## 1 Introduction  
Wireless networks provide a scalable and cost-effective solution for a large number of Internet of Things applications due to the prohibitively high cost and practical difficulty of employing cables in complex control environments [1, 2]. They provide considerable benefits such as simple deployment, low installation cost, and high flexibility of operation and monitoring of the control systems. There is a strong push from industrial control systems to apply wireless networks as critical infrastructure for enabling mission-critical control systems such as factory automation and power network protection [1, 3]. Mission-critical wireless networks must support the right decision at the right moment, even in the presence of unexpected congestion, network failures, or external manipulations of the environment. Unreliable communication may result in significant performance degradation, and even more severe problems such as critical economic loss, human injury, and/or environmental damage [2, 4]. The use of wireless networks for data transmission always introduces non-zero transmission error probability, since it is vulnerable to fading and interference. Several studies have shown the poor reliability in terms of received signal strength indication (RSSI), packet delivery ratio (PDR) degradation, and error burst length in harsh industrial environments [5, 6]. One of the major technical challenges faced by industrial control applications is the non-line-of-sight (NLOS) channel in which the transmitting (Tx) and receiving (Rx) nodes cannot see each other directly due to time-varying environments of mobile nodes and obstructions [7; 8]. Although several different techniques such as spread spectrum, time diversity, and channel diversity represent significant achievements of traditional networks, availability and reliability requirements of mission-critical control systems exceed what current networks can offer [1; 2]. An active array antenna system with a beam-scanning feature could be an effective solution but it also increases complexity and has a larger footprint compared to a simple and compact omni-directional antenna. Instead, the radiation pattern of the antenna at the node is worth reconsideration and should be tested as an alternative solution. The direction of the incoming signal under NLOS channels cannot be fixed or specified. Thus, a dipole antenna or a directive antenna, which have a null value at a certain point of the radiation pattern, may not be appropriate to use. That is, the antenna orientation and its polarization should have a significant effect on wireless networks [9; 10]. Therefore, an antenna with an isotropic radiation pattern, which is ideally a lossless source that can radiate the same intensity of linearly polarized electric field in all directions and receive electromagnetic energy equally from every direction in space, is desirable. This perfect isotropic antenna cannot exist, however, since the transverse electric field in the far-field region cannot have the same magnitude in all directions if the field is linearly polarized everywhere [11; 12]. Although a transverse electric field with a single, linearly polarized component cannot be uniform over a sphere in the far-field region, the sum of the electric fields with two polarizations is uniform [13]. The quasi-isotropic radiation pattern can be generated by the pattern synthesis of small dipoles such that the null point of one dipole is along the maximum-field direction of the other dipole, consequently minimizing the blind spots of each dipole. Cross-placement, proper wrapping of electric dipoles, or a combination of small electric and magnetic dipoles are common approaches [14; 15; 16; 17; 18; 19; 20]. The reported antennae designed under this principle show a measured gain deviation from 2.6 to 12.9 dB over the entire space [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. They are either in a volumetric or planar form.