The terminal designationsAnchor A17.8.2 A17.8.2
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The field strength must not exceed a maximum value Emax as described in P1546ITU-R P. 1546-1. In principle any correction which increases a field strength must not be allowed to produce values greater than these limits for the family of curves and distance concerned. However, limitation to maximum values should be applied only where indicated in Annex 6 of the ITU-R P.1546-4.
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For land paths less than 15 km one of , the following two methods should be method is used:
Terrain information not available
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m for d <= 3 km (Eq. 232)
m for 3 km < d < 15 km
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m (Eq. 233)
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If the value of h1 coincides with one of the eight heights for which curves are provided, namely 10, 20, 37.5, 75, 150, 300, 600 or 1 200 m, the required field strength may be obtained directly from the plotted curves or the associated tabulations. Otherwise the required field strength should be interpolated or extrapolated from field strengths obtained from two curves using:
dB(μV/m) (Eq. 234)
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The field strength resulting from extrapolation for h1 > 1200 m should be limited if necessary such that it does not exceed the maximum defined in § 2 of P1546Recommendation ITU-R P.1546-4.
This model is not valid for h1 > 3000 m.
Transmitting/base antenna height, h1, in the range 0 m to 10 m
For a land path, the field strength at the required distance d km for 0 £ h1 < 10 m is calculated using:
Interpolation of field strength as a function of distance
Figures 1 to 24 of Recommendation ITU-R P.1546-4 show field strength plotted against distance, d, the range 1 km to 1 000 km. No interpolation for distance is needed if field strengths are read directly from these graphs. For greater precision, and for computer implementation, field strengths should be obtained from the associated tabulations (see Annex 1 of ITU-R P.1546-4, § 3). In this case, unless d coincides with one of the tabulation distances given in Table 1, the field strength, E (dB(mV/m)), should be linearly interpolated for the logarithm of the distance using:
dB(μV/m) (Eq. 235)
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dB(μV/m) (Eq. 236)
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The field strength resulting from extrapolation for frequency above 2 000 MHz should be limited if necessary such that it does not exceed the maximum value given in § 2 of Recommendation ITU-R P.1546-4.
For sea paths where the required frequency is less than 100 MHz an alternative method should be used, based upon the path lengths at which 0.6 of the first Fresnel zone is just clear of obstruction by the sea surface. An approximate method for calculating this distance is given in § 17 of Recommendation ITU-R P.1546-4.
The alternative method should be used if all of the following conditions are true:
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If all of the above conditions are true, the required field strength, E, should be calculated using:
dB(μV/m) for d <= df
dB(μV/m) for d > df
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Emax : maximum field strength at the required distance as defined in § 2 of P1546-4
maximum field strength at distance df as defined in § 2 of P1546-4
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Field-strength values for a given percentage of time between 1% and 50% time should be calculated by interpolation between the nominal values 1% and 10% or between the nominal values 10% and 50% of time using:
dB(μV/m) (Eq. 237)
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The field-strength values given by the land curves and associated tabulations in this model are for a reference receiving/mobile antenna at a height, R (m), representative of the height of the ground cover surrounding the receiving/mobile antenna, subject to a minimum height value of 10 m. Examples of reference heights are 20 m for an urban area, 30 m for a dense urban area and 10 m for a suburban area.
Table 88: Default clutter height in the P1546ITU-R P.1546-4 model (when clutter height option not activated)Anchor T088 T088
Selected environment | Assumed height of local clutter |
Rural | 10 |
Sub-urban | 10 |
Urban | 20 |
Dense urban | 30 |
Where the receiving/mobile antenna is on land account should first be taken of the elevation angle of the arriving ray by calculating a modified representative clutter height R' (m), given by:
m (Eq. 238)
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Correction 
dB for h2 < R′ (Eq. 239)
dB for h2 ≥ R′
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v = 
m
degrees
f: frequency (MHz).
In cases in an urban environment where R' is less than 10 m, the correction should be reduced by Kh2 log(10/R′). Where the receiving/mobile antenna is on land in a rural or open environment the correction is given by equation 
for all values of h2 with R' set to 10 m.
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Correction = 
(Eq. 240)
where ha is the antenna height above ground (m) (i.e. height of the mast) and R is representative of the height of the ground cover surrounding the receiving/mobile antenna as defined in § 9 of P1546Recommendation ITU-R P. 1546-4, which also represents the height of ground cover surrounding the transmitting/base antenna. This correction only applies when d is less than 15 km and h1 − R is less than 150 m.
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dB (Eq. 241)
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dB (Eq. 242)
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