Transmission line structures

1. A composite waveguide termination structure, comprising at least two waveguide conductors having different geometries operatively located upon a common substrate, wherein each said waveguide conductor geometry includes a ground conductor on the same surface of the substrate, the ground conductors being electrically connected to each other, wherein a first of said geometries forms a microstrip transmission line and the second of said geometries forms a modified co-planar transmission line in which the ground conductor thereof is located on an opposite surface of the substrate to a conductor electrode thereof, and wherein the microstrip transmission line is arranged to feed into the co-planar transmission line.

2. The composite waveguide structure as claimed in claim 1 , wherein the ground conductors of both geometrics are connected directly to a packaging base for the lines.

3. The composite waveguide termination structure as claimed in claim 1 , wherein at least one of said geometries comprises a conductor electrode having two regions separated by a gap.

4. The composite waveguide termination structure as claimed in claim 3 , wherein said gap is of such a size as to allow placement of a DC block capacitor at said gap.

5. A composite waveguide termination structure, comprising

a first microstrip transmission line section including a substrate interposed between a first conductor electrode and a first ground conductor,

a second modified co-planar transmission line section including a substrate that is an extension of the substrate of the first section,

a second conductor electrode, and

a second ground conductor,

wherein the second ground conductor is provided on an opposite surface of the substrate relative to the second conductor electrode, and the second ground conductor is provided on the same surface of the substrate as the first ground conductor and is electrically connected therewith, and wherein the microstrip transmission line is arranged to feed into the co-planar transmission line.

6. The composite waveguide termination structure as claimed in claim 5 , wherein the first and second ground conductors are connected directly to a packaging base for the lines.

7. A composite waveguide termination structure, comprising a microstrip transmission line that is arranged to feed into a modified co-planar transmission line having the same substrate as that of the microstrip transmission line, such that a ground conductor of each said microstrip and co-planar transmission lines are connected directly to a packaging base for the transmission lines, wherein the modified co-planar transmission line geometry is such that the ground conductor thereof is located on an opposite surface of the substrate to a conductor electrode thereof, and the ground conductor of both lines are provided on the same surface of the substrate and electrically connected to each other.

8. The composite waveguide termination structure as claimed in claim 7 , wherein the co-planar transmission line includes a conductor electrode separated into two regions by a physical gap of such size so as to allow placement of a DC block capacitor at said gap.

9. The composite waveguide termination structure as claimed in claim 7 , wherein a characteristic impedance of the microstrip transmission line is maintained by the coplanar transmission line by an aperture provided in the ground conductors.

10. The composite waveguide termination structure as claimed in claim 7 , wherein a broadband termination is achieved by providing upon a first surface of the substrate a parallel combination of two sheet resistors that are placed adjacent to an outer edge of a two part main conductor provided upon said first surface, and wherein the sheet resistors are so positioned relative to each other as to enable the provision of at least one via whereby each sheet resistor and thus the main conductor parts can be connected to a ground plane provided upon a second surface of the substrate.