the –y direction, as in Figure 5). Because all reflections are
parallel to the y axis and have a constant time-of-flight
exiting the antenna, a nearly-collimated beam is produced.
Note that energy parallel to the
y axis coming into the paraboloid
(that is, received energy) will also
converge at the transducer.
Before going further in
discussing ultrasonic radar
antennas, let’s think about where
paraboloid reflectors occur in
One obvious answer is with
animals that rely heavily on their
sense of hearing for tracking prey
or communicating with other
members of their pack.
The outsized ears of the
African wild dog (Figure 6) are an
example. Note that their ears form
a slightly elliptical paraboloid in
which the horizontal axis is
somewhat smaller than the vertical
Making the Parabolic
Antenna More Practical
Okay, back to our ultrasonic radar antenna. The
parabolic reflector of Figure 5 has several problems that we
should consider. First, the
transducer (which in the figure is
shown just floating in space) will
have to be suspended somehow in
that position. This means adding
struts from the reflector to hold the
transducer at its focal position.
Second, once we have the
transducer and struts in place, we
will obstruct some of the reflected
energy that is supposed to be going
to or from the transducer.
Third, the parabolic reflector
will have to be supported to keep it
vertical. This is equivalent to
keeping a small bowl sitting on its
edge, and will require a fairly
substantial structure to reliably keep
the reflector in place.
Half a Reflector
than a Full
Since most ultrasonic
Figure 8. Ultrasonic transducer with
truncated paraboloid reflector.
Figure 7. The half-paraboloid reflector.
Figure 6. Parabolic (approximately) ears
of an African wild dog.
14 SERVO 05/06.2018