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Description of measurement methodology
BEMI – BÄTTRE ELMILJÖ
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Clas Tegenfeldt
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Copyright © 1994-2010 Clas Tegenfeldt
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NATURE OF RADIATION FROM ANTENNAS
A wave is a sort of oscillation, it could be
sound (mechanical vibration in a medium) or
electromagnetic (no medium necessary) such
as radio or light.
A wave has an amplitude which can be
described as strength or intensity.
The rate it oscillates, its frequency, is
measured in Hertz (Hz). Because it also
travels the waves peaks and zero crossings
also moves. The distance it moves while
oscillating one time is called the wavelength.
The wavelength is measured in meters. The wavelength depends on the propagation speed and the frequency.
The product of wavelength and frequency is equal to the propagations speed. For an electromagnetic wave in
free space the speed is equal to the speed of light (which also is an electromagnetic wave!).
Let c be the speed of light, f is the frequency in hertz and λ is the wavelength in meters, then
c 299792458
c=⋅ f , eller = =
f
f
The amount of power a wave carries is called Poyntings vector
Taking the magnitude of that vector gives the power density
S=
E2
2
[W / m ] in watt per square meter.
377
In fact, it is almost as simple as Ohms law and the definition of power (P=UI). What it tells you is that the
electric field and the magnetic field in combination carries the power in the travelling electromagnetic wave, or
if you prefer - the electromagnetic radiation. The number 377 Ohm is the impedance of free space.
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Since the wave/radiation can move in any direction one often refer it to some coordinate system, such as x, y and
z. The electric field E can then be described either as the vector in space or as three components along the x, y
ans z axis.
When measuring the electric field at some point, one either has to make sure the sensor antenna is directed along
the E vector to collect the full strength of the field/wave/radiation, or measure in the three orthogonal directions
x, y, and z and then use the above formula to find the E vector.
Antenna facts
An antenna is just something electrically conductive that is used to emit electrical energy into the air as a
electromagnetic wave. It could be in almost any shape and size, however, since the wavelength is dependent on
the frequency used, the size of the antenna has to be related to the wavelength.
For low frequency radios the antennas are large, for mobile phones using high frequency (microwave) the
antennas can be small.
The antennas used for basestations could be the same as for the mobile phones, however the mobile is driven by
battery and has limited power output at the same time the base station has no such limitation. The reciver in the
base station can be made sensitive (and expensive) at the same time the mobiles reciever should be simple and
cheap. This in combination makes it necessary for base station antennas to be larger, typically a meter or so high
and a decimeter or two wide. Also the power output through the base station antenna will be tens of times higher
than from the handheld mobile.
Antenna diagrams
An ideal antenna could radiate equally in all directions, this is called an isotropic radiator. This antenna isn't
physically implementable but can be approximated at larger distances. For mobile phone systems as well as
broadcasting there is actually no need to send signals straight up into the air or down directly into the ground.
Thus in actual use one want directional antennas., antennas that can throw the radiation in the needed direction
along the earth surface. To characterize antennas one uses antenna diagrams that describe the distribution of
radiated energy along different directions.
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The most of the energy is radiated along some direction, this is called the main lobe. If the directivity of the
antenna is high this main lobe is narrow, this is called a "high gain antenna" since the signal will be strong in that
direction compared to other directions. Gain is
a measure of how narrow an antenna radiates
its energy. High gain means that the field
levels and power density is increased in the
main direction (in the main lobe) while at the
same time decreases in other directions (back
lobe, side lobe and nulls).
To simplify the antenna diagrams are flattened
in 2D either as a linear or polar diagram.
Let us take a closer look at one of the antennas
used in Zambia. It is identified as "AP901213
120 degree directional panel antenna 890".
It is a GSM900 antenna and has a typical
antennadiagram in the horisontal and vertical planes as shown below:
The horisontal diagram (above left) shows that even though most energy is thrown in the main lobe along the xaxis (to the right), radiation also goes in other directions. Since a mobile phone user may be anywhere this is
often desirable. In the vertical diagram (above right) it is even more pronounced that the energy is along a line
parallel to the earth (ground). Some sidelobes will reach the ground near the antenna mast, but most of the
energy is thrown far away. GSM900 can reach 35 km and even 70 km if placed at high masts.
The antenna diagram implies that simulations in the vicinity of a base station must take both antenna diagrams
(or a complete 3D data set) into consideration to be able to make a proper prediction of field strengths. Farter
away, say 500 meter or more, the radiation more or less follows only the horisontal antenna diagram.
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Radiation around base stations
The image is from the ITU-K.61 document.
When an antenna transmitts electromagnetic energy this radiation can be reflected on the ground or other
surfaces such as buildings. Thus there exists multiple pathways for the electromagnetic radiation to reach the
body (or measurement point).
To be able to understand the various factors influencing the exposure one needs to begin with the antenna. In the
diagram below there is a 50% reflective flat ground but no buildings (no other reflections than from the ground).
140
Red line is spherical propagation in free space (no ground plane, a perfect isotropic antenna)
Blue line is with antenna diagrams, measured along the main horisontal lobe direction, and using
the vertical diagram for every distance point, above a groundplane.
130
120
110
100
90
80
0
100
200
300
400
500
600
700
800
900
1000
The red line is the simple square law propagation from an ideal isotropic radiator in free space. This is just a
simple reference line. The blue line is a GSM900 transmitter with 35 Watt output, neglecting cable losses, into
the antenna "AP901213 120 degree directional panel antenna 890" which had its diagrams shown earlier. The
antenna is at 20 meters height and downtilted 3 degrees.
What is interesting to note is the in real life the field strength do not increase when the antenna mast is
approached. This is due to the fact that the antenna height makes an increasingly steep angle upwards from
measureing point and the antenna. Reversely, the antenna "looks" more steeply down on the observer and thus
the vertical antenna diagram shows that less energy is radiated downwards than forward (horisontally).
The maximum field strength will occur at 50-500 meters depending on the antenna height and the down-tilt
angle. However, it is safe to say that the levels will be about 120 dBuV/m at maximum in most cases. Even with
a low (6 meters) wall mounted antenna, downtilted 10 degrees, and full 35 W transmitter power, the maximum
will be below 140 dBµV/m.
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Antenna height relative to the person matters
The illustration is from ITU-K.52 and shows a circular exclusion area around an antenna. The
need to actually use an exclusion zone depends on the height h and type of antenna and
transmitted power. For mobile phone basestations the exclusion area is mostly a concern for
roof mounted antennas. If the height is more than 10 meters and the system used is
GSM/UMTS then there is no need for an exlusion zone at ground level.
However, if the antenna is placed on a mast or another house at the same height h and the
distance d is short (less than 10 meters) then the exposure will be higher and the exclusion
zone need to be evaluated.
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Base station power variations in time
When a mesurement is done, is the value representing the "normal" field strength? In other words, are there
significant variations in the power output from the transmitter(s)?
For FM radio the answer is simply, no the radiation is almost perfectly constant.
For TV the answer is a little more complex, analog TV varies somewhat, but at a fast rate (image to image),
while digital TV is more like the FM case. The analog TV, if measured a few minutes will statistically collect
various image contents and thus the level is representative. However, if the transmitter is switched off during
mesurements then it is of course missed completely! FM is usually around the clock while TV may be switched
off certain during some hours each day.
For the GSM case, the transmitters are always
on. The total power variation for the GSM
case is small. This is due to the fact that the
first base station channel is always active with
all eight time slots, filled with dummy data if
necessary. This means that the transmitted
power will remain the same until all timeslots
are used by phone traffic. If there is more
traffic channels available at the site the power
will be increased by the transmitters output
power in steps of 1/8 (eight time slots), if
these are also filled with phone usage the
output power has doubled (equals +3 dB).
This variation is insignificant compared to
other variations when measuring the
exposure.
In real life long time measurements, such as
reported by Joe Wiart et al, or as BEMI has
performed in Sweden, shows variations from
80% to about 130% as compared to the mean level. This is smaller variations than the theoretical 200% above
base level. [See "Analysis of the influence of the power control and discontinous transmission on RF exposure.
GSM mobile phones, Joe Wiart et al. IEEE Transactions on electromagnetic compatibility, vol 42, no 4, pp 376385, nov 2000.]
For the 3G/UMTS case, the transmitters are also always on. The power variation is more complex than the GSM
case, however the levels do not change significantly more, just faster.
It is safe to assume that most power variations in time are accounted for during a 6 minute time, but bearing in
mind that high traffic times may increase the exposure by a few dB, and bearing in mind that other transmitters
such as TV may be switched off certain hours.
Uncertainty of selected measuring point
Waves may add or subtract onto eachother, creating interference, standing waves and something called fading.
The so called superposition principle is the basis for all these phenomena. The radiation from a transmitter that
may travel by many routes - by multipath propagation, will at the measurement point be blended.
By measuring in one point only the question arises if this point is on a maximum peak, lowest valley or
somewhere in between? How great difference is there to a point a few centimeters away?
There are two ways to answer this question, theory or simply by measuring many points!
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For GSM1800 the wavelength is
16 cm, by moving the antenna
and measuring the field strength
variation, it easy to see quite
large deviations due to fading. In
some
cases
with
nearby
reflections the differencies may
be even larger. Deviations of plus
or minus 10 dB is very common.
No matter how you coose one
point there is no way of knowing
if that is a "good choice".
If you know the wavelength you
can construct a measuring grid at
certain distances to make
statistically sure to catch a peak
with a high degree of confidence.
However, if you do not know the
frequency or if there are multiple
frequencies
from
different
sources there are no common grid
to use since it is frequency
dependent.
There have been many studies on this
problem, and by increasing the
measurement points at a location the error
can be estimated. The graph on the right
is one such attempt. For an average value
(NOT peak field strengths) only three
points are needed to get the error down to
about 3 dB. However, for peak field
strengths 20 points or more are needed.
BEMI has over many years developed the
method of a moving antenna. In fact, the
ONLY way to make sure that you find the
peak along one cycle is to actually
MOVE the antenna and measure during
the movement. This means that you
sample the waves spatially. By keeping
the instruments "peak-hold" value you
will make sure to find the peak field
strength at the location. By moving an
antenna a few wavelengths in each
direction the statistics gets very strong.
Furthermore, polarisation also differs point to point making it either necessary to do a measurement along x, y
and z-axis or tilting the antenna until maximum peak is found.
The graphs can be found in ["Analysis of electric field averaging for in situ radiofrequency exposure
assessment", Emmanuel Larcheveque, Christian Dole, Man-Fai Wong and Joe Wiart, IEEE VT 2005.]
Measurement uncertainties for measurements of fields are the results of errors due to system instrumentation,
field probe response and calibration, and the extrapolation, interpolation and integration algorithms used to
determine the averaged field. For evaluation and expression of uncertainties, see [ISO/IEC GUM], [IEC 62311],
[EN 50383], [EN 50400] and [b-IEC 62232], [b-FprEN 50492], [b-FprEN 50413] and [b-IEEE P.1597.1].
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