Revision of the Methodology for Processing Radio Noise Measurements in the Medium Wave Band

Transcription

1 Revision of the Methodology for Processing Radio Noise Measurements in the Medium Wave Band Iratxe Landa, Member, IEEE, Manuel Velez, Member, IEEE, Amaia Arrinda, Senior Member, IEEE, Pablo Angueira, Senior Member, IEEE and Iñaki Eizmendi All the authors are with the Communications Engineering Department of the University of the Basque Country, Faculty of Engineering, Alameda Urquijo s/n , Bilbao, Spain. Iratxe Landa: telephone: Manuel Velez: telephone: Amaia Arrinda: telephone: Pablo Angueira: , telephone: Iñaki Eizmendi: telephone: Abstract -This study discusses the relevance of the methodology applied to process indoor noise measurements in the Medium Wave band in order to provide reference noise levels. The knowledge of noise levels is necessary to plan radio services and to analyze the influence of radio noise in the human body. This work presents a description of the parameters to characterize the noise measurement antennas, and explains how these parameters are determined. The terms to specify the noise intensity and the methods to process radio noise measurements are described. The characteristics of an ideal and an actual antenna in the Medium Wave are presented, showing the relevance of the difference between them when radio noise levels are calculated. This work presents the differences in the noise levels given by different processing methods using an actual antenna and a theoretical antenna. Finally, the results of several recently published noise measurements, which have been processed by different methods are presented and compared with the values predicted by ITU-R P.372.

2 Keywords noise measurement, medium wave, noise figure, antenna factor, Gaussian noise, radio propagation, radio broadcasting, receiving antennas, antenna measurement. 1. Introduction The lowest part of the Medium Wave (MW) band, concerning to frequencies below 2 MHz, has been traditionally utilized for commercial AM broadcasting. However, new digital broadcasting standards, such as Digital Radio Mondiale (DRM) and HD-Radio, have been recently developed to be used in the Medium Wave band [1-6]. The radio noise level is one of the most important factors that should be considered to plan radio services. In the Medium Wave band the atmospheric noise and the man-made noise are the main contributions to be considered when calculating radio noise values. The man-made noise has obviously changed in these last years. In areas with high human activity, such as high populated urban areas and industrialized areas, the noise sources have increased. However, some of the predicted values in ITU-R P.372 [7] for MW band are still based on outdoor measurements carried out more than 30 years ago and there are no reference noise values for indoor locations in MW. Moreover, the methodology for processing radio noise measurements is not defined at all [8-9], and it is in this point, where this paper makes its main contribution. In this situation, the International Telecommunication Union (ITU) has asked to study the noise levels in all frequency bands, for indoor and outdoor locations and their temporal and geographical variations. The ITU has also asked to analyze how noise measurements should be done and how they should be processed [10]. At the same time a database has been created in order to collect all the noise measurements carried out around the world. Several recently published noise levels in frequencies below 2 MHz for indoor locations [11-17] are higher than the predicted values by ITU-R P.372 [7] for outdoor locations in city environments. However, other recently published noise levels [18-19] are lower. So, a clear conclusion is not easily obtained. It should be noted that the noise values given in previous works are highly dependent on the methodology used to measure and to process the results. Therefore, the goal of this paper is to provide an exhaustive analysis of the methods to process radio noise measurements in order to show the great importance of a detailed definition of the methodology to process radio noise data, and a good characterization of the measurement antenna in order to understand and compare the results of noise values from different authors. This work has been carried out after a deep analysis of the state of the art of radio noise characterization and a deep analysis of the characteristics of the antennas utilized in MW band. It has been found that there

3 is an important relation between the antenna characterization and the final noise levels. The paper is organized as follows. Section 2 describes the basics parameters used to characterize radio noise, including the antenna characterization and the parameters utilized to specify the noise intensity. In Section 3 the methods to process data are described. The characteristics of the actual antennas and ideal antennas in MW band are described in Section 4 and, finally, the discussion and conclusions are presented in Sections 5 and Parameters for Characterizing Radio Noise Radio noise may be classified into two categories according to its source. On one hand there is the natural noise, which is generated by radiation from lighting discharges, emissions from atmospheric gases and hydrometeors or radiation from celestial radio sources. On the other hand there is the man-made noise, which is generated by human activity such as the radiation from electrical machinery, electrical and electronic equipment, power transmission lines, and from internal combustion engine ignition [7]. In Medium Wave band the main contributions to the radio noise are due to the atmospheric noise, principally from the lighting discharges, and the man-made noise. The atmospheric noise depends on the geographical location and on the season. However the man-made noise is strongly dependent on the distance from the source. In the case of indoor radio noise measurements the main contribution is the manmade noise [7]. In order to characterize the radio noise in one location, the measurement equipment should consist on a test receiver and an antenna. The test receiver must exhibit a low noise figure together with high gain stability and must be calibrated [20-23]. The external measured radio noise should be 10 db higher than the internal noise of the receiver in order to have accurate measurements. Regarding the antenna, this is one of the critical elements in noise measurements in MW band. The measurement antenna and its calibration are the most important factors to perform radio noise measurements in the MW band [24]. 2.1 Antenna Characterization The antennas are characterized by their radiation pattern, directivity, gain, efficiency, impedance, antenna factor and reflection coefficient [25-29] that depend on the propagation mode in the frequency band. The antennas used for broadcasting in MW are physically large because its length is comparable to the

4 wavelength. However, the common antennas for reception in MW are physically smaller and its length is much smaller than the wavelength [30-32]. Usually the antennas for reception in low frequencies are mismatched. These antennas have a high reflection coefficient, so only a small fraction of the available power is transferred to the receiver [33]. The noise values given by the ITU [7] are referenced to a lossless antenna. Actually, lossless antennas do not exist. The most similar one is a matched vertical dipole [29, 34-36]. This dipole is the recommended antenna for measurements at frequencies above 30 MHz. For noise measurements at frequencies below 30 MHz, such a vertical dipole is too long and the recommended antenna is a short monopole [37-38]. In MW band, active elements are used to match the antenna to the receiver. However, for noise measurements the active elements are not recommended [8]. For noise measurements one of the most important factors is the proximity of the sources. When the antenna is close to the source, the antenna radiation pattern can be affected. This occurs quite often for indoor measurements in MW band [20, 39]. There are two parameters that characterize the antennas and which are mainly utilized to process radio noise data. These parameters are the antenna factor and the reflection coefficient. Antenna Factor The antenna factor is the most common parameter for characterizing antennas in field strength measurements and radio noise measurements [25]. This parameter gives the relationship between the external field strength in the antenna and the voltage induced in the receiver [25]. The antenna factor in linear units and in db(1/m) is given by expression (1), where k is the linear antenna factor, E is the electrical field strength in the antenna, V is the voltage induced in the receiver and K is the antenna factor in db(1/m). E k = V K ( db( 1 / m) ) = 20log k (1) There are three main ways to determine the K factor of the antenna: Theoretical antenna factor

5 The value of the antenna factor can be determined theoretically [9] if the antenna gain is known. First, the power density S due to an electric field E in the free space is evaluated with the impedance of the free space as 377 Ω. Then, the power captured by the antenna is calculated as the power density multiply by the effective area of the antenna. In this case, the impedance of the antenna is considered to be 50 Ω. The effective area of the antenna is expressed as a function of the gain and the wavelength. The expression (2) resolves the antenna factor in a theoretical way, where k is the linear antenna factor, g is the linear gain of the antenna, λ is the wavelength, K is the antenna factor in db(1/m), f is the frequency in MHz and G is the gain of the antenna in db k = λ g K ( db( 1/ m) ) = 20log f ( MHz) G( db) (2) If the antenna is not omnidirectional, the antenna factor calculated in this way concern only to the direction in which the antenna has this gain. Antenna factor given by the manufacturer The antenna factor provided by the manufacturer is determined according to the rules of a calibration standard. Usually this standard is for measurements of electromagnetic compatibility EMC. According to these rules, the K factor is computed placing the transmitting antenna and the receiving antenna at a specific distance and a specific height above ground. In the calibration process the antenna is exposed to a known electromagnetic field. The calculated K factor depends on the height of the antenna and on the distance. Antenna factor is supposed to be usable in far field and free space conditions when there is a dominant field or signal [15-17]. Antenna factor determined by comparison In case of having an antenna which is not calibrated by the manufacturer and another one calibrated, the antenna factor can be determined by comparison of both [9, 40]. Reflection Coefficient

6 The reflection coefficient is the relationship between the reflected wave and the incident wave. In other words, this term gives an idea about the losses due to the impedance mismatch between the antenna and the receiver. If the value of the reflection coefficient is zero, this means that the impedance of the antenna is matched to the impedance of the receiver (usually 50 Ω) and there is no mismatch loss. If the value is 1, this means that the antenna works like an open or short circuit and in this case the mismatch losses are theoretically infinite. The expression (3) gives the relationship between the power that arrives to the receiver P RECEIVER and the power in the antenna terminals P ANTENNA, being Г the reflection coefficient [41, pp ]. RECEIVER = P ANTENNA 2 ( 1 Γ ) P (3) The reflection coefficient is usually measured with a network analyzer. 2.2 Parameters to Specify of Noise Intensity There are two parameters to specify noise intensity. These are the noise figure and the electrical field strength. Noise Figure The noise figure F a is defined as the available noise power from an equivalent lossless antenna normalized to the receiver bandwidth in db. The noise factor f a is the linear value defined as expression (4), where p n is the available noise power from an equivalent lossless antenna, k = J/K is the Boltzmann s constant, t o is the reference temperature (K) taken as 290 K and b is receiver bandwidth (Hz). This parameter does not depend on the receiver bandwidth [7]. f F a a p = k t 0 n b ( db) = 10log f a (4) Taking into account the definition of noise figure, the considered noise power is from an equivalent lossless antenna. The common reference lossless antennas are an ideal short monopole or an ideal half-

7 wave dipole [7]. The reference antenna is regarded as lossless, ideal and matched to the receiver, as well as operating in the free space and in the far field conditions. Electrical Field Strength The relevance of noise measurements in radio engineering lies in the comparison between a radio signal and the background noise level. As the signal strength of the radio signal is expressed in field strength, the noise power can also be expressed as equivalent field strength, assuming that all the noise power at the antenna connector is received from the same direction (azimuth and elevation) as the radio signal was coming from [3]. But if the data are processed in this way, two problems appear. Firstly, the noise measurements assume that the noise sources are randomly distributed and can reach the antenna in any direction and not only in the direction of the radio signal. Secondly, if the noise is expressed as field strength, the value depends on the receiver bandwidth. These aspects should be taken into account in order to compare the results with other measurements or with reference values. 3. Methods to Process Radio Noise Measurements Four methods to process radio noise measurements could be considered in order to provide noise levels. Method 1- Noise figure according to ITU recommendation P [7] In this method, defined by the ITU [7], the parameter utilized to specify noise intensity is the external noise figure. This parameter is calculated with expression (5), where F a is the external noise figure, P R is the power measured by the test receiver, K is the antenna factor, f is the center frequency of the channel and b is the receiver bandwidth. ( db) = P ( dbm) K ( db( 1/ m) ) 20 log f ( MHz) 10 logb ( Hz) Fa R (5) Expression (5) has been extracted from expression (6) [7], where E n is the vertical component of the RMS field strength for a short vertical monopole above a perfect ground plane. ( db V / m) = F ( db) + 20 log f ( MHz) + 10 logb( Hz) En µ a (6)

8 This method considers an ideal short monopole antenna as a lossless reference antenna [7] in order to give the noise power from an equivalent lossless antenna. This antenna is regarded as lossless, ideal and matched to the receiver. This antenna is considered to operate in the free space and in far field conditions. The K factor of the ideal short monopole antenna is the theoretical K factor calculated with expression (3). To determine expression (5), first, from the power measured in the receiver and with the K factor of the antenna, the external field strength is estimated using expression (1). Then, the power that would be induced in the receiver with a lossless short monopole is calculated considering that the impedance of the antenna is 50 Ω, with the previously calculated external field strength and with the K factor of the ideal short monopole. At last, the noise figure in expression (5) is computed with this hypothetical power. This method has been used in recently published noise measurements in MW band [12, 14, 16, 17]. Method 2 - Noise figure using the reflection coefficient correction As in method 1, this method also computes the noise figure to specify the noise intensity. The reflection coefficient of the antenna takes into account the losses due to the mismatch between the antenna and the receiver [20, 41, 42-45]. This method gives the noise figure from the measured power in the receiver and the reflection coefficient of the antenna. The noise figure is calculated with expression (7), where F a is the external noise factor, P R is the measured power by the test receiver, Г ANT is the reflection coefficient of the antenna and b is the receiver bandwidth. 2 ( db) = P ( dbm) 10 log ( 1 Γ ) 10 logb ( Hz) Fa R ANT (7) The results of this method do not depend on the receiver bandwidth. Method 3 - Electrical Field Strength with the antenna factor This method gives the electrical field strength to specify the noise intensity. The noise is processed as the signal, and the equivalent electrical field is determined with expression (8), where E is the electrical field strength, P R is the power measured by the test receiver and K is the antenna factor. ( db V / m) = P ( dbm) K ( db( 1 m) ) E µ R / (8)

9 This is a useful method for calculating C/N ratio. However, as it has been commented before, the noise value calculated by this method depends on the receiver bandwidth. This method has been used in several recently published noise measurements in the MW band [9, 12, 13, 15]. Method 4 - Noise figure directly from the power measured in the receiver This method determines the noise figure to provide noise levels. This method considers that the measurement antenna is a lossless one, matched to the receiver. The noise figure, in this case, is computed directly from the power measured in the receiver, normalized to the receiver bandwidth with expression (9), where F a is the external noise factor, P R is the measured power by the test receiver, and b is the receiver bandwidth. ( db) = P ( dbm) 10 logb ( Hz) Fa R (9) This method has been used in recently published noise measurements in the MW band [18] and in a contribution of noise measurements in HF and LF to the ITU [19]. In Table 1 there is a summary of the methods to process radio noise and their main characteristics. Table 1. Methods to process radio noise measurements Method Method 1 Method 2 Method 3 Method 4 Parameter F a F a E F a Bandwidth dependent No No Yes No Measurement antenna considered Antenna parameter Yes Yes Yes No Г K factor Reflection K factor Coefficient

10 4. Considerations about Actual Antennas and Ideal Antennas in MW for Indoor Measurements There are many differences between actual antennas and ideal antennas in MW. The differences are higher at lower frequencies, and very high below 2 MHz. The theoretical K factor is calculated for lossless antennas that are matched to the receiver and operate in the free space in far field conditions. However, the actual antennas in MW do not fulfill these conditions. Moreover, the K factor could have been estimated by an EMC calibration standard [40]. This K factor depends on the specific conditions of calibration, heights of the antennas and distance between them, as it has been commented before. When a MW antenna is used for indoor measurements, the far field at 2 MHz for example is around one kilometer. Normally, in indoor noise measurements, the sources are closer to the antenna, and the radiation pattern of the antenna is usually modified by the furniture and nearby objects. So, the free space and the far field conditions are not fulfilled for indoor MW measurements. Considering that the theoretical K factor is not valid for actual antennas in MW, and that the K factor estimated by calibration methods does not fulfill the conditions for indoor radio noise measurements in MW, it may be concluded that the determination of the K factor for antennas in MW for indoor locations is not easy at all. The German quad and inverted V antennas, proposed for noise measurements in the Report ITU-R SM.2055 [9], are too large to be utilized indoor. In order to show the differences between actual antennas and ideal antennas in MW, two antennas have been taken as example, one actual and one ideal. These antennas are described next. Rod antenna, actual antenna in MW band The rod antenna is one of the antennas used for noise measurements in the MW [6-11]. An image of this antenna is shown in Figure 1 [46]. This is a passive broadband electric field monopole antenna. Its frequency range is from 1 khz to 30 MHz. It has been calibrated by the manufacturer according to IEEE std. 291 [47]. Its length is 1.04 m. The effective length of the antenna at these frequencies calculated as IEEE std [47] is 0.52 m.

11 Figure 1. Rod Antenna. Short monopole, ideal antenna The short monopole is considered as an ideal antenna that operates in the free space conditions, and that is matched to the receiver, so the value of its reflection coefficient is zero. The K factor of this antenna can be calculated in a theoretical way by the expression (2). The power in the terminals of the antenna is directly proportional to the effective area of the antenna. In the ideal case, this effective area is proportional to the wavelength. The wavelength in MW is high, so the K factor in db(1/m) is negative and very high. 4.1 Discussion on Antennas Comparing the rod antenna and the short monopole the Table 2 shows the K factor and the reflection coefficient from 0.5 to 30 MHz. At the lowest frequency, 0.5 MHz, the K factor in db(1/m) of the ideal antenna is negative and very high, db(1/m). This is because, as it has been mentioned before, the theoretical K factor is inversely proportional to the effective area, and the effective area is proportional to the wavelength. At lower frequencies the theoretical K factor in db(1/m) is negative and high. However, the actual antennas for indoor measurements in MW are usually no higher than 2 meters long. The electrical

12 length of these antennas is small and the effective area is proportional to the electrical length. For lower frequencies, the actual antennas in MW have smaller effective length. The calibration K factor, in db(1/m), of these antennas is positive and very high. This means that a very little part of the external field strength is transferred to the test receiver. For indoor noise measurements in MW band it is difficult to find more sensitive actual antennas. At 0.5 MHz the K factor of the rod antenna is 50 db(1/m) (see Table 2). The difference between the K factor in the ideal and the actual antenna is higher at lower frequencies, from 30 db at 30 MHz to 84.4 db at 0.5 MHz. Regarding the reflection coefficient, an ideal antenna is matched to the receiver, so its reflection coefficient has zero value. However, the actual measurement antennas in the MW band, at frequencies below 2 MHz, have reflection coefficients higher than 0.8 (see Table 2). Table 2. Characteristics of the antennas Antenna Rod Antenna Theoretical short monopole Frequency (MHz) K Factor db(1/m) Γ K Factor db(1/m) Γ Discussion of the Processing Method In this Section the four processing methods, explained in Section 3, are compared considering the two antennas presented in previous Section. The goal is to show the importance of the method and the antenna in order to provide reference values of radio noise. This is crucial in MW due to the characteristics of the antennas at these frequencies. Otherwise at higher frequency bands (UHF/VHF) the method to process radio noise measurements is easier because in this case the measurement antennas are more similar to ideal antennas, the noise sources are far from them and they do not affect their radiation pattern. Moreover, the

13 antennas at higher frequency bands are matched to the receiver, so the reflection coefficient is almost zero and the antenna factor is close to the theoretical factor calculated with expression (2). Table 3 shows a summary of the results obtained by the four processing methods with the ideal short monopole antenna. In the same way, Table 4 shows a summary of the results calculated by the four processing methods with the actual rod antenna. For this comparison, it has been considered that the power measured in the receiver has been -100 dbm, with a receiver bandwidth of 9 khz. The results are also shown in Figure 2. When an ideal antenna is considered, the results obtained by method 1, 2 and 4 are exactly the same (see Table 3 and Figure 2). This is because these three methods use noise figure to provide noise levels (see Table 1) and the ideal antenna is regarded as lossless and matched to the receiver, so its reflection coefficient is zero and the antenna factor is the antenna theoretical factor computed with the expression (2) (see Table 2). The results in Table 3 of methods 1, 2 and 4 are computed by the expressions (5), (7) and (9) respectively. Regarding method 3, the values are given in field strength (see Table 1), so these values depend on the receiver bandwidth and, they can not be compared directly with the values given in noise figure. The results in Table 3 of method 3 are calculated by the expression (8). The values in Table 4 concern the rod antenna. The method 4 gives the same results as the ideal antenna (see Table 4 and Figure 2). This is because method 4 assumes the antenna as it would be ideal, lossless and matched to the receiver. This method does not consider the actual characteristics of the measurement antenna (see Table 1). Regarding methods 1 and 2, although both methods calculate noise figure to specify noise levels (see Table 1) the results are different because whereas the method 1 applies the antenna factor to characterize the measurement antenna, the method 2 uses the reflection coefficient (see Table 1). The antenna factor has been provided by the manufacturer and the reflection coefficient has been measured with a network analyzer. At these frequencies the reflection coefficient is almost 1 (see Table 2) because of the mismatch between the antenna and the receiver. There is a relation between the antenna factor and the reflection coefficient: when the mismatch is high, the reflection coefficient is almost 1, the antenna factor (db(1/m)) is high. It can be seen that the differences between the results calculated by method 1 and 2 compared with the results of method 4 are higher at lower frequencies (see Figure 2). Regarding the differences for method 3 between Table 3 and Table 4, they are equivalent to the differences for method 1 between both tables, because method 1 and method 3 characterize the antenna with the antenna factor (see Table 1). However it must be considered again that method 3 gives noise levels in field strength units and these depend on the receiver bandwidth. It can be said that in Medium Wave band it is very important to specify which method has been applied to

14 give noise levels, which has been the measurement antenna and how it has been characterized, in order to understand the results and in order to be able to compare them with other noise values. Table 3. Results by different methods for ideal short monopole antenna Method Method 1 Method 2 Method 3 Method 4 Frequency F a (db) F a (db) E(dBµV/m) F a (db) 0.5 MHz MHz MHz MHz MHz MHz 8 Table 4. Results by different methods for rod antenna Method Method 1 Method 2 Method 3 Method 4 Frequency F a (db) F a (db) E(dBµV/m) F a (db) 0.5 MHz MHz MHz MHz MHz MHz

15 Figure 2. F a for the rod antenna and the short monopole antenna by method 1, 2, and 4 for a considered power measured in the receiver of -100 dbm in a bandwidth of 9 khz. As an example of the importance of defining the methodology to process radio noise measurements in order to compare noise levels, Figure 3 shows several of the recently published noise levels processed by method 1 and method 4, compared with the values predicted by ITU-R P.372 [7] for external urban environments. Method 1 [5-11] gives values higher than predicted by ITU, and method 4 [18-19] gives values lower than those predicted by ITU. The values predicted by ITU are based on measurements carried out more than 30 years ago. The antenna, its characterization and the data processing in these measurements are not completely known. When the measurement antenna has a very low reflection coefficient and a low k linear antenna factor, this antenna is more similar to an ideal, lossless and matched antenna. In this case, the results by method 1, method 2 and method 4 are the same. Antennas with these ideal characteristics can be easily found at higher frequencies, above 30 MHz. For lower frequencies, the actual antennas for indoor measurements in the MW are very different from an ideal antenna and method 1, method 2 and method 4 provide different results of noise levels. These differences are more acute at lower frequencies, below 2 MHz.

16 Figure 3. F a of recently published indoor noise measurements processed by method 1 and method 4, compared with the ITU predicted values for external urban environments. 6. Conclusions This study describes the methodology to process radio noise measurements in MW band. The basics for radio noise measurements have been described: the antenna characterization and the parameters to specify noise intensity. Then, four methods to process radio noise data have been described and the main characteristics and differences between the actual and ideal antennas in MW band are given. Finally a comparison of several recently published noise values and the values predicted by ITU-R P.372 has been presented. At higher frequencies, such as in VHF and UHF bands and higher, the antenna characterization and the method to process radio noise measurements are easier because the noise sources are far enough from the antenna to consider far field conditions. The antenna pattern is not affected by the proximity of the sources, so the behavior of the antennas is almost ideal [42-75]. Some noise reference values for indoor and outdoor locations have been included in the ITU-R P.372 [7] in 2006 and In these recently included measurements the antenna is not defined. The measurement

17 antenna is also omitted in other published noise measurements from the latest to the oldest reference values [57, 65, 74, 75, 76, 78, 80, 81]. As a result, it has been confirmed that the noise level values obtained by different process methods cannot be compared directly. Therefore, in order to understand and compare the results of noise measurements it is necessary to know, firstly, the process method to provide the noise figure or field strength, and secondly, the characterization of measurement antenna, its K factor and how this K factor has been calculated. After this exhaustive analysis, for frequencies below 2 MHz we recommend to specify noise levels using method 4. The characteristics of the antenna and the receiver bandwidth should be provided too. In this way noise levels could be compared with results from other authors, and the C/N ratio can be easily estimated for radio planning in the same measurement conditions. 7. Acknowledgment This work has been supported by the Basque Government GIC 07/110-IT , by the University of the Basque Country UPV/EHU (UFI 11/30), by the Spanish Ministry of Science and Innovation under the project NG-RADIATE, TEC , and by the Spanish Ministry of Industry, Tourism and Trade under the project ENGINES, TSI ENGINES project is under the Celtic Initiative (Celtic Label CP7-005). References 1. International Telecommunications Union ITU-R Recommendation BS , System for Digital Sound Broadcasting in the Broadcasting Bands Below 30 MHz, International Telecommunications Union ITU-R Recommendation BS , Planning Parameters for Digital Sound Broadcasting at Frequencies Below 30 MHz, International Electrotechnical Commission, Digital Radio Mondiale (DRM) Part1: System Specification, IEC , European Telecommunication Standard Institute, Digital Radio Mondiale (DRM) System Specification, ETSI ES v3.2.1, International Telecommunications Union ITU-R Report BS.2144, Planning Parameters and Coverage for Digital Radio Mondiale (DRM) Broadcasting at Frequencies Below 30 MHz, 2009.

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