1、STD-ITU-R RECMN P. B45-3-ENGL 1777 4855212 0527b40 231 E Rec. ITU-R P.845-3 RECOMMENDATION ITU-R P.845-3 HF FIELD-STRENGTH MEASUREMENT (Question ITU-R 223/3) 1 (1992-1994- 1995-1997) The KU Radiocommunication Assembly, considering a) field strengths against measured field-strength data of sufficient
2、 accuracy; b) spectrum, that determination of the accuracy of HF field-strength prediction methods requires comparison of predicted that accurate HF field-strength measurements are therefore indispensable for the effective use of the HF recommends 1 locations in the world; that HF field-strength mea
3、surements conforming to Annex 1 should be continued systematically at various 2 measurements; 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 Director, Radioc
4、ommunication 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 carefully controlled m
5、anner, 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 developing new methods. Id
6、eally, 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 seasons and for differe
7、nt solar epochs. While it is recognized that opportunities 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 results can be ob
8、tained in such cases. However, it is evident that data have their greatest value when measurements are carried 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 that other constra
9、ints permit. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesSTD-ITU-R RECMN P. 845-3-ENGL 1997 4855232 0527bLlL 178 m 2 Rec. ITU-R P.845-3 2 Choice of circuits and periods of operation Signal-intensity data are required from circuits
10、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-to-day distribution of signal intensity over all days of the month. Where it
11、 is not feasible to carry out measurements every day, uncertainties anse in estimates of these values. Assuming a log-normal law of variation with decile deviation from the median of D (dB), the standard error E in the median based on a sample of N days within a month of 30 days (see Fig. 1) is: E=
12、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 azimuthal coverage by means of vertical half-wavelength dipoles. These transmissions are particularly suitable for
13、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 intensities. However, even at low angles the performance is known more accurately than for most other types of
14、 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 receiver does not involve COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by In
15、formation Handling ServicesSTD-ITU-R RECMN P* 845-3-ENGL 1997 E 4855232 0527643 T4U 4 Rec. ITU-R P.845-3 reception of side-lobe signals. If propagation is over medium or long distances, ideally the antenna vertical polar diagram for elevation angles less than 20“ should approximate that of a referen
16、ce 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 dipole aligned for broadside radiation along the great-circle direction may be used. For greater range
17、s 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 height and should be avoided. Transmitting-antenna gain (like receiving-antenna gain) is best dete
18、rmined 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 possible to be able to arrange for such measurements to be carried out at a remote location, not un
19、der 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 certain assumptions concerning the type of ground involved. 4 Receiving antenna, receiver and recordin
20、g 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 like buildings and foliage and adjacent antennas which act as re-radiating structures, it is import
21、ant 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 angle from the horizontal at the centre of the antenna in excess of 5“. The separation from other an
22、tennas 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 lower frequencies during the daytime when there is much ionospheric absorption, threshold levels for s
23、ignal 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 assessing its performance. Accordingly, a short vertical active antenna or a grounded vertical monopole
24、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 direction to the transmitter. For long-distance paths where off-great circle propagation is likely to be
25、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 vertical antenna should be used. Some organizations use vertical monopoles for signal measurements but
26、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 th
27、e 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, both situated over average ground. For elevation angles below about 30 the monopole and loop antennas hav
28、e very similar polar diagrams but at higher elevation angles the loop-antenna pattern is preferable since the pick-up is relatively insensitive to angle. Figure 2b) shows the effect of antenna siting over ground of different properties. Signal pick-up for wet ground exceeds that for very dry ground
29、by some 2-6 dB with the largest differences occurring at low elevation angles. The marked dependence of the pick-up on the ground constants and on the elevation angle when this is low, which has been discussed already with regard to the transmitting antenna, leads to particular data interpretation d
30、ifficulties for long paths where elevation angles are not known accurately. In principle, the use of an artificial ground screen would lead to a receiving system performance less dependent on weather conditions which affect ground water content. The screen would improve the ground constants and so i
31、ncrease the signal pick-up, but to be effective in this role it would need to have dimensions of the order of tens of wavelengths and this is rarely practicable. On the other hand, short screens of length up to about five wavelengths can be implemented and are of value in stabilizing antenna impedan
32、ces to improve circuit matching. If a screen is used, it is desirable to assess its effect by carrying out near-site calibration measurements with signals radiated in the far-field region from an airborne transmitter. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed b
33、y Information Handling ServicesSTD-ITU-R RECMN P. 845-3-ENGL L977 = 4855232 0529644 787 Rec. ITU-R P.845-3 FIGURE 2 Difference between r.rns. equivalent-incident field strength, E, and r.m.s. sky-wave field strength, E, for short vertical monopole and for loop antenna at a height of 1 m. Frequency:
34、15 MHz 5 O s 3 N 1 -5 k.7 - 10 - 15 O 10 20 30 40 50 60 Elevation angle (degrees) aJ Monopole and loop sited over average ground of conductivity o = 0.01 S/m, relative dielectric constant E = 15 monopole - loop 5 O 3 N -5 8 I - 10 - 15 O 10 20 30 40 50 60 Elevation angle (degrees) 6) Loop sited over
35、 wet, average and very dry ground Curves A: wet ground cs = 0.02 Slm, E = 20 B: average ground cs = 0.01 S/m, E = 15 C: very dry ground o = 0.002 Slm, e = 5 0845-02 5 Horizontal half-wave dipoles for single-frequency operation or terminated dipoles for multiple-frequency measurements are sometimes s
36、uitable for reception of signals on short paths. In particular, pick-up is not strongly dependent on the ground constants. However, for medium distance and long paths when elevation angles are low, these antennas provide COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicense
37、d by Information Handling ServicesSTD-ITU-R RECMN P- B45-3-ENGL 1997 m 4855232 0529b45 813 = 6 Rec. ITU-R P.845-3 only limited pick-up, again markedly dependent on elevation angle, unless they are elevated to great heights. They should not be used for these paths because of calibration difficulties.
38、 Some organizations are equipped to make measurements using special antenna systems, such as rhombic arrays, designed for specific circuits to improve signalhoise ratios and to enable measurements to be made under conditions where a simple antenna would be unusable. It is difficult to interpret the
39、results obtained on an extended antenna system in the presence of a complex field built up of several waves incident at different angles, but measurements made with such antennas may be acceptable for the purpose in hand, if they can be related consistently to those that would be obtained at the sam
40、e time on a standard antenna. In making a choice between antennas responding either to vertical or horizontal polarization, it is prudent to check that, if propagation paths involve waves with markedly non-circular polarization, reception (or transmission) is predominantly that of the stronger ordin
41、ary wave. The receiving antenna should be connected to the receiver via a buried coaxial cable and appropriate matching circuitry. This latter may take the form of a transformer or a wideband pre-amplifier. The receiver bandwidth should be as narrow as possible consistent with the bandwidth of the t
42、ransmitted signals, in order to optimize the received signahoise ratio. For continuous-wave signals and for the monitoring of the steady tone sideband signals of standard-time transmissions, bandwidths of the order of 100 Hz or less are suggested. Received signal intensity depends on radiated power
43、within the receiver bandwidth. This is a function of the carrier, modulation and recording arrangement, For a receiver bandwidth which encompasses the carrier and all sidebands, the operative radiated power is equal to the sum of that of the carrier and all other components. Figures for different ty
44、pes of modulation are given in Recommendation ITU-R SM.326. In the case of narrowband reception of a single sideband of a standard time transmission of carrier power P where the amplitude modulation depth is m, the sideband power is m2P/4. Signals should be detected, applied to appropriate integrati
45、on-smoothing circuitry, and then recorded in suitable form. Some organizations monitor signals over oblique paths in order to note the occurrence of events like sudden ionospheric disturbances (SIDs) and magnetic storms, or to study fading statistics. In these cases, special recording procedures may
46、 be necessary. Where, however, the prime requirements are to collect representative hourly signal-intensity data, measurements are best made using a pen-chart recorder with a logarithmic amplitude scale (Le. linear in decibels) and a chart speed of about 2 cm per hour. The integration time constant
47、should be about 20 s. This arrangement provides a convenient length of record for manual smoothing whilst at the same time permitting the rejection of sections shown to be contaminated by interfering signals or strong atmospherics. It is often simpler to record the automatic gain-control voltage fro
48、m a commercial receiver after modifications to equate and lengthen the rise and decay time constants to the 20s noted above. However, this approach may lead to unacceptable errors under some conditions, even after continuous-wave calibration of the response. Output voltage is usually approximately p
49、roportional to the logarithm of the input voltage, but since this non-linearity is associated with the detection process and occurs prior to integration, recordings give the mean logarithm of the signal intensity and not the mean in logarithmic units as required. These quantities differ when there is signal fading present. An alternative acceptable form of recording involves digital quantization of instantaneous amplitudes at a convenient sampling rate so as to cover the known periodicity of typical fading components (with fading durations up to about 20min). Representative values ma
