# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 2 Distributed Multiple Antennas for V2X Communications
### Configuration of the ESPAR Antenna and Performance  
According to the investigations of References [29, 30], the upper plastic arm of the truck side mirror is selected as the optimum location for the antenna's installation. The first ESPAR antenna design attempt is carried out by employing conventional wire monopoles as the array elements. The proposed 5-wire monopole ESPAR antenna consists of one active and four parasitic elements in circular configuration, placed on top of a ground plane. Minimum radiation tilt is maintained with the use of a ground skirt. However, the size of the 5-element ESPAR antenna is relatively large compared to the limited available space of the upper mirror arm and an alternative, more compact solution should be followed. The second ESPAR antenna that is developed is 3-element printed ESPAR. The proposed ESPAR antenna is implemented in a planar structure and it was designed using an electromagnetic solver (CST 3D) [31]. Particularly, it employs one active and two parasitic monopole elements printed on a dielectric substrate (antenna panel). The two parasitic elements are located at the opposite ends of the antenna panel in a close distance from the actives (\(\lambda/5\)), enabling this way the beamforming capabilityof the ESPAR. Two PIN diodes are utilized as electronic switches between the parasitic elements and the ground plane in order to create an L-shaped reflector and control the beam of the antenna. In that manner, the three different PIN diodes' combinations (ON-OFF, OFF-ON, OFF-OFF) produce three different radiation patterns (two directive beams and one omni-directional pattern). Our antenna research team has great investigation experience on ESPAR antenna designs [28, 30, 32, 33]. However, there are a couple of technical issues that have been observed in the aforementioned designs. The first one is the observation of small deviations in the results of the antenna's return loss (\(S_{11}\)) between the operating antenna modes (OFF-OFF, ON-OFF) [32]. This is solved, in the current ESPAR design, by employing a reconfigurable impedance matching network also integrated in the antenna panel. The impedance matching network is realized by adding two open microstrip stubs in the microstrip structure. The stubs are connected to the quarter wavelength microstrip transformer line by two PIN diodes, providing this way suitable matching for the three antenna operating states. The second disadvantage of that type of ESPAR antennas is the significant tilt of the radiation patterns (E-plane) that is caused by the ground plane of the monopole. This is again addressed in this design by expanding the \"height\" of the ground plane and introducing the ground skirt effect that minimizes the radiation tilt. The fabrication of the ESPAR antenna was executed by photoetching and the components (SMA connector, PIN diodes, inductors, capacitor, DC pins) were soldered by hand. As far as the integration is concerned, two antennas were installed per side truck mirror of two trucks, which were proved to be optimal positions for V2V link performance for trucks, according to the outcomes of the antenna position investigations that our research team carried out in Reference [29]. The ESPAR antennas were positioned in a perpendicular arrangement in order to achieve better azimuthal coverage. Figure 1 shows the prototype of the 3-printed monopole ESPAR antenna along with the position of the two antennas inside the side mirror case. Figure 2 compares the simulated gain patterns in H-plane for both ESPAR antenna designs (5-wire monopole ESPAR and 3-printed monopole ESPAR) in red and blue colour respectively, for the omni-directional (Figure 2a) and the directive cases (Figure 2b). The planar structure of  
Figure 1: Installation of two reconfigurable electronically switched parasitic array radiator (ESPAR) antennas inside the truck side mirror casing. the printed ESPAR causes the non-perfectly (quasi) omnidirectional pattern (\"squeezed shape\").