Parallel-plane antenna with rotation of polarization

A parallel-plane antenna of the pillbox or cheese type in which the cylindrical-parabolic portion is constituted by a semitransparent reflector and the focal plane of which is constituted by a polarizing reflector. The cylindrical-parabolic semitransparent reflector is constituted by an array of wires parallel to the plane of polarization of the wave emitted by the source. The polarizing reflector is formed by a reflecting plate located at a distance of .lambda./4 from an array of parallel wires inclined at 45.degree. with respect to the plane of polarization of the incident wave.

BACKGROUND OF THE INVENTION 
This invention relates to a parallel-plane antenna known as a cheese 
antenna or pillbox antenna. 
In accordance with a generally accepted definition, a parallel-plane 
antenna of the cheese or pillbox type is essentially constituted by two 
parallel plates limited by a cylindrical-parabolic end-wall illuminated by 
a source placed on its focal line as shown in FIG. 1. The source can be a 
simple monomode horn or a multimode source. Depending on the height H or 
distance between the two parallel plates, a number of operating modes can 
be contemplated such as the TEM, TE and TM modes. Particulars on this type 
of antenna are given in "Les Antennes" by L. Thourel, published by Dunod, 
page 285, 1976 edition, and in "Microwave Antenna Theory and Design", Vol. 
12, page 459 of the "Radiation Laboratories Series" (1949 edition). 
A disadvantage of this type of antenna, however, lies in the fact that its 
radiating aperture is occulted by the source, thus having the effect of 
impairing the radiation characteristics of the antenna. 
One solution for eliminating this occulting effect without increasing the 
depth of the antenna consists in constructing a parallel-plane antenna of 
the cheese or pillbox type having a folded configuration as shown in FIG. 
2 in which the feed is separate from the radiating portion. This solution 
provides a higher gain than the simple pillbox or cheese antenna. 
This antenna is constituted by two pillbox or cheese antennas 41 and 43 
coupled together by means of a slot 44 extending over the entire length of 
the parabolic end-wall which is common to the two antennas. The slot 44 is 
so dimensioned that the assembly formed by the two cheese elements 41 and 
43 and the slot 44 constitutes an adapted folded waveguide. 
However, one drawback attached to these two antenna designs arises from the 
difficulties presented by the mechanisms employed for driving the antenna 
in rotation. In point of fact, the choice of an antenna having a long 
focal distance makes is necessary to place the excitation source at a 
substantial distance from the center of gravity of the antenna. This has 
the effect of complicating the supporting structure in proportion to said 
distance and increasing the capacity of the mechanisms employed for 
driving the turret on which the antenna is mounted. 
SUMMARY OF THE INVENTION 
The present invention makes it possible to overcome these disadvantages and 
is accordingly directed to a parallel-plane cheese or pillbox antenna of 
the rearfeed, rotatory polarization type. 
In accordance with a distinctive feature of the invention, the 
parallel-plane antenna has a cylindrical-parabolic end-wall constituted by 
a semitransparent reflector and a plane containing the focal line of the 
antenna, said plane being constituted by a polarizing reflector. 
One of the advantages of the invention lies in the fact that the optical 
path formed by means of the two reflectors is folded-back, thereby 
reducing the real focal distance of the antenna and therefore the overall 
length as well as both the volume and weight of the antenna to be driven 
in rotation. The semitransparent cylindrical-parabolic reflector has a 
long equivalent focal distance, thus permitting the use of sources which 
are capable of exciting the parallel-plane antenna under usual conditions 
of service. 
A further advantage of the invention lies in the construction of a 
parallel-plane antenna in which the excitation source is placed at the 
rear.

DESCRIPTION OF EMBODIMENTS 
FIG. 1 illustrates a conventional parallel-plane antenna constituted by two 
parallel plates limited by a cylindrical-parabolic end-wall 2 illuminated 
by a source 3 which is placed on its focal line F. The source 3 is either 
a monomode horn or a multimode source. The aperture of the horn 
constitutes an equiphase linear source which provides a number of 
operating modes according to the height H or distance between the two 
parallel plates 1. If the height H is less than one-half the wavelength at 
the center frequency of the operating band .lambda./2, only the TEM mode 
can exist. In such a case the aerial is mostly known as a pillbox antenna. 
If the height H is greater than .lambda./2, the TE and TM modes can 
propagate and the aerial is known as a cheese antenna. 
The fact that the source 3 is placed in front of the radiating aperture 
nevertheless produces a mask effect which impairs the radiation 
characteristics of the pillbox or cheese antenna. 
Another type of construction of a parallel-plane antenna of the pillbox or 
cheese type is shown in FIG. 2. 
The antenna feed source 40 is separated from the radiating portion 41 by a 
partition-wall 42 between the feed portion 43 and the radiating portion 
41. In fact, this folded pillbox or cheese antenna is constituted by two 
pillbox antennas or two cheese antennas coupled by means of a slot 44 
which extends along the entire length of the parabolic end-wall 45. The 
portion 43 of the folded antenna is excited by the source 40 such as a 
horn, for example. The dimensions of the slot 44 are such that the 
assembly formed by this latter and the two cheeses 41 and 43 constitutes 
an adapted folded waveguide. This system achieves good results especially 
in the TEM mode. 
FIG. 3 illustrates a parallel-plane antenna according to the invention. 
This antenna is constituted by two parallel plates 8 and 9 limited on the 
one hand by a semitransparent cylindrical-parabolic end-wall reflector 10 
and on the other hand by a plane polarizing reflector unit 11 located at 
right angles to the plates 8 and 9 and containing the focal line F of the 
antenna. The semitransparent cylindrical-parabolic reflector 10 is formed 
by an array of wires 12 which are parallel to the plane of polarization of 
the wave emitted by the source 13. The pitch of the array is small 
compared with the wavelength, namely 12 mm in the case of a frequency 
f=1300 MHz. The array therefore behaves as a waveguide which produces 
cutoff in respect of any wave which is polarized in a direction parallel 
to the wires and therefore behaves as a reflector. Thus the wave emitted 
by the source 13 is totally reflected from the cylindrical-parabolic 
reflector 10 before finally reaching the plane reflector unit 11. This 
plane reflector unit 11 is formed by a reflecting plate 14 and by an array 
15 of wires 16 which are parallel to each other and make an angle of 
45.degree. with the plane of polarization of the wave emitted by the 
source. Said unit accordingly ensures rotation of polarization. The 
distance between the plate 14 and the array 15 is substantially equal to 
one-quarter of the wavelength, namely .lambda./4 at the center frequency 
of the operating band. This condition is not imperative, however, if it is 
desired to operate over a wide band. 
Metal plates 17, 18, 19 and 20 constitute with the plane reflector unit 11 
a box having the shape of a rectangular parallelepiped. 
In the plane of the array 15 of parallel wires 16, the wave emitted by the 
source 13 and then reflected by the semitransparent cylindrical-parabolic 
reflector 10 can be split-up in two orthogonal directions, a first 
component being parallel to the wires 16 and a second component being 
perpendicular to said wires 16. The component which is perpendicular to 
the wires 16 passes through the array and is reflected by the reflecting 
plate 14. The parallel component does not pass through the array 15 and is 
reflected. Thus the perpendicular component is phase-shifted by 
180.degree. with respect to the parallel component after being reflected 
from the reflecting plate 14 and after having passed back through the 
array of wires 16. The effect of this phase lag is to produce a rotational 
displacement through an angle of 90.degree. of the plane of polarization 
of the resultant wave which is radiated a second time by the plane 
reflector unit 11 to the semitransparent cylindrical-parabolic reflector 
10. 
The cylindrical-parabolic reflector 10 has become transparent to said 
incident wave, the plane of polarization of which has rotated by 
90.degree. with respect to the wave emitted by the source 13, the electric 
field vector of the emitted wave being perpendicular to the parallel wires 
of the array 12 carried by the cylindrical-parabolic reflector 10. 
A so-called "folded" optical system has thus been formed. In other words, 
by folding-back the optical path, a cylindrical-parabolic mirror having a 
long equivalent focal distance can be placed within a space of small 
overall length, thus making it possible to employ sources which are 
capable of exciting the antenna under normal conditions of use. 
The two arrays of parallel wires 12 and 16 can be replaced by parallel 
reflecting plates or by wires embedded in a glass-resin complex. 
Another example of construction of a parallel-plane antenna of the cheese 
or pillbox type is illustrated in FIG. 4. In order to achieve enhanced 
directivity of the antenna, this latter is fabricated from a metallic box 
21 having the shape of a rectangular parallelepiped, the bottom portion of 
which is constituted by the plane reflector unit 11 whilst the metal 
plates 22, 23, 24 and 25 constitute the side walls which are perpendicular 
to the bottom end-wall. The metal plates 22 and 24 which are perpendicular 
to the bottom end-wall constitute the two parallel plates 8 and 9 of the 
antenna. The bottom portion of the box is formed by the reflecting plate 
14 located at a distance of .lambda./4 from the array 15 of wires which 
are parallel to each other and are inclined at an angle of 45.degree. with 
respect to the plane of polarization of the wave emitted by the source. 
Said bottom portion is illuminated by the source 13 placed at the center 
of this latter in order to radiate towards the interior. An aperture 26 is 
formed on the entire face remote from the reflecting end-wall and can have 
a flared-out portion in the shape of a horn 27 in order to improve the 
directivity. The array 10 of parallel wires are placed within the interior 
of the box and arranged on a surface of cylindrical-parabolic shape in 
order to form the semitransparent cylindrical-parabolic reflector of the 
antenna. 
By way of constructional example without any limitation being implied, the 
dimensions of the box in the case of an operating frequency of 1300 MHz 
are as follows: 
in the case of the polarizing end-wall reflector: 0.165 m.times.4 m 
in the case of the large side-walls perpendicular to the end-wall: 1.50 
m.times.4 m 
in the case of the real focal distance: 1.20 m 
In FIG. 5, the edges 29 and 28 of the cylindrical-parabolic array 10 are 
not in the focal plane but make a non-zero angle .PHI. with said array. 
The fact of closing said edges by solid metal plates 30 and 31 on the 
focal plane makes it possible to increase the gain of the antenna to an 
appreciable extent. This alternative embodiment is also applicable to the 
case of the antenna which is placed within a parallelepipedal box as shown 
in FIG. 6. 
It is in fact apparent from this figure that the edges 32 and 33 of the 
cylindrical-parabolic array 10 are inclined with respect to the focal line 
F at a non-zero angle .PHI.. 
By virtue of the principle of rotation of the plane of polarization of the 
wave emitted by the excitation source which permits the construction of a 
folded optical system, the antenna according to the invention is of small 
overall length along its focal axis. This accordingly results in a 
reduction both in volume and in weight while maintaining the same 
radioelectric characteristics as those exhibited by a conventional 
parallel-plane antenna. 
Potential applications of the parallel-plane antennas according to the 
invention include monitoring and target-locating. They have the same 
functions as the parallel-plane antennas of the prior art but are less 
costly to manufacture in the case of the antenna itself and its drive 
mechanisms.