Antenna Pattern Measurement: Concepts and Techniques
high frequencies become more common, understanding antenna
pattern measurement and how to obtain useful measurements
by TAISHA PAYTON
first article of this two-part series explores the basic
concepts and techniques of antenna pattern measurement
and evaluates the benefits and drawbacks of various measurement
methods. The concepts relating to near-field and far-field
pattern testing are discussed as well. The second article
(see page 34) presents the theory and equations governing
antenna properties and includes a complete description
of a site calibration for pattern-measurement testing.
pattern measurement refers to the determination of the
radiation pattern of an antenna under test (AUT). It is
the measurement of the relative magnitude and phase of
an electromagnetic signal received from the AUT. Although
highly directional antennas (i.e., horns) are often measured
by scanning a plane perpendicular to the bore-sight axis
of the antenna (i.e., parallel to the face of the horn)
at some distance, this article focuses on total spherical
pattern measurements. A subset of this is the simple polar
planar cut, in which the pattern is determined for a single
azimuth rotation around the antenna.
a passive antenna is reciprocal, the pattern information
could be obtained by using it as either the transmitter
or receiver. This is in contrast to an active antenna
system, in which transmit and receive behavior may be
considerably different, and thus both relative pattern
and absolute power information is required. In addition
to the relative information that makes up the antenna
pattern itself, and the various pieces of information
that can be determined from it, a variety of other results
can be determined from an active antenna system.
complex antenna-pattern measurement has been a common
requirement in the microwave antenna arena for many years,
it has only recently become more common to other areas
such as electromagnetic compatibility (EMC) and wireless
telecommunication. On the EMC front, the interest in pattern
measurements appears to stem from a range of sources.
The first is that, as EMC standards are forced to move
higher in frequency, the effects of narrow-beam radiation
from the equipment under test (EUT) and the corresponding
interaction with the receive antenna become increasingly
significant. It is important that the test antenna is
able to see all signals radiating from the EUT. In addition,
broadband antennas designed for EMC work are finding their
way into other applications in which concern for antenna
patterns has always been an issue. Finally, many engineers
with microwave backgrounds now must deal with EMC issues.
These engineers want more information than has traditionally
been provided on these antennas.
the wireless industry, base station antenna patterns have
always been important in ensuring coverage. Understanding
the pattern of each cell tower is critical to determining
the required spacing between them. However, lately the
industry has put considerable emphasis on handset pattern
measurement as well.
Cellular Telecommunications and Internet Association (CTIA)
has drafted a set of test plans aimed at verifying the
performance of cellular telephone handsets. One of the
CTIA plans provides tests for verifying radiated signal
cell phones were required to meet a peak-signal requirement,
but now they are required to meet a total radiated power
requirement. This requirement ensures that a cell phone
is transmitting energy in a broad pattern rather than
in a narrow beam and, therefore, is less likely to lose
contact with the cellular network.
tests are also designed to characterize both transmitted
and received power and pattern, as well as the minimum
signal that the phone can properly detect. There are also
calculations designed to determine the effectiveness of
the phone when the base station antennas are located along
the horizon (the typical configuration). The tests help
to ensure that not all of the radiated energy is directed
up into space or down into the ground.
cell phone manufacturers are often interested in the performance
of the phone by itself, CTIA also requires testing with
a liquid-filled phantom head or torso to simulate the
effect of human interaction with the phone.
addition to cell phones, other products with growing wireless
testing requirements include wireless personal digital
assistants, which are typically covered under the cellular
requirements, and home- and office-based wireless networks
such as wireless local-area networks and Bluetooth devices.
basic pattern-measurement technique that most people are
familiar with uses a single-axis rotational pattern. This
technique involves an AUT placed on a rotational positioner
and rotated about the azimuth to generate a two-dimensional
polar pattern. This measurement is commonly done for the
two principal axes of the antenna to determine parameters
such as antenna beam width in both the E and H planes.
Such data are typically only measured for the copolar
field component for simple horns or dipoles for which
the general polarization of the pattern is well known.
more-complicated radiators, for which the polarization
may not be known, or may vary as a function of angle,
it is important to be able to measure two orthonormal
(i.e., perpendicular) field components. This measurement
is usually accomplished by using a dual-polarized horn,
log-periodic dipole array, or dipole antenna as the measurement
antenna (MA). Although it provides the best result, this
technique requires two receivers or the ability to automatically
switch the polarization of a single receiver, which can
increase the cost of the test. A slower, and possibly
less accurate, option is to repeat an identical pattern
test for each MA polarization. This option could result
in time variations and alignment issues that could have
1 shows a typical polar-pattern test setup. The AUT (a
cell phone in this case) is placed on a rotating turntable,
and a dual-polarized antenna is placed level with the
AUT a fixed distance away. The turntable is rotated 360°,
and the response between the antennas is measured as a
function of angle. Normally, these measurements are performed
in a fully anechoic (simulated free-space) environment,
but sometimes it may be desirable to measure the pattern
over conducting ground, or in some other as-used geometry
to get real-world pattern information. Figure 2 shows
some polar patterns for typical antenna types and polarizations.
1. Test setup for single-axis polar pattern measurement.
2. Copolarized polar patterns for a vertically polarized
dipole, horizontally polarized dipole, and standard-gain
generate a full spherical-pattern measurement, it is necessary
to change the relationship between the AUT and the MA
and repeat the previous polar test for each new orientation.
The changes in orientation must be perpendicular to the
plane of measurement to completely cover a spherical surface.
In simpler terms, the second axis of rotation must be
perpendicular to and intersect the first axis of rotation.
two axes correspond to the q and f angles of the spherical
coordinate system and are typically referred to as elevation
and azimuth, respectively. Just as in the spherical
coordinate system, only one axis needs to be rotated through
360°, whereas the other is rotated only through 180°.
With the proper processing of the resulting data, it really
does not matter which axis is which. Either antenna can
be rotated around this second axis to generate the same
pattern, but each technique has both advantages and disadvantages.
conical-section method uses an elevated turntable to support
the AUT and rotates the MA around the AUT on an axis perpendicular
to the vertical rotational axis of the turntable (see
Figure 3). This method fits the geometric picture that
most people have for spherical coordinate systems, and,
therefore, it is often the method used for pattern measurements.
The turntable continues to provide the azimuth (f)
rotation, whereas the MA is raised (elevated) or lowered
in an arc around the AUT, and, thus, the term elevation
3. Illustration of the conical-section method for
spherical antenna-pattern measurement.
common misconception when visualizing this technique is
to consider moving the MA in a 180° arc across the
top of the AUT. However, a quick look at Figure 3 shows
that this would just duplicate the measurement across
the top half of the AUT and never measure the bottom half
of the pattern. The data points at (f
= 0°, q = +x°) and
(f = 180°, q
= x°), where q =
0° directly above the antenna, are the same.
method results in the MA describing circles of varying
diameter, and thus the reference to conical sections.
The circles may be thought of as latitude lines on a globe,
from the north (+z) to south (z) poles,
with the largest circle located at the equator. Only the
one circle where the MA is at the same height as the AUT
(i.e., the equator) results in a true polar pattern measurement.
the conical-section method is conceptually simple, it
has a number of drawbacks. A large pivot arm or arch support
is required to manipulate the MA. For long range lengths,
this requirement can be a difficult proposition. Similarly,
if this test is to be performed in a fully anechoic chamber,
the chamber must be much larger than would normally be
necessary to support the required range length because
the floor and ceiling must be the same distance away as
the rear wall behind the MA. This can dramatically increase
the cost of antenna measurement.
perform a full surface measurement, the turntable must
also be cantilevered out from a wall or other support
to allow the MA to be moved under the turntable. Otherwise,
there will be a dead zone where the antenna is blocked
by the supporting structure. In any case, the turntable
itself can significantly affect the pattern measured if
it is too massive or made of the wrong materials.
the great-circle method, the MA is fixed and the AUT is
repositioned on the turntable to generate each polar cut.
Because the MA is fixed, pointing perpendicular to the
rotation axis in this case, every cut is a true polar
pattern. Therefore, each rotation of the turntable provides
the greatest diameter circle possible.
compare the two methods, the AUT must be laid on its side
with respect to the setup for the conical-section method
to represent the associated shift in coordinate systems
(see Figure 4).
4. Great-circle configuration of antenna under test.
rotating the AUT about the horizontal axis between each
great-circle cut, the entire spherical surface can be
covered (see Figure 5). Each polar cut passes through
the others at the horizontal axis of rotation, and the
intersection points at the horizontal axis are equivalent
to the top and bottom MA positions in the conical-section
method. This is why the AUT was laid on its side, to support
the change in coordinates.
5. Illustration of the great-circle method for spherical
antenna-pattern measurement. The back sides of the
polar cuts have been removed for clarity.
the great-circle method, the circles can be thought of
as longitude lines, running from the north (+z)
to the south (z) pole and back around the
other side. As before, it is only necessary to rotate
the AUT (instead of the MA) through 180° to cover
the entire sphere because the great circles cover the
front and back of the sphere simultaneously.
the shift in coordinate systems, the turntable is now
an elevation positioner rather than an azimuth positioner
because it changes the MA position from pole to pole rather
than along latitudinal lines parallel to the equator.
The horizontal rotation axis of the AUT now provides the
great-circle method has the advantage of being relatively
easy to perform with a low-cost system by rotating the
AUT manually about the horizontal axis, but, as with most
such endeavors, it can be extremely tedious without additional
automation. The method has an added benefit. The path
between the AUT and MA is never obscured by the support
structure, although care must be taken to ensure that
the existing support structure does not have reflective
properties that could alter the antenna pattern, especially
if additional material is required to support the AUT
in different orientations.
because the MA is fixed, the chamber only needs to support
the required range length in one dimension. This opens
the possibility of using tapered chambers and the like
to obtain high performance and long range lengths affordably.
each method has advantages and disadvantages, it is important
to verify that they are both capable of producing the
same results. Figure 6 shows both conical section (a)
and great circle (b) results with the same step size between
measurement points and in which the coordinate systems
have been aligned. Overlaying the two plots (see Figure
6c) shows that the actual measured data points are identical,
regardless of the method used. Therefore, given just the
resulting data points (see Figure 6d), it is not possible
to determine which method was used to generate them.
6. Comparison of measurement points between (6a)
conical-section method and (6b) great-circle method.
(6c) shows the two results overlaid, and (6d) indicates
that it is impossible to tell which method was used
given only the resulting data points.
adopting the great-circle method and manipulating the
AUT in two axes, it is possible to automate the test such
that data can be acquired according to the measurement
sequence of either method. Figure 7 shows a simple two-axis
positioner that can automate the rotation of the AUT on
both axes. By rotating the turntable (elevation) 360°
and stepping the horizontal axis (azimuth) of the AUT
between each turntable rotation, the great-circle method
(see Figure 8a) can be duplicated. Alternatively, by rotating
the horizontal axis (azimuth) of the AUT 360° and
stepping the turntable (elevation), the conical-section
method (see Figure 8b) can be duplicated.
7. Example of a two-axis positioner setup for pattern-measurement
8. (a) Great-circle method and (b) conical-section
method performed using the same two-axis positioner.
two-axis positioner does suffer from one of the limitations
mentioned for the conical-section method. That is, for
some portion of the pattern (the south pole in Figures
7 and 8), the support structure is between the AUT and
the MA. This effect can be minimized by matching the support
structure to the load being rotated, thereby reducing
the amount of interposing material to a minimum. Controlling
the orientation of the AUT with respect to the support
can also improve results. By making sure that the support
is in a null or back-lobe, its effects on pattern-related
measurements can be minimized.
matter which method is used to acquire the data, the analysis
of the result is made easier by the use of a three-dimensional
spherical plot to graph the output. Figure 9 gives an
example of a dipole pattern (a) and a standard-gain horn
pattern (b) plotted in three dimensions. This type of
graphing capability allows the pattern to be rotated around
for different views to help get an idea of the relative
magnitude of the signal in various directions.
9. Three-dimensional spherical plot of (a) simple
dipole and (b) standard-gain horn. Note the expected
toroidal (donut) shape of the dipole pattern and
the strong directionality and sidelobes of the standard-gain
versus Far-Field Measurements
of how the data are acquired, one of the available system
variables is the range length. Usually, when one refers
to the properties of an antenna, be it antenna pattern,
gain, or another property, the reference is to the far-field,
free-space properties of the antenna. In the far-field,
free-space condition, the measured properties of the antenna
do not appear to vary as a function of separation distance
or antenna location. That is not to say that the measured
field levels themselves do not vary, but that the measured
gain or pattern does not vary. To state it simply, the
far-field, free-space condition is the condition in which
all of the theoretical equations typically used for calculating
antenna properties are valid.
a near-field or non-free-space environment, the antenna
properties that are measured appear to vary as a function
of their environment. Effects such as mutual coupling
between the AUT and the measurement antenna or the antennas
and other objects around them, as well as other near-field
perturbations, prevent the direct determination of the
desired antenna properties. Even assuming a good free-space
environment (i.e., a fully anechoic chamber), there are
still limitations to near-field testing.
readers will be familiar with at least one rule of thumb
for near- versus far-field determinations. In reality,
there are two very different definitions. The first, which
is usually more important at low frequencies, is represented
by the near-field term(s) of the electric and/or magnetic
field equations. These are the terms that behave as 1/rn,
where n > 1. These terms represent the nonpropagating
or evanescent electric and magnetic fieldsthose
caused by capacitively or inductively stored energy in
the antenna. Therefore, this region is referred to as
the reactive region of the antenna.2
reactive fields decay rapidly with distance from the antenna,
leaving only the
has a 1/r behavior. In this case, the far-field
condition is satisfied by l/r
<< 1, that is, where the measurement distance r
is much greater than wavelength l.
The reactive region is commonly defined as
D is the largest dimension of the radiating object.
For practical applications, a simple rule of thumb suitable
for most antennas is given by r < 2l.
Within this region, any measurement antenna or probe would
have a significant effect on the transmit antenna.
second far-field requirement, which is more familiar to
microwave engineers, is usually the dominant factor at
higher frequencies. In this case, the objects involved
(either the actual antennas or larger devices containing
small antennas) are large compared with the wavelength.
effects of scattering from different points on the object,
or from different emissions points in the case of an antenna
array or a leaky shielded enclosure with multiple openings,
result in wave fronts propagating in multiple directions.
The far-field condition is met when all of these different
wave fronts merge to form one wave front; that is, when
the multiple sources are indistinguishable from a single
source (when separation distance r > 2D2/l).
the bigger the object or the shorter the wavelength, the
farther away the receive antenna has to be for that object
to appear as a single source. The region inside the 2D2/l
distance, but outside the reactive near-field region,
is referred to as the radiating near-field or Fresnel
region, whereas the region outside this distance is
the far-field or Fraunhofer region.2
terms of antenna-pattern measurements, normally there
is little useful information to be gained within the reactive
region of an antenna. The one possible exception would
be when the antenna is to be used in the reactive region
as well. However, it would not be possible to eliminate
the effect of the measurement antenna on the AUT, and
therefore the usefulness of such data would be limited.
The Fresnel region contains propagating electromagnetic
energy, but not in a cohesive form. Therefore, pattern
measurements done in this region can readily determine
quantities such as total radiated power but may only provide
an approximation of the far-field pattern, gain, and other
from Near Field to Far Field
common practice in microwave antenna measurements, and
something of a Holy Grail for EMC measurements, is the
use of near-field measurements to predict far-field results.
In the Fresnel region, it is possible to scan the magnitude
and phase of the field along a closed surface (or, in
the case of planar near-field scanning, an open surface
intersecting the vast majority of the propagating energy)
and predict the far-field levels. Acquiring the relative
phase and magnitude at each point on the surface requires
the use of a reference signal in addition to the measurement
antenna signal. The fixed reference is needed to track
the relative phase of the signal in time because each
point in space is not sampled at the same instant in time.
passive antennas, a vector network analyzer is normally
used, which acquires both magnitude and phase information
against its own reference signal. Active devices are more
complicated, requiring the use of a fixed reference antenna
or sensor in addition to the measurement antenna to obtain
both phase and magnitude references (because an active
device may not maintain a constant magnitude or phase
relationship). In either case, the calculations required
to do the conversion are beyond the scope of this article.
EMC testing, the conversion of radiated-emissions measurements
from near field to far field is made much more difficult
by the nature of the electromagnetic signature of the
device under test and the frequency range required for
EMC testing. EMC emissions are far from being continuous
wave, often consisting of harmonics, broadband noise,
and spurious signals. Obtaining the same radiation signature
at each point of a near-field scan is very unlikely.
further complicate matters, low-frequency EMC measurements
are often performed in the reactive region of both the
EUT and the receive antenna. Although near-field reactive
terms can be easily determined for simple dipole elements,
such predictions for more-complicated antennas or emitters
are extremely difficult. The amount of data and processing
required to correctly separate the effects of the EUT
from the receive antenna and the rest of the environment
to truly predict a far-field result is far beyond the
current state of the art.
need for antenna-pattern information is increasing as
the EMC community moves to higher frequencies and more-advanced
techniques, and as wireless devices continue to pervade
our everyday radio-frequency (RF) environment. The techniques
for complex-pattern measurement are rather straightforward,
but there are some pitfalls. Useful pattern information
can be obtained using either the radiating near-field
or far-field, but not the reactive, region of the AUT.
The conversion of near-field pattern information to far-field
results is possible, but it requires specialized software
and measurement capabilities.
"Method of Measurement for Radiated RF Power and
Receiver Performance, Draft Revision 1.2" (Washington,
DC: CTIA, 2001).
CA Balanis, Antenna Theory, Analysis and Design
(New York: Harper & Row, 1982), 2223.
D. Foegelle, PhD, is senior principal design engineer
at ETS-Lindgren (Cedar Park, TX). He can be reached at
512-531-6444 or firstname.lastname@example.org.