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.