3 rd URSI AT-AP-RASC, Gran Canaria, 29 May – 3 June 2022 A Compact Active Monitor Antenna for HF Spectral Occupancy Measurements A. Constantinides * (1) H. Haralambous (1),(2) (1) Frederick Research Center, Nicosia, Cyprus (2) Frederick University, Nicosia, Cyprus, http://www.frederick.ac.cy/ Abstract This paper presents a simple short dipole monitor antenna which has been designed for HF Spectral Occupancy measurements. This short dipole is very lightweight and compact, as a result, it can be easily installed either vertically or horizontally on any mast in locations where limited space imposes a limitation. The active dipole’s efficiency has been optomized based on a two-stage low noise amplifier which is discussed in this paper below. The first stage of this LNA cancels the high capacitive reactance of the antenna’s impedance. The second stage operates as a common emitter amplifier in order to provide the required Gain to the receiver. It has been tested and justified that operates very satisfactory between 5-30MHz. 1. Introduction A directional monitor antenna was to be built last year as part of a research project for HF Spectral Occupancy and azimuthal interference monitoring over the Eastern Mediterranean[1]. It had to be compact with a High Front to Back ratio between 20-30MHz. Finally, a phased array antenna consisting of two active dipoles has been constructed with very good results [2][3]. One of the key parameters to the success of the mentioned antenna was the single element’s characteristics i.e. the active dipole which is analysed in depth in this report. From the test results presented in this paper, it has been confirmed that the dipole can operate very satisfactory between 5- 30MHz. The philosophy of the dipole’s design is discussed in the following chapters later. As it is well known in shortwaves, the dimensions of a dipole antenna are very important since the wavelengths are huge. That is, as the dimensions of a dipole become smaller in regards to the wavelength, its radiation resistance is constantly decreasing. Further, the impedance of the short dipole is dominated by a high capacitive reactance which has the effect of reducing its efficiency for the reasons that are discussed in the next chapters. One way to increase the efficiency of a short antenna is to cool it as to become superconductive[4]. Since this method is not of great practical use, different other ways should be explored. As a good begging in this research is to investigate the efficiency of a short dipole when is matched to 50Ω. To design a matching network for a shortwaves passive short dipole antenna like for instance the reactive L low pass illustrated in figure 1, it would be required an inductor with very high reactance since there is a large capacitance that must be cancelled. Such a highly inductive coil must be wound on a core which presents losses. Other serious losses will also result from the copper’s resistance and the A.C resistance of the inductor’s wire as well. All the losses mentioned above when they join will create a very high loss that in the end the importance of the matching network is now in question. This is one of the main reasons that passive small antennas have low efficiency. To make it clear such a circuit will be analysed along with other serious reasons affecting the reduction of antenna’s efficiency below. 2. The Passive Short Dipole Analysis As the first step, the Eznec antenna software has been used to simulate the impedance of the dipole with a length that is λ/12. The simulated impedance of the mentioned dipole at 5MHz is around Z= 1.2-j3659Ω. Without incorporating a matching network the coupling loss of the dipole when is connected directly to the 50Ω input port of a receiver is around 31dB over to a corresponding half-wave dipole which is fully matched. The equivalent circuit of the dipole is constituted by the source V1 in series with the resistor R1 and the capacitor C1 as is illustrated in figure 1. The loss resistance of the dipole has not been yet taken into account as is discussed later. Figure 1. The Output Impedance of the Short Dipole Matched to 50Ω According to figure 1, R1 represents the radiation resistance of the short dipole and C1 is its capacitive reactance. L1 and C2 constitute a typical matching network which matches the antenna’s impedance to 50Ω. The frequency response of the matching network has been