Single and dual band horns for multi-beam satellite antennas - application of artificially hard surfaces

Doktorsavhandling, 2005

The latest trend for the multimedia satellite systems is operation at the Ka-band, 17.7-20.2 GHz downlink and 27.5-30.0 GHz uplink. This will enable several new services and lead to cheaper and smaller terminals for the end-consumer. The common configuration for the onboard antennas for these systems are reflector antennas with a cluster of feeds, from which each feed creates a spot beam on earth. In order to realize many and efficient spot beams, the feeds in the cluster must be small and tightly packed. Furthermore, in order to increase the frequency usage, the complete frequency band (and thus also the total footprint of all spot beams) is subdivided into smaller frequency channels, which are shared between the spot beams. This leads to stringent requirements on isolation between beams with the same frequency. It is also evident that smaller feeds normally have low efficiency caused by increased spillover losses outside the reflector. The objective of this work is to address feed solutions that can satisfy the requirements on ground coverage and beam isolation in both the 20 and 30 GHz bands. This work was initiated with a trade-off study of different types of common and possible horns for these systems in which the bandwidth of aperture efficiency and cross polarization, mechanical manufacturability and usability in space were considered and discussed. The conclusion was that medium to high efficiency horns with wide or dual 20/30 GHz bandwidth are most suitable. One solution which was considered interesting was the hard horn, which was proposed as feed for the satellite cluster-fed reflector antennas already in 1988, but not truly investigated before now. The hard horn is most easily realized by using longitudinally corrugated walls filled with a dielectric. Therefore, the work continued by analysis of the corrugated hard horn and the use of it as feed in cluster-fed multi-beam satellite antennas for Ka-band. The study first considered hard horns with conical shape so that they support a single mode. They were investigated by using a classical-type open-ended waveguide model based on homogenization of the corrugated wall region. Thereafter, the multi-mode hard horns were introduced in order to reduce the size. They were studied using both an analytical approach, an in-house mode matching code (which is also based on homogenized hard walls), and commercial FDTD software. The finite period of the corrugated horn was investigated using a Finite Element Method program. A dual band multi-mode hard horn was designed, manufactured and measured. The present studies show that the hard horn is a good fed candidate for cluster-fed multi-beam satellite antennas in particular when dual band operation is needed, such as e.g. 20 and 30 GHz. The multimode hard horn is actually a compact new feed alternative in any single- or dual-band reflector system.