Page 297 - System on Package_ Miniaturization of the Entire System

Radio Fr equency System-on-Package (RF SOP) 271

To summarize, antennas of small physical size require materials with high permeability
and permittivity. However, such materials generate parasitic substrate modes that
diminish the gain of the antenna. Similarly, antennas with good gain require matching
at the antenna input port for the maximum transfer of energy from the transmission
line to the antenna. With high-permittivity and high-permeability materials, this
becomes difficult since very narrow traces are required to obtain an impedance of 50 Ω.
With such conflicting requirements, the design of antennas can be very tricky. Various
approaches have therefore been proposed by several researchers for the miniaturization
of these antennas such as combining multiple frequency bands into a single antenna
element [29–30], use of conformal antennas, use of magneto-dielectric materials , and
inclusion of electromagnetic bandgap structures to improve performance. Another
important element that requires special attention while integrating the antenna in the
module is the suppression of parasitic backside radiation or crosstalk, which can
otherwise couple to sensitive RF blocks in the substrate. This is measured as the front-
to-back ratio, which is the ratio of the maximum directivity of the antenna to its
directivity in the backward direction.
Three important antenna technologies (multiband antennas, conformal antennas,
and antennas on magneto-dielectric substrates) for WiFi applications are illustrated
here for miniaturizing the antenna size and integrating it with the rest of the RF front-
end module.

Multiband Antennas
As multiple communication standards are integrated into the RF front end, there is a
need for antennas that support multiple frequencies. Along with passing the required
frequencies, these antenna elements should also minimize interference by suppressing
the adjacent frequency bands. To minimize antenna size, a multiband antenna (instead
of multiple single-band antennas) can be constructed by controlling the length of the
antenna elements. This is illustrated in Figure 5.7, which shows a triband antenna
developed on RT-duroid substrate. The antenna resonates at 900 MHz (cellular), 2 GHz
(802.11b/g), and 5 GHz (802.11a) and has an almost omnidirectional radiation pattern.

Conformal Antennas
In SOP, technologies that combine rigid and flexible substrates are possible. This concept
can be used to embed the RF front end in the rigid part of the module, while the antenna
that is patterned on the flexible substrate can be folded or made to conform to the rigid
part. This approach reduces the size of the module containing the integrated antenna.
An example of a meander monopole antenna is shown in Figure 5.8, which has been
used due to its ability to achieve the required length of current path for a specific
resonant frequency within a compact size. The substrate of the antenna is composed of
two layers of dielectric, as shown in the figure. The top layer is a 25-μm-thick LCP layer
with a size of 18 mm × 25 mm and the bottom layer is a 508-μm-thick rigid, glass-
reinforced organic prepreg layer (core layer) with a size of 18 mm × 9 mm. The LCP
layer has a dielectric constant of 2.95 and a loss tangent of 0.002. The core layer has a
loss tangent of 0.0037 with a dielectric constant of 3.48. As can be seen from the figure,
the 16-mm-long portion of the LCP layer is not supported by the core layer and can be
easily conformed due to the flexibility of the LCP. The antenna is printed on the flexible
portion of the LCP, making it possible to bend, fold, and roll the antenna, as shown in
Figure 5.9, leading to a compact antenna design and integration with the module.