Figure 1. A 4-GHz horn antenna manufactured for Inmarsat by RYMSA
Contractors:
RYMSA (E), IRSA (E)
Funding:
Basic Technology Research Programme
Many satellite communication services employ horn antennas to achieve wide area coverage. These antennas radiate a symmetrical beam within a cone of 9 degrees from geostationary orbit. In particular, satellites used by both Inmarsat and Intelsat use such antennas at C-band. In the case of Inmarsat, where the bandwidth requirements are narrower, there is a strong possibility that a much more compact design can meet requirements.
Figure 1 shows a photograph of the 4-GHz horn antenna manufactured in carbon fibre material by RYMSA for Matra-Marconi. Although this antenna has low mass, its overall length, including the launching and polariser section, approaches one metre. The antennas described in this article are greatly reduced in volume and mass yet offer potentially similar performance.
A configuration based on the Von Trentini antenna (Figure 2) was studied. This antenna consists of a waveguide feed, positioned in the centre of a perfectly-reflecting circular ground plane. In front of the feed are placed one or more semi- transparent circular sheets of material, each of which may be a dielectric or, as is shown in Figure 2, an artificial dielectric screen, comprising a pattern of holes in a conducting plate. The gain of the openended waveguide feed is increased by selecting an appropriate value for the (effective) dielectric constant of the sheets. Rays incident on the screen are partially reflected and partially transmitted. The value of the reflection coefficient C determines the aperture distribution over the sheet. The distance between the screen and ground plane is chosen such that the rays transmitted into space have equal phases in the desired direction of maximum radiation, usually normal to the sheet.
Several configurations of this antenna were studied and prototypes were tested.
Figure 1. A 4-GHz horn antenna manufactured for Inmarsat by RYMSA
Figure 2. The configuration of a Von Trentini antenna
The study was based on requirements detailed in Table 1. These are comparable to those for the horn antennas of Inmarsat III, taking the receive antenna as a specific case. The launching section of the antenna is the same as that used for horn antennas and consists of a square septum polariser feed in the ground plane with two rectangular waveguide inputs, one for each direction of circular polarisation. The septum polariser has the following characteristics:
Axial ratio < 0.3 dB Isolation > 30 dB Return loss > -25 dB
The first prototypes used artificial dielectric screens consisting of holes drilled in a metal plate; the final design used a double screen. The antenna was analysed using the principles of geometrical optics. The finally selected prototype was 5.5 wavelengths in diameter and is shown alongside an equivalent horn antenna in Figure 3. The performance is summarised in the Table 2. The observed minor non-compliances in cross polarisation and gain are capable of correction.
The Von Trentini antenna has an open structure, and to obtain improved mechanical and electrical properties, it was decided to investigate a closed cavity design which offered a stiffer structure and a further reduction in aperture size. The electrical design of this antenna, based on Harrington's method-of-moments, led to the close cavity design shown in Figure 4 whose aperture was five wavelengths. The electrical performance shown in Table 2 complies with specifications.
Figure 3. Compact Von Trentini antenna (left) and global horn antenna (right)
Figure 4. A closed-cavity compact antenna
Central Frequency 6.44 GHz Bandwidth 32.6 MHz Polarisation Dual cicular Cross polarisation -31 dB Isolation at I/P ports -20 dB Coverage ±9° Edge of coverage gain -7.15 dB V.S.W.R. < 1.15
Compliance Parameter Requirements Von Trentini Closed-cavity ________________________________________________________________________ Bandwidth 32.6 MHz Yes Yes Polarisation --- --- Yes Cross polarisation -31 dB (±9°) -28 dB Yes (EOC) Isolation input ports -20 DB Yes Yes Gain (EOC) 17.15 dB (±9°) 16.5 dB Yes V.S.W>R. < 1.15 Yes Yes
It is feasible for a compact antenna to replace a narrow-band global coverage horn antenna. However future work is required to improve electrical performance. and to widen the bandwidth over which a compliant input match is obtained, creating additional margin to accommodate thermal variations. This activity together with a full development of a flight compatible design is strongly recommended.
Preparing for the Future Vol. 5 No. 2.