## 4 Discussion  
Measurements of RF electromagnetic field intensity parameters can be used to determine compliance with exposure guidelines. The great number of base stations are mostly installed in populated areas. Base stations operating in densely populated cities in the GSM 900, 1800 and UMTS 2100 MHz frequency bands are continuously evolving due to the conditions of competition and the need for a better response to increasing customer demand. This evolution is carried out by replacing classic antennas with new high-gain antennas or establishing a new base station in the targeted area. Naturally, the electromagnetic field density of the environment has also changed in parallel with this evolution process. Due to these changes, people who are working near base stations, especially on the roof, could be exposed to an electromagnetic field strength that is several times higher than for people working farther away from the base stations. Excessive exposure of people to electromagnetic fields can be avoided by correct positioning of the base station antennas. [PERSON] et al. [16] studied the distribution of the values of the electric field of a 35 m high mobile antenna at a height of 29-35 m (ERP = 1500 W) in the near zone (up to 30 m distance) and found that the values of the electric field strength were distributed between 0.15 and 42.47 V/m. The distribution of the values of the electric field strength depends on the distribution of the antenna diagram in the vertical plane. On comparing the results of the field measurements conducted with other results, it is obvious that the largest convergence of the results is obtained when the antenna height reaches 31 m, at which the values of the electric field strength at the distance of 1-30 m are distributed in the interval between 5 and 20 V/m. Field-measurement scientists carried out a study on mobile telephony bases station antennas located on roofs [13]. Their measurements established that the strength of the electric field emitted by a mobile antenna at a distance of 1 m reached 77.9 V/m; that emitted by a mobile antenna with ERP = 160 W at a distance of 1 m reached 26.5 V/m, and that emitted by a mobile antenna with ERP = 1350 W at a distance of 1 m reached 128 V/m. The values obtained for the electric field strength in our study are similar to these. The main difference is the mobile antenna's ERP, which is dependent on the electric field strength in the near zone. Some recent studies clearly illustrate the directional mobile antenna's feature of creating a local RF EMF power density at a certain distance, with values in the frame of the maximum exposure guidelines [37]. Mobile antennas 20 m high (with ERP = 800 and 1000 W, respectively) create an RF EMF power density exposure limit proportionately to distances of 60 and 85 m, respectively. In the first case, the power density value was equal to 1.8 \(\mu\)W/cm\({}^{2}\), and in the second it was equal to 1 \(\mu\)W/cm\({}^{2}\)  
Figure 14: Contributions of different antenna types to the total exposure. The discrepancy in the RF EMF power density values during the field measurements conducted can be explained by the fact that antennas with different levels of effective radiation were chosen for investigation. The distance at which the exposure limit is reached depends on the electric and mechanical downtilts. [PERSON] have studied RF EMR from mobile antennas. With his colleagues, he established that each antenna (using the GSM-900, GSM-1800 and UMTS-2100 technologies) creates unequal RF EMR values at particular frequencies [42]. His group established that 20%, 40%, and 40% of the total RF EMR is contributed by mobile antennas (ERP--800 W, height = 30 m) in the 900, 1800, and 2100 MHz bands, respectively. As can be seen from the comparison, the frequency ranges of different antennas, according to the percentage contribution to the total RF EMR found by a field measurement survey, differ from the results of this study.