frequencies above 1 GHz, a new site-qualification technique has
been needed to take the place of the normalized site attenuation
(NSA) test. This article provides a detailed description of this
new technique, noting areas for caution and care necessary to
ensure test repeatability. Testing was conducted to validate this
technique. The results are discussed in detail.
new procedure uses two identical horn antennas aimed at each
other. Test site is located at ETS-Lindgren (Round Rock, TX).
methods presented have been developed under the auspices of the
American National Standards Institute (ANSI) Accredited Standards
Committee (ASC) C63, Subcommittee 1, Working Group 1-13.2. This
work applies to site requirements for radiated measurements above
1 GHz of radio-frequency (RF) compliance test sites.
objective of any site-qualification test is to determine whether
the site has sources of reflection that will cause errors in product
measurements. The traditional NSA test described in ANSI C63.4
and CISPR 16.1 fails to perform this function as frequency increases
because of the natural beam forming that occurs in antennas above
1 GHz.13 Figure 1 illustrates the pattern of
a log-periodic dipole array at 7 GHz.
has also been shown that as the patterns of the transmitting and
receiving antennas narrow, only the space between the antennas
is being evaluated in a traditional NSA test.4 Obviously,
this is a case of diminishing returnsalthough the log-periodic
dipole array has a wide enough pattern to illuminate a sidewall
or an object, this pattern narrows with increasing frequency.
1. Log-periodic dipole array pattern (5 dB/div.) and 3-dB
beam width (46°) at 7 GHz, horizontal polarity.
the methods used at lower frequencies fail to meet the objective
of identifying reflecting objects at higher frequencies, a fresh
perspective must be taken. The site qualification test should
be as representative as possible of the actual measurement techniques
used to evaluate products. Product testing requires rotating the
product through 360° to find the emissions. During this process,
the site should not introduce errors to the measurement from reflection
of emissions at other azimuth angles of the product. Figure 2
illustrates this concern.
the latter half of the twentieth century, the location of aircraft
has been identified in aviation using radar. A radar method, or
rotational illumination technique, seems at first blush to be
analogous to the measurement methods currently used in EMC. It
would, therefore, appear to be appropriate for identifying reflecting
objects in EMC testing.
2. Illustration of errors from multilobe sources and site
proposed procedure has been thoroughly researched and is deceptively
simple.2 First, two identical horn antennas are bore-sighted
or aimed at each other at the desired measurement distance, usually
3 m. The transmitting antenna is located on a turntable with the
aperture over the center of the turntable. The ground plane between
the antennas is covered with absorbing material to ensure an anechoic
a network analyzer saves significant time and ensures adequate
dynamic range. The analyzer should be calibrated, and the maximum
output level should be set. Before beginning the measurements,
a through measurement and a simple dynamic-range test is conducted.
The through measurement is intended only to verify that the cables
being used are in good working order and do not have any unusual
power holes or narrow frequency ranges at which very high losses
occur. The dynamic range of the system is evaluated by terminating
the transmitting cable and taping the cable to the transmitting
antenna, near the antenna terminal. With this setup, sweeping
the analyzer over the frequency range of interest displays the
system dynamic range. Figure 3 illustrates typical results.
3. Typical measurement system dynamic range.
the dynamic range has been determined to be adequate, the antennas
can be connected to the cables and the bore-sight voltage measured
as a function of frequency. The site measurement can be conducted
starting with the transmitting antenna rotated 90°, or perpendicular
to the receiving antenna. The measurements begin at 90° because
the receiving antennas normally have sufficiently narrow beam
widths such that any testing at angles less than 90° would
require the signal to have multiple reflections in order to be
measured by the main lobe of the receiving antenna. The frequency
is swept, for example, from 1 to 18 GHz, and the received voltage
is recorded. The azimuth is then incremented 6°, and the
measurement is repeated. As a result, voltage is obtained as a
function of frequency and azimuth. This procedure is conducted
at both a calibration site and at a test site. It is, of course,
critical that the calibration site be free of reflections (a subject
of further study). Once this test has been conducted at both sites,
sufficient information is available to compare the sites. Any
variations in the patterns would be the result of reflections
and can be represented in linear terms by Equation 1:
question that must now be answered is how much error is acceptable
in a product measurement from a site reflection? The answer is
under study now. However, there is some guidance in ANSI C.63.6.5,6
This standard describes the derivation of the ±4 dB criteria
used for the NSA test. That criterion assumes 1 dB of error for
the site. An expression of 1 dB as the maximum acceptable error
contribution from sites above 1 GHz is shown in Equation 2:
linear terms, these criteria would be expressed as Equations 3
the log of Equation 4 defines the pass-fail criterion as: the
ratio of the reflected voltage to the bore-sight voltage should
not exceed 18.3 dB. This criterion as applied to the proposed
procedure is expressed in Equation 5:
criterion allows for 1 dB of error from one reflection source.
A product under measurement may have multiple emission lobes at
one frequency, and the site may have multiple reflection sources.
However, there is also a question as to the probability of the
reflections and sources aligning in a measurement. Also, consideration
still must be given to the uncertainty of the measurement. Piloting
(beta testing) the proposed criterion is the preferred method
for evaluating how stringent this criterion is.
Alert for Specific Hazards
first few attempts to apply this method and these criteria reveal
numerous issues that need to be considered. Most laboratories
have certain cables used for high-frequency measurements. These
cables often have narrow frequency ranges in which the cables
and fittings have very high losses. These abrupt changes in insertion
loss are often referred to as power holes. Power holes are far
more common than most laboratories suspect. After setting up the
equipment, a through measurement of the cables reveals any power
holes. Obviously, such cables should be replaced. Repeated measurements
of the through connection after making, breaking, and remaking
the connection will be required to ensure that intermittent problems
4. Log-periodic dipole array pattern (5 dB/div.) and 3-dB
beam width (46°) at 15 GHz, horizontal polarity.
important hazard to be avoided is antenna patterns exhibiting
large back lobes. Those antennas that have a large slope (dB/degree
of azimuth) on the back lobes will cause very large uncertainties
associated with the positioning of the transmitting antenna. An
example of such an antenna is shown in Figure 4.
validation testing was conducted at the National Institute of
Standards and Technology (NIST) in Boulder, CO, and at ETS-Lindgren
in Round Rock, TX. These initial tests involved the measurement
of the antenna patterns followed by repeating those measurements
with an intentional scattering object in the test environment.
Figure 5 illustrates one such measurement setup.
5. Photo shows setup for pattern measurement with a foil-backed
foam sample material.
test setup uses a 3-m measurement distance between the antennas,
and the scattering objects were located 3 m behind the transmitting
antenna. A second location for the scattering object was 3 m from
the bore-sight path and perpendicular to the center of that bore-sight
measurement path. Scattering objects were selected from typical
building construction materials. Applying Equation 5 to the measured
data and assigning a floor to the plot of 18.3 dB provides
a good illustration of the results. Figure 6 shows one such plot
of results when foil-backed foam was used as the scattering object.
This foam is commonly used as a vapor barrier in building construction
and would be an extreme example. Note in Figure 6 that the maximum
voltage displayed coincides with the path loss difference between
the bore-sight path, 3 m, and the reflected path, 9 m.
6. Graph provides example results of foil-backed foam scattering
object, horizontal polarity.
more realistic material test was made using a sheet of plywood.
Figure 7 illustrates the typical results from a 2 X 2-ft sheet
of plywood. In this test, it is clear that the plywood was located
directly behind the antenna (180°). The resonant frequencies
of the plywood are narrower than originally thought. Reflections
from the plywood exceeded the established criterion only in narrow
7. Graph shows example results with plywood scattering object,
materials were tested in this fashion. Those materials were particleboard,
fiberglass, plywood, foil-backed foam, roofing (plywood with shingles),
treated plywood, and chipboard. Testing these materials demonstrated
that the procedure worked well. Validating the procedure and the
criteria also requires testing real test sites. To date, three
semianechoic chambers have been tested using this procedure, and
as many as six more sites, including more chambers, open-area
test sites, weather-protected open-area test sites, and even gigahertz
transverse electromagnetic (GTEM) cells will be tested.
8. Graph shows example results of a 10-m semianechoic chamber,
8 shows the results of testing of a 10-m semianechoic chamber
with full absorber treatment on the walls and ceilings. The only
reflections found were at the 1-GHz frequency range and directly
behind the transmitting antenna. This is most likely the result
of the measurement axis being perpendicular to the rear wall.
Improvements could probably be realized by changing the measurement
9. Graph shows example results for a 10-m semianechoic chamber
with partial absorber treatment, vertical polarity.
of a 10-m chamber of the same size employing absorber only in
the spectral regions (partial treatment) yielded very different
results, as shown in Figure 9. These data were taken at 10°
from the centerline of the chamber. It is also important to note
that the turntable was not located in the center of the width
of the chamber, but rather a few meters to one side of the centerline
of the chamber. The results clearly show that the corner of the
chamber is reflecting as a result of the lack of absorber treatment
in that corner area.
10. Graph shows example results for a 5-m semianechoic chamber,
third semianechoic chamber tested was a partially treated 5-m
chamber. This chamber also had an offset turntable, and the measurement
angle was also approximately 10° from the centerline. Here
again, it appears that the corners are a source of reflection
that exceeds the 1-dB criterion.
three semianechoic chambers tested revealed an obvious problem.
Partially absorber-lined chambers may need modification to be
able to meet the proposed qualification test. Conversely, consideration
can be given to increasing the allowable reflections while recognizing
that the allowable error decided upon must be taken into account
in determining measurement uncertainty.
Caution must be made in conducting these tests to ensure that
the cables are free of power holes and that the antennas do not
have significant back lobes. Work remains to be done to evaluate
the performance of existing test sites. In addition, procedures
need to be developed to evaluate the adequacy of the ground-plane
absorber applied and the calibration site selected.
proposed procedure appears from all the validation tests to work
quite well. These results show that objects that can be a source
of reflection will be measured in this procedure. How significant
the reflections and the resulting errors in measurements can be
will probably be a topic of significant discussions in the EMC
community and certainly in ANSI ASC C63.
1. M Windler and D Camell, "Measuring Antennas above 1 GHz,"
in Proceedings of the 14th International Zurich Symposium on
Electromagnetic Compatibility (Zurich: Swiss Electrotechnical
Association, 2001), 91.
C63.4-2000, "American National Standard for Methods and Measurement
of Radio-Noise from Low-Voltage Electrical and Electronic Equipment
in the Range of 9 kHz to 40 GHz," American National Standards
Institute, New York.
16-1:1999-10, "International Special Committee on Radio InterferenceSpecification
for Radio Disturbance and Immunity Measuring Apparatus and MethodsPart
1: Radio Disturbance and Immunity Measuring Apparatus," International
Electrotechnical Commission, Geneva.
Johnk et al., "NIST Time-Domain Scattering Experiment Using Dual-Ridged
Horn Antennas and an Ultrawideband Measurement System," CISPR
A, Working Group 1, Ad Hoc Group on > 1 GHz, report dated January
Windler and D Camell, "Research on Site Qualifications above 1
GHz," in Proceedings of the 15th International Zurich Symposium
on Electromagnetic Compatibility, (Zurich, Swiss Electrotechnical
C36.6-1996, "American National Standard Guide for the Computation
of Errors in Open-Area Test Site Measurements," American National
Standards Institute, New York.
J. Windler is head of international EMC and NEBS services for
Underwriters Laboratories Inc. (Northbrook, IL). He can be reached
via e-mail at email@example.com.