FAQ
What is the size of your quiet zone?
The facility is a spherical near-field system. Therefore, the region where we have a planar wavefront is not important, as we can correct for that in the near-field to far-field transform. In that sense the size of the quiet zone is not relevant to this kind of facility. It is also not common to take quiet zone scans in this type of facilities, and we rely on the high quality absorbers to avoid reflections from the chamber walls. We have therefore chosen for the best possible absorbers from Emerson & cuming (please find a datasheet here: absorbers) with most of the chamber walls covered with 12" absorbers and 8" in the corners, which is sufficient above 2 GHz. Typical dynamic range at lower frequencies is around 60 dB, but the higher up in frequency the better of course. The RF system itself has a dynamic range of around 100 dB, and poses no practical limitations on the measurements. Isolation from the shielded room is also up at 100 dB all the way up to 40 GHz.
What is the largest size antenna you can accommodate?
The facility was designed for a nominal AUT diameter of 1m. If your antenna is larger, we may be able to fit it.
What is the load capacity of the antenna positioner?
As can be seen from the drawings on this page we have got our positioners designed to be heftier than usual for small SNFM systems. The specification of the facility allows an AUT load of 60 kg, fitting in a sphere of 1m diameter (an overturning moment of 300 Nm). The specification of the motors well exceed these values (maximum load of 450 kg and bending moment about mounting flange of 678 Nm). For eccentric loads, we need to look into the details to check whether we can accomodate the load with the motors and mounting options we have.
Can we hire the facility? If so, what do you charge for that?
YES! For more information, contact anechoic@ee.ucl.ac.uk
What software do you use?
The whole system has been delivered as a turn-key solution from MI-Technologies. So we also have the MI3000 software from them. It makes use of the spherical NF2FF engine from TICRA, which is the standard in industry. For what data presentation and postprocessing is concerned, we often use customised routines written in MATLAB. We are very flexible in the way we can deliver data to our customers.
What gain references do you use?
So far we have been using Standard Gain Horns (SGH) but as we just started we only have an S-band and an X-band SGH. We will extend our range of SGHs as we go along. We can also short-circuit the range for an accurate gain calibration. Therefore we invested in a $4000 K-type cable with excellent and well known characteristics well above 40 GHz.
What tolerances can you offer?
The directivity of a horn antenna was measured upon factory testing of the facility and compared with theoretical values. The difference between peak directivity was 0.09 dBi at 8.2 Ghz and 0.02 dBi at 10.3 GHz.There is more detailed information on the performance of the facility available on this page.
What is the advantage of the spherical near-field measurement (SNFM) system?
The idea of an antenna measurement facility is to simulate a free-space environment in which the radiation pattern is measured at a very large distance from the antenna under test (AUT), where the wavefront is almost planar (in the far-field). Unfortunately these requirements are not compatible. In an outdoor far-field range, the measurement probe is moved away a very large distance from the AUT. The problem here is that the environment is uncontrolled. Ground reflections, and especially weather conditions, can compromise or delay the measurements. With an outdoor range, one never knows for sure when the measurement will take place. Therefore, a controlled environment is preffered. An anechoic chamber can offer a controlled free-space environment, by absorbing the waves inciding on its walls. However, a realistic anechoic chamber is too small to place the measurement probe in the far-field. A compact range (CR) solves this problem by placing the measurement probe in the focal point of a parabolic reflector, so that a planar wavefront is produced in the so-called quiet-zone of the anechoic chamber. The drawback of a CR is that there are unknown errors from the shape of the reflector, diffraction from its edges and so forth. Therefore, for a CR to be useful, it needs to be a high-quality top-notch super-expensive facility. There are not many facilities around, and they run on a pretty tight schedule. Near-field systems then, are implemented in a much smaller, and therefore less expensive and easier to operate environment. As the name implies, they calculate the far-field from data gathered in the near-field. The transformation can be performed if complex samples (phase and amplitude) are taken in the near-field, as opposed to amplitude data only. Several near-field configurations are possible, most notably the planar, cylindrical and spherical configurations. Obviously, the spherical configuration is the only one that can actually obtain a full 3D pattern around the antenna. But also, it is necessary to obtain data over a full sphere surrounding the AUT to avoid truncation errors in the near-field to far-field transformation. A SNFM system is the only near-field system that can avoid these truncation errors, and can cope with low-gain antennas. Arguably, a SNFM system is the most cost-effective all-round solution for antenna measurements, that combines a controlled environment with 3D radiation patterns and reliable scheduling.
Can you also do RCS measurements?
Unfortunately it is not possible to do RCS measurements in a near-field facility. For proper RCS measurement you need a compact range instead (which is why RCS measurements are so expensive). For some basic CW reflectivity measurements (typically for research purposes) we may still be able to help, as we do this to support our own research.