RF power and in some cases on frequency. Traceability to basic SI units is achieved through measurements of RF power and length, or frequency, respectively.
The national standard comprises three transmission lines with a transversal electromagnetic wave (TEM cell) with different geometrical dimensions and construction, and sections of R14 and R22 waveguides. All above mentioned standards have been fabricated based on own designs. The transmission lines are terminated by well impedance matched power sensors or by absorbing materials. Other parts of the standard comprise signal generators, power amplifiers and RF power meters.
The field strength in the TEM cells is calculated as follows
field strength in the TEM cell,
power passing through the TEM cell,
nominal characteristic impedance of the TEM cell (50 Ω),
distance betwee the TEM cell wall and the septum.
Field strength in a rectangular waveguide is calculated as
field strength in the waveguide,
power of the incident wave,
waveguide width (longer dimension),
waveguide height (shorter dimension),
speed of light in vacuum,
In 2010, an anechoic chamber for field strength measurements up to 60 GHz was acquired. Currently measurements up to 18 GHz are performed.
The field strength E in the anechoic chamber can be calculated as (provided the distance d is sufficient)
power delivered to the antenna,
transmitting antenna gain,
distance between the transmitting antenna and the point with field strength E.
Tha national standard has successfully participated in one KCDB key comparison (measurement in TEM cells) and one KCDB supplement comparison (measurement in rectangular waveguides). Final reports for two other key comparisons (measurement in the anechoic chamber) are currently under preparation.