Page 296 - System on Package_ Miniaturization of the Entire System

270 Cha pte r F i v e

because of the inherent size advantages) and interference between the antenna and the
rest of the RF blocks in highly integrated modules.
With the integration of inductors and capacitors in the substrate, the integration of the
antenna now becomes the major size limiter. The gain of an antenna is a function of its
electrical size. Typical antennas such as patch antennas require an electrical size of l/2,
where λ is the electrical wavelength. This requirement makes the antenna have a very large
physical size. For example, at 2 GHz, the electrical wavelength in FR4 dielectrics (dielectric
constant = 4) is 74 mm, which translates into an antenna physical size of approximately
37 mm. Several technologies are available today where the RF front end from the antenna
port to the transceiver can be integrated in a size of 5 mm × 5 mm or less (see Chapter 4)
with multiple transmit-receive chains, which is approximately seven times smaller than
the antenna itself. Hence, the antenna now becomes the size limiter in the RF front end.
The important attributes of an antenna for consumer applications are (1) small
physical size, (2) suitable bandwidth, and (3) good gain.
1. Physical size. A printed antenna on a substrate has a size that is a function of the
material properties of the substrate. The physical size of the antenna scales as

λ
l ∝
με
rr
where l is the physical size of the antenna, l is the antenna’s electrical size in air,
m is the relative permeability, and e is the relative permittivity. With a dielectric
r r
material of e = 9, the size of the antenna can be reduced by a factor of / 3 (since
1
r
m = 1) as compared to the antenna in air. As the dielectric constant is increased,
r
electromagnetic fields get trapped in the substrate, leading to substrate modes
that diminish the bandwidth and gain of the antenna.
2. Bandwidth. The bandwidth (BW) of an antenna is typically described as a
percentage of the center frequency, which is calculated as:
F − F
BW = High Low × 100
F
Center
where F high is the highest frequency, F is the lowest frequency, and F center is the
low
center frequency. The bandwidth determines the range of frequencies where
the antenna would operate correctly. In consumer applications, both narrowband
(5 percent BW) and broadband (30 percent BW) are used depending on the
filtering technologies used.
3. Gain. Gain is always measured with reference to a standard antenna such as an
isotropic or dipole antenna. When the reference is an isotropic antenna, the units
used are dBi. Since an antenna only redistributes power that oftentimes is non-
uniform in space, the gain changes as a function of direction and therefore has
both positive and negative values. For consumer applications, an omnidirectional
radiation pattern is often preferred. The gain of an antenna is a function of many
factors, the most important of which is antenna matching. The transmission line
that feeds the antenna has to be well matched to the antenna input to ensure a
maximum transfer of power. This can be a very challenging task in antenna
miniaturization.