# Models of HF Interference over Cyprus
## 2 Materials and Methods
The MUF is the highest frequency that can establish communication for a link of a certain distance with a probability of 50% of the time, and it is calculated by multiplying the critical frequency of the F2 layer (foF2) by an MUF factor that is a function of the link distance. At noon (12:00 LT), the absorption of HF skywave signals as they pass through the lower ionospheric layer (D-layer) in the lower HF band causes users to shift to the upper segment of the HF band (>20 MHz). Due to maximum ionization, the MUF peaks at noon, and as a result, the higher HF band (i.e., 12-30 MHz) becomes congested, as shown in Figure 4c. At 18:00 LT, the ionization decreases, and subsequently, users migrate back to the lower frequencies, resulting in higher occupancy, as demonstrated in Figure 4d. Figure 5 depicts the 24 h variation of occupancy in the HF spectrum for interference signals exceeding a threshold of 20 \(\upmu\)V/m, which is in accordance with the temporal signal characteristics manifested in Figure 4. At noon, most HF interference signals appear in the middle of the HF band, whereas during the evening hours, HF spectral occupancy increases in the lower segments of the HF band. This is attributed to the decrease in solar radiation, which limits ionospheric ionization and, subsequently, supports HF signal propagation at higher frequencies. The HF spectral occupancy increase during nighttime is exacerbated by co-channel interference, which becomes more pronounced as a consequence of stronger skywave HF signals propagating for longer distances due to the disappearance of the D-layer absorption in the absence of solar radiation. These variability features enable the usage of a limited range of HF frequencies, causing an increase in occupancy levels within available allocations. These conditions become even more severe during low solar activity periods when the range of usable operating frequencies is limited, further causing higher spectral occupancy levels. The HF spectral occupancy variation in the lower HF spectrum frequencies is exactly opposite in comparison to the upper portion of the band. In the lower portion of the HF band, occupancy peaks during the evening hours since allocations follow the 24 h pattern of circuit LUF (Lowest Usable Frequency). The LUF is the lowest frequency that can provide the necessary Signal-to-Noise ratio (S/N) to ensure the minimum requirement of service  
Figure 4: Signal strength measurements across the HF band measured in Cyprus (**a**) at midnight (00:00 LT), (**b**) early morning (06:00 LT), (**c**) noon (12:00 LT), and (**d**) late afternoon (18:00 LT). for a certain link. It is highly correlated with absorption (due to D-layer ionization), which is inversely proportional to the square of the carrier frequency. The 24 h variation profile for allocations in the upper HF spectrum is the inverse of that of allocations in the lower portion of the band, as it is also characterized by significant diurnal variation, albeit with interference levels that peak during the daytime hours. Typical diurnal variation profiles for allocations in the lower and upper HF bands are shown in Figure 5(a,b, respectively. Figure 5: A diurnal plot of HF spectral occupancy at a threshold of 20 \(\upmu\)V/m. Figure 6: Diurnal variation profiles for broadcast allocations (**a**) 22 (5.950–6.200 MHz) and (**b**) 72 (21.450–21.870 MHz). Although the primary purpose of HF skywave propagation is long-range communication, short-range communication is also possible through near vertical incidence skywave (NVIS) HF propagation. According to NVIS, transmitted waves at high-elevation angles are reflected by the ionosphere to facilitate umbrella-like coverage. NVIS is suitable when ground-wave communication is not possible due to terrain-imposed limitations, such as in the case of Cyprus, for which NVIS communication is particularly important. The typical frequency range for NVIS is 2-10 MHz.