Power amplifier tank circuit

#1. Description of the Prior Art

#Resonant transmission lines, also commonly referred to as RF tank cavities or RF tank circuits have been built in the past in half-wave and quarter-wave types. These circuits usually require a plate blocking capacitor to insulate the high DC potential of an associated power amplifier anode from the cavity walls, and in addition, they require insulated mechanical sliding supports for tuning. Both the half-wave and quarter-wave cavities built along these principals are more costly and have less efficiency than the folded half-wave cavity which is the subject of the present invention. Due to the folded nature of this cavity, a plate blocking capacitor is not required, and the inner conductor of the cavity may be operated at a high DC as well as a high RF potential, since tuning is accomplished without contact whatsoever with this inner conductor.

#2. Field of the Invention

#FIG. 1 is a sectional view of a half-wave cavity using mechanical supports to adjust the transmission line and having sliding contacts between the movable sections of that line.

#FIG. 2 is another prior art device which is a quarter-wave length cavity also using sliding plate tuning contacts for tuning the cavity to a desired frequency. Both the devices in FIGS. 1 and 2 have plate blocking capacitors to insulate the high voltage of the RF amplifier tube from the walls of the transmission line.

#FIG. 3 is a sectional view of a folded half-wave cavity in accordance with the present invention which employs means for tuning the cavity without the use of sliding contacts. This cavity eliminates the necessity for a plate blocking capacitor since the section of the transmission line which is in contact with the anode of the tube has no sliding contacts and, in fact, is physically spaced from the external walls of the cavity.

#The present invention relates to a resonant transmission line or half-wave cavity for a power transmitter. The cavity of the present invention is simplified over the prior art, allows cost reductions and has increased efficiency for the power amplifier stage of a transmitter. All this is accomplished by an arrangement wherein the outer conductor is folded into itself such that the inwardly folded section, for a portion of the length of the cavity, is positioned axially centrally of the inner conductor. The inner conductor is mechanically and electrically coupled directly to the anode of the RF amplifier tube, and hence, is maintained at the same DC potential as the anode. This is possible since there are no contacts between this inner conductor and the external portions of the cavity.

#The folded portion of the cavity extends axially inside the inner conductor and is in the form of a hollow sleeve which is affixed to a honeycomb structure vent at the uppermost edge of the outer conductor. The end of the hollow sleeve has a berrilium copper bellows which is expandable by a drive mechanism which extends axially within the sleeve. Berrilium copper has a high conductivity and a long-term flex rate and therefore is particularly suitable for such a bellows. By adjusting the bellows, the tune frequency may be adjusted within a 2 MHz bandwidth. Coarse frequency adjustment is accomplished by presetting the position at which the hollow sleeve mounts to the honeycomb structure at the top of the cavity. The sleeve may be coarse-adjusted by loosening a hose clamp and then resetting the sleeve to a desired position for the frequency of operation. This, together with the tuning available by the bellows, covers the entire FM broadcast band.

#Prior to the present invention, many resonant transmission lines or cavities have been of the standard half-wave variety or of the quarter-wave length variety. In either case, the cavities have required sliding mechanical contacts at high RF potentials or currents in order to accomplish tuning. In addition, to assure that the sliding contacts were not at a high DC potential, the RF amplifier tube usually had a plate-blocking capacitor located between the anode of the tube and the transmission line which was affixed thereto. These designs (now being used) have comparatively less efficiency and considerably more complexity and more opportunity for failure than the device of the present invention. For example, in the prior art, the cavity was usually silver-plated to assure good contact with the sliding contacts. This is not needed in the device of the present invention.