# A Compact Wideband Active Two-Dipole HF Phased Array
###### Abstract  
The design and construction of an upgraded HF quarter-wavelength two-dipole active array with 90\({}^{\circ}\) difference feed was implemented in the course of a research project to perform a directional (azimuthal) investigation of interference at HF. The lack of affordable compact antennas to meet the project requirements was the incentive to develop a compact unidirectional antenna, with the maximum possible front-to-back ratio at frequencies of 20-30 MHz, where the dimensions of traditional passive antennas are enormous. By installing a low-noise very-high-input impedance amplifier in each dipole of the array, the effect of the mutual coupling between the two dipoles was reduced, improving the front-to-back (F/B) ratio over a wide frequency range. Electronic steering, easy polarization adjustment, and fast and easy deployment were the key requirements for the construction of the antenna. Therefore, a light and compact design was of the utmost importance to meet the space limitations at the monitoring site, which did not allow the deployment of a traditional HF directional antenna that employs a very long boom and elements.  
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Footnote †: journal: Applied Sciences  
because they consist of driven elements whose phase difference does not depend on their size. For example, the passive phased array, which is very widely used in the HF and in other bands, is a quarter-wavelength two-vertical-monopole array, with a 90\({}^{\circ}\) phase difference feed [5; 6; 7]. This array can produce a directional pattern with a very high F/B ratio but only under certain conditions. For instance, constructing the array without taking into account the effect of mutual coupling between the two driven elements produces an array radiation pattern in real conditions quite different from the simulated one based on Equation (6) below [5]. This occurs because the mutual coupling disturbs the phase difference between the two driven elements as well as their current balance, as it is discussed later. A traditional method to compensate for the effect of mutual coupling, but only at a certain frequency of operation, is to adjust the phasing of the array's elements by lumped-element phase shifters or delay lines. Another approach is to fine tune both driven elements to a 50 \(\Omega\) purely resistive impedance to ensure 0\({}^{\circ}\) phase shift between the voltage and current. Furthermore, the current magnitudes can be balanced by adjusting the length and the distance between the two driven elements [8; 9; 10]. For all these reasons, a passive phased array is not suitable for broadband operation and radiation patterns, as simulated in ideal conditions and discussed in the next chapter. For an F/B ratio broadband response, the phase difference and the current balance of the two driven elements of the array should be kept as undisturbed as possible throughout the entire frequency operating range. This has been implemented by the use of a very-high-input-impedance, two-stage, low-noise amplifier, which has been installed in each of the two driven elements of the array. The innovative aspect of this approach centres on the mutual coupling isolation that enables a compact array to operate over a wide range with a high sensitivity and F/B ratio. This linear directional antenna is very compact and light and does not require a boom. It can be programmed to steer at the desired direction by the use of a traditional relay switch. The array can be installed in a vertical or horizontal configuration as it employs elements with short size dimensions. Therefore, this HF active phased array has been designed and developed for monitoring sites that do not allow the deployment of a traditional HF directional antenna with a very long boom and elements. In Section 2, we discuss the underlying principles of the phased array; in Section 3, we provide information on specific design considerations and provide preliminary testing results; and in Section 4, we conclude the paper.