Comparison between anechoic chamber and open area test site through analyses with loop antennas

Introduction
Electromagnetic compatibility (EMC) is the branch of electrical sciences
which studies the unintentional generation, propagation and reception of
electromagnetic energy with reference to the unwanted effects (Electromag-
netic Interference, or EMI) that such energy may induce. The goal of EMC
is the correct operation, in the same electromagnetic environment, of dif-
ferent equipment which use electromagnetic phenomena, and the avoidance
of any interference effects. A particular definition of EMC, as given in the
International Electrotechnical Vocabulary is
”The ability of a device, equipment or system to function satis-
factory in its electromagnetic environment without introducing
intolerable electromagnetic disturbance to anything in that envi-
ronment.”
The main phenomena EMC deal with are indeed electromagnetic distur-
bances. First of all the conducted low frequency phenomena, that can be
related to the power supply, in particular with its phase when it is AC. They
can be divided in harmonics and interharmonics, signalling voltages, flickers
and induced LF voltages. Also radiated LF phenomena are considered and
they are mainly related to nearby power lines, buried or overhead. Con-
ducted HF phenomena are typical of wires exposed to RF fields that induces
a disturbance depending on the cable length, its separation from the ground
reference, loops formed and any resonance effects. Radiated HF phenomena
are given by nearby transmissions and can be divided in far-field or near-
field, according to the distance. On the other hand there is the need to avoid
unexpected emission of electromagnetic fields by the equipment under test
(EUT). Indeed in most countries of the world the radio spectrum is heavily
used for many kinds of traffic. Broadcasting and telecommunications are the
most obvious uses, but telemetry, radar, radionavigation and space research
are some other purposes. Spectrum users pay a licence for the privilege of
beingallowed totransmitandreceive andinreturntheyexpect thisprivilege
to be unaffected by interfering sources. For this reasons governments have
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found it necessary to regulate the spread of type of apparatus which, though
not licenced or intended as radio transmitter have the potential to disrupt
such services. To test the equipments, a particular setup is required. The
idealsituationisanopenareatestsite(OATS).AnOATSissupposedtobea
clear environment where no reflection can modify the free space propagation
ofelectromagnetic fields. Standardsalsorequires tohave aperfectconductor
as ground. Anyway, since it is impossible to create such a condition, these
OATS have many limitations, so indoor solutions have also been introduced
in EMC.
To simulate the OATS conditions in indoor situations, shielded and ane-
choic chambers areused. Shielded chambers are particular roomswith walls,
ceiling and floor made of conductor material which is supposed to be ideal,
at least in the interest frequency range (γ ≫ ωε) and inside them no elec-
tromagnetic field coming from outside can penetrate. However inside them
generated fields arereflected by the metallic surfaces. Anechoic chambers in-
stead are shielded from externals field and furthermore the inside generated
fields are not reflected because on walls, floor and ceiling there are special
materials, such as carbon-impregnated foam shaped into pyramids, that can
absorb the electromagnetic radiations.
The site attenuation (SA) of OATS should be equal to the SA of an
anechoic chamber. However there are some differences that must be known
to evaluate fields behaviour. These differences are called chamber factors.
Chamber factors are function of frequency and polarization, and depend
on EUT physical features, such as dimensions and radiating circuit type.
Conventionallytheparametersforanechoicchamberarebasedontheelectric
field values in μV/m or dB(μV/m) and use the far-field approximation, i.e.
the observer is supposed to be at distances greater than 2D
2
/λ ,being λ the
wavelength and D the maximum overall dimension of the source. This is not
agoodapproximation especially inthe VHFband(30-300MHz) inwhich the
wavelengths are comparable to the distances treated.
Open Area Test Site
The characteristics of a minimum standard OATS are defined in the CISPR
standard[8]. Such asite offers acontrolled radiofrequency (RF)attenuation
characteristic between the emitter andthemeasuring antenna(known assite
attenuation). In order to avoid influencing the measurement there should be
noobjects thatcould reflect RFnearby the site. The CISPR test site dimen-
sions are shown in Figure 1. The ellipse defines the area which must be flat
and free of reflecting objects, including overhead wires. In practice, for good
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repeatability between different test sites, a substantially larger surrounding
area free from reflecting objects is advisable.
Because it is impossible to avoid ground reflections, these are regularized
by the use of a ground plane. The minimum ground plane dimensions are
also shown in Figure 1. Again an extension beyond these dimensions will
bring site attenuation closer to the theoretical; scattering from the edges
contributessignificantlytotheinaccuracies,althoughthesecanbeminimized
by terminating the edges into the surrounding soil. The ground plane should
be made of solid metal sheet welded together. Bonded wire mesh is also
suitable, since it drains easily and resists warping. For RF purposes it must
not have voids or gaps that are greater than 0.1λ at the highest frequency.
Figure 1: The CISPR OATS
The measurement distance between the EUT and receiving antenna de-
termines the overall dimensions of the site and hence its cost. There are
three commonly specified distances, 3m, 10m and 30m. The measuring dis-
tance is defined between the boundary of the EUT and the reference point
of the antenna. Understanding measurements in OATS requires a grasp of
the fact that the signal at the observation point is the combination of signals
arriving directly and after reflection from the ground, we have to calculate
the incident field as will be shown in chapter one. The total field at the
observation point is that due to two antennas at distances d
1
and d
2
(see
Fig. 1.2). The two contributions differ in amplitude and in phase and it
is clear that under particular circumstances the two signals may be added
constructively or destructively. For a particular observation point and the
same radiator power, the response at low frequencies is higher for vertical
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polarization by approximately 10 dB. As d
1
and d
2
become comparable to
the wavelength, phase differences become important and the total signals for
vertical and horizontal polarizations become comparable in magnitude. At
even higher frequencies a strong interference pattern is observed.
Anechoic chamber
Open-area test sites have several disadvantages and this has led to a search
for alternative EMC test environments. Among these disadvantages are the
difficulty of ensuring a clean EM environment, dependence on the weather,
and land costs. In the case of immunity tests, it also difficult to avoid inter-
fering with other users of the EM spectrum. The solution is represented by
screened rooms, in particular by anechoic chambers.
An anechoic chamber is a shielded room designed to attenuate waves.
Anechoic chambers were originally used in the context of acoustic (sound)
echoes caused by reflections from the internal surfaces of the room, but more
recently anechoic chambers have been used to provide a shielded environ-
ment for radio frequency (RF) and microwaves. Shielding serves two basic
functions: first, preventing interference and second, preventing electronic
eavesdropping. The type of shielding required is a function of the purpose or
use of the equipment to be shielded. High-performance shielding is required
where sensitive equipment must be protected from nearby high power radia-
tion,butonlymoderateshieldingmay berequired tocontrolelectromagnetic
environment within an anechoic chamber.
Any EUT or antenna placed in this room interacts with the conducting
surfaces in a complex manner. For example, an empty rectangular cavity ex-
hibits resonances at specific frequencies obtained from the following formula:
f(MHz)= 150
null null m
a
null 2
+
null n
b
null 2
+
null p
c
null 2
where a, b, and c are the internal dimensions in meters and m, n, and p are
integers with no more than one being zero. Typical room dimensions range
from a few meters to a few tens of meters. The lowest resonant frequency
depends on room dimensions and is generally of the order of a few tens of
megahertz. The electromagnetic field structure at each resonant frequency
canbedetermined numerically oncegiventhem, n, andpparameters. Thus,
mode TE
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refers to a mode in which the vertical component of the electric
field has a maximum on a line running from the center of the floor to the
center of the ceiling (y-axis) just like in metallic waveguides. Higher-order
modes exhibit a more complex structure. These problems are over passed
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