# A Compact, Low-Profile, Broadband Quasi-Isotropic Antenna for Non-Line-of-Sight Communications
## 1 Introduction  
Owing to the rapid growth in wireless technology and the miniaturization of devices, the demand for compact, broadband, and full-spatial-coverage antennas has become popular in respect to many applications, including wireless sensor networks, energy harvesting, Internet of Things (IoTs), and monitoring and radio frequency identification (RFID) systems [1; 2; 3; 4; 5; 6; 7; 8]. However, such mission-critical wireless sensor network applications face challenges. First, in many realistic cases, the transmitting (Tx) and receiving (Rx) nodes cannot directly see each other due to the time-varying feature of mobile nodes and obstructions [2], representing the non-line-of-sight (NLOS) environment, as shown in Figure 1. Traditional networks offered noteworthy achievements using numerous techniques, such as spread spectrum, channel diversity, and time diversity. However, mission-critical control systems need reliable networks that surpass what existing networks can provide. The second issue is the need for frequency-hopping systems in wireless sensor networks, where the transmitter changes its carrier frequency to detect the signals from different channels and the interference during each hop [6]. This leads to a complex system requiring intricate and costly digital frequency synthesizers and an error detection mechanism. To overcome this, broadband operating antennas are preferred over narrow-band antennas. The need for batteries is the last, but not the least, predominant issue in regard to charging multiple devices in wireless sensor networks [9; 10]. Indeed, many wireless sensor nodes are encountered in inaccessible or burdensome places. The maintenance of the battery remains challenging; currently, many technologies are being developed that will resolve the battery life limitations in wireless devices, such as the advancement of rechargeablebatteries and the use of double-layer capacitors. In addition, the collection and conversion of ambient energy from diverse sources, including electromagnetic waves, vibration, and heat for energy harvesting technology, are being studied intensively [11,12]. Among these sources of energy, wireless energy harvesting has garnered significant attention due to the availability of several wireless signals such as cellular, satellite, and Wi-Fi. To provide reliable and robust NLOS communications with less cumbersome wireless connections and battery-less sensors, it is desired for the antenna to operate in broad frequencies and radiate electromagnetic energy uniformly in all directions. Such uniform radiation characteristics could be obtained by an ideal isotropic source [13]; however, due to its non-existence nature [14], researchers instead showed a way to obtain a nearly uniform radiation pattern in the name of quasi-isotropic antennas [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. These types of antennas can be achieved using radiation pattern synthesis, such as by a combination of electric and magnetic dipoles, a different orientation of dipoles, two electric dipoles with a 90\({}^{\circ}\) phase shift, etc. The performance of quasi-isotropic antennas is measured through the metrics of gain deviation, isotropic coverage, or beam width [15], and the practice of measuring the gain deviation is widely carried out. Gain deviation is defined as the difference between the minimum and maximum gains of the entire spherical radiating space. The remarkable achievements of quasi-isotropic antenna designs so far are that they mostly operate in a narrow band [16, 17, 18, 19] or in dual bands [20, 21, 22], and only very few of them show quasi-isotropic characteristics in broad frequencies [22, 23, 24, 25, 26]. In [23], an antenna made of a curved meandered dipole and two short-circuited strips was studied in an ultra-high-frequency (UHF) RFID band to provide a quasi-isotropic performance in a broad bandwidth of 20%.