Phase II Final Report - NASA's Institute for Advanced Concepts

Planetary Exploration Using Biomimetics
An Entomopter for Flight on Mars
tivity, the number of emitters must increase in order to get adequate angular coverage. In the
limit, this will approach the number of emitters found in the phased array, the difference being
that one is a conformal array while the other is a more planar array.
4.3.4.2 Requirements Driving the Antenna Design
The goal is to accomplish the described requirements with as much overlap in functionality as
possible for power conservation. For this application, the obstacle detection poses the most stringent
requirements, so the focus will be on this aspect of the project first. It would be beneficial
to use the same antenna to provide front hemispherical coverage for communications as well as
obstacle detection. Phased array antennas are commonly used for searching and tracking applications
where the 2D or 3D location of an object must be provided. A hemispherical domeshaped
phased array as described in [129] would fulfill all these requirements. However, a large
number of antenna elements would be necessary, each requiring a carefully designed signal path
to eliminate phase differences between different paths. In addition, each signal path requires a
phase shifter and gain-controlled low noise amplifier. The phase and gain are controlled to provide
the desired antenna pattern. The complexity of this design may prohibit its use for this
application where low power and reliability are critical.
Switched arrays, on the other hand, offer more simplicity, because pattern diversity can be
achieved by controlling the state of a number of RF switches but produce a limited number of
beam patterns. In this way, beam coverage and resolution can be controlled selectively on a
“need only” basis. In this way, power is conserved by illuminating only the required coverage
area for the functionality needed at a certain time (obstacle detection, communications, etc.).
The linearly tapered slot antenna (LTSA) circular array (sunflower antenna) shown in Figure 4-8
and described in [240 and 154] is one such switched array that could fulfill the Entomopter's
multifunctional needs. A photo of an operational K-band sunflower antenna is shown in
Figure 4-8. This antenna, proposed for mobile communications, is fed by a 1:16 microstrip line
power splitter composed of T junctions and right angle bends. A conventional microstrip-toslotline
transition is used to electromagnetically couple the output ports of the splitter to the slotline
of the LTSA. The measured radiation pattern of this endfire antenna is shown in Figure 4-9
at 19.8 GHz when the antenna is placed over a reflecting ground plane. From the figure it is
obvious that the antenna beam is omnidirectional in the azimuthal plane and is displaced about
28 degrees above the horizon in the elevation plane. The displacement in the elevation plane is
proportional to the distance between the antenna and the ground plane. As shown in Figure 4-10,
by removing the ground plane, this displacement above the horizon can be removed and the
antenna beam centered about the elevation angle φ = 0 o . The shape of the sunflower elements
can be varied to achieve the desired frequency, bandwidth, gain and 3 dB antenna beam bandwidth.
This antenna has advantages of wide bandwidth operation and compact configuration, making it
an attractive antenna for this mission where onboard space is limited and very wide band signals
are being considered. It resembles a hockey puck that can be mounted easily on the Entomopter
body. The diameter of the K-band design shown in Figure 4-8 is approximately 16 cm (6.3"). A
potential mounting scenario is shown in Figure 4-11, where half of the LTSA is mounted on the
front and half on the back of the Entomopter. In this way, when all elements are illuminated,
360 o of azimuthal coverage can be obtained by slightly widening the first and last sunflower pet-
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Phase II Final Report