Quasi-Optical MM-Wave/THz Power Splitters are of two general types – polarising and non-polarising. Each type has its own mix of advantages and disadvantages. The explanations given on this page mainly consider the example devices as power splitters. However it is worth noting that these devices can also be used as power combiners in the appropriate circumstances. The devices described on this page are a few examples from a wide variety of splitters/combiners which have been designed and supplied to order by the St. Andrews MM-Wave and THz Technology Group.
Polarising Power Splitters
Polarising splitters provide a well defined, uniform power split ratio over a wide frequency range. MM-Wave polarisers are made from a close spaced array of thin wires, arranged in a plane parallel array. The precise performance depends upon the size of the grid and the wire diameters and spacing. However a typical polarising splitter can provide accurate 50:50 power division over a range from below 50GHz to above 500GHz. Operating ranges can be extended to a few tens of GHz or above a THz if required. In plasma diagnostics applications, the splitter is often placed in a run of oversized waveguide. In such cases the low frequency limit of the splitter is set by the waveguide cut-off, not the splitter.
The picture on the left shows an example of a polariser grid used inside a power splitter or combiner. The UK penny is to give a sense of the size of the grid. In this example the wires were wound parallel to the long waveguide wall. Wire alignments in other orientations, and non-rectangular guide profiles can be produced as required. Click on the image if you wish to see an enlarged version.
In this example, the grid is mounted in WR-90 waveguide (23 by 10 mm) which is oversized for 180GHz. (It is common practice in plasma diagnostics systems to employ oversized guide to reduce the losses in long waveguide runs.)
The picture to the right shows an example of the kind of arrangement into which the above 180GHz polariser has been mounted. The picture also shows a pair of taper transitions. These can be employed if required to couple the splitter/combiner to single mode sources or detectors. Click on the image if you wish to see an enlarged version.
The waveguide tapers shown in the above illustration are to link between WR-90 and WR-5 (1.3 by 0·68 mm) guides. The arrangement can be used in either of two ways.
- As a splitter to divide power arriving along a WR-90 waveguide into two orthogonal polarisation components. The two outputs then being coupled into a pair of power detectors in WR-5 waveguide.
- As a power combiner to take power from two orthogonally polarised sources in WR-5 waveguide. The combined output then directed out along a WR-90 guide.
If required, a planar circulator can be added to convert such a system to a combined transmit/receive arrangement. As described on another page, the MM-Wave Group can provide suitable isolators and circulators for frequencies around 30GHz to 300GHz.
A typical polarising splitter can provide a very accurate 50:50 power division, with an insertion loss well below 0·01dB. The main advantages of this form of splitter are the wideband operation and low loss. The main limitations are that its properties depend upon the polarisation state of the input radiation, and the inevitability that the outputs will be plane polarised. In practice this is not a problem where the input is multi-mode unpolarised, or has circular polarised. Similarly, the polarised output is rarely a problem as it is acceptable for most power detectors, mixers, etc.
Where the input has a known plane polarisation it is normally possible to align the wires so as to obtain the required output power split. In addition, in such cases it is also possible to align the wires of the grid so as to obtain a well defined, controlled, power split of some ratio other than 50:50. So, for example, a wideband ‘10dB coupler’ is possible given polarised input.
Non-Polarising Power Splitters
Non-polarising splitters provide some degree of control over the power division ratio when the input polarisation state is either undefined or unknown. These splitters are of two general types – dielectric, and inductive mesh.
A simple dielectric splitter consist of a single thin sheet of a suitable dielectric material. They are relatively easy to manufacture and use, but have some significant limitations. The chief one being a relatively low peak reflectivity. Makes it difficult to achieve power division ratios that approach 50:50. Typical splitters of this general type can only provide power split ratios of the order of 10:90 or less. Multi-sheet splitters can offer a wider range of performance, but are harder to design and manufacture.
An inductive mesh splitter consists of a ‘crossed’ array of thin wires. The wire spacings and diameters are chosen so that the mesh ‘leaks’ to transmit a controlled fraction of the incident power. By suitable choice of wire diameters and spacings it is therefore possible to choose the required power division ratio over a wide range of values.
The picture to the right shows an external view of a non polarising, inductive mesh, splitter. This example was placed in WRJ-3 waveguide (34 by 72 mm) and was designed to act as a power splitter at 300 GHz. Click on the image to see an enlarged view.
The advantages of the mesh splitter are freedom to obtain a chosen power division ratio, and polarisation insensitivity. The main limitation is that the mesh has a frequency dependent reflectivity. As a consequence, the power division ratio changes with frequency. This makes the device unsuitable as a wideband power splitter. In some circumstances, however, it be exploited to make a form of diplexer. Low frequencies will be reflected and high frequencies transmitted. The device can therefore be used to overlay or separate power in different frequency ranges. It cannot, however, offer the sharp filter characteristics or high stop-band rejection of our drilled plate filters.
The picture on the left shows an example of a polariser grid used inside a power splitter or combiner. The UK penny is to give a sense of the size of the grid. In this example the wires were wound parallel to the long waveguide wall. Wire alignments in other orientations, and non-rectangular guide profiles can be produced as required. Click on the image if you wish to see an enlarged version.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)