Field strength intensity measurement of a Radio station for Frequency and coverage range inferences and human exposure index

In this research, the measurements of the field strength intensity of a radio station (ORISUN FM 89.5 MHZ) and its nearby locations within its coverage area were taken. This was taken to determine the frequency range, coverage area, field strength and power density measurements of the frequency modulation (FM) station and to ascertain whether its propagating frequency is within the radiofrequency (RF) protection guidelines. The result revealed that the frequency of the FM station did not reach all expected coverage areas in Ile-Ife and also that no mutual interference existed within the coverage areas. It also indicated that the RF propagation is within the protection guidelines of broadcasting stations and not dangerous to residents. The radiofrequency safety recommendations for the field strength and power density were not exceeded in all the locations covered. The highest values could be linked with other antennas. However, this research study provided radio station operators with information on environmental RF fields that will help ensure the prudent and safe operation of amateur facilities.

risk of exposure of humans to radiofrequency (RF) electromagnetic fields due to transmission from the transmitter (Ashwin, 2015;Holloway et al., 2001).
In Nigeria, more than 2000 licensed radio operators exist, and several others exist all over the world.Because of its responsibilities under the terms of the NBC and the NESDRA, it is necessary to ensure that the transmissions from these stations do not expose and render the community residents to high intensities of radiofrequency, which is not accepted by the regulating agencies and guidelines.The propagation profile (2 or 3 dimensions), which shows the relationship between distance, electric field and coordinates, gives the coverage of the landmass essentially by the transmitter (Akinbolatia et al., 2016; Gumusay et al., 2007;Deminco, 2000).
The exposure of humans to RF has been considered by some researchers as one of the major environmental factors to evaluate the potential impact from transmission and suggested guidelines for the evaluation of this exposure to RF energy.So, measurements of electromagnetic fields at a selected radio station in Ile-Ife, Osun State in July 2014 were taken in order to examine the potential impact of the transmission from the radio station.This data will help the regulating agencies in achieving their compliance mandate on radio station transmission in the future.The Orisun FM station was chosen for this study on the basis of accessibility of the workers, antennas and equipment range, approachability of the station transmitting site, and its proximity to the center of the city.Though, no two radio stations can be identical since they have several allocations of frequency bands between 1.8 MHz and 250 GHz and a 1500 watts peak cover power (Cleveland and Mantiply, 1991; ITU-R, 1992).
The signal quality and coverage area are very essential factors that attract high patronage from listeners.Thus, there is a need to carefully determine the rate of decay of electric field intensity alongside the propagation path because an amplified electric field strength and distance provide more signal clarity and reliability coupled with distance propagation (Akinsanmi, 2007;Ajewole et al., 2014;Akinbolati et al., 2016).In some literature, either one of the coverage area or profile of propagation were used to give details of the electric field strength variations near the transmitter and vice versa in radio and television stations in Nigeria, while some used propagation profiles to investigate the UHF and VHF signal strength variations (Ajewole et al., 2013;Oluwafemi and Femi-Jemilohun, 2017).Akinbolati et al., (2017) carried out some research work on UHF TV signal (source to destination) and concluded that the signal strength of UHF signal is seriously affected by the elevation pattern of its coverage.Ajewole et al., (2013) worked on spatial coverage of FM radio transmitters using the propagation of radio waves in the frequency-modulated (FM) band and relief on the very high frequency (VHF) band of the electromagnetic spectrum (30MHZ-300MHZ).
He concluded that the FM radio transmitter in Niger State does not give optimum coverage in the state.Phontus and Chotigo, (2008) proposed a humility correction factor of positive DC breakdown voltage of the sphere-sphere gap at h/δ lower than 13 g/m3.
Their results showed that the voltage measurement by means of standard air gaps which enables them to recognize the effects of electric barriers to the electric field of rod plane air gap.Calvente et al., (2016), Elder, (2003) and Ismail et al., (2010) investigations attempted to determine a correlation between non-ionizing radio frequency (RF) radiation and diverse health effects due to the intense increase in the use of wireless communication devices as well an increase in concerns about potential health risks.
Thus, the aim of the study is to investigate the relationship between the propagation profile and coverage area from the transmitter of a radio broadcasting station and the probable degrees of RF radiation exposure to the public (operators, and residents in the immediate vicinity of the station).The study will deduce the field intensity around the FM transmitting station's coverage area and provide data relating to the coverage area to avoid frequency channel interference.The study's objectives will classify the station's coverage area and determine the transmitted signal's intensity at the clarified coverage area.It will also investigate the radio station's compliance in accordance with NBC since all transmitting stations have their expected coverage area in order for their signal strength not to constitute interference with others.

Theoretical Background
The electric field intensity at a point can be obtained from the power provided to the transmitting antenna, its geometry, and radiation resistance.Consider an example of a center-fed half-wave dipole antenna in free space where the total length, L, is equal to one-half wavelengths (λ/2) (Figure 1).Assuming it is made of thin conductors, the current distribution is basically sinusoidal and the radiating electric field is given equation 1:

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Discovery 59, e108d1352 (2023) 3 of 9 ) 2  7 Applying a technique related to the above, the maximum electric field for a center-fed short dipole is given by equation 8: 8

METHODOLOGY
Measurements were made using an Industry Model EFS-1 field intensity meter at 1-2m above the ground surface at different distances from the station's antenna and also within the premises of the stations and operators building.The measurements of the electric field strength were obtained alongside the coordinates using the meter and a handheld GPS receiver with a total of seven routes around the radio transmitter.The radios transmitter limits and specifications are given in (Table 1).
The data were processed into contour map and profiles for proper visualization and interpretations.The values of the electromagnetic field intensity were adjusted for standardization inaccuracy and rounded off.Field strength meter is test meter equipment used to measure the strength or level of a radio frequency signal; it provides an indication of the presence of radio frequency (RF) energy.It is frequency sensitive and useful for both indication for a change in level and the actual strength of the signal indicated, it responds to signal at any selected frequency under its range.In an ideal open space, the electric field intensity created by a transmitter with isotropic space heater is estimated by equation 9 as: 9 Where E =electric field strength (V/m), P= transmitter power output (W), D= distance from the radiator (m).
From equation 9 it is obvious that the electric field strength and the distance are inversely proportional between the receiver and transmitter.Nevertheless, this relationship is applied for computing the electric field strength created on terrestrial transmitters in which reflections and attenuation caused by objects around the transmitter or receiver may affect the electrical field strength measured.The field strength of an antenna radiation at a given point in space is equal to the amount of voltage induced in a wire antenna 1m long located at that given point.This field strength is affected by a number of conditions such as the time of day, atmospheric conditions and distance from the transmitter.

RESULTS AND DISCUSSION
The parameters obtained from the work were used to determine the coverage areas of the FM radio signal carried out in Ile-Ife Osun State and the impact of the RF exposure to the public.The measurement was taken at the base of the transmitter which sends signal to the antenna.The parameter of the transmitter in which it sends signal to the antenna from the antenna to the receiving end is given in (Table 2).The electric field strength of the station was measured with a digitalized level meter over the range of 30-120dbµv within the hours of 9:00am and 4:00pm local time every day in the month of April 2014.Measurements were obtained at every neighboring town and village and at every location.The intensity meter will be switched on when the aerial on it is being elevated to a required level and it served as an antenna so that the meter can pick up the transmitting of the transmitter when the meter is switched on so it immediately begins to read the frequency of the programmed frequency from the based station.The resulting field data report is given in Table 3 which shows the location of each town and their respective frequency.The measurements were obtained at the beginning of rainy season.The contour map was obtained for the coverage areas for Orisun FM (89.5MHZ) radio signals to give a pictorial view of the coverage areas of the radio stations and to determine optimum coverage areas for the station based on the primary coverage area, secondary coverage area and fringe coverage area respectively (Figure 2).At the base station, the measured signal strength was averagely 112.4dbµv which was the highest across Ile-Ife and environs.
The signal level reduced from the base station.This conforms to the inverse square law (P1 is the power density and r is the distance from the transmitter).The signal dropped to about half of its base value around Ile-Ife round about.This sharp drop could be linked to the terrestrial factors on the signal path-tress, buildings and low-level area and around this place which caused attenuation in the signal's strength.Higher signal level was also measured and recorded at some distant locations such as the case at Omi-Okun, Ifelodun than expected as shown in the propagation profile across Ile-Ife and environs (Figure 3).However, a few places enjoy Grade B quality of service from the station.The signal enhancement in these locations can be attributed to the increased impact of space waves as the elevations of these places are more significant than that of the neighboring towns.The propagation profile of some signal along Ilesa, Moro and Edunabon revealed that signals along this route are grade C but occasionally moves to grade B, because of the changes in the troposphere around there and these changes can influence the way in which radio waves propagates from one point to another (Figure 3).
However, the signal strength of Orisun FM (89.5MHZ)Ife, as expected, varied significantly with distance from the transmitting antenna.This is due to the fact that the strength of the electric field of radio signals decreases with increase in distance from the transmitting antenna due to free space loss and attenuation by atmospheric condition.Attenuation due to free space loss is greater in these environments as a result of their respective distance from the transmitting antenna.Moro junction, Osu Ilesa Road, Akinlalu, Ile funfun areas may be shielded from the FM radio signals by the high-lands in the area as observed from the propagation profile.

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Discovery 59, e108d1352 (2023) 7 of 9 at a distance by high-tech means, mostly electromagnetic signals and waves.This modern equipment for lengthy-distance communication generally involves electromagnetic technologies like mobile phones, networks, radio, microwave transmission, fiber optics, and communications satellites.A radio transmitter's performance is a function of actions of the intensity of the electromagnetic field of the radio wave produced by the transmitter.This action includes differences in the electric field intensity propagation, which determines its transmission profile and coverage area, which are both related (Morris and William, 1973; Yaghjian et al., 1986; Seybold, 2005; Igbonoba et al., 2023).These two properties have their unique usefulness and play balancing roles in the performance of radio transmitter.Likewise, there is also the DISCOVERY To Cite: Sedara SO.Field strength intensity measurement of a Radio station for Frequency and coverage range inferences and human exposure index.Discovery 2023; 59: e108d1352 doi: https://doi.org/10.54905/disssi.v59i333.e108d1352

Figure 1
Figure 1 Distribution of current on length L of antennae equal to one half-wavelength (λ/2).

Figure 2
Figure 2Frequency distribution contour of the study area

Figure 3
Figure 3 Variations of the frequency obtained from each location of the study area

Figure 4
Figure 4 3D map of frequency distribution of study area

Table 1
Standard Frequency Band Range

Table 3
Summary of field strength measurement