In this paper, three possible approaches for definition of a highly accurate reference pattern of a reference antenna are described and their pros and contras are discussed. Following the most reliable approach, a dedicated measurement campaign was planned and carried out in 2007-2008 for definition of the highly accurate reference pattern of the VAST12 antenna. In planning the campaign, conclusions from the first comparison campaign with the VAST12 carried out within the ACE network in 2004-2005 were taken into account and these are also presented and discussed. Some typical measurement errors and uncertainties are listed and briefly discussed

Breinbjerg, Olav

Burgos Martínez, Sara

Eriksson, Håkan

Pivnenko, Sergey

Sierra Castañer, Manuel

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

DEDICATED MEASUREMENT CAMPAIGN FOR DEFINITION OF ACCURATE 
REFERENCE PATTERN OF THE VAST12 ANTENNA 
 
Sergey Pivnenko, Olav Breinbjerg 
Department of Electrical Engineering, Technical University of Denmark 
DK-2800 Kgs. Lyngby, Denmark, sp@elektro.dtu.dk 
 
Sara Burgos, Manuel Sierra-Castañer 
Universidad Politécnica de Madrid. E.T.S.I. de Telecomunicación 
Ciudad Universitaria, 28040 Madrid, Spain 
 
Håkan Eriksson 
SAAB Microwave Systems 
SE-412 89 Gothenburg, Sweden 
 
 
ABSTRACT 
In this paper, three possible approaches for definition of a 
highly accurate reference pattern of a reference antenna 
are described and their pros and contras are discussed. 
Following the most reliable approach, a dedicated 
measurement campaign was planned and carried out in 
2007-2008 for definition of the highly accurate reference 
pattern of the VAST12 antenna. In planning the 
campaign, conclusions from the first comparison 
campaign with the VAST12 carried out within the ACE 
network in 2004-2005 were taken into account and these 
are also presented and discussed. Some typical 
measurement errors and uncertainties are listed and briefly 
discussed. 
 
Keywords: accuracy, inter-range comparison, metrology, 
standards, reference. 
1. Introduction 
The 12 GHz Validation Standard (VAST12) Antenna was 
designed and manufactured at the Technical University of 
Denmark in 1992 under the contract from the European 
Space Research and Technology Center [1]. The VAST12 
antenna is shown in Fig. 1. The main purpose of the 
VAST12 antenna is to facilitate antenna test range 
intercomparisons for the European Space Agency (ESA).  
In 2004 ESA has permitted the use of the VAST12 
antenna for the European Union network "Antenna Center 
of Excellence" – ACE [2]. Within the Activity 1.2 of the 
ACE network, the First Facility Comparison Campaign 
was carried out with the VAST12 antenna during 2004-
2005, which involved several universities and private 
companies with total of 9 different measurement facilities. 
The results of the campaign are documented in the report 
[3] available from the Antennas Virtual Centre of 
Excellence portal [4]. 
A highly accurate reference pattern of a reference antenna, 
such as the VAST12 antenna, is clearly necessary, since 
its existence allows benchmarking of the antenna test 
ranges and estimating their measurement uncertainties. 
Potentially, it also gives a possibility to identify and 
correct errors in the applied measurement procedures by 
carefully analyzing the results of pattern comparisons 
supported by knowledge of pattern deviations from typical 
errors of the measurement setups. 
 
Figure 1. The 12 GHz Validation Standard Antenna. 
In this paper, three possible approaches for definition of a 
highly accurate reference pattern are described and their 
pros and contras are discussed. Following the most 
reliable approach, a dedicated measurement campaign was 
planned and carried out in 2007-2008 for definition of the 
highly accurate reference pattern of the VAST12 antenna. 
In planning the campaign, conclusions from the first 
comparison campaign with the VAST12 carried out 
within the ACE network in 2004-2005 were taken into 
account and these are also presented and discussed. Some 
typical measurement errors and uncertainties are listed, 
while their extensive investigation and effective 
compensation, including the residual uncertainty estimate, 
are presented in a companion paper “Characterization of 
measurement systems through extensive measurement 
campaigns”. 
2. The 12 GHz Validation Standard Antenna 
The VAST12 antenna represents an offset shaped 
parabolic reflector with circular aperture, see Fig 1. The 
feed is a corrugated horn with a polarizer allowing 
changing from linear to circular polarization. In order to 
make the antenna rigid and thermally stable, the support 
structure is made of carbon-fiber reinforced plastic 
(CFRP) sandwich, while the mounting flange and dish 
suspension points are made of stainless steel. The 
reflector is made of high density acrylic foam covered in 
front and in back with several layers of fibers in four 
directions. The reflecting surface is silver painted with a 
protective paint on top. The dimensions of the VAST12 
are 842mm x 508mm x 939mm and the weight is about 20 
kg. The operating frequency is 12 GHz. A complete 
description of the design, manufacturing and testing of the 
VAST12 antenna is given in [1]. 
For the purpose of facility comparisons, three coordinate 
systems (CS) are defined: the optical CS is defined by a 
mirror cube attached at the top of the reflector, the 
mechanical CS is defined by the mounting flange and 
using a level placed on the support arm, and the electrical 
CS is defined by the peak of the far-field co-polar pattern 
and the minimum of the cross-polar pattern [1] 
3. Establishment of the Reference Pattern 
The difficulty of establishing a reliable reference pattern 
is associated with insufficient trust to the results obtained 
at some particular facility. In the case that some non-
identified or unknown errors are present in the 
measurement procedure, the obtained uncertainty estimate 
is wrong as well as the results of the benchmarking. 
Several approaches can be considered for the definition of 
the reference pattern.  
Approach N1: Measurements at several facilities 
One obvious way of improving the reference pattern is to 
measure the antenna at several facilities, preferably by 
different techniques, and average the results. The purpose 
of this approach is, obviously, reducing the risk of non-
identified errors and thus increasing the trust to the 
results. However, effective decreasing of the resulted 
uncertainty is only possible, if the uncertainties of the 
involved facilities are similar, which should be 
demonstrated by carrying out the uncertainty estimates at 
each facility with a standardized approach. The last does 
not exist yet, though several attempts have been made for 
its development, for example, the well known 18 terms 
NIST error budget for planar near-field measurements [5]. 
If the estimated uncertainties are different, proper 
weighting can be applied, such that the resulted 
uncertainty is decreased. 
There are some disadvantages of this approach. For 
example, uncertainties of reproducing the coordinate 
system (both orientation and origin location) add to the 
list, standardized uncertainty estimates are not developed 
yet for many measurement techniques, antenna traveling 
takes time, facilities willing to participate should be found 
and agreements with them should be made. 
Approach N2: Several measurements at single facility 
Another way of improving the reference pattern and thus 
increasing the trust is the following. An antenna pattern 
measured at only one facility can be completely trusted, if 
it is proven that all errors are identified and properly 
corrected, uncertainties are minimized, and their residual 
effects are properly estimated.  
Obviously, the main difficulty lies in ensuring that all 
errors are identified, which represents a serious challenge. 
To this end, facility comparisons play a key role in 
solving this problem as these provide effective means to 
identify any possibly yet unknown errors of some 
particular technique. The comparison campaign carried 
out within ACE in 2004-2005 [3] allowed identifying and 
helped to correct several mistakes and errors in the 
measurement procedures of the participating facilities. 
Some conclusions of the above campaign, relevant for this 
discussion, are given in the next section. 
Special techniques can be applied for minimizing various 
uncertainties and their effect on the radiation pattern. 
Averaging of two patterns in which the effect of some 
uncertainty is represented differently, most desirably in an 
opposite way, is proven to be an effective technique of 
suppression or reduction of the effect of that uncertainty. 
Well known examples are the following: multiple 
reflections and averaging the results measured at 2 
distances differed by a quarter of wavelength, wall 
reflections or compact range reflector edge diffraction and 
averaging the results measured at two or more AUT 
locations in the chamber, averaging two nominally 
identical measurements to improve signal-to-noise ratio 
and reduce the drift effects, etc. Making several 
measurements in the way described above and averaging 
the results, the uncertainty of the determined pattern is 
decreased.  
Approach N3: several measurements at several facilities 
A third way of improving the reference pattern is using a 
combination of the above two approaches. Several 
measurements are performed at each facility and the 
results are averaged, at each facility, aiming on reduction 
of the effect of uncertainties known to be the largest for 
that particular facility. The obtained best patterns from 
several facilities are then averaged in between thus 
forming the most reliable reference pattern with the 
remaining residual uncertainties reduced even more. The 
key requirement in this approach is the existence of a 
clear and easy repeatable definition of the antenna CS. 
This last approach is being used in definition of the 
reference pattern of the VAST12 antenna within the ACE 
network. 
4. Conclusions from the Facility Comparison 
Campaign with the VAST12 in 2004-2005 
As it was already mentioned above, the complete results 
of the campaign are documented in the report [3]. Only 
few conclusions important for the present discussion are 
given below. 
One lesson learned from the campaign was that even small 
errors can result in noticeable deviations of the obtained 
far-field pattern of such complicated antennas as the 
VAST12. For example, despite of a very careful 
mechanical alignment, slight bending of the horizontal 
axis under the weight of the AUT in a spherical near-field 
setup may result in deviations up to ±0.2 dB in the main 
beam of the co-polar pattern. Another example is that 
even using the calibrated probe, but forgetting to check its 
precise polarization alignment, can result in deviation of 
the polarization tilt angle in the peak of the AUT field up 
to 1° and thus deviations of the cross-polar pattern up to 
few decibels. One more example is that transformation 
from the measurement CS to the optical CS or to the 
electrical CS, which involves additional optical 
measurements and determination of the Euler angles, adds 
few uncertainties to the final result, which increases the 
deviations between patterns from different facilities even 
more. This last example clearly shows that the proper 
choice of the CS ensuring minimum amount of 
uncertainties is essential for the definition of accurate 
reference pattern. 
Another lesson learned was that mistakes can always 
happen, especially in the complicated measurement 
procedures, and thus some self-checks are the very 
necessary means to ensure the quality of the results. This 
conclusion seems to be obvious, but in view of the high 
importance of the accurate reference pattern these self-
checks must always be performed for the key alignments 
in the system. 
Finally, it should be said that it was not possible to define 
an accurate reference pattern from the results of this 
campaign due to several reasons:  
• Few mistakes made by the participants resulted in not 
quite accurate obtained patterns from some facilities. 
• Only few facilities could provide uncertainty estimates 
and these were compiled according to rather different 
approaches, which did not allow their direct 
comparison. 
• Results were provided in different CS, which did not 
allow collecting large enough amount of data in some 
particular CS. 
5. Dedicated Measurement Campaign 
Taking into account the conclusions mentioned above, a 
dedicated measurement campaign for definition of 
accurate reference pattern of the VAST12 antenna was 
planned. It was decided that only few facilities should 
participate and each involved facility should carry out a 
series of measurements with small modifications in the 
setup aiming on reduction of the effect of the largest 
uncertainties. In addition, thorough checks and additional 
adjustments, if proven necessary, should be performed in 
order to ensure absence of mistakes and highest quality of 
the results. 
The chosen CS for the reference pattern was the 
mechanical CS, which can be directly implemented in 
many facilities and does not require any additional 
transformations. 
It was agreed that the uncertainty estimates for the 
obtained patterns should be carried our according to a 
unified approach developed in the work package 1.2-2 
“Standardization of Antenna Measurement Techniques” 
of the ACE network 2006-2007[6]. 
Three facilities participated in this dedicated campaign: 
the measurements at the Technical University of Denmark 
(DTU) were carried out in June 2007, at SAAB 
Microwave Systems (SAAB MS) in September 2007, and 
at the Polytechnical University of Madrid (UPM) in 
January 2008.  
The measurements at DTU (spherical near-field facility) 
consisted of in total 14 full-sphere near-field acquisitions, 
which were aimed at estimate of influence and at 
reduction of the following uncertainties: 
• Axes intersection and pointing of the mechanical setup 
• Amplitude and phase drift and noise 
• Receiver non-linearity 
• 
• Multiple reflections between the AUT and probe 
• Mounting structure interference 
The final result is formed as a complex far-field average 
of 12 of the available full-sphere data. 
The measurements at SAAB MS (compact range facility) 
consisted of in total 12 directly measured far-field patterns 
as cuts in the main and diagonal planes, which were aimed 
at estimate of influence and at reduction of the following 
uncertainties: 
• Wall reflections (is this correct?) 
• Edge diffraction at the compact range reflector  
• Multiple reflections between the AUT and the 
compact range 
The final result is formed as a complex far-field average 
of all 12 available patterns. 
The measurements at UPM (spherical near-field facility) 
consisted of in total 18 full-sphere near-field acquisitions, 
which were aimed at estimate of influence and at 
reduction of the following uncertainties: 
• Mechanical uncertainties of the setup 
• Chamber reflections 
• Mounting structure interference 
• Receiver non-linearity 
• Multiple reflections between the AUT and probe 
• Amplitude and phase drift and noise 
The final result is formed as a complex far-field average 
of ??? available patterns. 
While data processing and forming the averaged patterns 
is completed, compilation of the uncertainty estimates is 
still on-going at the facilities. The last are necessary for 
the forming of the reference pattern by averaging the 
results from different facilities with weights inversely 
proportional to the estimated uncertainties. 
Some results of investigation of the above uncertainties in 
the participating facilities are presented in a companion 
paper “Characterization of measurement systems through 
extensive measurement campaigns”. 
6.  Summary 
The existence of an accurate reference pattern of a 
reference antenna allows benchmarking of the antenna test 
ranges and estimating their measurement uncertainties. 
Three possible approaches for definition of a highly 
accurate reference pattern were discussed. A dedicated 
measurement campaign carried out in 2007-2008 for 
definition of the highly accurate reference pattern of the 
VAST12 antenna was described. Conclusions from the 
first comparison campaign with the VAST12 carried out 
within the ACE network in 2004-2005 were taken into 
account and these were also presented and discussed. 
Some typical measurement errors and uncertainties are 
listed and briefly discussed. 
7. REFERENCES 
[1] J. E. Hansen, “Definition, design, manufacture, test 
and use of a 12 GHz Validation Standard Antenna”, 
Executive Summary, ESTEC contract No. 7407/87/NL 
/PB, Technical Report R672, Tech. Univ. of Denmark, 
1997. 
[2] Antenna Centre of Excellence Network web-page, 
www.ist-ace.org 
[3] J. E. Pallesen, S. Pivnenko, and O. Breinbjerg (Eds), 
“EU Antenna Centre of Excellence. WP 1.2-2: First 
Facility Comparison Campaign. Final Report”, Doc. No. 
FP6-IST-508009-A1.2D2, Tech. Univ. of Denmark, 
Report R728, 2006. 
[4] Antennas Virtual Centre of Excellence portal, 
www.antennasvce.org 
[5] A. S. Newell, “Error Analysis Techniques for Planar 
Near-Field Measurements,” IEEE Trans. Antennas 
Propagat., vol. AP-36, no. 6, pp. 754-768, June 1988. 
[6] L. J. Foged (Ed), “Deliverable A1.2D2. Recommen-
dations and comparative investigations for near-field 
antenna measurement techniques and procedures”, Doc. 
No. FP6-IST-026957-A1.2D2, SATIMO, 2008. 
8. ACKNOWLEDGMENTS 
This work was supported by the European Union under 
the contract FP6-IST-026957 “Antenna Centre of 
Excellence” 2006-2007. 
 


Dedicated measurement campaign for definition of accurate reference pattern of the VAST12 antenna

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