Microwave polarizing lens structure

A microwave polarizing lens structure having two concentric hemispherical arrays of metallic linear scattering elements (dipoles) supported by thin walled dielectric shells. It has the property of controlling the sense of polarization, the ellipticity ratio and shape of the radiation pattern of the antenna contained within it.

FIELD OF THE INVENTION 
This invention relates to the field of microwave antennas and in particular 
to vehicle antennas used in mobile satellite communication systems. 
BACKGROUND OF THE INVENTION 
In mobile satellite communication systems, the satellite is circularly 
polarized to overcome the effects of Faraday Rotation and to simplify 
polarization alignment at the ground terminal. The vehicle directive 
antenna must track the satellite under all the dynamic conditions of the 
host vehicle. In the case of a system employing a geostationary satellite, 
the elevation angle of the satellite subtended at the vehicle is a 
function of the latitude of the vehicle and the position of the satellite 
on the geostationary orbital arc. With the satellite optimumly located, 
the satellite elevation angles at vehicle latitudes of 70.degree., 
45.degree. and 20.degree. North are about 10.degree., 45.degree. and 
65.degree. respectively. The signal strength margins in geostationary 
mobile satellite communication systems are relatively small, and the 
coverage must be sufficiently high to maintain good communications. 
One such antenna is described in U.S. patent 4,700,186 issued Oct. 13th, 
1987, invented by R. Milne. The antenna is elegantly simple, inexpensive 
to manufacture and has negligible RF loss. It generates, electronically, a 
number of fixed beams in azimuth and elevation and is designed to meet the 
requirements of mobile satellite communications systems providing regional 
coverage i.e. the North American continent. The antenna is however 
linearly polarized and there is a nominal 3 dB loss in gain when operating 
with a circularly polarized satellite. There is a requirement, in global 
mobile satellite communication systems, for higher antenna gain. A 
polarized lens structure has been invented that converts the linearly 
polarized signal radiated by the antenna to circular polarization and 
extends the elevation angular coverage. 
DESCRIPTION OF THE PRIOR ART 
U.S. Pat. No. 3,089,142 describes plural layers of wires and dipoles 
respectively to achieve a 90.degree. phase shift differential and to 
minimize reflections. U.S. Pat. Nos. 2,978,702 and 3,267,480 describe 
structures that utilize a combination of multi-layer dipoles, wires or 
plates with different refraction coefficients to enhance the operational 
bandwidths. The performance of the polarizers are described in terms of 
refraction coefficients vs frequency or differential phase shift vs 
bandwidth. The polarizers must function in conjunction with antennae. The 
patents do not, however, address a wide range of antenna parameters of 
common interest, namely, non-planar geometries; the resultant radiation 
patterns in terms of sidelobe levels, beam width and pointing; ellipticity 
ratio and antenna return loss. They are essentially polarizers and do not 
address the potential beam shaping properties of such structures. 
SUMMARY OF THE INVENTION 
The present invention converts the linearly polarized signal radiated by 
the patented antenna design to circular polarization and extends the lower 
and upper limits of its elevation angular coverage. In addition, the 
present invention provides no RF loss and hence no increase in antenna 
noise temperature, no significant increase in antenna VSWR or return loss, 
and no significant increase in relative antenna sidelobe levels. 
In the present invention a polarizing lens structure enhances the gain of 
the antenna contained within it. A preferred embodiment is comprised of 
two hemispherical arrays of metallic linear dipoles supported by thin wall 
dielectric shells. The length of the dipole elements, their physical 
separation and orientation are predetermined such as to create a 
differential phase shift of 90.degree. between two equal orthogonal 
electric vectors radiated by the antenna. 
The result is that the linearly polarized signal of the antenna is 
converted to circular polarization. The structure also shapes the antenna 
patterns in the elevation plane by controlling the net phase shift through 
the structure. The radial spacing between the two hemispheres is adjusted 
so that their reflections cancel thus reducing their effect on the antenna 
VSWR.

DETAILED DESCRIPTION OF THE INVENTION 
A perspective, partly phantom view of an inner hemispherical shell 1 is 
shown in FIG. 1. A concentric separate overlying shell 2 is illustrated in 
section for ease of description. The shells can be made from dielectric 
materials such as ABS and PVC plastics. The thickness of the shells are 
sufficiently small as to introduce a relatively small phase shift 
(&lt;10.degree.). An array of dipole elements 3 (only a few being shown) are 
disposed on the surface of each shell. The separation of the arrays should 
be such that their reflections cancel at midband frequency thus minimizing 
their effect on an antenna VSWR. The dipole elements are fixed in position 
and orientation such as to impart a differential 90.degree. phase shift to 
two equal orthogonal electric vectors of the microwave signal passing 
through the structure. By this means the linearly polarized signal 
radiated by the antenna is converted to circular polarization and the 
circularly polarized signal from the satellite is converted to linear 
polarization, thus increasing the antenna gain. 
Turning now to FIG. 2, an antenna such as that described in U.S. Pat. No. 
4,701,917 (although other antennas could be used) is disposed as follows. 
A driven element 4 and electrically enabled reflectors 5, are located 
above a ground plane 6 and are protected by a radome 7, as described in 
the aforenoted U.S. patent. The ground plane typically has a diameter of 
between 2 and 4 wavelengths and the antenna is contained within the 
polarizing lens structure described above. 
The theory of operation will now be described using the co-ordinate system 
of FIG. 3. The differential phase shift through the arrays is a function 
of dipole element length, width and spacing. Each hemispherical array 
produces a nominal differential phase shift of 45.degree. at midband 
frequency resulting in a total differential phase shift of 90.degree.. To 
achieve the required differential phase shift, the dipole elements are 
inclined at 45.degree. relative to a local line of longitude (see FIG. 3). 
The required locus to achieve this condition is given by 
EQU .phi.=log.sub.e (tan(.theta./2+.pi./4)) 
where .phi. and .theta. are the angular position of the dipole element in 
azimuth and elevation respectively. Because the polarizing structure is a 
curved surface and lies within the Near Field of the antenna contained 
with it, the relative improvement in gain is limited to about 2 dB. The 
preferred length and width of the dipole elements are 1/3 and 1/40 
wavelengths respectively. The thickness of the dielectric shells is less 
than 1/60 wavelength. In one successful embodiment, the array of elements 
was generated by incrementing the locus by 22.5.degree. in azimuth 
generating a total of 16 locii. Four rows of dipole elements were 
generated centered at .theta.=10, 30, 50 and 70.degree. respectively. To 
maintain the same nominal physical separation between elements at 
.theta.=70.degree. only 8 dipole elements were used spaced every 
45.degree. in azimuth. 
It is important that the reflections from the dipole arrays do not 
significantly affect the sidelobe levels and return loss of the antenna. 
To achieve low reflections, the arrays are separated by 1/8 wavelengths. 
The reflections from each array substantially cancel. 
FIG. 4 are graphs of antenna return loss for the antenna described in the 
aforenoted U.S. patent in combination with the dipole element array 
structures. Graphs of antenna return loss for the antenna itself, a short 
circuit reference, the antenna plus one array, and the antenna plus two 
arrays are illustrated. It can be seen that there is a significant 
increase in return loss when one array is added. By adding the second 
array the reflections cancel and the return loss is only slightly greater 
than the antenna itself. 
The antenna described in the aforenoted U.S. patent has two design 
limitations. Because of the fundamental limitations of the antenna 
radiating elements, the antenna gain drops off rapidly above 65.degree. 
elevation and is zero at 90.degree. elevation. Between 30.degree. 
elevation and 0.degree. elevation there is also a 6 db reduction in gain 
because of the finite size of the antenna ground plane. It is desirable to 
enhance the gain in these regions to extend the operational elevation 
angular coverage. 
It is possible to enhance the gain at the expense of some increase in 
ellipticity ratio of the circularly polarized signal. Antenna gain is 
relatively insensitive to ellipticity ratio. A 6 dB ellipticity ratio 
would result in a loss of gain of only 0.5 dB. A perfect polarizer with 0 
dB ellipticity ratio introduces a net phase shift of -45.degree. i.e. the 
mean of -90.degree. and 0.degree.. By controlling the net phase shift 
through the structure it is possible to extend the upper and lower limits 
of elevation angular coverage. 
FIG. 5 shows the low and high elevation beams of a linearly polarized 
antenna and the resulting patterns when the polarized lens structure is 
added. At 70.degree. elevation an improvement of 4 dB in antenna gain is 
realized which is about 2 dB higher than can be achieved by polarization 
alone. At 0.degree. elevation the improvement in gain is 3.5 dB. Because 
of the limitations in polarizer design and the boundary conditions imposed 
by the ground plane, about 2 dB of the improvement can be attributed to 
beam shaping alone. 
It should be noted that the invention is not restricted to hemispherical 
shells, and as long as the general design criteria are maintained, shells 
of elliptical, cylindrical and conical cross-sections can also be used. 
The invention can significantly enhance the antenna gain of the linearly 
polarized antenna design and extend its elevation angular coverage. As the 
downlink system margins i.e. from satellite to ground terminal, are more 
critical than the uplink i.e. from ground terminal to satellite, the 
polarizing structure is optimized for the downlink frequencies, i.e. 
1530-1560 MHz. 
A person understanding this invention may now conceive of alternative 
structures and embodiments or variations of the above. All of those which 
fall within the scope of the claims appended hereto are considered to be 
part of the present invention.