Communication and Network, 2010, 2, 65-68
doi:10.4236/cn.2010.21010 Published Online February 2010 (
Copyright © 2010 SciRes CN
How to Measure in the Near Field and
in the Far Field
Tomasz Dlugosz, Hubert Trzaska
Wroclaw University of Technology Institute of Telecommunications, Teleinformatics and Acoustics,
Wyspianskiego, Wroclaw, Poland
Received November 5, 2009; accepted December 24, 2009
Abstract: A background of the electromagnetic field (EMF) measurements is presented in the work. A spe-
cial attention is given to the specificity of the measurements performed in the Near Field. Factors, that should
be taken into consideration as during the measurements as well during their analysis, are discussed. Without
their understanding and considering a comparison of the measurements’ results, meters’ calibration and EMF
standards comparison between different centers is impossible.
Keywords: electromagnetic fields measurements, the near field, the far field
1. Introduction
Surfing on the World Wide Web, when in one of the most
popular browsers we enter the words: “electromagnetic
field” (EMF), we obtain over 1.5 million answers. In
various libraries we also can find a few hundred thou-
sand documents, publications and books pertaining to
EMF measurements. It would seem that one more publi-
cation on this subject is superfluous, but experience
shows something totally different. In reality, in many
cases the manner in which EMF measurements are per-
formed is an affront to any forms of correctness and has
nothing to do with accuracy and engineering diligence.
Even people familiar with this domain forget about some
conditions which have to be met in order to carry out
EMF measurement correctly, which means, with the re-
quired accuracy [4]. A good example that there is no un-
derstanding of the EMF metrology fundamentals is the
EMF measurement in a room with the use of a log-peri-
odic antenna, described in [5]. You can wonder what in
fact has been measured?
Why is EMF metrology so important? The answer is
relatively simple, because it consitutes a sine qua non
condition of the activities associated with protection of
electromagnetic environment, as well as of fundamental
research, especially research on EMF impact on the ani-
mate matter, in particular, on human beings. Such re-
search is an initial step leading to determination of pro-
tective regulations, pertaining both to the safety of work
as well as protection of the general population. As an
interesting side note, we shall remind here that in spite of
the poor EMF measurement accuracy and even lesser
accuracy of biomedical research based on them, the pro-
tection standards are determined with an amazing accu-
racy. And the EMF metrology is not counted among the
easiest and the most accurate. If the achievable accuracy
in the far field amounts to 1 dB, in the near field it is
only 3 dB, and even 6 dB! This fact shows that the exist-
ing measurement methods need to be analysed and their
accuracy increased and that new measurement tech-
niques should be pursued, e.g. photonic sensors [1].
2. Is It Still the Near Field or Already the Far
Prior to discussing the differences existing in EMF me-
trology in the near and in the far field, meaning of these
notions should be defined. What does “the near field”
mean? The authors propose two new definitions. The
first one, more general and less rigorous, can be as fol-
lows: the near field is the field surrounding primary and
secondary radiation sources where measurement accu-
racy is limited (e.g.) to 5 %, as compared with the far
field. The second definition is more demanding: the near
field exists everywhere where we carry out measure-
ments. This definition results from the experience and it
refers to measurements in urbanized areas where multi-
path propagation may occur and we have to do with in-
terference and reflections – sometimes reflected rays can
be stronger than the direct ray. This shows that it is nec-
essary to act with due caution even during measurements
in the far field, where directional antennas are used,
which may not “catch” all transmitted rays. And here we
encounter a paradox – a correctly calibrated meter does
not ensure the expected measurement accuracy.
In the traditional approach (Figure 1), in order to dis-
Copyright © 2010 SciRes CN
Figure 1. Near and Far Field around an antenna
tinguish the specificity of measurements in the near field
and in the far field, a criterion was adopted which en-
ables delimitation of these two areas, although there is no
clearly defined and discrete boundary between the near
field and the far field [8].
If D is adopted as the largest antenna size and the
emitted wave length is designated, the boundary (R) be-
tween the near zone and the far zone can be determined
from the following relationship:
R (1)
In order to demonstrate that the far zone can be the
same for different types of antennas, operating on dif-
ferent frequencies, two examples will be given (Trzaska,
Example No. 1:
For a parabolic antenna with 3 m dish diameter oper-
ating on 10 GHz frequency the far field boundary is at
600 m,
Example No. 2:
For the antenna of the former transmission centre in
Gąbin (Poland) having a height of 0.5 λ and operating on
227 kHz frequency the Far Field boundary is at 660 m.
As you can see, the Far Field zone is not something
assigned permanently to a given antenna operating on the
preset frequency. As the above two examples show, the
same far zone boundary exists for extremely different
antennas. Also the Near Field can be a function of elec-
trical size [2].
3. Measured Quantities
In the Near Field, the mutual relationship between elec-
tric field (E) and magnetic field (H) components depends
on the type of EMF source and on the distance between
the source and the observation point. Therefore, deter-
mination of one of them is not sufficient for computing
the other.
Situation is different in the case of the far field where
knowledge of one of the field components, e.g. of elec-
tric field vector – E, enables determination of the other
(magnetic field vector – H), using the relationship in
which these two quantities are interrelated by means of
the impedance of free space (Z):
In both cases, i.e. in the near and in the far field, when
we know the E and H components, we are in a position
to determine the power density. With this aim, the mean
value of the Poynting vector (S) is determined:
Re5.0 (3)
In the Far Field metrology it is not necessary to carry
out an additional measurement of quantities other than
the E, H or S, contrary to the near field metrology in
which the temperature increase and current density,
caused by the EMF impact, are also measured.
Measurement of the temperature increment (ΔT), re-
sulting from the EMF impact, of a material which has a
given specific heat (cw), makes it possible to determine
the Specific Absorption Rate SAR:
The SAR is commonly used for examination of the
EMF impact on human body. However, there are some
limitations of its use, which are discussed in detail in [8].
In this paper we shall only note that the SAR parameter
can be used for the frequencies higher than 300 MHz due
to too small sensitivity. In the lower frequency ranges an
essential parameter is the density of the current induced
into tissues [7]. Knowing the conductivity (σ) of the ex-
amined medium and the density value of electric field (E)
existing in this medium, the current density (J) can be
The manner of measurements of the current flowing
through a human body is described in [7]. Often meas-
urements of the currents flowing through legs or feet are
presented, neglecting the currents appearing in other
parts of the body, or unmeasurable eddy currents.
For electric field measurements in the near field elec-
trically-short dipole antennas are used, while magnetic
field is measured by means of small frame antennas. In
the far field directional antennas are used. An essential
problem faced in EMF measurements, regardless of what
sensor is used, is the sensor’s presence in the measured
field, which causes deformation of this field and mutual
interaction between the sensor and the neighbouring ma-
terial objects. This interaction constitutes a serious factor
affecting the measurement accuracy, both during EMF
measurements and EMF sensor calibration, as well as in
cases when we use exposure kits for examination of the
Copyright © 2010 SciRes CN
features of any material object [6].
In the measurements polarization is also important.
After all, the E, H and S vectors can have three spatial
components each (quasi-ellipsoidal polarization caused,
for example, by rotation of the polarization plane in
space). In such a case isotropic sensors have to be used.
4. Measurement Accuracy
Measurement accuracy constitutes the biggest problem in
EMF measurements. For calibration of EMF meters, as
well as for examination of equipment and matter sensi-
tivity to EMF impact, EMFs of known parameters are
used – standard EMFs. For generation of a standard EMF
knowledge of not only the values of generated parame-
ters is necessary, but also of the accuracies of their gen-
eration. EMF standards are among the least accurate as
compared with the standards of other physical quantities.
Many of such quantities are determined with an accuracy
of 10-10 % or higher, while the error of standard EMFs
generation in renowned centres ranges from 5 % to 10 %.
In other words, even before we commence field meas-
urements, from the very beginning the measurement re-
sult is burdened with an error which amounts to 5 % in
the best case, and this is not all.
The main factor which limits EMF measurement ac-
curacy in the near field is the antenna dimensions. Point
antennas would be the best to use, because otherwise an
antenna causes averaging of the measured EMF values.
Variations of the spatial field strength, resulting from
either amplitude or phase variations, are subject to aver-
aging. These variations depend on the curvature of the
EMF field which surrounds the source [3,8]. Some ex-
amples of error graphs, both amplitude and phase errors,
are shown on Figures 2 and 3 (where: Ro – the distance
between the source and the measuring antenna centre, α
– an exponent characterizing field curvature, h – the
length of dipole arm, k – propagation constant). The pre-
sented curves refer to a dipole antenna but identical con-
siderations are applicable to a frame antenna as well [8].
Passing over the impact of the meter used and of the
person performing the measurements on the disturbances
of the measured EMF, you should not forget the error
Figure 2. Amplitude error δ A
Figure 3. Phase error δ f
which is contributed by the measuring person, which we
shall call a “human factor”. This factor also depends on
the conditions in which measurements are performed and
its importance is essential, as it is shown in [4]. This fac-
tor is described on the basis of two measurement series,
performed by four persons in the same measuring points,
by means of two meters: MEH-25 with 3AS-1 probe and
PMM 8053A with EP-300 probe. This simple experiment
has shown (see Table 1) how diversified the measure-
ment results can be if the measurements are performed
by different persons. Therefore, the “human factor” is a
gross error but, unfortunately, it is not taken into account
when measurement results are worked out.
Table 1. “Human factor” measurement results [4]
Series I Series II
Position of
measurements 1 2 3 4
1 2 3 4
1 14.9 16.5 17.6 15.5 16.1 8.3 16.6 15.8 14.2 16.9 15.9 8.7
2 17.6 16.2 18.5 19.0 17.8 8.0 16.5 16.5 19.2 18.5 17.7 7.6
3 9.2 7.3 6.6 8.8 8.0 16.5 5.8 8.8 8.2 6.1 7.2 20.5
4 9.1 8.2 10.2 8.2 8.9 10.9 9.6 10.4 9.9 8.6 9.6 9.5
5 9.9 10.2 11.6 11.0 10.7 7.9 11.0 10.4 14.0 10.4 11.5 14.8
Copyright © 2010 SciRes CN
Table 2. Comparison of measurements in the near field and in the far field
Parameter Near Field Far Field
measured EMF component E, H & S E or H, and
S on mwaves
other magnitudes
I, T, (SA, SAR)
“HESTIA” unnecessary
spatial components 3 1 or 2
polarization quasi-ellipsoidal linear or elliptical
complex, multipath propagation & inter-
usually simple
frequency spectrum wide, often unknown, many fringes usually single frequency
antennas small, omnidirectional resonant, directional
temporal & spatial EMF alternations significant usually negligible
uncertainty 3, 6 or more dB around 1 dB
temperature sensitivity significant unessential
susceptibility significant ommitable
influence of surroundings significant usually ommitable
procedures complex simple
agreement with theory reasonable good
measured levels V/m, kV/m mV/m, mV/m
5. Summary
The paper presents a comparative analysis of EMF me-
trology in the near field and in the far field. Measure-
ments in the near field are more difficult and burdened
with a considerably larger error than measurements per-
formed in the far field. As you can see there are many
factors which have an impact on measurement accuracy
and the selection of a measurement zone should involve
proper selection of adequate tools and measurement
It is not feasible to present all aspects of EMF meas-
urements in the near field and in the far field. Due to
practical limitations of this paper only most important
aspects of this metrology are discussed herein, supple-
mented by Table 2.
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