60-GHz LTCC Antenna Arrays - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 207

Compared with a series feed, a parallel feed can offer a wider bandwidth. Several
types of transmission line can be used as the feeding network, such as microstrip line,
stripline (SL), coplanar waveguide (CPW), substrate-integrated waveguide (SIW),
or hybrid structures. However, the gain of the antenna array is limited by the loss
caused by its feeding network, such as a microstrip line, especially at the 60-GHz
band applications. Stripline and SIW with lower transmission loss and better isolation
than microstrip line have been widely employed at the 60-GHz LTCC antenna array
designs.
Furthermore, due to the higher permittivity of LTCC substrate, the surface wave
could introduce significant loss in LTCC antenna array design at millimeter-wave
60-GHz bands. Enhancing the gain performance of the 60-GHz antenna array based
on LTCC technology becomes much more challenging. The strong surface wave
propagation is the primary reason of producing the mutual coupling between the
elements. Some methods of suppressing the surface-wave loss to reduce the mutual
coupling and enhance the gain of the LTCC antenna have been reported, such as
embedding an air cavity inside the substrate, removing the partially substrate around
the radiating elements, loading the uniplanar-compact electromagnetic band-gap
(UC-EBG), loading the artificial soft-surface structure, using open-ended SIW cavity
structure, and using metal-top via fence structure, etc. [ 14 - 18 ].
In this chapter, the antenna arrays based on LTCC technology for 60-GHz short-
range wireless communication applications are presented. The antenna elements and
compact feeding networks for achieving different polarization performances of the
antenna arrays are studied. For measurement, the transition of the grounded coplanar
waveguide (GCPW) to the SL is designed based on LTCC multilayer structure. Also,
for suppressing the surface wave propagation in the array, the soft-surface structure,
and open-ended SIW cavities are used to improve the radiation performance of
antenna arrays with different polarization characteristic. The antenna arrays with
compact structure and good performance, including a wideband linearly polarized
L-probe feed patch antenna array with soft-surface structure, and a dual-polarized L-
probe feed patch antenna array with open-ended SIW cavities structure are presented
and studied.

60-GHz LTCC Linearly Polarized Antenna array

A4 4 L-probe patch linearly polarized antenna array with a soft-surface structure

using multilayer LTCC technology is presented for 60-GHz band applications. The
technique of employing an L-shaped probe to feed a thick patch antenna for achiev-
ing a wideband antenna of bandwidth > 30 % has been widely reported [ 19 - 21 ].
However, all these antennas were designed on traditional substrate materials, and
the operating frequency bands were below 10 GHz. The L-probe feed patch antenna
as an element is designed and fabricated using LTCC technology at 60-GHz band.
On the other hand, the artificially soft-surface structure was employed to improve
antenna performance as it can force the field intensity for any polarization to be

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