## 1 Introduction  
The design and development of a compact active phased array dipole antenna under limited available space was explored as part of a recently funded postdoctoral project [1, 2]. This antenna was designed to monitor the azimuthal interference characteristics over Cyprus in the high-frequency band (HF) by electronically steering its beam at the four cardinal directions without mechanical rotation [3]. The array comprised two 0.41 \(\lambda\) (25 MHz) active dipole antennas with a 90\({}^{\circ}\) phase difference feed, placed a quarter wavelength apart. The resulting design confirmed that the inherent characteristics of the low-noise amplifiers (LNAs) employed in each active dipole effectively mitigated the mutual coupling effect, thus significantly enhancing the front-to-back ratio compared to other traditional two-dipole quarter-wave passive antenna designs with a 90\({}^{\circ}\) phase difference feed. Unlike these passive arrays, which require the fine tuning of each dipole to eliminate mutual coupling, the proposed active dipole configuration with high-impedance LNAs reduces the need for such precise adjustments [4, 5]. In fact, traditional two-dipole or monopole quarter-wave antennas with a 90\({}^{\circ}\) phase difference feed have several disadvantages. For optimal performance, the two driven elements of a quarter-wave array must typically be set to specific lengths, with dipoles typically adjusted to half a wavelength and monopoles to a quarter wavelength. These dimensions are particularly significant for low-frequency applications where wavelengths are relatively long. Moreover, maintaining a fixed distance of 90\({}^{\circ}\) (\(\lambda/4\)) between the two driven elements is imperative and cannot be reduced for low-frequency applications [6]. Notably, the phase difference between the two driven elements varies inversely proportional with the distance between them as a function of the wavelength, resulting in a significant phase error when the array operates off the center frequency. This implies that as the wavelength increases, the phase difference decreases, and vice versa, consequently reducing the front-to-back ratio over the center frequency of operation, even if the driven elements are perfectly balanced [7]. Additionally, it is crucial to consider that a two-dipole array operates at a linear polarization, necessitating installation either vertically or horizontally [7]. On the other hand, using multi-element HF traditional antennas is not always the best option. For example, Log periodic and Yagi-Uda antennas, which are very popular for HF monitoring, consist of large-sized elements mounted on very long heavy booms and therefore require expensive infrastructure and adequate installation space at the monitoring site. For this reason, traditional circularly polarized directional antennas are very rarely used for HF monitoring because they are composed of very large vertical and horizontal crossed elements. In this study, we introduce an upgraded, superior end-fired helical phased array antenna with reduced dimensions to enable the monitoring of both linear and circularly polarization waves. This compact and lightweight design, compared to traditional HF quarter-wavelength arrays and directional antennas, offers a viable alternative to stationary or mobile antennas. A key design goal was to improve the front-to-back ratio of the corresponding array across the operating frequency range. To reduce the size of the array, the two radiators were designed around the normal-mode helical antenna (NMHA) of an extremely short size, i.e., 0.05 \(\lambda\) at 25 MHz. As established in the literature, the circularly polarized NMHA enables the simultaneous reception of vertical and horizontal polarization signals [8]. The input impedance of low-noise amplifiers (LNAs), utilized in each active helical antenna, was optimized to enhance the efficiency and bandwidth of the array by eliminating the parasitic input capacitances of the solid-state amplifiers (LNAs) used in each helical antenna.