## 1 Introduction  
Isotropic antennas are capable of transmitting or receiving uniform power in all three dimensions. Such antennas are useful in applications where the relative position of the receive and the transmit antennas is uncertain or keeps changing. Radio Frequency Identification (RFID) systems, wireless endoscopy, energy harvesting, wireless Access Points (APs), Internet of Things (IoTs), and antennas for Satellites and drones are examples of such applications [1, 2, 3, 4, 5, 6]. Isotropic antennas are also ideal elements in arrays that are designed for direction of arrival (DoA) estimation [7]. This is because using antennas with nulls in beam pattern can result in an array that is blind to signal sources in the direction of null. In addition, any gain variation in beam pattern of individual antenna element can cause a bias in DoA estimation when multiple signal sources are present [8]. Although an ideal isotropic antenna can be useful in a number of applications, it is known to be impossible to achieve [9]. As a result, the design of _near_ isotropic antennas has remained a challenging problem with many different designs being proposed in the literature. Amongst the designs reported in the literature, the best design [10] achieves a simulated variation of 0.91 dB and measured variation of 1.95 dB in total gain. This antenna comprises of a three-dimensional structure having upper (radiator) and lower patch (ground) connected via a sidewall which is fed through a coaxial probe. A three-dimensional folded spherical wire antenna (SWA) proposed in [11] has reported a maximum simulated variation of 3 dB in total gain. Another three-dimensional (3D) antenna has a \(\lambda/4\) monopole configuration, wrapped on a box [12], it achieved a design variation of 9.46 dB in total gain by generating equal orthogonal vector potentials at an operating frequency of 900 MHz. Circular arraying of unidirectional antennas [13] attains quasi-isotropy, but needs complex antenna construction and feeding network. The design in [5] exploits the fractional order radiating behavior of a circular cavity sector and achieves isotropy of 5.7 dB at an operating frequency of 2.45 GHz having a bandwidth of 0.1 GHz for a reflection coefficient of \(\leq\)\(-\)10 dB. Another approach [14] uses a band of frequencies in such a way that the nulls of one frequency are filled up by the maxima of the other with multiport antenna or by many antennas operating at different frequencies (independent feeds), such that they have different polarizations. The concept of combining magnetic and electric dipoles presented in [15] involves a complex feed network with four quadrature signals that were fed to four L-shaped monopoles to achieve quasi-isotropy of 5.75 dB. In [16], a combination of two slots and a monopole provides half-space quasi-isotropy due to large plane ground. A total gain variation of 3.8 dB is realized in all three principal planes [17] through the combination of a pair of printed 1.4 turn loops and printed dipole. The dielectric resonator antenna (DRA) that was proposed in [18] achieved a theoretical variation of 3 dB by combining magnetic dipole with small ground plane that also serves the purpose of electric dipole. In [19], the length of two curved dipoles is adjusted to obtain an isotropic radiation pattern that exhibits a simulated variation of \(\leq\)3.7 dB in total gain and an isotropic bandwidth of 80 MHz for a total gain variation of \(\leq\)6 dB. A 3D U-shaped slot antenna design is proposed in [20], which had total simulated gain variation of \(\leq\)3 dB, with 10 dB impedance bandwidth of 27 MHz. Designs of planar antennas for achieving quasi-isotropic radiation pattern have been proposed in [21; 22].