IEEE Geoscience and Remote Sensing Magazine - March 2022 - 242

Patch (PCB 1 Top Side)
FR-4 Substrate (PCB 1)
Slotted Ground (PCB 2 Top Side)
Taconic TLX-8 Substrate (PCB 2)
Antenna Feed, Phase Shifter, and
dc Bias Line (PCB 2 Bottom Side)
Driving Electronics (PCB 3)
Metallic Back Reflector
FIGURE 9. The overall layout of the L-band 4 × 4 phased array. PCB: printed circuit board.
the phase shifters and control network integration in the
feedline layer.
Figure 10 shows the details of the antenna feedline layer
design. In this design, a 1-16-way power divider with nonuniform
amplitude distributions has been adopted to suppress
the antenna SLL down to -25 dB. With the 2-bit phase
shifters integrated into the feedline of each patch antenna
and each phase shifter bit being independently controlled
by dc bias lines, this phased array is capable of generating
nine beams in 2D scanning. Those are ±20° beams in the
x-direction, ±15° beams in the y-direction, and diagonal
beams at angles (±19° x, ±17° y).
Figure 11 depicts the simulated radiation pattern results
of the ±20° beam in the x-direction, and this antenna array
exhibits a peak directivity of 18.5 dBi
with a 3-dB beamwidth of 22°. The
SLL is better than -27 dB on the H
plane. The highest SLL on the electric
field (E) plane is -25.2 dB at a 0°
steered beam and -14.2 dB at ±20°
steered beams.
The simulated diagonal beam at
(-19° x, +17° y) is demonstrated in
Figure 12. The corresponding phase
assignment of each antenna element
in the phased array is shown in Table 5,
where the top left patch is assigned
0° of phase shift, and the bottom
right patch is assigned 180° of phase
shift. Overall, this simulation shows
that this phased-array design can
achieve a beam efficiency of >80%
for all 1D steering beams and better
than 77% for all diagonal beams at 1.4 GHz. The total loss
is less than -1.85 dB for all beams at 1.4 GHz. This antenna
design has been fabricated and is now under preparation
for measurement.
DBF
Another way to achieve electronic beam forming is through
performing the phase shift and amplitude tapering in the
digital domain. Multiple antenna beams can be obtained
simultaneously in the digital domain [36], [62], which allows
smoother beam scanning as compared to that of a
phased array [36].
The ability to accurately control multiple and simultaneous
antenna beams directly benefits the push-broom
scanning scheme, which is famous for its high sensitivity;
more importantly, no moving parts are required
for this scheme, which can significantly reduce the system's
weight [85]. Additionally, the high sensitivity requirement
is critical in ocean observations [85] whose
parameters can also be measured by L-band radiometers
like SMOS.
The main drawbacks of this scanning scheme are
similar to those of a digital beam former and phased array
operating in a multibeam configuration. They all need
complex antenna systems and/or a large number of receivers
[34], [85]. However, as technology progresses,
it will be possible to develop cheap and lightweight receivers
in the future [34]. Therefore, digital multibeam
antennas in combination with a push-broom scanning
scheme should be considered for compact and lightweight
radiometers.
In DBF, the signal from each antenna element is sampled
and multiplied by a corresponding complex weight, and
these are then summed together to give the desired beam
shape. Assuming input
Xx xx mM1== ,
[, ,.,] ,, ,
T
12
FIGURE 10. The feedline layer of a 4 × 4 L-band phased-array antenna.
242
ff
M
where m is the mth antenna, a combining matrix W can be
defined according to the following equations [36]:
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE MARCH 2022

IEEE Geoscience and Remote Sensing Magazine - March 2022

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