PROJECT 245 : SOME KEY ACHIEVEMENTS

Working Group II: Radiating Elements and Arrays

Mobile communication satellites require circular polarized radiating elements to feed reflectors or for direct radiating arrays.

In addition to launch and space environmental conditions, one very critical requirement, also present for cellular base station antennas used for mobile telephony, is that of very low generation in high power transmitting antennas of passive intermodulation that would interfere within the receiving band.

Fig. 8 shows feed elements developed in conjunction with the COST project 245 for the ESA satellites EMS and ARTEMIS with extremely low passive intermodulation. This technology is to be exported to equip several US satellites.

Fig. 8: Feed array for ARTEMIS L-band antenna (coutresy of SES-Sweden)

The L-band reflector antenna on the ARTEMIS satellite is used for mobile communications. It has a combined receive/transmit feed array. The many carriers at the transmit band and the small frequency separation to the receive band makes it very sensitive to passive intermodulation (PIM). A very low PIM design has been achieved by using very few, silver plated parts. The elements are patch-excited cups, which give very high aperture efficiency and low cross polarisation radiation. (WS Proc. p 303) The antenna uses the COST Project 245 semi-active feeding concept. (WS Proc. p 295)

For satellite communications and broadcasting, dual polarization antennas operating over large frequency bands are required. Several designs have been successfully developed in conjunction with COST project 245. The electric field is concentrated in the air below the suspended substrate and extremely low losses can be achieved. Another successful development of dual polarized radiators is illustrated in fig. 9, which shows microstrip radiators in a novel suspended technology.

Fig.9 Low loss "suspended technology" dual polarized radiating elements. (Courtesy Saab Ericsson Space, Sweden).

Fig. 10 Dual polarisation toroidal antenna (Courtesy Tech. Univ. of Denmark)

The support structure for a C-band dual linearly polarized antenna with toroidal pattern for TT&C purposes developed in cooperation with Saab Ericsson Space and the Technical University of Denmark is shown in fig. 10. All elements are dual polarized and use a fork shaped microstrip feeding network for each polarization. Each fork excites one arm of a crossed slot, while the perpendicular slot is not excited. The crossed slot couples to a rectangular patch, which in turn radiates the microwave energy into free space. This new feeding techniques ensures that large bandwidth can be achieved simultaneously with extremely high isolation between the two ports feeding orthogonal polarizations. (WS Proc. p 71). On the support structure photographic films are mounted. These films correspond to the circuits used for characterization of the embedded microstrip antenna elements on the cylindrical surface. The dielectric layers are organized as follows : - On the innermost dielectric layer (suspended and shielded)microstrip lines which comprise the feed network for the TC element(s) (f0 = 6.3 GHz) is shown. - The central dielectric layer contains the etched crossed slots facing inwards and the feed network for TM element(s) (f0 = 4.1 GHz) facing outwards. - On the outermost dielectric layer the suspended rectangular patches facing inwards are shown. For the final array (not shown in the figure) the upper and lower 16 element cylindrical arrays operates independently. Hence, a set of redundant ports are available for each band/polarization as often required in space applications.

Another COST-245 development is a 4-element active array for satellite TV reception at 12 GHz. Every patch includes its own low noise amplifier.

In fig.11a, a 4x4 Butler matrix is build in low dielectric stripline. The matrix is designed in copper foil and supported by foam. The circuit has very low losses and is inexpensive to manufacture. The complete array is shown in fig. 11b, as designed and built by the mm-wave group at Chalmers University.

Fig. 11a Novel Butler matrix fed array for mobile communications (Courtesy Chalmers University & Allgon, Sweden)

Fig. 11b A complete 4 element antenna for PCS 1900 system designed with the previously shown 4x4 Butler matrix. The antenna has 4 independent lobes to cover a 120=B0 cell. (Courtesy Chalmers University & Allgon, Sweden)

Fig. 12 Dual polarisation slotted waveguide antenna (Courtesy Chalmers University & Saab Ericsson Space, Sweden)

In parallel with the above work, several waveguide antennas have been developed at Saab Ericsson Space to generate dual polarization, and dual and triple beams from one aperture. As an alternative at lower frequencies, large microstrip arrays have also been developed. An extremely lightweight and low-cost design uses a thin suspended carrier for the radiating patches. The fig. 12 shows a laboratory model of a dual polarized X-band waveguide slot antenna developed for use in future synthetic aperture radars (SAR) in space. Such SAR antennas will be used for remote sensing of the land and see surface. The antenna group at Chalmers University, Gothenburg, Sweden has within the COST 245 project developed analysis methods and software tools for design of such antennas. The model above consists of longitudinal slots in the broad walls of a set of parallel rectangular waveguides, and transverse wire-excited slots in the narrow walls of another set of parallel waveguides interleaved with the first set. The work has included the calculation of radiation patterns, slot admittances and mutual coupling of different slot geometries as illustrated above. The developed methods are very general and they can handle any cross-sectional shape of the structure from which the slots radiate. The method is referred to as using a spectrum of two-dimensional solutions (S2DS).(WS Proc. p 17).

COST-245 Summary report

- 12 -

Mars 1997 Content Responsible : Technical Manager : Webmaster@lema.epfl.ch