Dual-band System composed by a Photonics-based Radar and a Focal-Point / Cassegrain Parabolic Antenna

This work reports a dual-band radar system composed of aphotonics-based transceiver and a unique dual-band antenna. The proposed antenna consists of an integration of conventional focal-point and Cassegrain parabolic antennas in the same structure, ensured by using a subreflector based on a frequency selective surface. Numerical and experimental results in terms of the antenna reflection coefficient, radiation pattern and gain are reported with excellent agreement over both frequency ranges. The innovative dual-band photonics-based radar transceiver operates simultaneously in the Sand X-bands. The radar system has been properlyvalidated by multiple detections of helicopters and airplanes in real conditions.


I. INTRODUCTION
Currently, most transceivers operate only in a single band.New architectures have been proposed to simultaneously operate in multiple bands, by using independent hardware for each band [1], [2].
Future generations of radar systems aim to meet reconfigurability, multi-functionality and small footprint characteristics in order to provide improved sensors networks [3].In this context, in the past few years, researchers have proposed photonics-based radar technologies to achieve coherent multiband and multi-functionality capability, by applying software-defined radio (SDR) [4]- [6].In this way, multiple radiofrequency carriers are combined for enabling short-and far-range detections.
Since higher frequencies are more vulnerable to the weather conditions, they are usually used for short-range applications, whereas lower frequencies are desirable for far-range detections [7]- [9].Therefore, a multiband radar transceiver increases the system robustness in terms of climatic variations, providing enhanced detection capability even in adverse conditions.Finally, combining multiple functions in a unique hardware allows a general size, weight and energy consumption reduction.
Several approaches for designing dual-band antennas are found in the literature [10]- [13].For

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based antennas for radars [11]- [13] and communications [14], [15].FSS-based subreflectors have also been applied for multi-band antennas [16]- [18].In [16], an inductive FSS subreflector has been employed on a dichroic subreflector to electromagnetically separate the S-and K-band feeders for satellite communication applications.A flat dichroic FSS subreflector for Cassegrain antennas has been proposed in [17] for achieving high electromagnetic transparency between 1.9and2.3GHz and high reflectivity between 3.6and4.2GHz.Additionally, other authors have proposed a four-band response FSS which reflects in the Ka-band frequency range, whereas in S-, X-, and Ku-bands, it behaves as an electromagnetic transparent element [18].
The current manuscript represents an extension of two previous works from our research group [12], [13], which reported numerical and preliminary experimental results of a dual-band Cassegrain parabolic antenna based on frequency selective surfaces (FSS), designed to simultaneously operate at 2.525 and 9.925 GHz [19].Here, we present the antenna full characterization and application in conjunction with a photonic-based radar transceiver, previously proposed by our group [5], for multiple radar detections of helicopters and airplanes in the S-and X-bands.
The manuscript is structured in five sections.Section II is regarding the FSS-based Focal-Point/Cassegrain parabolic antenna design.Its numerical and experimental results in terms of reflection coefficient, radiation pattern and gain are presented in Section III, whereasthe photonicsbased radar experiment is described in Section IV.Finally, conclusions and future works are outlined in Section V.

II. FSS-BASED FOCAL-POINT/CASSEGRAIN PARABOLIC ANTENNA DESIGN
Frequency selective surfaces consist of conducting or aperture elements, arranged in a planar periodic array for creating a band-pass or band-stop filter [20]- [22].The elements can be placed in one or more dielectric layers, according to the desired frequency response.The FSS properties can be varied by choosing the element type, dimensions and volumetric structure, as well as the dielectric material electromagnetic properties [21].In dual-reflector systems, a FSS can be applied to the subreflectorin order to combine focal-point and Cassegrain parabolic antennas into a single structure [12], [13], [23].
The antenna design, including the dielectric support, horn antennas andFSS-based subreflector are depicted in Fig. 1(a).

III. ANTENNA PROTOTYPE CHARACTERIZATION
The FSS-based Focal-Point/Cassegrain parabolic antenna prototype (Fig. 4) was based on a sixpieces main reflector, two horn antennas milled with aluminum and the FSS-based subreflector.The upper feeder (S-band) and subreflector were sustained using dielectric bars to avoid phase variations, 5 dBi, respectively.The antenna gain at 9.925 GHz was expected to be approximately 12 dBi higher than that at 2.525 GHz, according to reflector antenna theory [24], since the main reflector is  Finally, we have performed a detection of a non-cooperative helicopter using S-and X-bands concurrently.It was coincidentally flying over the building rooftop at lower velocity, which made easier to be precisely detected at both frequency ranges simultaneously.
electrically large at higher frequencies.Such behavior has not been observed due to the FSS-based subreflector lower efficiency.The efficiency is decreased due to losses inserted by the dielectric and fabrication imprecisions, such as air gaps between the dielectric pieces.In other words, the Focal-Point Parabolic antenna and the Cassegrain Parabolic antenna present distinct radiation and aperture efficiencies due to the FSS-based subreflector presence in the latter one.
Fig. 11 displays the experimental results at 2.525 and 9.925 GHz in terms of Doppler map and normalized received power.An excellent agreement between the two bands has been observed, with a target detection at 2.65 km away from the transceiver and velocity of 11.86 m/s.