1、 STD-ITU-R RECMN S*b?Z-LI-ENGL 3997 4855232 0530301 521 Rec. IT-R S.672-4 1 RECOMMENDATION ITU-R S.672-4 SATELLITE ANTENNA RADIATION PATTERN FOR USE AS A DESIGN OBJECTIVE IN THE FIXED-SATELLITE SERVICE EMPLOYING GEOSTATIONARY SATELLITES (Question IT-R 41/4) ( 1990- 1992- 1993- 1995- 1997) The ITU Ra
2、diocommunication Assembly, considering a) use of the radio-frequency spectrum and the geostationary orbit; that the use of space-station antennas with the best available radiation patterns will lead to the most efficient b) space stations; that both single feed elliptical (or circular) and multiple
3、feed shaped beam antennas are used on operational cl required before a reference radiation pattern can be adopted for coordination purposes; that although improvements are being made in the design of space-station antennas, further information is still d) fabrication and use of orbit-efficient anten
4、nas; that the adoption of a design objective radiation pattern for space-station antennas will encourage the e) interference for coordination purposes; that it is only necessary to speciQ space-station antenna radiation characteristics in directions of potential 0 effective predictions; that for wid
5、e applicability the mathematical expressions should be as simple as possible consistent with g) adaptable to emerging technologies; that nevertheless, the expressions should account for the characteristics of practical antenna systems and be h) angles; that measurement difficulties lead to inaccurac
6、ies in the modelling of spacecraft antennas at large off-axis j) particularly at lower frequencies such as the 614 GHz band; that the size constraints of launch vehicles lead to limitations in the DA values of spacecraft antennas, k) may be used to define a space-station reference antenna pattern, a
7、re found in Annex 1 ; that space-station antenna pattern parameters such as reference point, coverage area, equivalent peak gain, that 1) that two computer programs have been developed to generate coverage contours (see Annex 2), recornmen for service areas as large as Canada, United States or China
8、 the value of 6 is generally one to two beams at 614 GHz band and about four beams at 1411 1 GHz band, in the application of this model. Thus, for most of the systems the value of Q is normally less than 1.1. That is, the beam broadening effect is generally about 10% of the width of the elemental be
9、amlet of the shaped-beam antenna. Neglecting the main beam broadening due to blockage and reflector surface error, and assuming a worst-case value of 0.35 for F/Dp ratio of the reflector, the beam broadening factor Q can be simplified as: Q= 10 0.0037 (6 - 1/2) STD-ITU-R RECMN Smb72-4-ENGL L997 4855
10、232 0530333 243 Rec. ITU-R S.672-4 14 In the 6/4 GHz band, a -25 dB side-lobe level can be achieved with little difficulty using a multi-horn solid reflector antenna of about 2 m in diameter, consistent with a PAM-D type launch. To achieve 30 dB discrimination, a larger antenna diameter could be req
11、uired if a sizeable angular range is to be protected or controlled. In the 14/11 GHz fured-satellite bands, 30 dE3 discrimination can generally be achieved with the 2 m antenna and the use of a more elaborate feed design. The above equations for the reference pattern are dependent upon the scan angl
12、e of the component beam at the edge of coverage in the direction of each individual cut for which the pattern is to be applied. For a reference pattern to be used as a design objective, a simple pattern with minimum parametric dependence is desirable. Hence, a value or values of Q which cover typica
13、lly satellite coverages should be selected and incorporated in the above equations. A steeper main beam fall-off rate can be achieved for a typical domestic satellite service area as compared to very large regional coverage areas; and conversely a reference pattern satisfjing a regional coverage wil
14、l be too relaxed for domestic satellite coverages. Therefore it is proposed to simplifj Model 1 into the following two cases for the FSS antennas. For these cases a -25 di3 side-lobe plateau level is assumed. a) Small coverage regions (6 2 for I WO O I Av I 0.9794 WO GdBi(AW) = Gep - 25 for 0.9794 W
15、O e Av 5 2.1168 WO for 2.1168yo 3.5) Examples for wide coverage regions are the hemi-beam and global coverages of INTELSAT and INMARSAT. In order to represent the pattern degradation due to large scan, a value of 1.3 is taken for the Q factor. The reference patterns applicable to these coverages (6
16、3.5) are defmed as: 7.73 WO Gep + 0.256 - 7 (Av + 0.65 02 for O S Av I 1.1575 for 1.1575 WO 2.75 where: S : (angular displacement from the centre of coverage) / 2vb vb = 40 UD K2 = 1.25 It should be noted that the values proposed for shaped-reflector antennas correspond to available information on s
17、imple antenna configurations. This new technology is rapidly developing and therefore these values should be considered tentative. Furthermore, additional study may be needed to verify the achievable side-lobe plateau levels. STDmITU-R RECMN Smb72-4-ENGL 1997 m q855232 0530338 825 19 RW. ITU-R S.672
18、4 Use of improvement factors Ki and K2 The improvement factors K1 and K2 are not intended to express any physical process in the model, but are simple constants to make adjustments to the overall shape of the antenna pattern without changing its substance, Increasing the value of K1 from its present
19、 value of 1, will lead to an increase in the sharpness of the main beam roll-off. Parameter K2 can be used to adjust the levels of the side-lobe plateau region by increasing K2 from its value of unity. 2.5 Shaped beam pattern roll-off characteristics The main beam roll-off characteristics of shaped
20、beam antennas depend primarily on the antenna size. The angular distance AWL from the edge of coverage area to the point where the gain has decreased by 22 dB (relative to edge gain) is a useful parameter for orbit planning purposes: it is related to the antenna size as: For central beams with littl
21、e or no shaping, the value of C is 64 for -25 dl3 peak side-lobe level. However, for scanned beams C is typically in the range 64 to 80 depending on the extent of main beam broadening. 2.6 Reference pattern for intermediate scan ratios Recommendr 2.1 and 2.2 have two reference patterns for the satel
22、lite antennas in the FSS, one for small coverage areas with scan ratios less than 3.5 and the other for wide coverage areas with scan ratios greater than 5.0. However, the radiation patterns for intermediate scan ratios (3.5 112) and the pencil beam (6 = 1/2). For intermediate scan ratios in the ran
23、ge 3.5 2yb (see Fig. 7). If the coverage polygon can be included in a circle of radius Yb, this circle is the coverage area contour. The centre of this circle is the centre of a minimum radius circle which will just encompass the coverage area contour. If the coverage polygon cannot be included in a
24、 circle of radius Yb, then proceed as follows: Step I: For ali interior coverage polygon angles 1 80, construct a circle of radius Yb which is tangent to the lines connected to the coverage point whose centre is on the exterior bisector of the angle. b) If this circle is not wholly outside the cover
25、age polygon, then construct a circle of radius tangent to the coverage polygon at its two nearest points and wholly outside the coverage polygon. which is Step 3: Construct straight line segments which are tangent to the portions of the circles of Steps 1 and 2 which are closest to, but outside the
26、coverage polygon. Step 4: If the interior distance between any two straight lie segments from Step 3 is less than 2vb, the controlling points on the coverage polygon should be adjusted such that reapplying Steps 1 through 3 results in an interior distance between the two straight line segments equal
27、 to 2vb. An example of this construction technique is shown in Fig. 7, 1.2 Gain contours about the coverage area contours As also noted in Annex 1, difficulties arise where the coverage area contour exhibits concavities. Using a Ay measured normal to the coverage area contour will result in intersec
28、tions of the normals and could result in intersections with the coverage area contour. In order to circumvent this problem, as weil as others, a two step process is proposed. If there are no concavities in the coverage contours, the following Step 2 is eliminated. Step I: For each Av, construct a co
29、ntour such that the angular distance between this contour and the coverage area contour is never less than Av. This can be done by constructing arcs of Ay dimension from points on the coverage area contour. The outer envelope of these arcs is the resultant gain contour. Where the coverage area conto
30、ur is straight or convex, this condition is satisfied by measuring normal to the coverage area contour. No intersections of normals will occur for this case. STD-ITU-R RECMN S-bTZ-q-ENGL 1997 = 4855232 U530321 3LT D 22 Rec. ITU-R S.672-4 Using the process described in Step 1 circumvents these constr
31、uction problems in areas of concavity. However, from a realistic standpoint some problem areas remain. As noted in Annex 1, side-lobe control in regions of concavity can become more difflcult as the degree of concavity increases, the pattern cross-section tends to broaden and using the Step 1 proces
32、s, discontinuities in the slope of the gain contour can exist. It would appear reasonable to postulate that gain contours should have radii of curvature which are never less than (Yb + Ay) as viewed from inside and outside the gain contour. This condition is satisfied by the Step 1 process where the
33、 coverage area contour is straight or convex, but not in areas of concavity in the coverage area contour. The focal points for radii of curvature where the coverage area contour is straight or convex are within the gain contour. In areas of concavity, the use of Step 1 can result in radii of curvatu
34、re as viewed from outside the gain contour which are less than (Wb -k Ay). FIGURE 7 Construction of a coverage area contour -._-_- I I I l , I I , I I I l ! , over rag area contour Figure 8 shows an example of the Step 1 process in an area of concavity. Semi-circular segments are used for the covera
35、ge area contour for construction convenience. Note the slope discontinuity. To account for the problems enumerated above and to eliminate any slope discontinuity, a Step 2 is proposed where the concavities exist. Rec. ITU-R S.672-4 FIGURE 8 Gain contours from Step 1 in a concave coverage area contou
36、r Coverage area contour discontinui 23 Step 2: in areas of the gain contour determined by Step 1 where the radius of curvature as viewed from outside this contour is less than (yb + Ay) this portion of the gain contour should be replaced by a contour having a radius equal to (ipb + Ay). Figure 9 sho
37、ws an example of the Step 2 process applied to concavity of Fig. 8. For purposes of illustration, values of the relative gain contours are shown, assuming yq, as shown and a value of B = 3 dB. This method of construction has no ambiguities and results in contours in areas of concavities which might
38、reasonably be expected. However, difficulties occur in generating software to implement the method, and furthermore it is not entirely appropriate for small coverage areas. Further work will continue to refuie the method. To fmd the gain values at specific points without developing contours the foll
39、owing process is used. Gain values at points which are not near an area of concavity can be found by determining the angle Ay measured normal to the coverage area contour and computing the gain from the appropriate equation: (lo), (1 i), (12), (1 3) or (14). The gain at a point in concavity can be d
40、etermined as follows. First a simple test is applied. Draw a straight line across the coverage concavity so that it touches the coverage edge at two points without crossing it anywhere. Draw normals to the coverage contour at the tangential points. If the point under consideration lies outside the c
41、overage area between the two normals, the antenna discrimination at that point may be affected by the coverage concavity. It is then necessasr to proceed as follows: Determine the smallest angle Ay between the point under consideration and the coverage area contour. Construct a circle with radius (y
42、b + Av), whose circumference contains the point, in such a way that its angular distance from any point on the coverage area contour is maximized when the circle lies entirely outside the coverage area; call this maximum angular distance Ayr . The value of Ay may be any angle between O and Ay; it ca
43、nnot be greater than but may be equal to Ay. The antenna discrimination for the point under consideration is then obtained from equations (lo), (1 i), (12), (13) or (14) as appropriate using Ay instead of Ay. STD-ITU-R RECMN Smb72-q-ENGL 3997 = 4855232 0530323 192 24 Rec. ITU-R S.672-4 Two computer
44、programs for generating the coverage area contours based on the above method have been developed and are available at the Radiocommunication Bureau. FIGURE 9 Construction of gain contours in a concave coverage area contour - Step 1 plus Step 2 r = vb + Av ra = 1.9vb ro: radius of curvature of coverage contour concavity r: radius of curvature !&wy
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