Results of an intercomparison for free space antenna factor measurements within the German Calibration Service (DKD)

In this paper we discuss the results of an intercomparison for free space antenna factor measurements performed within the German Calibration Service (DKD). Three different types of antennas covering the frequency range from 30 MHz to 26.5 GHz have been calibrated in five different laboratories using different methods and calibration sites to obtain the free space antenna factor. The results agree well within the uncertainties specified by the laboratories suggesting that different approaches and different measurement sites to obtain the free space antenna factor are well compatible.


Introduction
Antennas are extensively used in communication systems, remote sensing and navigation but also in wireless energy transmission and electromagnetic compatibility testing and measurement.New antenna applications foster more and more the integration as well as the expansion of the frequency range.Both, the increasing importance of interoperability aspects and the need to decrease energy consumption in complex systems, require the accurate and reliable knowledge of antenna factors for a wide range of antenna types.This requires determination of antenna factors traceable to the International System of Units (SI) including the specification of measurement uncertainties (GUM, 2008).In Germany, three laboratories are accredited for antenna factor measurements by the German Accreditation Service (Deutsche Akkredi-tierungsstelle -DAkkS).To assess the technical competence of the accredited laboratories an intercomparison was organised by the Physikalisch-Technische Bundesanstalt (PTB), the German National Metrology Institute, within the framework of the German Calibration Service (Deutscher Kalibrierdienst -DKD), which is the association of the accredited laboratories in Germany (PTB-Mitteilungen, 2015).
Among different antenna quantities the free space antenna factor is the most suitable quantity for a measurement comparison of different antenna types.As different approaches for its determination are available and different types of measurement sites are used, goal of the intercomparison was to find out whether this leads to comparable results.Measurements can either be performed on open-area test sites (OATS) or in semi-anechoic chambers (SAC).In both cases, the conductive ground-plane leads to height-dependent antenna properties.The standard-site method (SSM) follows the approach to determine free space antenna factor from site-attenuation measurements in horizontal polarization (determining the minimum transmission attenuation between two antennas with a fixed antenna height of the transmitting antenna and a height scan of the receiving antenna) taking into account the ground reflection.The SSM is used in different national (ANSI C63.5, 2006) and international standards (CISPR 16-1-6, 2014).Alternatively, the calibration can be performed in such a way that the indirect transmission path is minimized, e.g. in a fully anechoic room (FAR) or above a ground plane in vertical or diagonal orientation at a sufficient Published by Copernicus Publications on behalf of the URSI Landesausschuss in der Bundesrepublik Deutschland e.V. height.Standards such as IEEE 149-1979(ANSI/IEEE 149-1979, 2002) and SAE ARP958D (SAE ARP958D, 1999) use these simpler approaches.In all cases, the antenna calibration can be based on two-antenna methods (calibrating two equal antennas or using a known reference antenna) or the three-antenna method (measuring pairs of three unknown antennas).Finally, three accredited and three non-accredited laboratories took part in the intercomparison.As measurement artefacts three different antenna types were chosen covering the most common types of measurement antennas: the biconical antenna Schwarzbeck VHBB 9124 with antenna elements BBAK 9137, the logarithmic-periodic antenna Amplifier Research AR ® AT1000B and the logarithmic-periodic antenna Rohde & Schwarz ® HL050.
The free-space antenna factors were measured in the frequency range between 30 MHz and 26.5 GHz using different calibration methods (SSM and free-space calibration with vertical or diagonal orientation) in different calibration sites (SAC and OATS).The compatibility according to CEI IEC 60359 (IEC 60359, 2001) of the free-space antenna factors of the participants was ensured.
In the next chapter we describe the travelling standards, the intercomparison schedule and the approach taken for data evaluation.In the following three chapters we then present the results for the three different antennas and draw conclusions in the final chapter.

Travelling standards
For the lower frequency range between 30 and 300 MHz a biconical antenna Schwarzbeck VHBB 9124 with antenna elements BBAK 9137 as shown in Fig. 1a was measured with a step width of 1 MHz.
For the intermediate frequency range between 200 and 1000 MHz a logarithmic-periodic antenna Amplifier Research AR ® AT1000B was circulated.The antenna is shown in Fig. 1b.As this antenna type has a phase center position that depends on frequency, the antenna factor was determined with regard to a mark on the antenna.The antenna was measured with a frequency step width of 5 MHz.
In the upper frequency range between 800 MHz and 26.5 GHz, the logarithmic-periodic antenna Rohde & Schwarz ® HL050 shown in Fig. 1c was measured with a frequency step width of 50 MHz.As this antenna has a frequency dependent phase center position as well, the antenna factor was determined with regard to the antenna tip.

Time schedule
The antennas were first measured at Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, then at  campaign took place from September 2011 until January 2012.

Data evaluation
The intercomparison was evaluated based on the measurement results reported by all laboratories that were able to calibrate the distinct antenna.According to Cox et al. (2002)

CRV
The weighting factor w i results in a higher contribution of the laboratories with lower uncertainties to comparison reference value CRV.From the antenna factors AF i and the measurement uncertainties U(AF i ) reported by the individual laboratories and the comparison reference value CRV a degree of equivalence DoE and its uncertainty U(DoE) has been calculated using as a measure for the deviation from the expected result of each laboratory.
3 Results for biconical antenna Schwarzbeck VHBB 9124 with antenna elements BBAK 9137 The calibration results for the antenna VHBB 9124 with antenna elements BBAK 9137 are shown in Fig. 2. The antenna has been measured with the SSM according to ANSI C63.5 at OBL, R&S and BV, whereas it has been measured in vertical or diagonal orientation by R&S, Schwarzbeck, BV and PTB.The SSM measurements at a distance of 10 m between the antennas were performed in OATS (OBL, R&S) and in an SAC (BV).The free space measurements were performed at a distance of 3 m between the antennas with vertical orientation (R&S, PTB, BV -according to IEEE 149) and at

Conclusions
The intercomparison between different laboratories calibrating antennas in Germany showed that the calibration results for the free space antenna factor are compatible although different calibration methods and different types of calibration sites are used.The specified measurement uncertainties are realistic.