Andy Pye takes a brief look at a new method of keeping down the cost of antenna systems in the microwave and millimetre ranges.
Electromagnetic waves are at the heart of our daily lives, through smartphones, computers, tablets, cars, trains and planes – none would work without them. Since its creation in 1987, The Microwave Vision Group (MVG) has developed expertise in the visualisation of electromagnetic waves, while evaluating the characteristics of antennas, and helping speed up the development of products using microwave frequencies.
A Compact Antenna Test Range (CATR) is a facility used to provide convenient testing of antenna systems at frequencies where obtaining far-field spacing to the antenna under test (AUT) would not be feasible using traditional free space methods. This is typically for microwave and millimetre wave frequencies where the far-field distance is large, such as with high-gain reflector antennas. It was invented by Richard C. Johnson at the Georgia Tech Research Institute.
The CATR uses a source antenna which radiates a spherical wavefront and one or more secondary reflectors to collimate the radiated spherical wavefront into a planar wavefront within the desired test zone. A typical embodiment might use a horn feed antenna and a parabolic reflector.
The size of the range that is required can be much less than the size required for a full-size far-field anechoic chamber, although the cost of fabrication of the specially-designed CATR reflector can be expensive, due to the need to ensure precision of the reflecting surface and to specially treat the edge of the reflector to avoid diffracted waves which can interfere with the desired beam pattern.
Accurate testing of low cross-polar antennas in CATRs requires a quiet zone with high polarisation purity. In the testing of electrically large antennas fitted to large platforms, this requirement is often the main motivation to choose the more complex and thus more expensive compensated dual reflector CATR as opposed to the simpler and cheaper single reflector CATR.
Now, with the CXR feed, MVG has developed and validated a new plug-and-play component that performs cross-polar reduction, performing similarly to the second reflector in dual compensated CATR. The CXR feed is designed to significantly improve the cross-polar accuracy of side/corner-fed single reflector systems, but is equally suitable for dual cylindrical reflector systems.
The concept behind the CXR feed is the cancellation of the Geometrical Optics (GO) cross-polar component induced by the offset reflector of any single reflector CATR configuration. This is achieved by means of an innovative architecture providing conjugate field matching over a bandwidth of 1.5:1.
Here, the central element produces a highly pure horizontal component. The cross-polar component at the Quiet Zone is fully induced by the reflector Geometrical Optics. The lateral elements are cross-polarised with respect to the central element. The two-element array is designed to generate a conjugate matched pattern. The overall effect is the cancellation of the Geometrical Optics cross-polar component at the Quiet Zone, while the co-polar performance is fully preserved.
While maintaining all other quiet zone performance parameters, the feed provides uniform low cross-polarisation levels, typically 40dB cross-polar discrimination. This is directly translated to an improvement of the uncertainty of the system.
The dual linear polarisation of the CXR shortens the feed alignment time and can greatly reduce the measurement time.
Talking about this latest development, MVG’s Scientific Director Lars Jacob Foged, explains: “High polarisation purity of the test area undoubtedly impacts measurement accuracy. This novel feed concept is a breakthrough in CATR systems as it extends their measurement capabilities beyond the traditional limitations. And this at the affordable cost of a feed replacement.”