60-GHz LTCC Antenna Arrays - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 215

Fig. 13 Simulated electric field distributions on the top surface of the substrate for the mid-
dle two-row eight elements of the antenna array with and without the soft-surface structure.
a Without the soft-surface structure, b With the soft-surface structure (from [ 10 ], reprinted with
permission from IEEE)
When the antenna is fabricated on a large-size substrate with higher dielectric
constant, the radiation performance will be much affected by the diffraction of sur-
face waves. As Fig. 13 a shows, there is a strong electric field from the surface waves
in the area between the antenna elements, in particular along the y -axis. As the in-
trinsic electric field of the L-probe patch antenna element is heavily affected by the
strong surface waves, the radiation performance of antenna array is deteriorated. The
proposed soft-surface structure with the shorted metal strip and via fence blocks the
surface wave propagates. As a result, the surface waves can be substantially sup-
pressed; hence, the radiation performance of antenna array is improved. To confirm
this finding by checking the field distribution in the substrate shown in Fig. 13 b, we
can see that the surface waves is suppressed in the substrate and the field distribution
of the L-probe radiation patch is improved obviously.
The other factor contributing to the radiation performance improvement is the
fringing field along the metal strips of the proposed soft-surface structure. As Fig. 13 b
shows, the fringing field along the metal strips forms a radiation array. The formed
array acts as a broadside array. Although the magnitude of the fringing field along
the soft-surface structure is much lower than radiating edge of the L-probe patch
antenna, the size of the soft-surface structure is much larger than the patch. As a
result, the contribution from the soft-surface structure to improve the performance
of antenna array is significant.