Within the ISAS project the design and measurements of low scattering posts for Ku-band DGR antenna applications was addressed. Three companies were involved in this study: the prime contractor HPS GmbH (Germany) and the subcontractors Astrium Les Mureaux (France) and MDA (Canada).
HPS was mainly in charge of the mechanical design and analyses of the low scattering posts, their fabrication and the mechanical measurements. Astrium Les Mureaux was responsible to provide a dual gridded reflector for the measurements and to perform the DGR antenna mechanical analyses. Finally, MDA was responsible for the RF design and measurements of the low scattering posts as well as to perform the antenna RF analyses and measurements.
Two types of low scattering posts have been studied: a dual-band design and a single wideband design. For both a suitable design was assessed in detail and their performances was compared to the ones of a reference conventional post consisting of a dielectric honeycomb sandwich. For the wideband design a RF test campaign was performed in a DGR antenna setup.
The overall objective of this activity was to define and assess novel methods for DGR intercostal posts that minimize the impact on the RF-path i.e. mitigate the reflection, diffraction and blockage induced by them.
Validation of the objective was performed by designing, manufacturing and testing an antenna demonstrator. An assessment of the improvement achieved has been done by comparing RF performance with an antenna implemented using conventional technology. Mechanical analyses showed that RF improvements are obtained while maintaining (and partly improving) the structural properties of the antenna.
Fig: Low scattering posts developed within the ISAS project
click for larger image
Dual gridded reflector antennas (DGR) are used in space communications applications that require linear polarization with good cross-polarization isolation performance. A DGR is usually composed of a gridded front reflector and a solid rear reflector. To ensure the structural integrity of the DGR antenna and to minimize the thermo-elastic distortions of the front reflector, intercostals support structures are used in between these reflectors. These posts are located inside the rear reflector aperture where field strengths are relatively high. They generate scattered fields which significantly degrade the RFperformances. The effects of the scattering on the antenna performances can include: increased losses, increased sidelobe levels and crosspolarization levels, increased gain variation over the frequency band.
Conventional posts used in DGR antennas are made of tubes of dielectric or dielectric sandwiches made from Kevlar composites. Several studies have been done to reduce the scattering of posts. However, not much work has been done to try to design low scattering posts for DGR antennas. Within the project low scattering posts to improve the performances of Ku-band DGR antenna for a geostationary Tx/Rx communications satellite application have been developed.
Two different post designs have been elaborated and RF-tested on coupon level: For the option 1 post structural and RF related characteristics were separated from each other by covering a structural CFRP rod with an RF guiding metamaterial stack. Option 2 was designed such that structural and RF related characteristics were combined at the same time into a sandwich construction.
The study was performed in six major work packages:
WP1000 State of the Art and Requirements,
WP2000/3000 Preliminary Design of Antenna and Posts,
WP4000 Critical Technology, Manufacturing and Test of Posts,
WP5000 Antenna Demonstrator Detailed Design,
WP6000 Antenna Demonstrator Manufacturing and Test, and
WP7000 Conclusions and Development Plan.
The challenges addressed in the development of the post were mainly split into two parts: On the one hand the RF functionality development and on the other the structural parts development.
Within the RF development a so called meta-material stack was developed guiding the incidence RF waves around the post. Herein, special care had to be taken on proper RF modelling and prediction in order to elaborate a suitable design considering the structural and mechanical constrains.
For the structural development a post design had to be elaborated incorporating the RF relevant parts and properties but also fulfilling the thermo-mechanical requirements relevant for the performance of the dual gridded reflector. Thereby, special care had been taken regarding requirements such as weight, thermo-elastic behaviour, mechanical capabilities, mechanical integration to the dual gridded reflector, space compliant materials and especially structural integrity of the meta-material stack under space environment (CTE mismatch, cryogenic as well as high temperature capability, adhesion).
The most important conclusion of the work performed during this project is that it is possible to improve the performance of DGR antenna by using advanced post designs based on hard surfaces or layered construction. The simulations and measurements showed that the DGR with the low scattering posts exhibits better isolation performance than the DGR with conventional composite Kevlar posts. The main conclusions of the study are:
- The deterioration of the isolation performance due to the posts in the DGR can be reduced by using invisible posts. The measured improvement in isolation is significant but lower than predicted,
- The measured performance in the coverage area with the invisible posts is well predicted and equivalent to the one achieved with the reference posts,
- The agreement between the measurements and the predictions is fairly good in the isolation regions given that the gain levels in these regions are very low. There are however discrepancies between the predictions and measurements that cannot easily be explained by measurement uncertainties or fabrication tolerances. The most probable cause for these discrepancies is modeling inaccuracy,
- Better software tools would be needed in the future to fully develop the low scattering posts technology for DGR applications.
Fig.: Integrated power in an isolated region (ISO-2) as a function of frequency: (left) simulations, (right) measurements
The project was finalized December 2011.
Last Update: 24 May 2012