II. DESIGN OF THE ANTENA AND RESULTS
The antenna characteristics are specified by the normative
documents [1], the link-budget, the receiver architecture, and
the width of a toll lane (around 3.5m). The needed half-power
beam width is 35 degrees, while 70 degrees are required when
the radiation pattern falls 15 dB below boresight. The antenna
should be left hand circularly polarized with 15 dB of crosspolarization
rejection at boresight, and 10dB at half power.
To ensure the required radiation pattern, the use of antenna
arrays with uniform excitation and structure does not achieve
the goal. The proposed structure consists of a non-uniform
antenna array with four elements at the corners, with a fifth
element at the center, as shown in Fig. 2. Each corner element
is fed with the same amount of power, while the power of the
center element is fourfold. This structure can be analyzed as an
approximation to a binomial configuration (N=3) [3], looking
at the array as two linear arrays with three elements showcasing
a binomial excitation, being the central element ”shared” by
both linear arrays, as illustrated in Fig. 2.
Fig. 2 Binomial based excitation
To achieve circular polarization the corners of each element
are chamfered. Moreover, to improve the circular polarization,
the technique of sequential rotation was used in the four
external elements [4]. This technique applies a physical
rotation of the elements and a corresponding phase shift for
every feed.
In the development of the array feed network, apart from
attention to create the correct phase delays for each feed, power
splitting was done using T-junction. The first splitter has three
unequal outputs, as can be seen in Fig. 3. The output that feeds
the central elements has double the power of the other two.
The antenna with the proposed structure was simulated and
manufactured using a Arlon CuClad 217 substrate, whose maincharacteristics are dielectric constant εr=2.17, thickness
h=0.787mm and a loss tangent of tan δ=0.0009.
Fig. 3 Manufactured prototype antenna array
The manufactured prototype presented in the Fig. 3 was
tested for return loss, polarization and radiation pattern.
Fig. 4 Measured and simulated return loss (S11)
Concerning to the return loss, Fig. 4 shows a good
agreement between simulation and the measurements.
Considering a good matching for a S11 level below -10dB, the
antenna has a bandwidth of 260MHz. Fig. 5 Measured and simulated axial ratio
As can be seen in the Fig. 5 the measured bandwidth is
narrower than simulated. Assuming an axial ratio below 3dB as
an acceptable value to characterize a good circular polarization,
a 150MHz band was obtained.
II. DESIGN OF THE ANTENA AND RESULTS
The antenna characteristics are specified by the normative
documents [1], the link-budget, the receiver architecture, and
the width of a toll lane (around 3.5m). The needed half-power
beam width is 35 degrees, while 70 degrees are required when
the radiation pattern falls 15 dB below boresight. The antenna
should be left hand circularly polarized with 15 dB of crosspolarization
rejection at boresight, and 10dB at half power.
To ensure the required radiation pattern, the use of antenna
arrays with uniform excitation and structure does not achieve
the goal. The proposed structure consists of a non-uniform
antenna array with four elements at the corners, with a fifth
element at the center, as shown in Fig. 2. Each corner element
is fed with the same amount of power, while the power of the
center element is fourfold. This structure can be analyzed as an
approximation to a binomial configuration (N=3) [3], looking
at the array as two linear arrays with three elements showcasing
a binomial excitation, being the central element ”shared” by
both linear arrays, as illustrated in Fig. 2.
Fig. 2 Binomial based excitation
To achieve circular polarization the corners of each element
are chamfered. Moreover, to improve the circular polarization,
the technique of sequential rotation was used in the four
external elements [4]. This technique applies a physical
rotation of the elements and a corresponding phase shift for
every feed.
In the development of the array feed network, apart from
attention to create the correct phase delays for each feed, power
splitting was done using T-junction. The first splitter has three
unequal outputs, as can be seen in Fig. 3. The output that feeds
the central elements has double the power of the other two.
The antenna with the proposed structure was simulated and
manufactured using a Arlon CuClad 217 substrate, whose maincharacteristics are dielectric constant εr=2.17, thickness
h=0.787mm and a loss tangent of tan δ=0.0009.
Fig. 3 Manufactured prototype antenna array
The manufactured prototype presented in the Fig. 3 was
tested for return loss, polarization and radiation pattern.
Fig. 4 Measured and simulated return loss (S11)
Concerning to the return loss, Fig. 4 shows a good
agreement between simulation and the measurements.
Considering a good matching for a S11 level below -10dB, the
antenna has a bandwidth of 260MHz. Fig. 5 Measured and simulated axial ratio
As can be seen in the Fig. 5 the measured bandwidth is
narrower than simulated. Assuming an axial ratio below 3dB as
an acceptable value to characterize a good circular polarization,
a 150MHz band was obtained.
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