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1. (WO1993019497) ANTENNE A REFLECTEUR A LENTILLE DE FRESNEL POUR HYPERFREQUENCES
Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

Fresnel-lens reflecting antenna for microwave frequency use

The present invention is related a Fresnel-lens reflecting antenna according to the preamble of claim 1 for microwave frequency use.

Particularly at the northern latitudes, conventional paraboloid antennas become expensive, bulky in their depth dimension, and consequently, difficult to mount. The large dimensions of such antennas cause additional problems in mechanical mounting, which must be designed capable of bearing high loads from wind and snow, whereby the antenna construction requires support and bracing structures.

The art is familiar with planar transmissive antennas, that is lens-type Fresnel-zone antennas, which can be deployed to their operating position in the same manner as a roll curtain, for example. Such an antenna is comprised of concentric planar zones that are alternatively transmissive and nontransmissive to microwaves. For a plane wave which impinges on the antenna plane orthogonally, the projections of the zones are circular rings. When the width of the rings is dimensioned so that the difference of the distance from the outer and the inner rim, respectively, of each ring from a certain point, the focal point, situated on the center axis of the antenna plane is a half-wavelength (λ/2), the components of the impinging wave which are diffracted through the transmissive rings add essentially in phase (to an accuracy of ±λ/4) in a receiving antenna (hereinafter "feed horn") placed to said focal point. That impinging component, which would be essentially in opposite phase if allowed to reach the focal point, is reflected back from the nontransmissive (metallic) rings. Such a lens structure thus operates as a focusing element for electromagnetic waves, causing, however, a loss of half the incoming signal intensity due to reflection alone.

The above-described construction has also been employed as a focusing reflector by placing the feed horn to the incoming-wave side of the reflector plane. In this case, too, the signal is attenuated by 50 % already as the transmissive zones allow a portion of the wavefront to pass through the plane without reflection.

The above antennas based on adding wavefront components by half-wavelength phasing steps have the advantage of shallow structure, even down to a thickness of a few micrometers in the form of a partially metallized polymer film. Their disadvantage is in the poor efficiency due to the adding principle by the half-wavelength phasing steps and losses from reflection or transmission.

If the rear side of the above-described reflector is complemented with a contiguous, planar, reflecting metal plate placed at a quarter-wavelength (λ/4), a portion of the transmitted wave component can be reflected and diffracted with a delay of a halfwavelength to the focal point. As this component would initially have been phased out by an odd multiple of half-wavelengths, it can thus be made to add at the focal point in phase with the wave components reflected from the nontransmissive zones. Also this kind of solutions are known from the literature, but generally their efficiency remains inferior to paraboloid reflectors of even appreciably smaller size due to the inefficiency of the half-wavelength stepped phasing scheme.

The literature of the art also describes transmissive embodiments, that is, structures with a lens action having a dielectric material shaped in stepped zones so as to delay the transmitted wave in increments of a wavelength, that is, in integral fractions i*λ/n (i = 0...n and n = 2, 3, 4, ...) of a wavelength, whereby the wave is diffracted adding at the feed horn with an improved in-phase accuracy thus improving the efficiency of the antenna. Phasing in wavelength increments of λ/8, for instance, gives approx. 95 % efficiency when dielectric losses are neglected. This solution is restricted by the high requirements imposed on the accurate control of the dielectric constant and homogeneity of the diffracting material. Extra costs are also caused by high material consumption which is further multiplied by the high pricing of low-loss materials.

In some disclosed antenna structures, the consumption of the dielectric material has been halved by providing the stepped structure with delay elements of i*λ/(2*n) and then using said structure of zoned delaying elements in conjunction with a planar reflector plate placed behind it. This arrangement causes the wave to pass twice through the same material, whereby the phasing of the wave components stay correct despite the halving of the material thickness. Such an arrangement still has the disadvantages of the construction described last above, though the material consumption is slightly reduced.

It is an object of the present invention to overcome the drawbacks of the abovedescribed conventional technology and to achieve an entirely novel type of Fresnel-lens antenna for microwave frequencies.

The planar Fresnel-zone antenna according to the invention is based on the utilization of planar reflecting zones alone. The zones are dimensioned and placed in a stepped manner so that the entire wave is directly reflected from the zoned pattern so as to add in phase within the resolution of a chosen scheme of phasing steps.

Characteristically, the structure of the reflector is formed into a stepped polymer material workpiece comprised of adjacent, stepped, planar zones. Such planar zones are coated with a metallic layer made by vacuum evaporation, sputtering, chemical or galvanic electrodeposition, printing with a conducting ink, spraying or any other suitable metallization technique.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention provides significant benefits.

The reflecting zones are typically made by coating an inexpensive yet rigid support structure with a thin metal layer of high electrical conductivity. The metal layer is located facing the incoming wave and the feed horn, whereby signal attenuation by transmission losses in the lossy dielectric material are avoided. Thus, a reduced material consumption and lightweight structure are attained. In addition to the property that the thickness (height) of the phasing step is independent from the antenna material employed, a low-loss embodiment is attained as the signal need not pass through the antenna structure. Low losses also result from the fact that the reflecting zone rings form a relatively dense reflecting surface which is nontransmissive to radiation. Furthermore, the structure can be made at lower cost as the stepped reflecting zones permit the omission of a separate reflecting plate.

In the following the invention is examined in greater detail with reference to the exemplifying embodiments illustrated in the annexed drawing in which

Fig. 1 shows an antenna structure according to the invention in a longitudinally sectional side view.

Fig. 2 shows the antenna structure illustrated in Fig. 1 in a front view.

Fig. 3 shows the antenna structure illustrated in Fig. 1 in an enlarged sectional view.

The antenna according to the invention is suited to the reception of microwave signals from geostationary satellites. With reference to Fig. 1, the antenna 2 is principally intended for mounting in a substantially vertical position, whereby an angle θ is subtended between the vertically aligned antenna plane (in the elevation direction) and the incoming plane wave received from a satellite. In reality, the antenna 2 is tilted slightly forward to avoid accumulation of snow and contamination. In the horizontal direction (azimuth direction), the antenna 2 is aimed toward the satellite. The scope of the invention does not, though, pose any restrictions for an orientation of the antenna 2 differently from that described above. To simplify the description, the antenna structure is, however, discussed on the basis of the default orientation mentioned above.

In Fig. 1, the depth dimension of the antenna 2 is exaggerated for greater understanding of its structure. The active surface of the antenna 2 is comprised of planar rings 5 coated with a highly reflecting material such as a metal. The antenna 2 is designed with a stepped structure so that the phasing is implemented using wavelength increments of λ/8. The height difference between the adjacent annular zones is, however, λ/16 as the incoming plane wave reaches the feed horn via the reflector after travelling this path difference twice thereby adding up to said λ/8. Thus, the height of the steps is dependent on the receiving frequency. The function of the stepped structure is to add the plane wave components in phase at the focal point 1 to the accuracy determined by the appropriate design of the stepped structure. Each annular step 5 forms a separate zone and operates with respect to its adjacent neighbour at a phase difference of λ/8, or in phase angle, 45° ± 22.5°. The cyclic repetition o the stepped zone pattern in a sequence of 8 consecutive annular zones is made possible therein that after such a sequence the phase angle has completed a full cycle as 8*45° equals 360°, or a full wavelength. Obviously, each complete sequence always returns phasing to its origin. The adjacent stepped zone sequences thus start from and end at the same depth level. When a substantially half-wavelength depth shift, corresponding to a full wavelength in the wave propagation path, is made at the depth level corresponding to a phase shift angle of 360° or a multiple thereof, the shallowest possible structure is obtained; in principle, however, the stepped zone sequence can be situated in the depth direction so as to be shifted by any multiple of half wavelength in front of or behind the reference plane. If the major downward depth shift is omitted at the points where the phase shift angle reaches 360°, a shallow antenna structure cannot be attained.

Strictly, the total height of the stepped zone sequence is dimensioned to be a half wavelength only in the case when the incoming plane wave is received from the direction of the normal of the reflector, and respectively, the reflected spherical wavefront propagates back in the direction of the normal of the reflector toward the feed horn. At other angles the total height of the stepped zone sequence should be dimensioned to be a half-wavelength multiplied by average sines of the angles of incidence and reflection. In practice the sine function of said angles is essentially 1 resulting in minimal error even if the sine correction is neglected. For large-diameter antennas the correction in the height dimensioning of the stepped zone sequence is advantageously made, whereby the steps become the shallower the farther they are located from the projection of the feed point on the reflector plane.

An essential principle in the dimensioning of the antenna geometry is that each wavefront element travels from the satellite to the focal point along a path which can be written as n * λ + δ where n = any integral number (can be different from component to component), λ = wavelength and δ = differential path travelled by the wavefront after an integral number of full wavelengths. This differential path should be identical for all elements of the propagating wavefront. Besides in millimeters, the differential path δ can also be expressed in degrees of phase shift, or θ = 360°*δ/λ. The geometric dimensioning rules for the antenna 2 dictate that the stepped zone sequences become more elevated toward the edge areas of the antenna, whereby each stepped zone sequence has a total height of a half-wavelength and each stepped zone sequence starts from the same depth level. If the shallowness of the antenna is not a crucial property, the base levels for the stepped zone sequences can also be freely located provided that their depth relative to the reference plane is shifted by an integral number of half-wavelengths at the operating frequency.

According to Fig. 2, the antenna 2 is comprised of ellipses situated within each other. The larger the elevation angle θ, the more elongated the ellipses will be and the more asymmetrical the antenna will look. If the normal of the antenna plane can be pointed directly toward the satellite, whereby the antenna plane is aligned parallel to the impinging wavefront, the stepped zone sequences can be simply formed by concentric circular zones.

According to Figs. 1 and 2, the reflector thus reflects four components of the wavefronts that add at the focal point 1. With reference to Fig. 2, the first wavefront component meets the upper part of the reflector 2 at area 11, the next component at area 7, the third component at area 6 and the fourth component at area 8. The shape of zoned areas and the location thereof, the stepped pattern inclusive, is dimensioned so that despite the different distances of the various points of the reflector from the focal point, all wavefront components entering the feed horn are in phase, that is, the stepped pattern equalizes the different paths of the wavefront components for one wavelength to within λ/8. The stepped zone sequence of the center area 8 performs an equal function with that of the other areas 11, 7 and 6. Herein, the center area 8 is simply chosen as a reference for one case study of the phasing scheme. The size of the area is dimensioned in the exemplifying embodiment illustrated in the diagrams so that the signal reflected at the rim of the area is out-of-phase maximally by λ/ 16, or by an angle of 22.5°, relative to the signal reflected at the middle of the area.

The discussed exemplifying embodiment of the antenna is based on a printout shown in Appendix 1 and produced by way of a program listed in Appendix 2. The program is written in the GFA Basic language. The initial values for the program are the lati tude and longitude of the receiving site, the longitude, bearing and azimuth of the satellite, and the orientation of the antenna. In addition to these, the program requests for the operating frequency and focal length. On the basis of given information, the program computes in a conventional manner from formulas of elementary geometry the loci of such points from which the focal point "sees" the reflected components of the incoming plane wave to add in phase at the focal point. According to theoretical considerations verified in the prior art such loci form a set of ellipses within one another; for special case of direct aiming toward the satellite, the ellipses degenerate into concentric circles (which, in fact, are special cases of ellipses). Thus, the program computes a desired sequence of ellipses for which the offset x of the ellipse's focal point is shown in the first column, the ellipse's major half-axis a in the second column and the ellipse's minor half-axis b in the third column and the step height d between the zones in the fourth column. According to the Fig. 3 the step height d is constant. As discussed above, the step height is wavelength divided by 2*N where N is the number of zones in the stepped zone sequence 6, resulting in 26.44 mm/16 = 1.65 mm in the exemplifying case (f = 11.343 GHz). As a rule it can be stated that the step height can be expressed as d ≈ λ/(N*2) where N is the number of zones in each stepped sequence. The ellipses are computationally formed about a point appropriately called the initial point 3. In the embodiment shown in the diagram, the major half-axis al of the first ellipse is 50 mm and the distance x1 of the ellipse's focal point k1 from the initial point 3 is 1.1 mm. Correspondingly, the ellipse e9 has a major half-axis a9 of 209.4 mm and 19.2 mm distance of the its focal point k9 from the initial point 3. The ellipse e17 has a major half-axis a17 of 296.2 mm and 37.4 mm distance of the its focal point from the initial point. Thus, the outer rim 10 of the first zone 6 is situated in the depth direction of the antenna 2 at a height of approx. λ/2 from the inner rim 9 of second zone 7. The denser the stepping scheme, the closer this height dimension will be to λ/2.

The program computes from the entered information also the distance y of the focal point 1 shown in Fig. 1 from the initial point 3. According to the diagram, this distance is 284 mm. Further, the program computes the distance h of the focal point 1 from the antenna plane. The greatest dimension L of the antenna structure 2 shown in Fig. 1 is 732.6 mm.

As is the case in the Fresnel-zone planar antennas discussed in conjunction with the prior art embodiments, the zones in the antenna according to the present invention degenerate into concentric circles provided that the feed horn which performs the adding of the incoming wavefront components is situated in the focal point of the stepped zones and the incoming plane wave meets the antenna from the direction of the normal of the stepped zones.

When design aspects dictate an orientation of the stepped zones different from perpendicular to the propagation direction of the incoming plane wave, the zones assume the shape of a set of ellipses situated within one another due to the geometrical design rule stating that the components of the reflected wavefront must be in phase at the feed horn within the tolerance of the stepped phasing scheme. So, if the antenna is horizontally aimed toward the satellite, but vertically not, the major axes of the ellipses are vertical. Correspondingly, if the antenna is vertically aimed toward the satellite, but horizontally not, the major axes of the ellipses are horizontal. When the aiming of the antenna deviates in both directions, the axes will be inclined.

Thus, the design of the antenna can be modified for downward tilted mounting, whereby rain, snow and ice cannot accumulate on the antenna surface so as to cause signal attenuation. Due to aesthetic aspects, it is also possible to design the reflector plane of the antenna to suit mounting in a parallel position with a wall or a cornice of the roof.

Due to the geometry of the Fresnel-zone lens or reflector, such a structure can provide relatively high gain also for other signal sources than those directly focused at the focal point of the structure. The prototype antennas constructed for tests have been adapted for reception of signals from satellites deviating ±15° from the bearing to the main signal source by transferring the feed horn from the main focal point to the focal point of the wave components received from the secondary source, or alternatively, providing both focal points with separate feed horns.

The invention sets no limitations to the number of steps in the stepped zone sequence. Theoretically, the width of the annular zones, as well as their height, can be decreased to an infinitesimally small value, whereby the number of steps grows infinitesimally large. Then, the stepped zone sequence according to the invention approaches a smooth, curved surface with a total height of a half- wavelength.

Further, the invention sets no limitation to the variation of the number N of the steps in the adjacent stepped zone sequences. The stepped phasing scheme of fractional wavelengths designed for the different zones can be varied within the main lobe of a single feed horn provided that the width of each zone is dimensioned for the desired phase shift.

In some cases the angle of incidence of the plane wave can be so much deviated from the normal of the reflector plane that the use of annular zones is not practical, whereby only segments of the zones will suffice. Particularly in large antenna structures mounted in the terrain or as parts of the surfaces in a building, the stepped zone sequences can be located relatively far from each other, even on surfaces of different orientation, provided that wave components reflected from each of the different parts of the antenna reflector fulfill the design rule given on rows 27 ... 34 of page 5 and rows 1 ... 7 of page 6.

Appendix 1
Annular zone #1 at λ/16 distance, next zones at λ/8 spacing
RECEPTION SITE:
Latitude (°) = 52 Longitude (º) = 10
SATELLITE LOCATION: Longitude (°) = 10
Azimuth (º) = 180 Elevation (°) = 30,53
ANTENNA ORIENTATION:
Azimuth (°) = 180 Elevation (°) = 0
θ (°) = 30,53 ɸ (°) = 0
FREQUENCY:
Center f (GHz) = 11 ,343 Lower Limit f (GHz) = 10,95 Upper Limit f (GHz) 11 ,75

FOCAL LENGTH (mm) = 560; FOCAL POINT y (mm) = 284, h (mm) = 482
Focal length correction @:10,95GHz -20 mm, @: 11 ,75 GHz 21 mm
n x (mm) a (mm) b (mm) Step height (mm)
1 1.1 50.0 43.1 11.57
2 3.4 86.8 74.7 9.92
3 5.7 112.3 96.7 8.27
4 7.9 133.1 114.6 6.61
5 10.2 151.2 130.2 4.96
6 12.5 167.5 144.3 3.31
7 14.7 182.4 157.1 1.65
8 17.0 196.3 169.1 0
9 19.2 209.4 180.4
10 21.5 221.8 191.1
11 23.8 233.7 201.3
12 26.0 245.0 211.0
13 28.3 255.9 220.4
14 30.6 266.4 229.5
15 32.8 276.7 238.3
16 35.1 286.6 246.8
17 37.4 296.2 255.2
18 39.6 305.6 263.3
19 41.9 314.8 271.2
20 44.1 323.8 278.9
21 46.4 332.7 286.5
22 48.7 341.3 294.0
23 50.9 349.8 301.3
24 53.2 358.1 308.5
25 55.5 366.3 315.5
26 57.7 374.4 322.5
27 60.0 382.4 329.3
28 62.3 390.2 336.1
29 64.5 398.0 342.8
30 66.8 405.6 349.4
31 69.0 413.1 355.9
32 71.3 420.6 362.3
33 73.6 428.0 368.6
34 75.8 435.3 374.9
35 78.1 442.5 381.2
36 80.4 449.7 387.3
37 82.6 456.7 393.4
38 84.9 463.8 399.5
39 87.2 470.7 405.5
40 89.4 477.6 411.4
41 91.7 484.5 417.3 Appendix 2
' PHASING AT λ/8 STEPS (Annular zone #1 at λ/16 distance)

PRINT "RECEPTION SITE:"
INPUT "Latitude (°) =
Longitude (°) =",lat
INPUT " ",long
PRINT
'
La=RAD(lat)
Lo=RAD(long)
'
PRINT "SATELLITE LOCATION:"
INPUT "Longitude (°) = ",sat
PRINT
st=RAD(sat)- lo
'
ca=COS(st)*COS(la)
sa=SQR(1-ca*ca)
az=ASIN(SIN(st)/sa)
IF La>0
az=PI-az
ENDIF
el=PI/2-ASIN(sa/SQR(1.02277-0.30178*ca))
IF ca<0.15089
el=-el
ENDIF
'
PRINT "Azimuth (°) = ";ROUND(DEG(az),2)
PRINT "Elevation (°) = ";ROUND(DEG(el),2)
PRINT
'
PRINT "ANTENNA POINTING:"
INPUT "Azimuth (°) = ",aaz
INPUT "Elevation (°) = ",ael
PRINT
'
daz=az-RAD(aaz)
del=el-RAD(ael)
cth=COS(daz)*COS(del)
cth2=cth*cth
xk=SQR(1-cth2)/cth2
th=ACOS(cth)
IF del==0
ph=PI/2
ELSE
ph=SGN(daz)*ACOS(1/SQR((SIN(daz)/TAN(del))Λ2+1))

IF del<0
ph=PI-ph
ENDIF ENDIF
'
PRINT "θ (°) = ";ROUND(DEG(th),2)
PRINT "ɸ (°) = ";ROUND(DEG(ph),2)
PRINT
'
INPUT "LOWER FREQUENCY (GHz) = ",gh1
INPUT "UPPER FREQUENCY (GHz) = ",gh2
ghz=SQR(gh1*gh2)
PRINT
wl=300/ghz
qwl=wl/16
INPUT "FOCAL POINT (mm) = ",focus
PRINT
df=2*focus
focy=-focus*SIN(th)
foch=focus*COS(th)
'
INPUT "N = ",n&
PRINT
'
OPEN "",#1 ,"con:"
DO

PRINT #1,,"Ahti Aintila, ";DATE$,TIME$ Appendix 2

'
PRINT #1,,"Annular zone #1 at λ/16 distance, next zones at λ spacing"

PRINT #1
PRINT #1,,"RECEPTION SITE:"
PRINT #1,,"Latitude (°) = ";ROUND(lat,2),
PRINT #1 ,"Longitude (°) = ";ROUND(long,2)
PRINT #1
PRINT #1,,"SATELLITE LOCATION:",
PRINT #1,"Longitude (°) = ";ROUND(sat,2)
PRINT #1,,"Azimuth (°) =";ROUND(DEG(az),2),
PRINT #1,"Elevation (°) = ";ROUND(DEG(el),2)
PRINT #1
'
PRINT #1,,"ANTENNA POINTING:"
PRINT #1,,"Azimuth (") = ";ROUND(aaz,2),
PRINT #1,"Elevation (°) = ";ROUND(ael,2)
'
PRINT #1,,"θ (°) = ";ROUND(DEG(th),2),
PRINT #1,"ɸ (°) = ";ROUND(DEG(ph),2)
PRINT #1
'
PRINT #1,,"FREQUENCY:"
PRINT #1,,"Center (GHz)= ";ROUND(ghz,3);
PRINT #1," Lower (GHz)= ";ROUND(gh1,3);
PRINT #1," Upper (GHz)= ";ROUND(gh2,3)
PRINT #1
'
PRINT #1,,"FOCAL LENGTH (mm) = ";ROUND(focus);
PRINT #1,"; FOCAL POINT y(mm) = ";ROUND(focy);", h(mm) = ";ROUND(foch) wl1=300/gh1
wl2=300/gh2
foct=focus*wl/wl1+0.25*(wl+wl1)*(wl-wl1)/wl1
foc2=focus*wl/wl2+0.25*(wl+wl2)*(wl-wl2)/wl2
PRINT #1,,"Focal length correction @:";ROUND(gh1,3);"GHz ";
PRINT #1,ROUND(-focus+foc1);" mm, @:";ROUND(gh2,3);"GHz ";
PRINT #1,ROUND(-focus+foc2);" mm"
PRINT #1
'
PRINT #1,," n x (mm) a (mm) b (mm) Step (mm)"
FOR ϊ&=1 TO n&
d=(2*i&-1)*qwl
x=d*xk
b=SQR(d*(d/cth2+df))
a=b/cth
'
PRINT #1,,STR$(i&,3);STR$(x,8,1);STR$(a,9,1);STR$(b,8,1);
IF ϊ&<=8 THEN
PRINT #1," ";ROUND((8-i&)*qwl,2);
ENDIF

PRINT #1
NEXT i&
'
PRINT #1
CLOSE
'
EXIT IF p&
'
PRINT "Print ? (K/E)"
DO
p&=INSTR("KE",UPPER$(INPUT$(1)))
LOOP UNTIL p&
EXIT IF p&=2
'
OPEN "",#1,"prn:"
LOOP
'