VHF / UHF Filters and Multicouplers: Application of Air Resonators

Chapter 6

The Combline Filter

6.1. Architecture

6.1.1. General structure

The word combline is formed by the combination of the terms “comb” and “line”. This pleonastic expression designates a filter where the resonators are aligned in a common plane-parallel space. It is thus no longer a question of juxtaposed profiles: we can say that it is the evolution of a compact filter where we progressively increased the coupling windows, by moving the tubes slightly more apart each time, in order to compensate for the increasing of the coupling, doing so until the walls were completely suppressed. In comparison to the compact filter constituted by the same tubes, it will thus be a little longer, but on the other hand the suppression of the walls implies a decrease of material that will translate into a reduction in losses and weight.

The combline is necessarily a bandpass. Figure 6.1 shows the typical structure of a 4-pole. The connectors are placed on the tube-holder plate, but they could just as easily be found on the sides, to the right and to the left (Figure 6.2): what is essential, from an electrical point of view, is to be able to make a closed loop with the tap, the resonator and the mass. We also note that the taps are not linked to the tubes directly, but by means of a silver-plated strap that runs along the tubes over approximately the lower third of their height, parallel to their axis and very close to it so that it will be under the same potential. The fastening is done easily ...