Research interest: In recent years we have seen the emergence of
commercial applications at high frequencies, such as the top part of
the microwave band and the millimeter and sub-millimeter bands,
and it is expected a big increase in the coming years. This growing
demand requires a rapid development of low-cost technology
with good performance at these frequencies, where common technologies,
such as microstrip and standard waveguides, have some
shortcomings. In particular, existing solutions for high-gain planar
scanning antennas at these frequencies suffer from the disadvantages
of these technologies giving rise to high-cost products not suitable
for high volume production.

Objectives: The main objective of this thesis is to study the feasibility
of a new proposal to improve existing solutions to date for
low-cost high-gain planar scanning antennas at high frequencies.
This overall objective has resulted in another central objective of
this thesis, which is the research of new quasi-TEM waveguides that
are more appropriate than current technologies for the realization
of circuits and components at these frequency bands. These guided
solutions make use of periodic or artificial surfaces in order to confine 
and channel the fields within these waveguides.

Methodology: The work follows a logical sequence of specific tasks 
aimed at achieving the main objective of this thesis. Chapter 2
presents the proposed guiding solution and shows its performance
numerical and experimentally. The optimized design of high-gain
antennas based on waveguide slot arrays requires the development
of efficient ad-hoc codes. The implementation and validation of this
code is presented in Chapter 3, where a new method for the analysis
of corrugated surfaces is proposed, and in Chapter 4, which extends
this code to the analysis of waveguide slot arrays. The process
design and optimization of a two-dimensional array is described
in Chapter 5, where a preliminary experimental validation is also
described. Moreover, the proposed guiding solution has inspired
the development of a new guiding technology of wider bandwidth
and more versatile for the realization of circuits and components at
high frequencies. Chapter 6 presents the contributions to the study
of this technology and its application to the design of circuits.

Results: It has been shown numerical and experimentally that the
two guiding structures, which have been studied here, one designed
to support a slot array, and the other one especially interesting for
the realization of circuits and components, have particular characteristics
that give them great potential for application in antennas
and circuits at high frequencies. The feasibility study culminates
with the design of a two-dimensional slot array fed by the proposed
guide. For this design an efficient ad-hoc code based on a
new method for the analysis of corrugated surfaces has been used.
This numerical implementation is one of the main contributions of
this work. The application of the second guiding structure to the
realization of circuits has been demonstrated through the design,
fabrication and measurement of some microwave circuits.