Planar flat plate scanning antenna

A scanning antenna (1) has, a first waveguide (2) with parallel metal plates (10, 11), a waveguide feed (19) substantially at a focus (15) of the first waveguide (2), the plates (10, 11) being partially filled therebetween with a biconvex dielectric body (21) emanating a wave of planar phase front, and a planar waveguide bend (4) having a planar profile (13) redirecting an incident wave of planar phase front for propagation in a second waveguide (3) having an array of radiating antenna elements (5).

BACKGROUND OF THE INVENTION 
A known scanning antenna is disclosed in PCT Application Number WO 91/17586 
having first and second, flat plate waveguides connected by a waveguide 
bend. Waveforms at a focus of the first waveguide propagate as a beam of 
plane waves having a nonplanar, thin cylindrical, phase front, and become 
redirected by a parabolic profile of the waveguide bend to a beam for 
propagation in the second waveguide having an array of antenna apertures 
to be illuminated by the redirected beam. 
In the known antenna, the waveguide bend has a parabolic profile that 
redirects the nonplanar phase front in reverse, by a 2 .pi. change in 
phase, to propagate in the second waveguide. The parabolic profile of the 
waveguide bend has a detrimental effect on the redirected phase front. A 
parabolic profile must be selected so as to provide a redirected beam 
pattern that may vary from, a narrow beam pattern with low side lobes, 
characteristic of a deep parabolic profile, to a broader beam pattern of 
more uniform illumination of the array of antenna elements with higher 
scattering, characteristic of a shallow parabolic profile. Accordingly, 
attainment of a redirected beam with a planar phase front is difficult to 
attain by a waveguide bend with a parabolic profile redirecting a beam 
having an nonplanar phase front. 
In the known antenna, the first waveguide has parallel metal plates filled 
therebetween with dielectric material. The dielectric material extends 
from the focus of the waveguide to the parabolic waveguide bend. The 
dielectric material is without a desired feature that would emanate a 
planar phase front toward the waveguide bend. A wave of planar phase front 
would simplify the waveguide bend to a planar profile, and would reduce 
the detrimental effect of a parabolic waveguide bend on the redirected 
phase front. 
SUMMARY OF THE INVENTION 
According to the invention, the first waveguide has a waveguide feed 
substantially at a focus of the first waveguide, the focus being the focus 
of a biconvex dielectric body partially filling between parallel metal 
plates of the waveguide and emanating a wave of planar phase front 
incident on a planar waveguide bend having a planar profile directly 
opposite the dielectric body from the focus, the planar profile 
redirecting an incident wave of planar phase front for propagation in a 
second waveguide superposed with the first waveguide. 
The planar profile reduces a need to focus a redirected beam of planar 
phase front, and reduces beam scattering due to off-axis propagation of a 
redirected phase front.

DESCRIPTION 
With reference to each of FIGS. 1, 2, 3 and 7, a scanning antenna 1 
suitable for mm-wave, millimeter-wave, 77 gHz., for example, has a flat 
plate, first waveguide 2 and a flat plate, second waveguide 3 connected by 
a waveguide bend 4. In the first waveguide 2, a wave form propagates from 
a focus 15 of the first waveguide 2, and becomes redirected by the profile 
13 of the waveguide bend 4 for propagation in the second waveguide 3 
having an array of antenna elements 5 in the form of apertures 6, FIG. 1, 
or, alternatively, antenna patch elements 7, FIGS. 2, 3 and 7. 
With reference to each of FIGS. 1, 2, 3 and 7, the first waveguide 2 is 
fabricated with a suggested construction as a metal conducting body 8 
providing side walls 9 unitary with a conducting base plate 10, together 
defining a cavity for mm-wave propagation. The side walls 9 are 
magnetically lossy material absorbing misdirected, mm-waves. A conducting 
top plate 11 covers the cavity, and registers on the side walls 9. The top 
plate 11 has a single row of apertures 12 for the waveguide bend 4. The 
side walls 9 meet the reflecting profile 13 of the waveguide bend 4 at an 
end of the first waveguide 2. The side walls 9 meet respective walls 14 
that taper to a focus 15 of the first waveguide 2. 
With reference to FIG. 6, the second waveguide 3 has copper cladding 16 
surrounding a sheet of dielectric material 17 such as 
polytetrafluoroethylene. The copper cladding 16 forms the conducting body 
8, similar to the conducting body 8 of the first waveguide 2, with a base 
plate 10 and a parallel top plate 11. The dielectric material 17 fills 
between the base plate 10 and the top plate 11, and serves as the 
propagation medium. An exemplary antenna element 5 is shown in FIG. 6. As 
shown in FIGS. 4 and 5, a waveguide opening 18 through the base plate 10 
is aligned with the apertures 12 of the waveguide bend 4. 
With reference to FIG. 4, a waveguide feed 19 is substantially at the focus 
15 of the first waveguide 2. For example, the waveguide feed 1 comprises 
at least one waveguide duct 20, FIGS. 1, 2, 3 and 7, or multiple waveguide 
ducts 20 substantially at the focus 15, for example, feeding a wave in 
opposite, transmitting and receiving directions, depending upon which duct 
20 is connected to a transmitter portion or receiver portion of a known 
electronic transceiver apparatus, not shown. The direction of propagation 
applies in reference to a transmit mode, when a wave propagates from the 
focus 15. The direction of propagation further applies in reference to a 
receive mode, when a wave propagates toward the focus 15. 
With reference to FIGS. 1, 3 and 7, a biconvex dielectric body 21 will now 
be described. The biconvex dielectric body 21 is spaced from the focus 15 
and from the waveguide bend 4, and, thereby, partially fills between the 
metal plates 10 and 11 of the first waveguide 2. The remaining area 
between the plates 10 and 11 is filled with air as a dielectric medium of 
propagation. 
The dielectric body 21 is biconvex in the direction of propagation, serves 
as a mm-wave lens, and has, as its lens focus 15, the focus 15 of the 
first waveguide 2. Because the first waveguide 2 is very thin, a wave 
propagating from the focus 15 toward the dielectric body 21 will have a 
nonplanar, thin cylindrical, phase front, and will be incident on the 
dielectric body 21. The dielectric body 21 is biconvex in the direction of 
propagation to emanate the wave with a planar phase front propagating 
toward the waveguide bend 4. An incident wave of planar phase front 
simplifies the shape of the waveguide bend 4 to a planar profile 13. 
The planar phase front becomes redirected by the planar profile 13 of the 
waveguide bend 4 for propagation, in reverse, in the second waveguide 3 as 
a beam of planar phase front. Due to the planar profile 13, the redirected 
beam of planar phase front is focused at infinity, which avoids a need to 
focus the redirected beam to reduce scattering by off-axis propagation of 
redirected phase fronts. The planar phase front provides uniform 
illumination of the array of antenna elements 5, with reduced scattering 
loss due to the beam spreading laterally. 
The planar profile 13 of the waveguide bend 4 reduces the detrimental 
effect of a known parabolic waveguide bend 4 on the redirected phase 
front. A parabolic profile must be selected so as to provide a redirected 
beam pattern that varies from, a narrow beam pattern with low side lobes, 
characteristic of a deep parabolic profile, to a broader beam pattern of 
more uniform illumination of the array of antenna elements 5 with higher 
scattering, characteristic of a shallow parabolic profile. Accordingly, a 
redirected beam with a planar phase front, and with minimum skattering, is 
difficult to attain with a parabolic profile being used to redirect a beam 
having a nonplanar phase front. 
According to one embodiment, in FIG. 7, the dielectric body 21 is of 
uniform thickness, transverse to the direction of propagation, and of 
uniform dielectric constant, and is biconvex in the direction of 
propagation. The thickness bridges between the parallel plates 10 and 11 
that have a plate spacing sufficiently small for propagation of a wave of 
TEM mode. Propagation of a wave in the dielectric body 21 can be modeled 
by an analysis of an optical wave propogating through an optically 
transmitting biconvex lens. 
According to another embodiment, in each of FIGS. 1, 2 and 3, the 
dielectric body 21 is of uniform dielectric constant and is cylindrical 
about an axis transverse to the direction of propagation, with a thickness 
progressively increasing concentrically from a minimum thickness at a 
cylindrical edge 22. 
The effective index of refraction for this mode is expressed as: 
EQU tan [k.sub.1 (s-t)]+k.sub.2 /.epsilon..sub.r [tan(k.sub.2 t)]=0 
where 
EQU k.sub.1 =2 .pi./.lambda.(1-n.sup.2).sup.1/2, k.sub.2 =2 
.pi./.lambda.(.epsilon..sub.r -n.sup.2)1/2 
s is the spacing between the plates 
t is the thickness 
.epsilon..sub.r is the dielectric constant of the body 
.lambda. is the free space wavelength 
A slight deformation from parallelism of the interface between air and the 
dielectric body 21 has an insignificant effect on the velocity of the wave 
at any point when the thickness varies, so that a known Luneberg relation 
is satisfied, according to the equation: 
EQU n.sup.2 =2-(r/R).sup.2 
where: 
n=dielectric constant 
r=radial distance from center axis 
R=radius of waveguide at the center of the waveguide, r=0, and n=2, its 
maximum value. When r=R, n=1. 
With reference to FIG. 6, the antenna patch elements 7 are unitary with 
respective, conducting, secondary feed lines 23 linked to a unitary 
primary feed line 24. As shown in FIG. 4, the patch elements 7 and the 
secondary feed lines 23 and each primary feed line 24 are fabricated, for 
example, as a pattern of plated metal adhered to dielectric material 25, 
in turn, adhered to the top plate 11. The plated metal extends through an 
opening in the top plate 11, and is encircled by the dielectric material 
in the opening to provide a coaxial feed 26. Further details of the patch 
elements 7 and their construction are described in U.S. Pat. No. 
5,712,644, incorporated herein by reference. 
In summary, a scanning antenna has first and second, flat plate waveguides 
2 and 3 connected by a waveguide bend 4. Wave forms at a focus 15 of the 
first waveguide 2 propagate with a nonplanar phase front, and emanate with 
a planar phase front from a biconvex dielectric body 21 toward the profile 
13 of the waveguide bend 4 for propagation in the second waveguide 3 
having an array of radiating antenna elements 5. 
The first waveguide 2 has parallel metal plates, a waveguide feed 19 
substantially at a focus 15 of the waveguide, the plates being partially 
filled therebetween with a biconvex dielectric body 21 emanating a wave of 
planar phase front, and a planar waveguide bend 4 having a planar profile 
13 directly opposite the dielectric body 21 from the focus 15, the planar 
profile 13 redirecting an incident wave of planar phase front for 
propagation in a second waveguide 3 superposed with the first waveguide 2. 
Other embodiments and modifications of the invention are intended to be 
covered by the spirit and cope of the appended claims.