1. Field of the Invention
The present invention relates to a method for producing radio frequency transmission line beamforming networks and antennas having improved electrical properties and increased strength-to-weight or mass ratios, for use in beamforming networks for satellite antennas.
2. State of the Art
Satellite communications multibeam antennas or phased arrays have been developed to provide precisely tailored beams to cover multiple designated coverage areas on the earth without wasting antenna beam coverage or radiated power on regions where there are no users of interest.
Space-borne antennas were individually designed and assembled for a particular satellite, usually launched for a specific purpose. Each element of the many elements of the antenna had to be individually fabricated and assembled. Thus, the antenna was very expensive to fabricate and assemble.
Commonly-assigned U.S. Pat. No. 5,539,415 discloses an inexpensive, small, compact, light weight, easy to assemble, multibeam or phased array device useful as a direct radiating array or as a feed for a reflector or lens antenna. The device employs an array of planar radiators coupled to radio frequency (R.F.) transmission lines to form individual feed or antenna strips. The feed or antenna strips are coupled into a filter to pass the desired band of frequencies and reject undesirable bands of frequencies. The filters are coupled to Monolithic Microwave Integrated Circuit (MMIC) amplifiers that contain N amplifiers with an integral isolator. Amplifiers are Solid State Power Amplifiers (SSPA""s) or Low Noise Amplifiers (LNA""s). SSPA""s are used for the transmit mode and LNA""s are used for the receive mode. Amplifiers are utilized to amplify the aforementioned RF signals.
Previously-known RF strip transmission lines for use in antenna(s), antenna feed(s) and beamforming network(s) of the types disclosed by U.S. Pat. No. 5,539,415 have a wide variety of disadvantages.
A microwave transmission line configuration known as bonded strip transmission line is formed from two solid dielectric substrates with metal foil (usually copper) bonded or plated to one or both flat sides of each substrate. The substrates are bonded together under pressure and at elevated temperature to form a sealed package. The foil is configured (usually by photo etching) to have two metal ground planes with one or more metal strip conductors in the middle to form a strip transmission line. The middle conductor can be formed (usually by photo etching) to produce various microwave circuits. Channelization may be used to prevent unwanted parallel plate modes.
Other previously-known microwave transmission lines are referred to as bar line and suspended air strip transmission lines. They are composed of one or more metal bars or thin dielectric supported strips located midway between two metal ground planes forming a microwave transmission line. The middle bar in bar lines is suspended between the ground planes using low dielectric constant honeycomb or foam. The package for bar line and for suspended air strip transmission lines is held together by mechanically clamping, usually with bolts or screw fasteners.
The previously-known structures for microwave transmission lines of the aforementioned types have important disadvantages with respect to their electrical and/or their physical properties such as strength and weight. The barline and suspended air strip transmission lines are composed of unbonded layers, mechanically fastened together, requiring thicker structural panels and numerous mechanical fasteners to provide a good degree of mechanical performance under sheer and tension forces. The resulting barline and suspended air strip transmission line configurations are heavy and have a high weight to strength ratio. Also, they are composed of individual components: machined ground planes, conducting bars for the barline and photo etched strips for the suspended air strip transmission line, and foam or honeycomb cores mechanically held together with fasteners requiring substantial assembly time and labor intensive methods used to fabricate and assemble the barline and suspended air strip transmission line packages.
Also, the prior known bonded strip transmission line beamforming networks suffer high RF energy insertion loss.
The present invention provides a novel method for producing improved RF transmission lines for satellite antennas and beamforming networks, comprising the steps of (1)bonding together (a) a central conductor strip or trace strip comprising a dielectric layer or circuit board having on one or both surfaces thereof conductive strip circuitry; (b) upper and lower core layers of lightweight closed-cell plastic foam bonded to (a), and (c) upper and lower surface layers or faceskins of conductive metal foil or of dielectric material bonded to metal foil layers inside and/or outside, or solid metal plates, to form ground planes, bonded to the surfaces of the adjacent foam core layers, (2) boring holes or vias through the ground plane or through the central conductor strips, core layers and ground plane, layers and the core layers, and (3) plating the bores or vias with an electroconductive metal, usually copper, to provide a plurality of electroconductive connections between the ground planes, for parallel plate propagating mode suppression, or between central conductor strips to connect independent assemblies.