# A Compact Wideband Active Two-Dipole HF Phased Array
## 3 Design Considerations
### Final Validation Using Other Antennas  
To validate the HF two-dipole active array antenna, we used a Rohde & Schwarz (R&S) EM510 HF digital wideband receiver and dual polarization (HE016) active antenna already installed at the validation site [19]. Since the developed array is horizontally polarized, the omnidirectional turnstile component of the R&S HE016 antenna, capable of receiving HF signals at high incident angles, was used.  
Since the dipoles are placed 90\({}^{\circ}\) apart, the physical distance between the two dipoles that constitute the antenna at 25 MHz is only 3 m as is illustrated in Figure 13 where the two-dipole phased array is in its final form. The phased antenna steers to the desired direction by the use of an electronic switch that can be controlled by a dedicated software.  
Figure 11: Automatic control switch that interfaces with the 90\({}^{\circ}\) hybrid.  
Figure 12: Test results of the phased array between 16 and 30 MHz.  
The HF scans were obtained by the use of an R&S EM510 receiver and an R&S ZS129 hardware switch controlled by ARGUS software [20; 21] illustrated in Figure 14 below.  
Figure 15 presents the free-space azimuthal response for the quarter-wavelength two horizontal 0.41 \(\lambda\) length dipole phased-array antenna at 25 MHz and 30\({}^{\circ}\) elevation angle, as has been simulated by EZNEC based on pattern multiplication principle i.e., beyond the array factor, the array's radiation pattern is based on the dipoles' pattern as well. In comparison to Figure 6, the pattern of the 0.25 \(\lambda\) array which was simulated employing isotropic radiators showing HPBW 180\({}^{\circ}\) and a 3 dB gain. However, the pattern in Figure 15 showing a HPBW 44.5\({}^{\circ}\) and a 7.23 dBi gain. Furthermore, taking advantage of the HE016 antenna which was used as a reference, the sensitivity of the array was evaluated. The phased antenna directions under test were East-West and North-South as illustrated in Figure 16 where the dipole phased-array system is deployed. The results are given in Figures 17 and 18. The scope of this test was to ensure that the active dipoles enabled a good reception below 20 MHz.  
Figure 14: The set-up used in the testing and validation phase.  
Figure 13: The two-dipole phased array in its final form.  
Figure 16: Deployed dipole phased-array system.  
Figure 15: The free-space azimuthal response for the horizontal two-dipole phased-array antenna at 25 MHz and 30\({}^{\circ}\) elevation angle as simulated by EZNEC with gain = 7.23 dB, F/B = 19.79 dB, HPBW = 44.5\({}^{\circ}\).  
It is evident that the proposed array presents a very good performance in terms of receiving signals across the entire HF band, as illustrated in Figure 17. Therefore, the array can be used as a monitor unidirectional antenna for applications below 20 MHz by adjusting the distance between the two dipoles.  
The results of testing the front-back ratio of the array between 20 and 30 MHz are given in Figure 18. These results indicate that the array provides a very good F/B ratio between 20 and 30 MHz for air signals, as clearly some of the received signals are suppressed when the array is electronically steered. For instance, based on Figure 18, the F/B ratio is almost 20 dB at 30 MHz as indicated by charts (b) North-South and (d) South-North. The F/B is also of the order of almost 20 dB between 24 and 28 MHz as indicated by charts (a) East-West, and (b) North-South. Between 20 and 24 MHz, signals are also suppressed by switching the arrays in the opposite direction.