## 3 Design Principles of the Helical Phased Array
### Final Assessment of the Helical Phased Array  
To validate the performance of the helical phased array, tests were conducted under real conditions. A vertically polarized monopole served as the transmitting antenna, installed at a distance of 15 \(\lambda\) from the helical array. The transmission signal, generated by an AGILENT N9310A frequency generator and amplified by a 2 W HF amplifier, was utilized for the tests. Meanwhile, the helical array underwent testing in both vertical and horizontal polarizations (as shown in Figure 25). Signal strength measurements of on-air HF signals were recorded before the test using a spectrum analyzer (Tiny SA Ultra), as shown in Figure 26. Based on multiple tests, the optimum performance of the array was achieved with the two helical antennas placed 1.87 m apart. Although the helical phased array was originally designed to operate in the 20-30 MHz range, final performance tests were conducted from 15 MHz to assess its operational bandwidth at lower frequencies, where antenna dimensions play a critical role.  
Figure 26: The helical array monitoring on-air HF signals using Tiny SA Ultra.  
Figure 25: Helical array tested (**a**) vertically and (**b**) horizontally.  
Figure 27a illustrates array measurements in a vertically polarized configuration. The blue plot at the top represents forward radiation, while the orange graph at the bottom shows backward radiation. Figure 27b indicates array tests in a horizontally polarized configuration. As depicted in the figures, the axial ratio of the array deviates from unity only below 15 MHz.  
The measured values of the amplitude difference between the two polarizations to determine the axial ratio of the array are depicted in Figure 28a. The axial ratio exhibits a maximum deviation from unity of around 6 dB at 26 MHz within the mid-range frequencies. Additionally, at 16 MHz, 18 MHz, and 20 MHz, the axial ratio ideally approaches 0 dB. The poorest performance was observed at 15 MHz, where the axial ratio reached 30 dB, indicating that the helical array behaved as a linearly polarized antenna as was expected.  
The front-to-back ratio of the helical array in vertical and horizontal polarization between 15 and 30 MHz was determined from measurements in Figure 28b. The blue and orange plots correspond to vertical and horizontal polarizations, respectively. As observed in Figure 28b, the two plots are in good agreement. For vertical polarization, the front-to-back response was 33 dB at 21 MHz and 22 MHz, respectively. At 20 MHz, the front-to-back ratio was 13 dB for vertical and 8 dB for horizontal polarization.  
It is important to underline that all final tests of the array were conducted outdoors in a reflection-free environment. However, some of the measurements were affected by incoming on air signals. Nevertheless, the front-to-back ratio response of the helical array is very satisfactory, particularly when considering its extremely compact size.  
Figure 28: (**a**) The axial ratio response of the array; (**b**) The front-to-back ratio response of the array.  
Figure 27: (**a**) Array tested vertically polarized. (**b**) Array tested horizontally polarized.