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SG 3 Databanks - Databanks of VHF/UHF measurements relating to terrestrial broadcasting

​​​Preamble​​

1. Layout of the databank of VHF/UHF measurements​​

  • The format of the data has, in general, followed that prescribed in Recommendation ITU-R P.311 (Part VI, Tables VI-1 and VI-2), although the analysis of the substantial volume of measurements has revealed the need for certain changes, some of which have been incorporated in the information now presented. Annex 1 shows the data tabulation which has been used. The Annex also compares the layout with the format proposed in Recommendation ITU-R P.311. The reasons for significant modifications, i.e., those not involving a minor alteration such as the elimination of unnecessary detail, are described in the next section.​​
  • With regard to general layout, it is observed that although the measurements which have been processed nominally relate to Part VI of Recommendation ITU-R P.311, covering terrestrial broadcasting services, many were originally obtained during studies of mobile radio systems. The information they contain will therefore apply in part to Part V, dealing with terrestrial land mobile. Similarly, some of the data relate to Part III as regards clear air paths.​​​
  • Furthermore, having regard to the frequency range covered (30 MHz - 3 GHz), neither the existing nor the proposed layouts contain any provision for measurements outside those limits, yet the Tables refer to "terrestrial broadcasting".​​
  • The measurements have been sub-divided for entry into Table VI-I (signal level variation with time) and Table VI-2 (signal level variation with location) of the SG 3 databanks. In the case of Table VI-1, each entry represents a result for a stated period, i.e. it reports the statistics of temporal variation during that time. Both transmitting and receiving antennas are at fixed points in space, hence each entry involves one unique propagation path, i.e. the influence upon reception of spatial movement of the antennas is eliminated. These are point-to-point, or "site-specific", measurements. In Table Vl-2, although there are some site specific results, most entries record the spatial variation of the signal within a small area, and there are likely to be differences in the propagation paths between the fixed transmitting antenna and the receiving antenna moved within the boundaries of the selected space. Previously known as point-to-area measurements, these are now referred to as "site general" results. For these entries, the effects of temporal variation are ignored.​​

2. Comments on the Contents​
  • The number of entries contained in the databank are listed, by country, in Annex 2. Results of 3 328 temporal and 104 666 spatial measurements have been recorded.​​​
  • Each entry in the databank is referenced with an "experiment", rather than a "station" number. This avoids confusion with the subsequent "station" code, and appears at the start of the entry, rather than under "measurement" (see 2.2). The first number of the six character entry is used to identify the broadcasting frequency band from which the data are drawn, although the actual limits of these are extended to ensure continuity across the spectrum.​​
  • Thus:​
    • ​​1 = Band I (nominally ​30-75 MHz)​​
    • 2 = Band II (nominally 75-150 MHz)​
    • 3 = Band III (nominally 150-300 MHz)​​​
    • 4 = Band IV (nominally 300-600 MHz)​
    • 5 = Band V (nominally 600 MHz-1.0 GHz)​​​
    • 6 =1.0-2.0 GHz​
    • 7 = 2.0-3.0 GHz​​​​​​
  • Comments on the detailed contents now follow in the order in which items appear in the data tabulation shown in Annex 1.​​

2.1 Transmitter​​
  • Because it contains useful information about the type of transmission and the method of modulation, it was decided to include the Radio Regulations designation of emission data as a new entry. Unfortunately there is some confusion in the evidence now stored, partly because the methods of recording this information have changed over the years; if possible, it is desirable to have the modern reference.​​
  • With regard to e.r.p. information, Recommendation ITU-R P.311 asks for values on both horizontal and vertical polarizations. However, in the layout now adopted a figure has been shown for the "wanted" polarization only. Information describing both polarizations can be of use when the discrimination of the transmission is being examined in connection with features in the propagation path, for example in a study of multipath. In these circumstances the relationship between the orthogonal powers actually radiated must be defined by measurement in the immediate vicinity of the transmitting antenna, and with angular displacement precisely defined - a design specification for the antenna is virtually useless.​​
  • Probably because of the difficulties of determination, few of the thousands of measurements gave details for both polarizations. Furthermore, there was some confusion amongst contributions between the single result, which should report the actual e.r.p. on the wanted polarization towards the receiving point, and the power reference of the measured field strength, given later in the table. Superficially there seems little point at all in reporting the actual e.r.p., although this can give some indication of the complexity of the vertical radiation pattern (v.r.p.) of the transmitting antenna, and provide a clue to one source of anomaly which occurs in measurements made at short ranges (i.e. < 5 km) from high transmitting antennas. Such terminals often have complex, frequency-sensitive v.r.p.´s, creating doubts about the actual power radiated towards immediate receiving sites, underlining a possible unreliability of these particular results.
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  • Provision has been made for recording the nature of the environment in the vicinity of the transmitting antenna, having regard to the possible influence of this upon low-height installations. Although provision is made in Recommendation ITU-R P.311 for a total of 38 types of terrain cover, the data so far contributed do not extend beyond a maximum of five, i.e. "rural (10)", "wooded (20)", "suburban (33)", "urban (35)" and "dense urban (35)".​​

2.2 Measurement​
  • As mentioned above, in the revised layout the experiment number is quoted at the beginning of the entry, and consequently is not repeated here.​​

2.3 Receiver
  • No significant changes, although it is noted that the entry in the original layout requesting field strength in the plane of the received polarization has been dropped. Classification of receiving site environment is identical to that adopted for the transmitting site.​

2.4 Meteorology​
  • Entries for rain zones and refractivity information to be extracted from ITU-R publications have been retained, although very few of the contributions contained these details.​​​​

2.5 Path data​
  • A reference to the profile data, which are separately stored (using the format prescribed for the SG 3 databanks), is given at this point.​​
  • Path data are important - without them, full interpretation of the results is impossible. However, relatively few of the measurements now recorded contain tabulated profile data. In some cases parameters have been supplied for a particular prediction model and these could be included in the databank, if required. With other contributions, drawn profiles have been provided, and wherever possible these have been digitized. However, the majority of the data remains without any terrain information.​​
  • A major reason for omission of terrain details is source data copyright. It may eventually be possible to provide adequate details using a Worldwide terrain database which is becoming available.​​
  • Of course, where terrain information is available under local licence to those using the measurements´ databank, the required profiles can be modelled, because the locations of the path terminals are stored. However, an obvious caution must be emphasized here. Inevitably, variations will occur between profiles derived using terrain databases of varying densities and/or different modelling methods. This could result in disagreements between the assessment of parameters needed for a particular prediction program. If the objective is to check a prediction method against results produced by the originators of that method, then it will be necessary to confirm the technique used by them. Standardization both of the density of profile data and of the method used for profile construction seem desirable features of terrain data storage in the longer term.

2.6 Measurement data
  • This is the section within which the most substantial changes in layout are proposed.​​
  • One general point relating to the entries for both Tables - all measurements are quoted for an e.r.p. of 1 kW. This was not explicitly stated in the original Recommendation and, as mentioned in 2.1 above, was a source of confusion in some of the contributions.​​
  • Dealing firstly with the temporal results for Table VI-I, an original criticism was that little account had been taken of the duration of these experiments in the development of prediction methods. For example, results obtained from a recording lasting 24 hours were given equal status to those extending over periods of four years or more. Furthermore it was not easy to obtain information concerning certain important features of each experiment - as examples, seasonal changes, worst month statistics or the influence of receiving location upon the temporal statistics. Some progress has been made wherever possible by giving transmission timings, and by classifying the type of result. Thus entries appear as before for each full experiment (F), and to these have been added the component monthly analyses (M), and any measurements intended to reveal the effect upon the temporal distribution of small changes in the receiving location (V).​​​
  • At present, entries concerning the measurement of the interrelationship of time and location are limited to the UK. Nevertheless, these are important, and have revealed some significant information needing further investigation. When these measurements were originally made they were intended to correct long-term results obtained at fixed, usually exposed, receiving sites for conditions likely to exist at "average" (50% location) sites. The technique adopted underestimated the short-percentage time field strengths at the majority of receiving locations, and there is no doubt that the methods used can be improved. For the present, the results recorded for these entries are confined to those known to be reliable, and no presumed corrections have been made to the measurements recorded at the fixed sites.​​
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  • Greater attention has also been focused upon the diurnal results, especially for overland measurements, because it has been shown that analyses of measurements obtained throughout the 24 hour day can be significantly higher than those confined to the original "broadcasting hours". Again, where the information is available, the differences have been recorded.​
  • Turning now to the spatial contents of Table Vl-2, some changes have been made to provide more detail describing location variation, but the end result is still not completely satisfactory. The problem lies with the need to establish area standards for the results, so that direct comparison is permissible, but this has yet to be achieved. Ideally, for the purposes for which these data were originally assembled, individual entries should define the "microscopic" distribution of field strength within an area, e.g. a 100 m square. However, relatively few of the contributions do provide this information, little being known about many of the remainder. Unlike the temporal variations, spatial changes are largely unaffected by regional conditions, although the influences of time and path length upon the spatial results is an issue requiring further clarification.
  • For both temporal and spatial results, a reference to the source of the measurements is recorded wherever this information is available. Further description may be needed, because at this stage the detail is very cryptic.





ANNEX 1 - COMPARISON OF DATA TABULATION
ANNEX 2 - CONTENTS OF DATABANK​​​
Data files​​​

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