Abstract

The present thesis is concerned with the design of compact antenna arrays.
The terms "compact" and "miniaturised" are used interchangeably for antenna
arrays where the element separation is smaller than the conventional
lambda/2 distance. Ultimately, the goal is to make more efficient use of
the frequency spectrum by improving the diversity capabilities of the
antenna arrays used.

Theoretical and practical limits of antenna array miniaturisation are
discussed. The physical phenomenon of superdirectivity is examined, which
accounts for the strong coupling seen between closely spaced antenna
elements. An analysis of the physical limits based on spherical harmonics
reveals that six orthogonal polarisation modes exist even in the case of
electrically very small arrays. Properly exploiting these degrees of
freedom can therefore lead to an improved performance of compact antenna
array designs.

It is found that an antenna array is optimally described in terms of its
overall eigenmode efficiencies. Eigenmodes can be interpreted as an
orthogonal basis for radiation. Practical examples demonstrate that
eigenmode matching as well as the eigenmode quality factors are suitable
measures for the successful optimisation of compact antenna arrays.

The major contribution of this work is the development of a new design
method for decoupling and matching networks. Decoupling and matching
networks counteract gain reductions caused by reflections due to eigenmode
mismatch. The new method takes advantage of all degrees of freedom
available for network design. Furthermore, effects of network losses are
taken into account during the design procedure. For the first time it is
demonstrated how a set of desired port beam patterns can be orthogonalised
and subsequently realised as part of a decoupling and matching network.

Methods for reducing network complexity and optimising network element
tolerances are introduced, which make practical implementations feasible.
Diversity gain improvements due to the decoupling and matching network are
shown to reach between two and three decibels.

Another question addressed in this work deals with the advantages and
disadvantages associated with different network realisations.
Theoretically, the specific set of orthogonal beam patterns is shown to be
of secondary concern. More important is minimisation of ohmic losses as
well as parasitic energies stored within the network.