[0001] The present invention relates to an antenna for transmitting and/or receiving electromagnetic
radiation comprising a cavity, an aperture arranged in front of the cavity, a feed
element arranged in the cavity and a dielectric arranged in connection with the cavity.
[0002] Cavity antennas with a dielectric according to the above paragraph are well known
to the expert in the antenna field. In this connection, reference can be made to the
article by E H Newman and G A Thiele, "Some Important Parameters in the Design of
T-Bar Fed Hot Antennas", IEEE Trans Ap, January 1975, in which a cavity antenna according
to the above is disclosed. The measurement results shown certainly apply to an airfilled
cavity but it is apparent from the article that many cavity antennas have a dielectric
filling.
[0003] The cavity antennas have a directional effect and are suitable for use as communication
antennas within, for example, the UHF band.
[0004] However, the known cavity antennas have a number of characteristics in common which
are unwanted, at least with certain applications. Thus, it can be said that the antenna
has a relatively large radar target cross-section. The large radar target cross-section
is primarily caused by the corner reflectors inside the cavity. To prevent detection,
it is more and more important, above all in military applications, that the equipment
has the smallest possible radar target cross-section. Stealth technology is becoming
more and more important in the construction of military equipment. Moreover, the antenna
cavity is bulky and, in consequence, heavy.
[0005] It is the aim of the present invention to produce an antenna which eliminates, or
at least reduces, the above-mentioned unwanted characteristics through its design.
The aim of the invention is achieved by an antenna which is characterised by the fact
that the dielectric is provided with hollow spaces and that these hollow spaces contain
electrically conducting shells.
[0006] By introducing a dielectric with the specified construction, the radar target cross-section
of the antenna is significantly reduced to a level comparable to the level of a plane
plate. The dielectric provided with holes forms a frequency-selective volume with
low-pass characteristics. The radar target cross-section produced by the corner reflectors
within the cavity has been reduced to an acceptable level.
[0007] The dielectric, provided with holes, with an electrically conducting shells and also
called artificial dielectric hereinafter exhibits a change both of the dielectric
constant and the permeability constant. This entails that also the index of refraction
is changed, or, more accurately, increases. The artificial dielectric also exhibits
changed transmission and reflection characteristics. Low frequencies are transmitted
and high frequencies are reflected. By varying the periodicity of the location of
the shells in the dielectric and the size and shape of the shells, the artificial
dielectric can assume different frequency characteristics. For example, the artificial
dielectric can be made to be mainly reflecting over a very large frequency range.
[0008] The dielectric used, provided with holes, entails that its size and the size of the
antenna cavity can be reduced without reducing the antenna frequency bandwidth. This
thus results in a more compact antenna with unchanged performance. The more compact
format also makes possible a significant reduction in the weight of the antenna. A
further weight reduction is produced by the dielectric according to the invention
which, through its hole structure, has a lower weight per volume unit than the dielectric
used earlier.
[0009] The hole spaces in the dielectric are advantageously periodically arranged in a threedimensional
matrix, which results in a dielectric with adequate low-pass characteristics.
[0010] According to an advantageous embodiment of the antenna, the dielectric is divided
up into a number of layers, each layer consisting of two part-layers with indentations
arranged opposite one another in opposite surfaces of the part layers for forming
hollow spaces. A dividing of the dielectric into layers and part layers according
to this embodiment provides the antenna with great flexibility and makes the installation
of the electrically conducting shells relatively uncomplicated. The shells are installed
in the indentations of one part layer. After that the other cooperating part layer
is installed. Each pair of part layers will contain a plane with conducting shells.
The number of pairs of part layers comprised in the dielectric determines and corresponds
to the number of planes with conducting shells.
[0011] In the text below, the invention will be described in greater detail with reference
to the attached drawings, in which:
Figure 1 shows an embodiment of a cavity antenna according to the invention in a front
view,
Figure 2 shows a section according to 2-2 in Figure 1 through the embodiment of the
cavity antenna according to the invention,
Figure 3 shows for comparison a section corresponding to Figure 2 for a known cavity
antenna, and
Figure 4 shows a layer of a dielectric included in the cavity antenna according to
the invention in a perspective view and divided into two separate part layers.
[0012] The antenna 1 shown in Figure 1 and 2 comprises a cavity 2 mounted in a frame 3 in
its open end. The aperture 4 of the antenna is defined by the open end of the cavity
and is rectangular in the embodiment shown. A feed element 5 is arranged in the inside
of the cavity 2 in its front part and has the shape of a T-shaped bar. A feed cable
6 connects the feed element 5 and the cavity 2 to external units and is preferably
constituted by a coaxial cable.
[0013] The major part of the cavity 2 is filled with a dielectric 7. In the embodiment shown,
the dielectric 7 is divided into four layers 8, 9, 10, 11. However, the number of
layers can vary from only one to significantly more than the four layers shown, depending
on what is suitable for the actual antenna. The choice is determined by the size of
the cavity 2 and the requirements for the characteristics of the dielectric 7.
[0014] Figure 4 shows layer 11 in a perspective view. Each layer 8, 9, 10, 11 is in turn
divided into part layers, part layers 11a, 11b being shown for layer 11 in Figure
2 and 4. The part layers 11a, 11b have a plane surface 12a and 12b, respectively.
In the plane surfaces, symmetrically arranged indentations 13a and 13b with an essentially
hemispherical shape are located. When two part layers with opposite part layers 11a,
11b provided with indentations are assembled with the indentations opposite one another,
a layer 11 with essentially spherical hollow spaces 14 is formed. Before two part
layers 11a, 11b are assembled, one part layer 11a is provided with electrically conducting
spherical shells 15 in the hemispherical indentations 13a. In an assembled layer 11,
the shells 15 fill out the hollow spaces 14. The shells are made of metallic or metallized
shells and can be made, for example, of silver-coated celluloid balls. The layers
which enclose the electrically conducting shells are suitably made of material with
low electromagnetic transmission losses for frequencies up to about 10 GHz and, for
example, a material sold under the trademark Roasell, can be used.
[0015] By filling or covering the antenna cavity with artificial dielectric, the unwanted
radiation is reduced which is otherwise reflected by the cavity in the direction of
the incident radiation and is mainly caused by the corner reflectors in the interior
of the cavity. The artificial dielectric is placed in front of the cavity bottom.
The incident radiation is then reflected against the dielectric instead, since this
is constructed for reflecting the frequencies or frequency ranges for which it is
desired to reduce the radiation reflected by the cavity. If the normal to a three-dimensional
matrix formed by the shells in the dielectric is not directed towards the incident
radiation, the reflected radiation will be strongly reduced in the direction of incidence.
[0016] As comparison, it is shown in Figure 3 how a dielectric 7 is arranged in a known
cavity antenna. Corresponding components in the known cavity antenna have been given
the same reference designations as in Figures 1-2 and 4 for the cavity antenna according
to the invention. As shown in Figure 3, the known antenna contains a homogeneous dielectric
without division into layers and inhomogeneity-creating hollow spaces filled with
electrically conducting shells. As indicated in the figure, the cavity 2 has a significantly
greater depth than the antenna according to the invention shown in the same section
in Figure 2.
[0017] By introducing an artificial dielectric according to the invention with hollow spaces,
in which electrically conducting shells are placed, a number of positive effects are
created. The size of the cavity antenna can be reduced whilst retaining the frequency
bandwidth, mainly due to the effect that the depth of the cavity can be reduced.
[0018] The antenna can be made lighter due to its reduced size and due to the fact that
the artificial dielectric has a lower weight than the homogeneous dielectric previously
used. The artificial dielectric also reduces the monostatic radar target cross-section
of the antenna.
[0019] To reduce the radar target cross-section of the cavity antenna further, the antenna
aperture can be covered with a plane frequency-selective structure constructed of
one or several parallel layers provided with metallic periodic patterns. Such a structure,
also called radome, has been shown as a plane layer 16 covering the aperture 4 of
the cavity antenna in Figure 2. The radome 16 is ideally transparent to the operating
frequency band of the antenna and reflecting for all other frequencies.
[0020] By filling the antenna cavity with the artificial dielectric described above and
covering the antenna aperture with a frequency-selective radome, the radar target
cross-section of the antenna has changed appearance from being considered as a four
corner reflector to being considered as a plane plate.
1. Antenna for transmitting and/or receiving electromagnetic radiation comprising a cavity,
an aperture arranged in front of the cavity, a feed element arranged in the cavity,
and a dielectric arranged in connection with the cavity, characterised in that the
dielectric is provided with hollow spaces and that these hollow spaces contain electrically
conducting shells.
2. Antenna according to Claim 1, characterised in that the hollow spaces are periodically
arranged in a three-dimensional matrix.
3. Antenna according to any of the preceding claims, characterised in that the dielectric
is divided into a number of layers.
4. Antenna according to Claim 3, characterised in that each layer consists of two part
layers with indentations arranged opposite one another in opposite surfaces of the
part layers for forming hollow spaces.
5. Antenna according to Claim 4, characterised in that the indentations have an essentially
hemispherical shape.
6. Antenna according to any of the preceding claims, characterised in that a radome plate
is arranged in front of the aperture of the antenna.