single-band antenna would have been
easier to optimize, with better performance
in just one band of interest and
smaller size, there was not enough
space on the device to accommodate
four single-band antennas (two bands
times two antennas each). In addition,
using multiple single-band antennas
would require the addition of diplexers
that would increase loss, size, and
complexity and might not perform as
well for diversity/MIMO channelization
as coupling might increase.
About a dozen AntSyn software runs
were conducted to explore the trade
space and determine what size, height,
and RF parameters would yield the best
result. Many of the runs were conducted
to optimize the tradeoff between
size and performance. AntSyn software
uses a proprietary advanced genetic
algorithm to synthesize antennas that
employs an iterative process, enabling
the tool to search very difficult and general
design spaces with multiple performance
criteria. Full 3-D simulations are
used to calculate the performance of
each design. (Note: at the time this design
was synthesized, each antenna had
to be optimized separately, but AntSyn
software can now optimize multi-port
antennas like this all at once and can include
coupling as a metric.) The AntSyn
results were almost sufficient for design
closure, however the antennas were fed
individually and orthogonally through the
PREPERM substrate, which had a solid
ground plane. This introduced some performance
changes that required slight
printed-circuit modifications.
The synthesized antenna design was
then exported to Microwave Office circuit
design software and simulated using
the AXIEM planar method-of-moments
(MoM) EM solver. A matching network
(Figure 3) was designed using Optenni
Lab and then fine-tuned with the AXIEM
simulator.
It was decided to place the matching
circuitry on an inverted substrate on the
opposite surface from the antennas. Addition
of the supporting substrate layer
detuned the antennas, thus Optenni Lab
was again used to synthesize and retune
5G – Antenna Design
Figure 3: Optenni Lab synthesizes automatically many microstrip matching
circuits that optimize the total efficiency. The circuits proposed by Optenni Lab
were implemented in the prototype.
Figure 4: AXIEM layout showing the placement of the antennas (green) and
matching circuit with feed network.
Figure 6: S11 simulation versus measured performance.
Figure 5: Anritsu ShockLine MS46322B
series 2-port VNA used for the measurements
of the prototype.
www.mwee.com March - April 2019 MW 13
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