This study deals with a Ku/Ka Frequency Selective Surface. Envisaged missions are broadcast or/and Multimedia in Ku and Ka band. Antenna S/S allows the use of two frequency resources to save one or several apertures. On one antenna, it is achieved by means of FSS system introduced between feeds and reflector. The final antenna concept becomes a combined Single Offset antenna (with a RF transparency of the FSS) with a Cassegrain Offset antenna (with a RF reflecting of the FSS).
The generation of low level of cross polarisation and a low loss for a reflected wave or a going through wave are the design target for FSS. The wave could be circularly or linearly polarized.
The objective of this activity is demonstrate the capabilities of Frequency Selective Surface based multifrequency Ku/Ka band antenna and to design, manufacture and test an Engineering Model of a dual Polarised (CP or LP) Frequency Selective Surface.
The EM hardware expected tests are:
- Thermal cycling,
- Mechanical testing
- RF patterns
The RF behaviour in cross-polarization for a commercial frequency reuse, the losses for an efficient antenna and a high power capability are main telecom challenges.
Several missions were imagined as:
ESA reference broadcast mission over Europe in Ku/Ka – Tx/Rx
Broadcast Tx only over USA
Broadband Ku User Multibeam + Ka feeder
Broadband Ku User for low capacity + Ka User/GW for high capacity
A reference scenario broadcasting over USA over the full Ku and Ka bandwidth (Tx and Rx) for dual circular polarization was selected.
Due to the limitations on the achieved performance over the full Ku/Ka band, a narrower band scenario, in Tx only using Circular Polarization, was redefined.
FSS allows combining on a same antenna system two different bandwidths, avoiding a very complex combined feed system. Combining two antennas in one saves room for other possible mission or at least saves mass.
1 shaped beam over USA is generated by means of 2 frequency slots in circular polarisation.
Ku-band Tx: 11.70 – 12.75 GHz
Ka-band Tx: 17.30 – 20.20 GHz
The study is divided into four parts:
- Part 1 (up to BDR): state of the art, antenna requirement consolidation and architecture trade off, requirement consolidation.
- Part 2 (up to PDR): FSS preliminary design (antenna baseline RF design, FSS on composite support, FSS on quartz support, thermal and mechanical analysis), FSS sample manufacturing and testing.
- Part 3 (up to DR): FSS detailed design and analysis, EM manufacturing, and testing
- Part 4 (up to TRB): test report and design verification.
Study stopped at after PDR.
On the FSS preliminary design on quartz technology, it is not considered relevant to continue with further analysis (specification not reached).
Considering the RF results on Kevlar samples, the QUB feedback and the poor thermal capability, it is not considered relevant to continue with further activity.
Kevlar is lossy and with an effective permittivity >1. Both are a major drawback mainly at Ka band.
Above 16GHz it becomes impossible to have identical response for TE/TM. The operation in linear polarization would be simpler, but the insertion losses would remain. It is not possible to compensate by design the difference in Amplitude and Phase over a wide bandwidth (phase is constraining and sensitive). Therefore, the Axial Ratio becomes very bad and only usable on a small band (target ~0.5dB for few hundred of MHz).
For a Ku Ka FSS, a less lossy glue/material and thicker metallisation is needed.
The exact knowledge of the permittivity of each layer of the sandwich (skin, honeycomb, adhesive for the required frequency) and their exact size/thickness is major and key to designing an FSS.The RF behaviour of the honeycomb (with a detailed description) is also a big challenge at SW/HW levels.