The low power efficiency (5-15%) of solid state power amplifiers, working in back-off to guarantee a good linearity, could become the show stopper for some future space and terrestrial communication ventures at Ka-band and above. Semi-active reflector and conformal antennas, using Butler-like matrices between the power amplifiers and the feed elements, were first introduced to directly generate flexible multiple beams most efficiently.For linear and planar arrays, such phase-only control at power amplifier inputs can provide beam zooming or amplitude tapering, but no proper beam steering. The paper discusses and analyses the extension of the semi-active principle to linear and planar arrays, by use of multiport amplifiers with amplitude and phase input control to power the elements most efficiently.Performances and limitations of such systems are analyzed and demonstrated for a few examples.

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Semi-Active Multiple Beam Arrays

The latest generation of mobile link antennas for mobile communication satellites are characterised by the requirement to generate a lattice of similar pencil beams with good mutual isolation. To satisfy this requirement complex antennas with distributed amplification are necessary. The optimum antenna solution in any particular case may be either a direct radiating array or a reflector system, depending on various system characteristics. The paper focuses on direct radiating arrays for the mobile link transmit function and examines the application to such antennas of the latest developments in semi-active architectures. Some fundamental issues concerning beamforming, frequency reuse and grating lobe structure are addressed. A typical active antenna design is introduced for comparison, a semi-active solution for the same example is derived, and preliminary pattern plots are given.

S. Stirland

Papers

Active and semi-active array antennas for mobile communications satellites