February 2016
6
C
over
S
tory
Radar in every bumper
By Barry Manz,
Mouser Electronic
As the automotive industry
aggressively marches toward
huge advances in vehicle safety and
ultimately to autonomous vehicles,
major enablers will be RF and
microwave technology, and
millimeter-wave radars in particular.
Not long ago, autonomous vehicles were
the stuff of science fiction. Today, they’re
being tested throughout the world. Google
autonomous vehicles alone have racked up
more than 1.8 million miles (with a human on
board for now) and logged only 12 incidents.
There’s little doubt that in the not-too-dis-
tant future, autonomous vehicles will become
a commercial reality in some form. When
that day comes, it will be thanks in no small
measure to the versatility of radar, which is
increasingly auto manufacturer choice for use
in blind spot and side-impact detection sys-
tems and for adaptive cruise control.
Of course, radar is not alone in enabling
nowadays rapidly-advancing vehicle safety
systems. Infrared and visible-light cameras
augment radar capabilities and have their own
unique benefits, acoustic sensors are used in
parking-assist systems, and radar optical
counterpart (LIDAR) is also used in some
vehicles. (LIDAR stands for Light Detection
and Ranging.) However, even LIDAR-centric
Google Prius test cars have two radars in the
front bumper and two in the back. In less than
a decade, radar has become available even in
lower-cost compact cars, either as an option
or increasingly as standard equipment. Radar
accounts for more than 35% of the collision
avoidance sensor market according to Grand-
view Research, and its market penetration is
increasing, surpassed only by cameras. One
major characteristic that differentiates vehicle
safety radars (from radars used for more tra-
ditional applications) is their much higher
operating frequencies (76 to 80GHz), which
were allocated internationally for this pur-
pose. They were chosen because of their
signal propagation characteristics and are
defined as millimeter wave for their very
small wavelengths. The millimeter-wave
region is generally considered to begin above
30 GHz and until now frequencies above
about 40 GHz have been devoid of activ-
ity other than by some scientific and mili-
tary systems. There are several reasons for
this. Foremost is that millimeter-wave sig-
nal propagation severely limits range and
decreases in range as frequency increases.
Tiny millimeter-wave signals are susceptible
to attenuation by virtually anything in front
of them, from rain and snow and even fog,
to foliage and any solid structure. Even with
a clear line of sight, range is much less than
at the lower frequencies used by applications
such as wireless communication and radio
and television broadcast.
Millimeter-wave systems have traditionally been
very expensive to manufacture, as their mechan-
ical components such as antennas are very small
and require precision machining. There are also
only a few semiconductors that provide accept-
able performance (or work at all) at such high
frequencies. All of these drawbacks could have
been surmounted if there was a huge commer-
cial market to drive down costs and fund inno-
vation, but as none existed, the millimeter-wave
region remained a frontier waiting to be con-
quered. Auto safety is that conquering hero.
Although the aforementioned characteristics
make millimeter-wavelengths unappealing for
most applications, they’re actually beneficial
for use by car safety systems. For example, the
range-limiting characteristics of millimeter
wavelengths in general do not apply to every
millimeter-wave frequency because atmo-
spheric absorption at some frequencies is less
than at others. Reduced absorption increases
usable range -- but not so much as to cause
widespread interference, and as these spec-
tral snippets are narrow in bandwidth. Con-
sequently, frequencies between 71 and 81GHz
are excellent regions for auto radar to operate.
Earlier, some applications operated at 24GHz
where system cost was lower (although system
size was larger) but the higher frequencies are
now almost universally accepted. Another
benefit of millimeter wavelengths is that only
very low RF output power is required for
radar systems, which for the auto cost-sensi-
tive industry is essential and high power levels
are very difficult to generate as well.
Figure 1. Estimates (in billions)
from Grandview Research
clearly show the steady upward
march of radar in vehicles.
