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The objective is to design and manufacture VSAT Waveguide Lens Antennas (WGLA) for the Ku-satellite band, 10.75-14.50 GHz.
The antenna is receiving TV and internet at 10,75-12,75 GHz and transmitting at 13,75-14,50 GHz. This large bandwidth is made possible with a new approach of the geometry of the WGLA compared to previous designs for TVRO application.
The main benefit of using the WGLA compared to a conventional dish is its ability to multi-satellite capture. The antenna systems will provide communication also through wide offset angles. Feed horns especially designed to match the WGLA are being developed which results in increased overall gain of the system.
The WGLA system is seen to be especially beneficial for applications where multi-satellite communication is required. A potential application is to integrate the antenna on a stabilised platform and several satellite links can then be maintained from one single antenna system.
The key issue is to achieve sufficient bandwidth for reception and transmission in the Ku satellite band, 10.75-14.50 GHz, with a WGLA. Special care is taken on the antenna geometry for minimised frequency sensitivity.
The side-lobe levels needs to be within strict envelopes and the cross polarisation to be >35 dB. Feed horns are designed to especially match the WGLA and as a result maximise the gain. The manufacturing of the antenna is seen as a key issue as dimensional accuracy is required in order to fulfil the specification for transmission.
Modelling of the antenna with computer codes is also a comprehensive task due to its complexity and high size/wavelength-ratio. Successful antenna simulations with FDTD code have been made using a 72 processor super-computer.
The biggest benefit of the WGLA is its scanning performance and ability to capture multiple geostationary satellites. The mentioned filtering functions as well as the behaved electro-magnetic performance and low noise temperature is also seen as a particular advantage.
As the feeds and RF packs are situated behind the WGLA there is no blocking of the signal. The curvature of the geostationary satellite orbit for different latitudes is easily mirrored on the feed arm. For e.g. toroidal dual reflectors the sub-reflector shape is only valid for a latitude region i.e. the efficiency will be reduced closer to the equator or at northerly/southerly positions. Low values of VSWR have been measured making it suitable for transmission. As a result overheating, which is occurring for phased array systems, is not a problem for the WGLA.
As we have seen there is a wide range of benefits with WGLA technology compared to conventional single and dual reflector antennas. The solution is also considerably more cost effective than phased array systems.
The WGLA is an array of waveguides. The lens is transforming a spherical wave from the feed to a plane wave. This is done through the phase modulation through the waveguides where the guide wavelength is a function of frequency and waveguide widths.
The feeds are fixed at the focal plane of the lens and signal with down to 2 degrees separation can be handled simultaneously The scanning properties of the WGLA is seen as one of its strongest features. The feed horns are especially designed to match the WGLA in terms of efficient illumination and also the small variations of focal point shift over frequency that is present for the lens. This is compensated for by designing the feed horns with a phase centre shifting with frequency that matches the focal shift of the WGLA.
The feed horns are of corrugated type and the geometry of the antenna allows very low system cross polarisation to be achieved which is a requirement for transmission. As the feed horns are pointing upwards to the satellites in the cold sky low noise temperature figures are obtained. The cut off frequency of the waveguides in the lens means that the antenna acts as a filter where frequencies below 9 GHz e.g. radar are blocked by the lens and frequencies above 15 GHz will not be focused.
Figure 1: Principal operation of lens with feeds. Spherical to plane wave representation for the nominal position and an offset angle.
The project is to be done through a number of cycles. The WGLA's are designed, simulated and tested in parallel with the feed horns. After tests refinements are done before the final product is completed. Development and optimisation of production methods of the WGLA are done in parallel. The outcome of the project shall be two VSAT antennas with matched feed horns and the system shall concur to present VSAT specifications e.g. Eutelsat type approval.
Antenna simulations with FDTD code have been done on a number of different lens variations. An example of simulated radiation patterns of a WGLA is shown below. Two antenna prototypes are now under production and will be field tested during summer 2008. Lab tests at ESTEC for radiation patterns are planned for September 2008.
In FDTD code simulated radiation patterns at 13.75 GHz of a WGLA