# Models of HF Interference over Cyprus
## 2 Materials and Methods  
The monitoring system is based on a R&S EM 510 digital wideband HF receiver connected to a switch that can select either the low- or high-elevation angle signal components of a R&S HE016 omnidirectional antenna (Figure 1). In this modeling study, the low-elevation angle dataset was used. The dataset was obtained by connecting the monopole HE016 antenna component to the antenna input of the R&S EM 510 measurement receiver using a 100-ms dwell time so that the full HF spectrum could be sampled in less than 30 min. By shifting a 1 kHz filter in 2 kHz increments in the frequency range of 1.606-30.000 MHz, we were able to determine the RMS field strength (in dB\(\upmu\)V/m) in 14,198 channels corresponding to 95 ITU-defined frequency allocations for different user types in accordance with Table 1. Each 1 kHz channel was defined as occupied at a particular field strength threshold (from \(-20\) to \(45\,\mathrm{dB}\upmu\)V/m in \(5\,\mathrm{dB}\upmu\)V/m steps) if the signal RMS value determined over the 100 ms observation period exceeded the corresponding calibration threshold set at the receiver's input. The fraction of such channels across each user allocation then defines the congestion, \(Q\) for the particular field strength threshold, and allocation using the particular antenna (Figure 2) [13]. The HF spectrum is divided by the International Telecommunications Union (ITU) into frequency allocations for 9 different user types to account for the fact that there is an international agreement that defines its utilization. Interference characteristics for distinct types of users exhibit significant differences; therefore, the dataset was divided into 95 ITU-defined frequency allocations for different user types in accordance with Table 1. This is because different user types employ different transmission powers, bandwidths, modulation schemes, and operating procedures. ITU rules and guidelines dictate that transmissions in the HF spectrum must adhere to specific operating frequency ranges from designated frequency assignments, with the frequency assignment selected on the basis of several aspects such as average propagation conditions, expected attenuation due to D-layer absorption, atmospheric noise background, and last but not least, anticipated spectral occupancy levels emanating from local groundwater or long-distance skywave signal transmissions. Figure 3 depicts measured field strength measurements received from the monopole antenna over a frequency range of 5.060-5.480 MHz, which corresponds to a fixed/mobile allocation 19 (Table 1). The bar chart in Figure 3b below illustrates occupied channels with black bars at a threshold of 20 dB\(\upmu\)V/m on top of a white background, which corresponds to unoccupied channels. Full HF spectra of signal strength measurements at midnight (00:00 LT), early morning (06:00 LT), noon (12:00 LT), and late afternoon (18:00 LT) are shown in Figure 4. At midnight, due to the absence of solar radiation, the ionization of the F layer is at its minimum, and therefore the maximum usable frequency (MUF), which dictates the range of HF frequencies that can be supported for skywave propagation, is also at its minimum. Consequently, HF users must occupy the lower end of the HF band, causing increased spectral occupancy.