The invention relates to a transmission-line network, comprising an assembly of a first conductive base, a first dielectric sheet having an at least substantially constant thickness, an inner conductor structure having a given form, a second dielectric sheet having an at least substantially constant thickness and a second conductive base. Such transmission-line networks are for instance used as distribution networks in phased array or multibeam antennas. A drawback of these well-known transmission-line networks is their relatively limited capability for power transmission owing to heat developing in the dielectric sheets near the inner conductor structure as a result of dielectric losses, which could cause excessive heating of the transmission-line network.

A possible solution is given in patent specification EP-A- 0.402.052, in which it is proposed that the dielectric sheet be provided with a number of cavities located between the inner conductor structure and the first and second conductive base. On the one hand, this method provides sufficient support for the inner conductor structure and on the other hand, it reduces the volume of dielectric material and the consequent development of heat.

This well-known method has the drawback that a relatively large volume of dielectric material remains present between the inner conductor structure and the first and second conductive base and that the alternate presence or absence of dielectric material impairs the wave properties of the transmission-line network.

The present invention has for its object to realize a transmission-line network that does not experience these

drawbacks and is characterised in that at least one dielectric sheet is provided with a cavity whose form substantially conforms to at least a part of the form of the inner conductor structure. Both dielectric sheets will as a rule be provided with identical cavities.

Thus, it is ensured that a small volume of dielectric is present between the inner conductor structure and the first and second conductive base, without this affecting the wave properties of the transmission-line network.

A favourable embodiment of the invention is characterised in that the cavity extends over the entire thickness of the dielectric. In this case, at least substantially no dielectric is present between the inner conductor structure and the first and second conductive base. It is then required to make provisions in support of the inner conductor structure. This inner conductor structure will generally include a stripline network.

In support of the stripline network, a first exceptionally favourable embodiment enables the width of the cavity to be selected smaller than the width of the stripline network at that position. Thus, the volume of dielectric that will cause the occurrence of dielectric losses will be strongly reduced, any remaining dielectric only being present on the edge of the inner conductor structure where the occurring electric field strengths and, consequently, the occurring dielectric losses will usually be smaller. Moreover, it will generally not be required to support the inner conductor structure over its entire length as a result of which, in those regions where support is desired, the presence of a limited volume of dielectric between the inner conductor structure and the first and the second conductive base, may suffice.

In a second exceptionally favourable embodiment the stripline-network can be placed on a dielectric film, which is at least substantially contained between the first and the second dielectric sheet. It will then be possible to select the width of the cavity to be larger than the width of the stripline, resulting in the complete elimination of dielectric losses occurring in the dielectric sheet.

This entails the additional advantage that the stripline network can be implemented as a printed circuit placed on the dielectric film. It furthermore enables complex inner conductor structures to be manufactured at relatively low cost.

Transmission-line networks thus obtained are capable of high-power transmission, the ohmic losses in the inner conductor structure and dielectric losses occurring in the dielectric film constituting the restrictive factors. By for instance using kapton or polyamide, these dielectric losses can be kept low.

An additional advantage of the invention is that the power to be transmitted can be further increased by cooling the inner conductor structure. A further favourable embodiment of the invention is thereto characterised in that means have been provided for the supply of a fluid flow along at least a part of the stripline network for cooling the network. For this purpose, additional cavities may be provided in a dielectric sheet. The use of dry air as fluid flow also prevents moisture from penetrating into the transmission-line network; this moisture might give rise to further losses.

This is particularly relevant if the dielectric sheet is made of synthetic foam that is noted for its absorptive capacity.

The invention will now be explained in more detail with reference to the following figures, of which: Fig. 1 schematically represents a top view of a transmission-line network; Fig. 2 represents a cross-section of a first embodiment of a transmission-line network according to the invention; Fig. 3 represents a cross-section of a second embodiment of a transmission-line network according to the invention; Fig. 4 represents a cross-section of a third embodiment of a transmission-line network according to the invention.

Fig. 1 schematically presents a top view of a transmission- line network, provided with an inner conductor structure 1. It can for instance be used for the distribution of microwave energy supplied to input 2 over the four outputs 3a, 3b, 3c and 3d. Such transmission-line networks are frequently applied in for instance antennas, with a transmitter or a receiver being connected to input 2 and radiating elements to the outputs 3a, 3b, 3c and 3d. Inner conductor structure 1 will generally be a stripline network contained in a sandwich structure 4 that will be further described below.

In accordance with the line AA' shown in Fig. 1, Fig. 2 represents a cross-section of a possible structure of a transmission-line network according to the invention. Inner conductor structure 1 is sandwiched between a first conductive base 5, a first dielectric sheet 6, a second

dielectric sheet 7 and a second conductive base 8. To prevent dielectric losses, the dielectric near the inner conductor structure 1 has been partly removed by means of cavities 9 and 10 provided in the first dielectric sheet 6 and the second dielectric sheet 7 respectively. The cavities 9 and 10 are such that they faithfully conform to the form of the inner conductor structure 1 as shown in Fig. 1, while still maintaining a sufficient support for the inner conductor structure 1. It should also be noted that inner conductor structure 1 normally has a far more complex form than is indicated in Fig. 1, which not only results from more complex functions having to be realized, but is also due to the incorporation of a variety of compensation structures for obtaining a large bandwidth.

In accordance with the line AA' in Fig. 1, Fig. 3 also represents a cross-section of a further possible construction of a transmission-line network according to the invention. Cavities 9 and 10 are now bottomless, which means that inner conductor structure 1 is floating, except for the edges which are only just clamped between the first dielectric sheet 6 and the second dielectric sheet 7. By tapering the edges of cavities 6 and 7, as indicated in this figure, dielectric losses can be further reduced. This construction is particularly useful if the inner conductor structure 1 is light, for instance a thin stripline network. This may give rise to ohmic losses in inner conductor structure 1, although this can be countered by supplying sufficient cooling by for instance blowing cold air into the duct-shaped cavities 9 and 10 near input 2, the air being subsequently capable of escaping from outputs 3a, 3b, 3c and 3d. This can also be realised with more complex forms of inner conductor structure 1, for instance by means of additional cavities in dielectric sheet 6 and 7 specifically provided for the transport of a cooling fluid.

Additional cavities are also necessary if the cavities 9 and 10 do not follow the entire inner conductor structure 1 but only a part of it, for example near input 2. When connected to a transmitter this part tends to develop problems to heating first.

Fig. 4 also represents, in accordance with the line AA' in Fig. 1, a cross-section of a further possible construction of a transmission-line network according to the invention. Inner conductor structure is implemented as a stripline network placed on a dielectric film 11, for instance as a printed circuit. This method has the advantage that also complex stripline networks can be manufactured at low cost. Cavities 9 and 10 can now even be wider than inner conductor structure 1 at that location, since support is provided by the dielectric film 11. Dielectric film 11 introduces dielectric losses, although these may be reduced through a suitable material selection such as kapton or glass-fibre reinforced polyamide. Also here it is possible to use the ducts formed by cavities 9 and 10 for cooling the inner conductor structure 1, including the dielectric film.

Although in said embodiments, the sandwich 4 has been represented as a planar structure, the invention can also be applied to curved surfaces, for instance for feeding a conformal antenna array.