IEEE Microwave Magazine - October 2016 - 68

Reflection Coefficient (dB)

0
-5

The 2-D pattern cuts in Figure 11 show
that the Vivaldi has 1.2-dB less directivity,
resulting in a broader pattern caused by the
planar nature of the antenna. But the slightly
lower gain of the Vivaldi antenna is more
than compensated for by the additional PA
gain and the lower through-loss in the new
20-dB coupler as opposed to the 3 dB in the
original OCW radar. Additionally, the Vivaldi has a significant advantage in terms of
planar form factor in size, cost, ease of assembly, and integration into a system.

Vivaldi Antenna Performance
DB(|s(1,1)|)
Vivaldi_Antenna_AXIEM
DB(|s(1,1)|)
Vivaldi_Antenna_IRL_Meas

-10
-15
-20
-25
1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5
Frequency (GHz)

System Simulation RF
Link Budget Results

After the subsystem components were designed and selected, a system simulation was
performed to provide a budget analysis for
the FMCW radar's power and gain at its center frequency of 2.45 GHz. This analysis allows the system integrator to see the contribution of the different
individual component's RF metrics as the signal propagates through the system-a capability extremely useful for system engineers who must consider which
components need upgrading for higher performance.
For this example, Figure 12 shows the comparison
between the original system (in blue) and the recaffeinated system (in green). The OCW system starts
out with a higher output power from the original VCO
than the new SMT VCO, but as the signal travels to
the transmit antenna, the redesigned system achieves
a higher equivalent isotropic radiated power (EIRP).
This is mainly due to the lower insertion loss of the
20-db coupler and the higher PA gain of the SMT
replacement part driving the antenna. EIRP is defined
as the amount of power that a theoretical isotropic

Figure 10. The performance for the Vivaldi planar antennas, which
replaced the original coffee-can antennas.
ANSYS's HFSS, Keysight's EMPro (all based on FEM),
or CST's Microwave Studio (based on finite difference
time-domain methods). Figure 11 displays the 3-D radiation pattern and a two-dimensional (2-D) cut along
boresight for the FEM simulation, including the discrete PCB substrate effects.
Although alternative 3-D EM tools, as previously
mentioned, could be used to derive the radiation patterns, this demonstration used Analyst, which supports the ability for the pattern to be directly read into
the VSS diagram for system-level analysis. Having the
entire pattern in the system tool allows the designer
to vary the θ and φ locations of the target and see the
effect on the system performance metrics when the target is off boresight. For comparison, the original coffeecan antenna was also analyzed in Analyst to provide a
simulated comparison pattern to the Vivaldi.

Antenna Pattern Comparison

Mag Max
10 dB Comparison of
Simulated Coffee
Can and Vivaldi
Patterns
Mag 7.3 dB
Ang 90°

Coffee Can Pattern
Vivaldi Pattern
10 dB
Per Div

Mag 8.5 dB
Ang 90°
Mag Min
-30 dB

Figure 11. The Vivaldi planar antenna pattern versus the "cantenna" patterns. Mag: magnitude; Ang: angle; Div: division.

October 2016

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