Microwave antenna transmission device having a stripline to waveguide transition via a slot coupling

A device for the power transmission of microwaves between a strip-line and a number of parallel cavity waveguides arranged in a group antenna. The strip-line includes H-shaped slots. These slots are centered with respect to a central conductor. Opposite each of the slots, a corresponding slot is arranged through the wall of the cavity waveguide. Electrically conducting seals are arranged to follow immediately outside the contours of the slots. The strip-line is fixedly fastened to the seals and the ridge waveguide, whereby good electrical coupling is achieved. Simultaneously, small cavities are formed between the slots. These cavities have a leveling effect such that the demands on mechanical precision is appreciably lowered, such that the tolerance to placement of the slots opposite to each other is increased substantially as compared to the case of the waveguides directly abutting the strip-line.

TECHNICAL FIELD 
The invention concerns devices for power transmission between two 
transmission conductor devices for electromagnetic microwaves, such as a 
cavity waveguide and a strip-line, via radiation slots. The invention also 
concerns a microwave antenna coupled by means of such devices. 
BACKGROUND AND PRIOR ART 
Group antennas for microwaves comprising a desired number of parallel 
cavity waveguides are known. The cavity waveguides are thereby placed 
adjacent to each other and on the front sides of the cavity waveguides, a 
great number of short slots are arranged one after the other, through 
which microwave energy is emitted to and/or is taken up from the 
surroundings. The slots are normally evenly spaced along the cavity 
waveguides. The cavity waveguides may according to a suitable point of 
view be looked upon as resonance chambers, from which microwaves may be 
emitted through said slots. 
In U.S. Pat. No. 5,028,891 an antenna of this type is described, in which 
the cavity waveguides, which preferably are comprised of ridge waveguides 
are fed via a number of adaptation chambers in which a central conductor 
is arranged in a substrate. Each adaptation chamber is fed by a coaxial 
cable and is arranged in direct communication with one of the cavity 
waveguides in such a way that one of the walls of the same is formed by 
one of the walls of the cavity waveguide. In this wall a preferably 
H-shaped slot is arranged through which microwaves are transmitted from 
the adaptation chamber to the cavity waveguide. 
The construction described in U.S. Pat. No. 5,028,891 having adaptation 
chambers is, however, expensive and relatively complex. High demands are 
for instance made on the adaptation chamber fitting tightly against the 
cavity waveguide. Each adaptation chamber for the group antenna needs 
individual mounting and adjustment with small tolerances. 
The shown construction also demands relatively much space depthwise, which 
presents a substantial drawback in antenna constructions where the 
available space often constitutes a limiting factor. This fact is 
accentuated in mobile applications. 
Power transmission of microwaves between different transmission conductor 
devices using slots is also known in other contexts. U.S. Pat. No. 
5,539,361 shows a transition section between a cavity waveguide and a 
microstrip conductor. The cavity waveguide exhibits a continuously 
tapering form up to an aperture around which the cavity waveguide 
preferably is tightly applied to an earth plane on the microstrip card. A 
slot is arranged in the earth plane opposite this aperture. This slot is 
the same size or smaller than the aperture in the cavity waveguide. The 
cavity waveguide is adapted to transmit microwaves in its longitudinal 
direction up to the aperture. As the slot is small in comparison to the 
cross-section of the cavity waveguide reflections tend to arise. To try to 
counteract this effect the cavity waveguide exhibits a slowly tapering 
cross-section. 
Also for the construction described in this document it is true that much 
care is required to accomplish a tight transition in order to avoid power 
losses. Further, this construction is sensitive to a possible displacement 
of the aperture in relation to the slot in the earth plane. This is 
especially so, when the aperture is approximately as big as the slot. If 
the slot is smaller than the aperture, problems arise with reflections 
giving less efficiency. 
SUMMARY OF THE INVENTION 
As is mentioned above, it is desirable to achieve a device for power 
transmission of electromagnetic microwaves between a first and a second 
transmission conductor device, e.g. a cavity waveguide and a strip-line in 
which high efficiency may be combined with low complexity and small 
requirements as to space. Especially desirable is the possibility to 
achieve a power transmission device for antennas where the antenna 
elements are constituted by cavity waveguides, in which high efficiency 
may be combined with low complexity and small requirements as to space, 
especially depthwise, without the requirements on the mechanical precision 
becoming too great. It has earlier been a problem to fulfil these 
requirements. 
The present invention solves this problem by arranging said first 
transmission conductor device and the second transmission conductor device 
adjacent to each other in such a way that the first transmission conductor 
device is delimited or bounded in the direction of the second transmission 
conductor device by a first electrically conducting wall, and the second 
transmission conductor device is delimited or bounded in the direction of 
said first transmission conductor device by a second electrically 
conducting wall. To accomplish this, a first radiation slot in the first 
electrically conducting wall and a second radiation slot in the second 
electrically conducting wall are used for the power transmission, whereat 
the first electrically conducting wall belongs to the first transmission 
conductor device and the second electrically conducting wall belongs to 
the second transmission conductor device. These two radiation slots 
exhibit essentially the same form and elongation, and are arranged 
adjacent and essentially opposite each other. An electrically conducting 
sealing means is arranged in electrical contact with said first 
electrically conducting wall and that the second electrically conducting 
wall, around said first and second radiation slots such that a 
electrically essentially closed cavity (10) from the environment is 
created between said first and said second wall, through which cavity the 
microwave effect may be transmitted. 
Said first transmission conduction device preferably consists of a cavity 
waveguide, such as a ridge waveguide in a group antenna. The second 
transmission conduction device is arranged adjacent to the first 
transmission conduction device in such a way that the electrically 
conducting walls are essentially plane-parallel, and the slots arranged 
essentially opposite each other. Two adjacent, cooperating slots 
implicitly demands an exact centering of the slots in order to achieve 
good efficiency. This effect is, however, counteracted by the electrically 
conducting sealing means, which abuts both the first and the second 
electrically conducting wall such that a substantially, towards the 
environment, electrically sealed cavity is created between said first and 
said second transmission conduction devices. This cavity has a levelling 
effect, such that the demands on the mechanical precision is considerably 
lowered. The cavity is preferably small in comparison to the transmission 
conduction device and in comparison with the wavelength of the microwaves. 
One object of the present invention is to achieve a device for power 
transmission of electromagnetic microwaves between a first transmission 
conduction device and a second transmission conduction device in which 
high efficiency may be combined with low complexity and small demands on 
space. 
Another object of the invention is the possibility to achieve a device for 
power transmission in microwave antennas, preferably group antennas, where 
the antenna elements are achieved by means of cavity waveguides, in which 
high efficiency may be combined with low complexity, moderate demands on 
mechanical precision and small demands on space, especially depthwise. 
One advantage of the present invention is that a device for power 
transmission of electromagnetic microwaves between a first transmission 
conduction device and a second transmission conduction device is achieved 
in which high efficiency may be combined with good bandwidth and small 
demands on space. 
Another advantage of the present invention is the possibility to achieve a 
device for power transmission of electromagnetic microwaves to and/or from 
group antennas, which is adapted to mobile applications where strict space 
requirements are required. 
A further advantage of the present invention is the possibility to achieve 
a device for power transmission of electromagnetic microwaves between a 
first transmission conductor device and a second transmission conductor 
device, in which the mutual relationship of all elements demands high 
mechanical precision, may be realized in one and the same building 
element, thus all these demands may be fulfilled without difficulty. 
Yet another advantage of the present invention is the possibility to 
achieve a device for power transmission for microwave group antennas, in 
which the antenna elements are achieved by means of a cavity waveguide, 
wherein one and the same transmission conductor device, e.g. being a 
strip-line card, may be used for power transmission to and from several of 
the cavity waveguides comprised in the antenna.

PREFERRED EMBODIMENTS 
FIG. 1 a shows a cavity waveguide for microwaves as described in U.S. Pat. 
No. 5,028,891. The cavity waveguide designated 31 is formed of 
electrically conduction material and exhibits a rectangular cross-section. 
The cavity waveguide designated 31 supports an adaptation chamber 32 which 
is coupled to a coaxial conductor 34 having a rotationally symmetric 
cross-section. The cavity waveguide 31 has on its front side a set of 
slots 37, through which microwave energy may radiate to the environment. 
The adaptation chamber 32 is built around a dielectric substrate 36 (See 
FIG. 1b). This substrate is on five of its six sides surrounded by 
electrically conducting walls. The sixth side of the substrate 36 abuts 
the side of the cavity waveguide 31 which is opposite to the side having 
said set of slots 37. Centrally in the substrate a central conductor 33 
arranged in the longitudinal direction of the cavity waveguide. The wall 
of the cavity waveguide abutting the adap-tation chamber 32 is provided 
with a resonance slot 35, which is arranged perpendicularly to the 
longitudinal direction of the cavity waveguide. Via this resonance slot 35 
the microwave energy in the adaptation chamber 32 is coupled to the cavity 
waveguide 31. 
FIG. 1b shows a cross-section A--A through cavity waveguide 31 and the 
adaptation chamber 32 in FIG. 1a. Here may be seen that while the 
substrate 36 in the adaptation circuit on all sides but one is surrounded 
by conducting walls, the substrate directly abuts the cavity waveguide 31, 
whereby the wall of the cavity waveguide is used as a sixth delimiting 
wall for the adaptation chamber 32. The adaptation chamber is used as a 
resonance chamber. By means of the central conductor an electromagnetic 
wave is generated in the adaptation chamber 32, which via the resonance 
slot 35, is coupled to the cavity waveguide 31. 
The construction described in U.S. Pat. No 5,028,891, having an adaptation 
chamber feed via a coaxial conductor is expensive and exhibits a rather 
high complexity. Every adaptation chamber demands individual mounting and 
adjustment using small tolerances. High demands are in this respect made 
upon the adaptation chamber walls being tightly fitted to the wall of the 
cavity waveguide in order to keep the effect losses down. The coaxial 
coupling also leads to the adaptation chamber demanding a rather big space 
depthwise. In the construction of microwave group antennas the available 
space is often a limiting factor. Especially considering a mobile antenna, 
such as an antenna mounted in an aircraft for mobile reconnaissance radar, 
the demands on space, especially depthwise, is a critical factor. 
In the present invention the power transmission to and from a cavity 
waveguide is accomplished using a strip-line arranged in the orthogonal 
direction as related to the power transmission direction in direct 
connection to the top face of a cavity waveguide. Hereby the space demands 
depthwise are considerably reduced since the coaxial connection can 
totally be left out. Further this construction makes it possible to 
arrange, in one strip-line card, several power transmission devices, 
arranged parallel to each other, for several cavity waveguides, e.g. to 
all cavity waveguides in a group antenna. 
However, at the same time new problems arise. The topside of the cavity 
waveguide and the earth plane which is situated on the underside of said 
strip-line adjacent to the cavity waveguide must fit tightly to each other 
in order to avoid power losses. Further, in this construction there must 
be a radiation slot in both the strip-line, the earth plane and the cavity 
waveguide. The position of these slots, must for good efficiency, be 
adapted to each other with a high degree of accuracy and repeatability. 
This leads to very high demands on tolerance, i.e. permissible variations, 
especially if the same strip-line card is used for several adjacently 
arranged cavity waveguides. This tends to lead to unreasonably high costs. 
In the present invention this is solved by an electrically conducting 
sealing device between the waveguides around the slots, whereby good 
isolation is guaranteed. This sealing device is arranged according to the 
invention such that a small cavity is formed between the two transmission 
conduction devices. This cavity has a levelling effect such that a device 
having good transmission characteristics is obtained, without high demands 
on mechanical precision in relation to the transmission conduction devices 
and the slots. 
However, it is essential that symmetry is achieved between the strip-line 
guide and the slot in the earth plane which is associated with this 
strip-line guide. It is further important to achieve a well-defined 
distance between the slot and the strip-line guide. This distance 
determines the transition impedance. By using a slot in the earth plane of 
the strip-line card, this slot and the strip-line guide will be found in 
the same structure, whereby a desired positioning of this slot in relation 
to the guide may be accomplished without problems. 
FIG. 2a shows a perspective view of a preferred embodiment of the 
invention. A strip-line 12 is arranged to transmit microwave signals, in 
this case in the frequency band 3 to 3.5 GHz, to and/or from a number of 
essentially identical ridge waveguides being part of a group antenna. One 
of these waveguides denoted 11 is shown in FIG. 2a. In the Figure is also 
shown in outline an adjacent ridge waveguide 20. The ridge waveguide 11 is 
equipped with a ridge 18 along one of its sides, said ridge protruding 
into the waveguide and extending in the longitudinal direction of the 
waveguide. 
The ridge waveguide has the advantage of allowing a relatively broad 
bandwidth in the fundamental mode of a microwave which propagates in the 
waveguide. The ridge waveguide also has the advantage of having a width B 
which is relatively small in comparison to the wave-length .lambda. of the 
microwave, e.g. of the size B=0.4.multidot..lambda., which may be compared 
to a known rule of thumb stating that in order to avoid the appearance of 
grid lobes for a group antenna, d&lt;.mu./2, wherein d designates the 
distance between two adjacent antenna elements. These characteristics may 
be used with the above mentioned type of group antennas, which have many 
parallel waveguides closely adjacent each other. By using the relatively 
small width it is possible to achieve phased microwave antennas according 
to known technology. 
FIG. 2b is a sectional view through said strip-line 12 along a plane which 
is shown by the line C--C in FIG. 2a. This strip-line 12 is equipped with 
an upper earth plane 12b and a bottom earth plane 12a. Between these two 
earth planes an electrically isolating substrate 12c is arranged. In the 
substrate, on a well-defined distance from the earth planes 12a and 12b, a 
central conductor 13 is arranged. In this example the central conductor is 
arranged in the middle between the two earth planes. The earth plane 12a 
facing towards the ridge waveguide 11 is provided with a H-shaped slot 14. 
H-shaped slots are especially well adapted in such cases in which the 
wavelength of the signal is large relative to the maximum length of the 
slot. The H-shaped slot 14, which in this example is produced through 
etching, is arranged centered in relation to the central conductor 13. The 
slot has in this example a width b (See FIG. 2a) of approximately 32 mm 
and the width B (See FIG. 2a) of the waveguide 11 is approximately 43 mm. 
Right opposite this slot 14 is a corresponding second H-shaped slot 15 
arranged, as shown in FIG. 2a, through the wall 11a of the ridge waveguide 
on the side where the ridge 18 is arranged. The ridge 18 may, from one 
standpoint, be looked upon as a fold protruding into the cavity waveguide. 
Looked upon from the outside of the cavity waveguide 11, the ridge 18 
appears as a longitudinal recess in the waveguide. As can be seen from 
FIG. 2a, this recess is filled with a conducting material, on a level with 
the slots 14, 15. 
As shown in FIG. 2b, an electrically conducting seal 19 is arranged in a 
groove 11c in the outer wall 11a of the ridge waveguide. The seal 19 is in 
this example of the type O-ring seal and is made from silicon rubber with 
a coating of silver-plated aluminium spheres vulcanized onto it. The seal 
is adapted to follow immediately outside the contours of the slots, as 
shown by a distance d in FIG. 2b. As outlined in FIG. 2b, the seal 19 in 
this example is hollow. Hereby swelling of the seal at compression is 
counteracted. In this example the distance d between the outer contours of 
the slots and the seal is approximately 1 mm. Outside the groove 11c, a 
flange 11d is arranged directly adjacent the groove with an associated 
seal 19. The flange 11d has in this example a height h of 0.5 mm and runs, 
as does the groove 11c, around the whole slot 15. However, it is not 
necessary that the flange runs around the whole slot. The flange may also 
be interrupted or solely support the strip-line card in a limited number 
of points. Another conceivable possibility is to arrange the seal 19 
outside the flange 11c. 
The strip-line 12 is fixed to the seal 19 and the ridge waveguide 11 by 
means of fixing devices, which in this example consist of a number of 
screws (not shown in the figure). 
Around these screws the waveguide is provided with flanges of the same type 
and the same height as the flange 11d. Said strip-line 12 will hereby be 
pressed against the elastic seal 19 whereby the seal is hermetically tight 
to the environment, and a good electrical coupling is guaranteed between 
the strip-line-earth plane 12a and the ridge waveguide wall 11a. Hereby 
the risk of airgaps being formed between the two transmission conduction 
devices and possible leakage, is essentially removed. The strip-line 12 
will in this case bear upon the flange 11d and also upon the flanges 
surrounding the screws. Hereby a small cavity 10 between said strip-line 
12 and the cavity waveguide 11 is formed. The height of the cavity will 
then be decided by the height of the flanges, which in this case is h=0.5 
mm. Its extension in the two other dimensions is delimited by the seal 19. 
The cavity 10 has a levelling effect. Thereby the demands on the mechanical 
precision is decreased so that the tolerance towards the placement of the 
slots in relation to each other is essentially increased as compared to 
the case wherein the strip-line-earth plane would directly abut the cavity 
waveguide. The slots 14 and 15 may be allowed to be displaced up to 1 mm 
relative to each other in longitudinal and/or lateral direction without 
detrimental effect on the power transmission. One example of such a 
displacement is shown in FIG. 2c, which shows the cross-section of FIG. 2b 
through the cavity 10. The displacement is shown in the longitudinal 
direction of the ridge waveguide 11 by a distance f. In the same way it is 
possible to let the central conductor 13 be displaced approximately 1/2 mm 
askew relative to the slot 15 in the cavity waveguide. Put in relation to 
the width b of the slots being approximately 30 mm and the conductor width 
of the strip-line, i.e. 1.92 mm, this implies very low tolerance demands. 
The height of the cavity 10 is, as mentioned above, 0.5 mm in this 
embodiment of the invention. For achieving the best power transmission of 
microwave signals in the frequency range of this example, the height h 
should preferably be chosen between approximately 0.3 and 1.0 mm. 
FIG. 2b shows how the above mentioned slot 15 in the ridge waveguide wall 
has been broadened in the longitudinal direction of the ridge waveguide 
into a tunnel-shape. This tunnel-shape, however, is only formed in the 
filled-up ridge 18. As can be seen more clearly in FIG. 2a, the ridge 
waveguide slot 15 also extends on both sides of the ridge. Here the slot 
is characterized by a simple opening in the wall of the waveguide. 
In the above described embodiment of the present invention a power 
transmission is shown between a strip-line card and an essentially 
rectangular cavity waveguide. The invention can also be realized using a 
cavity waveguide having a circular cross-section, or using completely 
different combinations of transmission conductor devices where these may 
be so arranged that they are delimited toward each other by electrically 
conducting and essentially plane-parallel walls. An example of this is a 
cavity waveguide-to-cavity waveguide transition, a 
strip-line-to-strip-line transition, where one or both of these 
strip-lines may even be made using microstrip technique, or a 
strip-line-to-coaxial conductor transition. 
FIG. 3 is a perspective view of an alternative embodiment of said 
strip-line 12 in FIGS. 2a and 2b. This strip-line, here denoted 22, is 
according to prior art per se equipped with an upper earth plane 22b and a 
bottom earth plane 22a. The bottom earth plane is equipped with an 
H-shaped slot 24. A number of through-plated holes 25 connecting the upper 
and the bottom earth plane 22b,22a are arranged along the sides of an 
imaginary rectangle, essentially symmetrically around the slot 24. The 
distance between these through-plated holes is small compared to the 
microwave wavelength .lambda.. In said strip-line substrate a central 
conductor 23 is arranged. It is arranged to pass between two adjacent 
through-plated holes and to extend in the longitudinal direction of the 
cavity waveguide past the center of the slot 24. 
In the transmission conduction transition, there occurs a transition from a 
transversal electromagnetic wave (TEM), coming into said strip-line, to a 
transversal electric wave (TE) in the cavity waveguide According to a 
strongly simplified view the TEM-wave sees the slot 24 as an unsymmetrical 
interference, which causes TE-waves to arise. As these are not bound to 
the central conductor in the same way as the TEM-wave, part of the 
microwave power could show a tendency to propagate freely through the 
strip-line substrate. This phenomenon is counteracted by the 
through-plated holes 25 which, somewhat simplified, can be said to form an 
earthed cage around the slot 24. 
Owing to the fact that one and the same strip-line card may be connected to 
several adjacent cavity waveguides at the same time, where the power 
transmission preferably is executed at several locations of the same 
cavity waveguide, the invention offers a mechanically simple construction 
for power transmission in a group antenna constituted by cavity 
waveguides. Preferably the strip-line card comprises at least a 
distribution network, by which the power is distributed to the several 
slots-transitions. Preferably other components, such as impedance 
attenuation circuits and filters may advantageously be integrated on the 
strip-line card according to known technique. 
FIG. 4 shows an over-arching and somewhat simplified view of an antenna 
device 40 where this is illustrated. The antenna device 40 in this case 
comprises a group antenna realized by means of a number of parallel cavity 
waveguides. Three of these cavity waveguides 41,42,43 are shown in the 
Figure. An adjacent fourth cavity waveguide 44 is indicated with dashed 
lines. Each cavity waveguide has a longitudinal ridge 41a,42a, 43a. 
Further, the cavity waveguides are each provided with a number of slots, 
of which two slots 51 can be seen in the figure. As is indicated in the 
figure, the ridges of the cavity waveguides are filled on level with these 
slots 51. The slots are in this example are Z-formed, whereat they 
comprise a longer section of approximately 30 mm, which is perpendicular 
to the longitudinal direction of the cavity waveguides, and in each end of 
this longer section a shorter section of approximately 10 mm, which is 
oriented in the longitudinal direction of the cavity waveguides. Many 
other slot-forms are, however, possible. 
Around each of the slots 51 in the cavity waveguides an electrically 
conducting, elastic sealing device 53 is arranged in a groove in the outer 
wall of the cavity waveguides. The sealing devices 53 comprise a set of 
short sealing elements which are arranged one after another and are 
adjusted to follow right outside the contours of the slots. In this 
example the distance between the outer contours of the slots and the 
sealing devices 53 is approximately 1 mm. The distance between two 
adjacent sealing elements is small in comparison to the wavelength of the 
microwave signals, such that the sealing devices 53 may be considered 
electrically sealed in the meaning that leakage of signal effect through 
the interspaces between separate sealing elements essentially can be 
totally ignored. 
A strip-line card 45 is arranged across all of the cavity waveguides in the 
group antenna. This strip-line card 45, which in the figure is shown as 
severed in order to show the underlying cavity waveguides, is arranged to 
conduct the microwave signals to, and/or from, the cavity waveguides 
through said slots 51 in the cavity waveguides. Essentially straight above 
each of these slots 51, the strip-line card has a corresponding slot 49 in 
that one of the two earth planes which faces towards the cavity 
waveguides. These earth plane slots 49 have mainly the same form and 
extension as the slots 51 in the cavity waveguides. The slots 49 and 51 
therefore form pairs of adjacent similar slots. 
A set of through-plated holes 50 is symmetrically arranged in a rectangular 
form around each slot 49 in the strip-line card. These through-plated 
holes 50 connect the two earth planes of the strip-line card electrically. 
The distance between two adjacent holes is small in comparison to the 
microwave signal wavelength. Each set of through-plated holes act together 
with the two earth planes as a mode suppressor the extension of which is 
adapted to the microwave signal wavelength .lambda.. Into each such mode 
suppressor, formed by through-plated holes, a strip-line conductor 48 
leads, oriented in the longitudinal direction of the cavity waveguides, 
which strip-line conductor, after having transversed its respective slot 
49, ends as an open stub conductor. The strip-line conductor 48 may, 
according to one point of view, be seen as a sond, a so-called probe, 
which propagates into the mode suppressor and there produces an 
electromagnetic wave, which is transferred via the slots 49 and 51 to the 
respective cavity waveguides. 
Each cavity waveguide is fixed to the strip-line card 45 by means of a 
number of screws of which two screws 52 for each of the cavity waveguides 
41, 42 and 43 are shown in this FIG. 4. By means of these screws, said 
strip-line card 45 is forced against the elastic sealing devices 53. 
Thereby, good electrical coupling is obtained through each sealing element 
in the sealing devices 53 between the strip-line-earth plane and the 
cavity waveguides. These sealing devices hereby is electrically sealed 
towards the environment so that the risk of leakage of signal power to the 
environment is minimized. At the same time, in the same way as in earlier 
described embodiments of the invention, a small cavity between the slots 
in each pair of slots if formed, where the cavity has a levelling effect. 
Through this, the demands for mechanical precision is decreased so that 
the tolerance towards the placement of the slots opposite to each other 
essentially can be increased in comparison to the case where the 
waveguides 41,42,43 would bear directly against the strip-line card 45. 
On the strip-line card 45 a power distributing network is indicated by 
which signal effect is conducted to the strip-line conductor 48, which 
transfers the signal effect via said slots to the cavity waveguides. The 
power distribution net comprises a set of power distributors 46 in the 
form of Wilkinson-distributors, which distribute the incoming effect to 
two outgoing strip-line conductors. In this example, the effect is 
distributed in equal parts. The power distributing net further comprises a 
set of adaptation circuits 47. Such an adaptation circuit 47 is arranged 
for each pair of slots. The adaptation circuits 47 are, according to known 
technique per se, realized by means of a pair of stub conductors 54, the 
length and positions of which being adapted to give a good adaptation at 
the transitions. 
The description of the antenna device 40 in this embodiment has been made 
from the point of view that the antenna device is used for sending, at 
which effect/power is transferred from the strip-line card 45 to the 
cavity waveguides. The antenna device 40, however, equally well is suited 
for receiving. 
The strip-line card 45 is in this example manufactured in the traditional 
strip-line technique having two earth planes on each side of a substrate 
comprising a strip-line conductor. This is an advantageous embodiment 
since good power transfer to the cavity waveguides with small losses is 
possible using this technique. It would, however, also be possible to make 
the strip-line card in microstrip technique. Further, the power is fed to 
the whole antenna by means of one and the same strip-line card in this 
embodiment. It is of course possible, and when using large antennas 
possibly advisable, to use a set of strip-line cards arranged parallel to 
each other for the antenna connection, where each strip-line card feeds a 
number of slots in a number of the cavity waveguides comprised in the 
antenna. In this case, these strip-line cards can of course transfer power 
both to and from the cavity waveguides.