Design of Antennas for Handheld DVB-H ... - Lunds tekniska högskola

Major operational disadvantages of microstrip antennas are their low efficiency,
low power, high Q and very narrow frequency bandwidth. These drawbacks are
avoided by building antennas on thick low-permittivity substrates, where larger
bandwidth and efficiency are achieved.
The four most popular configurations that are used to feed microstrip antennas are
the microstrip line, coaxial probe, proximity and aperture coupling. These are
displayed in figure 3.2. The microstrip line feed (figure 3.2 a) is easy to fabricate,
simple to match by controlling inset position and rather simple to model.
However, this structure cannot be optimised either as an antenna or as a
transmission line, because the requirements for both are contradictory. This means
that some compromise must be made, in which the feed line does not radiate too
much at the discontinuities.
Coaxial line feed (figure 3.2 b), where the inner conductor of the coax is attached
to the radiation patch and the outer conductor with the groundplane, are also
widely used. The coaxial probe feed is also easy to fabricate and match. However,
it has narrow bandwidth like the microstrip line feed and it is more difficult to
model, especially for thick substrates.
Of the four different feeds described here, the proximity (figure 3.2 c) has the
largest bandwidth and is somewhat easy to model. However its fabrication is
somewhat more difficult. The length of the feeding stub and the width-to-line
ratio of the patch can be used to control the match.
The aperture coupling (figure 3.2 d) is the most difficult of all four to fabricate
and has narrow bandwidth. The aperture coupling consists of two substrates
separated by a ground plane. On the bottom side of the lower substrate there is a
microstrip feed line whose energy is coupled through a slot on the ground plane to
the patch. This arrangement allows independent optimisation of the feed
mechanism and the radiating element. Matching is typically performed by
changing the width of the feed line and the length of the slot [28, 29].
28