WS5: Antenna Measurements at Rohde & Schwarz: The New Test Antenna Chamber
Organiser
Yvonne Weitsch, Rohde & Schwarz, Germany
Abstract
Rohde & Schwarz develops high-quality microwave measurement equipment, antennas and diverse test solutions. The company is member in various bodies in the field of mobile communication to collaborate to standards. In the area of EMC, R&S offers highly accurate measurement antennas as well as the service to characterise and calibrate antennas of customers according to CISPR Standards. The measurements are hampered by the coupling between the environment and the antenna. Therefore, the measurements are carried out under specific known conditions and the measurement results can be corrected by this systematic influence. In the past, 2D radiation diagrams have mostly been sufficient for the characterisation of antennas. However, high-tech and ever advancing antenna systems require the knowledge of 3D radiation characteristics. This applies e.g for direction finding antenna systems to deliver precise bearings. The measurement of 3D patterns in the far field, depending on size of the antenna under test (AUT) and frequency, cannot be handled over a wide bandwidth. An anechoic measurement chamber not only enables the possibility of near field measurement technique with subsequent transformation into the far field. Besides other advantageous, it also provides improved measurement accuracy and after all reproducible test conditions. The comprehensive antenna portfolio of R&S, covering not only broadband antennas but both low and highly directive types, has not allowed for a test system off the shelf. Instead the company needed a tailored test chamber supporting all types of measurements.
The company’s choice of measurement system has been a combined near field / far field antenna chamber measuring 14 m x 10 m x 8 m. The 8-axis positioning system with a positioning accuracy of ±0.02 dB can move AUTs with a weight up to 200 kg and with a length up to 2 m. An exceptionally wide frequency range from 200 MHz to 40 GHz can be measured. The measurement distance can be varied from 1m to 8.5 m.
The workshop will open with the introduction into standards applying for antenna measurements and associated test requirements. Future applications demand for high precision 3D measurement systems. Different test systems are presented as well as the company’s decision for a spherical test system with the option to measure in the near field as well as in the far field. Before having gone into operation, knowledge about the performance of the new antenna test chamber has been gathered by evaluation measurement tests. The workshop will present the range of near field and far field measurements and will explain R&S’ experience in it. The near field measurement technique reveals new topics such as first and full order probe correction. Together with the TU München, R&S investigates the influence of probe correction. The TU München has developed on top an algorithm based on equivalent current sources. This algorithm leaves many degrees of freedom such as full probe correction. Another advantage of using an equivalent current source representation is that the current distribution close to 2 / 2 the aperture of the AUT can be visualised. This is a diagnostic tool which is helpful in the development process. Another topic is to ever enhance the accuracy of the measurements. The presentation gives insight in the technologies used in the company’s test chamber such as the gating function implemented in the ARCS software by March Microwave Systems. Ongoing research at the TU München is echo suppression or the method of the principal component. A singular value decomposition is behind it where the patterns can be split into major and minor components and can be filtered accordingly.
In conclusion, the new antenna chamber enables high-precision 3D measurements from 200 MHz to 40 GHz with great development potential due to continuing research on further increasing accuracy by novel techniques.
