## 3 Directional Antennas Suitable for WSN Nodes  
The physical layer of a wireless sensor network is in charge of bit-stream transmission/reception over wireless communication channels, performing a series of tasks that includes carrier frequency selection and generation, signal detection, modulation or data encryption. A central role in this context is played by antenna devices which basically transform electric power into electromagnetic waves, or _vice versa_. In order to be used in WSN nodes, directional antennas have to possess four basic features: they must be small, reasonably priced, consume low power and able to operate in licensed frequency bands: 315 MHz, 433 MHz or 868 MHz in Europe, 915 MHz in North America, 2.45 GHz Industrial-Scientific-Medical (ISM) band or within the millimeter-wave spectrum [25; 26; 27; 28]. These requirements drastically limit the number of directional antenna construction types adaptable for sensor nodes [29]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristic** & **Omnidirectional Antenna** & **Directional Antenna** \\ \hline Energetic efficiency & Lower & Higher \\ Broadcasting direction & All & Desired \\ Transmission range & Lower & Higher \\ Node orientation & Not required & Required \\ Price & Lower & Higher \\ Dimensions & Smaller & Bigger \\ Transmission security & Lower & Higher \\ Collisions & More & Less \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Omnidirectional _vs._ directional antenna comparison. The directional antenna prototypes specifically designed to equip sensor nodes are briefly presented in Table 2.  
[PERSON] and [PERSON] [30] indicated the need and usefulness of smart antenna integration into WSN nodes by analyzing the overall network performance and nodes' power consumption. They proposed a small, inexpensive and modular sensor node hardware platform, termed SensorDVB. This platform, built from commercial-off-the-shelf components and occupying no more than 33 cc in volume, provided onboard processing, sensing, and radio communication using smart antennas operating in the 868 MHz radio frequency spectrum. [PERSON] [31; 32] identified three construction types as plausible candidates to equip WSN nodes: the adcock-pair antenna, pseudo-Doppler antenna, and electronically switched parasitic element antenna. He proposed a variant of electronically switched parasitic element antenna, named SPIDA 2.44-GHz prototype, and demonstrated its efficiency through numerical simulations and lab experiments. [PERSON] _et al._[38] presented a real-world evaluation of the SPIDA prototype. They interfaced this electronically switched directional antenna with a TMote Sky (an off-the-shelf sensor node), obtaining a fully functional real-world WSN node with improved performances in terms of communication range, and wireless link quality. A 2.4 GHz four-beam patch antenna prototype meeting the size, cost and energy constraints of sensor nodes was proposed by [PERSON] _et al._[33]. This directional antenna uses a box-like structure of four coaxially fed planar patch antennas. Experiments involving TelosB motes demonstrated the substantial benefits of using such antennas, the communication range being extended from 140 to more than 350 m while suppressing the interferences due to multipath fading. [PERSON] _et al._[34] developed a beam-switching WSN node using the VirtualSense platform. They enclosed the VirtualSense mote with an active cylindrical frequency selective surface. By this action, the antenna's radiation pattern was converted from omnidirectional into a directional one by modifying the configuration of active PIN diodes. As a direct result, the miniaturization and ultra-low-power features of the VirtualSense node were preserved. [PERSON] _et al._ proposed and tested two cost-effective and compact switched-beam antenna prototypes, in the ISM band. The first one employs a radiation structure made of eight microstrip antennas using rectangular two-element patch antenna arrays and a vertical half-wavelength dipole antenna [12].