Far-field near-field – Atec Narda-EMR-20-30 User Manual

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in the cost of regular re-calibration, which is recommended for all

field measuring instruments.

Fields of application

The diagram shows some typical applications where electro-

magnetic radiation occurs or is utilized. The frequency spectrum

is normally divided into two areas:
1: Low frequencies up to about 30 kHz.

This region includes some railway system overhead power

supplies running at 16

Hz, domestic a.c. power at 50/60 Hz

and extends up to VDU workstations at 30 kHz (see EFA data

sheets).

2: High frequencies above 30 kHz.

Typical frequencies encountered here are FM radio (88 to 108

MHz), television signals (40 to 900 MHz), mobile radio (400 to

1800 MHz) and satellite communications (up to 18 GHz). Other

frequencies which are often used in industry and medicine

are 27, 433 and 2450 MHz. Knowledge of the frequency is

important when monitoring limit values for electromagnetic

fields because these limit values depend on the frequency.

Frequency ranges of electromagnetic radiation encountered

in everyday life.

Limit values

Work on defining legally binding limit values for electromagnetic

radiation is currently being done at national and international

levels. The limit values specified in the draft CENELEC European

standard are quoted here as an example.

Limit values for common industrial and medical frequencies,

derived from the above-mentioned draft standard:

27 MHz

433 MHz

2.45 GHz

Workplace

61.4 V/m

0.16 A/m

10 W/m

63 V/m

0.17 A/m

11 W/m

137 V/m

0.36 A/m

50 W/m

Public areas

27.5 V/m

0.07 A/m

2 W/m

28 V/m

0.08 A/m

2.2 W/m

61.4 V/m

0.16 A/m

10 W/m

Near-field and far-field

Electromagnetic fields can be split into two components: the elec-

tric field E [measured in V/m] and the magnetic field H [measured

in A/m]. The E-field and H-field are strongly interdependent for the

far-field, i.e. anywhere more than a certain distance from the

source (see diagram). If, say, the H-field is measured in this

region, the magnitude of the E-field and the power density S

[W/m] can be calculated from it. In contrast, the H-field and E-field

must be measured separately in the near-field region.

Near-field and far-field definition. Measurements at a

distance d of 1 x wavelength ll (better: 3 x ll) from the source

are made under far-field conditions.

Applications and tips

± Induction heaters, RF welding equipment and erosion

machines: Electric fields are less important here, the magnetic

fields need to be monitored. Use EMR-10/EMR-10C Magnetic

Field Meter

± Radio and TV transmitters/antennas: As long as the location

is in the far-field region, an E-field sensor is preferable due to

the large bandwidth (EMR-20/EMR-30). When working close

to antennas (near-field) separate checking of the E-field and

the H-field is unavoidable (use EMR-20/EMR-30 for E-field,

EMR-10 for H-field)

± Diathermy equipment (RF equipment for medical therapy):

Very high field strengths are present at the electrodes and on

the connecting leads to the electrodes. The main component

is normally the electric field (use EMR-20/EMR-30).

± Microwave ovens: The very short wavelength means that

exposure is normally in the far-field. E-field measurements are

therefore sufficient (use EMR-20/EMR-30)

3 Limit values for electromagnetic radiation.

Further details are found in the draft European standard

CENELEC 50166-2.

Radio waves

Industry

Microwaves

Television

Cellular radio

Satellite radio

and

medicine

line

voltage

Long

wave

Medium wave

Short

wave

VSW/VHF

UHF

Centimeter waves

(EHF)

Millimeter waves

(SHF)

Power density,

workplace [W/m

]

Power density,

public areas [W/m

]

H-field,

workplace [A/m]

H-field,

public areas [A/m]

0.1

E-field, workplace [V/m]

E-field,

public areas [V/m]

Far-field

Near-field