1、ITU-R RECMNrP. SERIES 95 4855232 0527437 202 Rec. ITU-R P.845-2 57 SECTION P-E: IONOSPHERIC ASPECTS RECOMMENDATION ITU-R P.845-2 HF FIELD-STRENGTH MEASUREMENT (Question ITU-R 223/3) (1992-1994-1995) The ITU Radiocommunication Assembly, considering that determination of the accuracy of HF field-stren
2、gth prediction methods requires comparison of predicted a) field strengths against measured field-strength data of sufficient accuracy; b) spectrum, that accurate HF field-strength measurements are therefore indispensable for the effective use of the HF recommends 1 locations in the world; 2 measure
3、ments: that HF field-strength measurements conforming to Annex 1 should be continued systematically at various that, where possible, the standard measurement method described in Annex 2 should be applied to the 3 that the field-strength data obtained from such measurements should be forwarded to the
4、 Director, Radiocommunication Bureau (BR) to permit the development of a data base containing uniformly consistent field-strength data. ANNEX 1 Measurement of sky-wave signal intensities at frequencies above 1.6 MHz 1 Introduction Measurements of sky-wave signal intensities, if undertaken in a caref
5、ully controlled manner, are of value in assessing the accuracy of methods for estimating field strength and transmission loss. Such measurements may also yield an indication of sources of error in existing prediction methods and may be used either to improve these methods or as a basis for developin
6、g new methods. Ideally, the requirements are for measurements to be carried out systematically over as wide a range of conditions as possible at a series of frequencies over paths of different lengths in all regions of the world. Measurements are needed at each hour of the day in the separate season
7、s and for different solar epochs. While it is recognized that opportuni ties to make measurements for particular circuits often arise only incidentally, with transmission schedules and system parameters such as the choice of antennas being determined by operational considerations, nonetheless useful
8、 results can be obtained in such cases. However, it is evident that data have their greatest value when measurements are canid out under standardized conditions and when uniform analysis and tabulation procedures are followed. This Annex presents the desirable criteria to be adopted to the extent th
9、at other constraints permit. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMNxP- SERIES 95 4855212 0527438 149 58 Rec ITU-R P.845-2 2 Choice of circuits and periods of operation Signal-intensity data are required from circui
10、ts of different ranges in all geographical regions. Recordings of a given transmission should be made for as many hours as possible every day. The objective should be to derive the median and other percentile values of the day-today distribution of signal intensity over all days of the month. Where
11、it is not feasible to carry out measurements every day, uncertainties arise in estimates of these values. Assuming a log-normal law of variation with decile deviation from the median of D (a). the standard error E in the median based on a sample of N days within a month of 30 days (see Fig. 1) is: 1
12、5 10 5 O FIGURE 1 Standard error in monthly median (E) as a function of number of days sampled (N) for different deciie deviations from the median (O) 30 N :m: “, 1 10 20 Clearly the standard error increases as the number of days of recording decreases. While there is no limiting sample size giving
13、an abrupt increase in error, as a general rule 10 or more measurements are required for the calculation of the medians, 14 for the quartiles and 18 for the deciles. It is seldom feasible to embark on a measurement programme extending over a significant part of a solar cycle but to ease data interpre
14、tation and to be statistically meaningful, measurements should cover a minimum period of one year at a given fixed frequency. There are particular advantages in attempting to record signals simultaneously over a path at a series of different frequencies, both to aid the understanding of propagation
15、effects and to permit quantitative data to be obtained by night when maximum usable frequencies are low, as well as by day when there is much absorption at the lower frequencies so that signals are masked by background noise. 3 Transmitter and transmitting antenna The transmitter should be unambiguo
16、usly identifiable so as to be sure that what is recorded is the wanted transmission and not co-channel signals, adjacent channel signals. or interfering noise. It is useful if the signals are interrupted at some periodic rate, say for 5 min once every hour, both as an aid to transmitter identificati
17、on and to determine received background levels as confirmation that there is no significant signal contamination. The transmitter should operate COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMN*P. SERIES 95 = 4855232 0527439
18、 085 m Rec. ITU-R P.845-2 59 preferably for 24 h per day. It must be stable in both frequency and radiated power, and these two parameters must be known accurately. For reception over short paths it should desirably have a radiated power in excess of 1 kW and over medium distance and long paths a po
19、wer of 10 kW or more, Where a special transmitter is operated, this would normally radiate continuous waves, although other waveforms may be used to study the characteristics of individual propagation modes. If use is made of commercial transmitters carrying modulated signals, it is important that t
20、he type of modulation should be constant and the mean percentage modulation should not vary. Narrow-band transmissions (approximately 1 kHz bandwidth or less) or a narrow-band component of a composite signal are most appropriate to record. Wider bandwidth signals are liable to interference contamina
21、tion. Standard-frequency transmissions have been employed in the past, but in many receiver locations there is now serious interference between signals from different transmitters operating this service and sharing the same frequency. Nevertheless, interference can be avoided to some extent by means
22、 of a narrow-band receiver capable of resolving the different audio modulation frequencies of each co-channel transmitter. Transmitters for point-to-point telephony or telegraphy services offer the advantages of providing channels which are relatively free from interference, and a detailed log of tr
23、ansmission schedules is usually available. On the other hand, these transmitters often employ high-gain antennas, which tends to be a disadvantage. A suitable category of transmitters meeting nearly all of the above criteria is weather-chart (FAX) transmitters using frequency shift-keying (k4 Hz). A
24、s there are numbers of receivers (ships) with unknown position, these transmitters use omnidirectional antennas and transmit mostly for 24 h per day. Receiving systems should be very sensitive especially when recordings are made for very long paths. Inspection of the International Frequency List mai
25、ntained by the BR is of value in the selection of suitable transmitters to monitor. In particular, this usually gives information concerning transmitter radiated power, form of modulation and period of the day of operation. Sometimes it also yields details of the antenna type and orientation. The In
26、temational Frequency List is useful additionally in providing a list of co-channel and adjacent-channel transmitters which should be considered further to assess the likelihood of possible interference. However, before embarking on a programme of systematic measurements it is recommended that after
27、selecting a potentially suitable transmitter in this way, firstly a series of monitoring measurements should be carried out at various times over a period of about a month to determine the order of signal intensities encountered, the time coverage over which such signals can be detected, and the amo
28、unts of interference experienced. Then, a direct approach should be made to the organization operating the transmitter to verify the International Frequency List entries, to supply such additional details as are required - for example, concerning the type of antenna used and the associated ground pr
29、operties. In particular, it should be checked that the radiated power is maintained constant, that different antennas are not used by night and day, and that the transmitter is not part of a network of transmitters operating at the same frequency from geographically separated sites - a procedure ado
30、pted in the HF broadcasting services in some countries, It is important to confm also that it is proposed that the transmitter will remain operational throughout the whole period for which it is intended to make measurements. Only then should a decision be reached to carry out systematic recordings
31、of the transmissions. Whilst ideally it would be desirable to receive details of the transmitter log, noting in particular any malfunctions or temporary changes in technical characteristics which might influence the measurements, it is rarely feasible to obtain such data and to apply variable correc
32、tions to results in retrospect. Instead, every effort should be taken at the outset to avoid the monitoring of transmitters whose characteristics are known to fluctuate. For a particular transmitter to be suitable for signal-intensity measurements, the performance of its transmitting antenna needs t
33、o be known accurately. Transmitters coupled to antennas with little directivity have advantages over those with highly-directional antennas because radiation patterns usually approximate more closely to theory, because the relative strengths at the receiving site of signals travelling via different
34、modes are then determined mainly by propagation effects, and because valid deductions may be made with a single allowance for transmitting-antenna gain in the absence of a knowledge of wave launch directions. Unfortunately though, low-gain transmitting antennas are seldom used for other applications
35、. Most point-to-point HF land-fixed communication circuits employ high-gain rhombic or log-periodic antennas; for sky-wave broadcasting, arrays of horizontal dipoles, also with significant directivity, are popular. The exception is with standard-time transmitters which aim to provide all-round azimu
36、thal coverage by means of vertical half-wavelength dipoles. These transmissions are particularly suitable for monitoring purposes. Radiation patterns for a vertical-dipole antenna may be estimated fairly accurately, except at low elevation angles where the particular ground constants control signal
37、intensities. However, even at low angles the performance is known more accurately than for most other types of antenna. If no such transmitter is conveniently positioned for use, then before monitoring transmissions from a directional antenna it should be checked that the great-circle path to the re
38、ceiver does not involve COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services1TU-R RECMN*Pl SERIES 75 4855232 0527420 dT7 60 Rec. ITU-R P.845-2 reception of side-lobe signals. If propagation is over medium or long distances, ideally the ant
39、enna vertical polar diagram for elevation angles less than 20“ should approximate that of a reference short vertical radiator sited over average ground (see Fig. 2a). Where a special transmitter is operated, a short vertical antenna is to be preferred. Alternatively, for short paths a horizontal dip
40、ole aligned for broadside radiation along the great-circle direction may be used, For greater ranges corresponding to low elevation angles, the direct and ground-reflected components of the sky wave nearly cancel one another so that a horizontal antenna is very inefficient unless elevated to a great
41、 height and should be avoided. Transmitting-antenna gain (like receiving-antenna gain) is best determined from near-site measurements in the far-field region, but it is recognized that these rarely form part of the normal programme of work at a transmitting installation and that it is not generally
42、possible to be able to arrange for such measurements to be carried out at a remote location, not under the control of the receiving organization. Accordingly, transmitting-antenna gain must usually be calculated from theoretical relationships in terms of the known antenna geometry, and by making cer
43、tain assumptions concerning the type of ground involved. 4 Receiving antenna, receiver and recording techniques Since existing methods of prediction of signal intensities do not take account of field distortion effects due to local features at the receiving site such as undulating ground, obstacles
44、like buildings and foliage and adjacent antennas which act as re-radiating structures, it is important to site the receiving antenna so that these effects are kept to a minimum. The ground should have a slope not exceeding 2“ out to a distance of five wavelengths and no obstacles should subtend an a
45、ngle from the horizontal at the centre of the antenna in excess of 5“. The separation from other antennas should be not less than ten times the antenna length. It is more important that the receiving-antenna performance should be known accurately than that it should have high gain. Except at the low
46、er frequencies during the daytime when there is much ionospheric absorption, threshold levels for signal detection will normally be determined by external noise intensities whatever receiving antenna is used. In general, the greater the antenna gain, the more likely the possibility of error in asses
47、sing its performance. Accordingly, a short vertical active antenna or a grounded vertical monopole antenna not exceeding a quarter wavelength high or a small loop antenna are most appropriate to employ. The loop antenna would normally be aligned in a vertical plane containing the great-circle direct
48、ion to the transmitter. For long-distance paths where off-great circle propagation is likely to be important, the vertical-monopole antenna is preferable since this provides omnidirectional azimuthal pick-up. If several transmissions from different azimuths are recorded with one antenna, only a vert
49、ical antenna should be used. Some organizations use vertical monopoles for signal measurements but standardize results by means of calibration data involving comparisons for selected sample signals with the pick-up indicated by a portable “field-strength“ meter incorporating an integral loop-receiving antenna. Figure 2a) shows the variation with elevation angle of the term Eo -E (a measure of the signal pick-up resulting from a downcoming sky-wave of constant intensity and its associated ground-reflected wave, defined in 0 6.2) for a short vertical grounded monopole and a loop antenna, bo
