Schemes for combining power from coherent microwave sources of arbitrary (unequal or equal) amplitude have been proposed. Most prior microwave-power-combining schemes are limited to sources of equal amplitude.
Quasi-optical components suitable for implementing these schemes include grids of parallel wires, vane polarizers, and a variety of waveguide structures. For the sake of brevity, the remainder of this article illustrates the basic principle by focusing on one scheme in which a wire grid and two vane polarizers would be used.
Wire grids are the key quasi-optical elements in many prior equal-power combiners. In somewhat oversimplified terms, a wire grid reflects an incident beam having an electric field parallel to the wires and passes an incident beam having an electric field perpendicular to the wires. In a typical prior equal-power combining scheme, one provides for two properly phased, equal-amplitude signals having mutually perpendicular linear polarizations to impinge from two mutually perpendicular directions on a wire grid in a plane oriented at an angle of 45° with respect to both beam axes. The wires in the grid are oriented to pass one of the incident beams straight through onto the output path and to reflect the other incident beam onto the output path along with the first-mentioned beam.
In the ideal case, the output beam contains the sum of the input beam powers and has linear polarization at an angle of 45° with respect to either of the input polarizations. Although ordinarily used to combine input signals of equal amplitude, this scheme still works when the amplitudes are unequal, except that undesirably, the output polarization is not fixed at 45°: instead, the angle of the output polarization, relative to the polarization of input signal 1, is given by arctan(E1/E2), where E1 and E2 are the electric-field amplitudes of the first and second input signals, respectively. According to the scheme now proposed, one would use a wire-grid combiner as in the equal-power case described above, in combination with two vane polarizers (see figure) that, as described below, would ensure that the output beam had the desired 45° linear polarization.
A vane polarizer (also known as a venetian-blind polarizer) consists of a number of thin metal strips that are parallel to each other. When the electric field of an incident beam is perpendicular to the strips, the field does not induce any electric current in the strips and so the beam passes through the strips, unaffected, in the transverse electromagnetic (TEM) mode. When the electric field is parallel to the strips, the beam is forced into the first transverse electric (TE1) mode, which has a wavelength longer than that of the TEM mode. The thickness of the vane polarizer is chosen so that the TEM mode is delayed by a phase difference of 90° more than that of the TE1 mode. When the incident beam is polarized at 45° to the vanes, the beam is split into two equal components, one of which is delayed by 90° relative to the other. Upon recombination of the components at the output, the resultant beam is circularly polarized. Conversely, when the incident beam is circularly polarized, the output beam is linearly polarized at 45°.
In the proposed scheme, the two vane polarizers would be placed in the output path of the wire-grid combiner. The first vane polarizer would be oriented with its vanes at 45° with respect to the polarization of the combined input beam, so that the output of the first vane polarizer would be circularly polarized. The second vane polarizer would convert the circular polarization to linear and would be oriented to place the final output polarization at a desired, standard angle. Fixing the polarization at a standard angle would facilitate the assembly of multiple stages to combine power from more than two sources.
Proper phasing is essential to the success of the proposed scheme. The phasing problem is somewhat more complex than in the case of a simple equal-power combiner because propagation through and between the vane polarizers introduces additional phase shift. However, this is not a serious problem because the majority of the phase shift is a predictable function of the positions and orientations of the vane polarizers, and each power-combining stage could be designed to incorporate an adjustable phase shifter for fine-tuning. There is also an analog of this combining technique in waveguide.
This work was done by Bruce Conroy and Daniel Hoppe of Caltech for NASA’s Jet Propulsion Laboratory. NPO-44532
This Brief includes a Technical Support Package (TSP).
Microwave Power Combiners for Signals of Arbitrary Amplitude
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Overview
The document titled "Microwave Power Combiners for Signals of Arbitrary Amplitude" from NASA's Jet Propulsion Laboratory discusses innovative techniques for combining microwave power from multiple sources, particularly when the sources have unequal amplitudes. This technology is significant for applications in aerospace and telecommunications, where efficient power management is crucial.
The core concept revolves around a quasi-optical power combiner that utilizes a wire grid to reflect and transmit waves based on their electric field orientation. When two sources of equal amplitude are properly phased and positioned at right angles to each other, the output is a linearly polarized wave at a 45° angle. This principle can be extended to sources with unequal amplitudes, where the polarization angle of the resultant wave is determined by the ratio of the amplitudes of the two sources.
A key component discussed is the vane polarizer, which consists of thin metal strips that allow for the manipulation of the wave's polarization. When illuminated by a wave polarized at 45°, the vane polarizer splits the wave into two components, delaying one by 90° to produce circular polarization. Conversely, circularly polarized waves can be converted back to linear polarization at a fixed angle, facilitating further power combining stages.
The document also addresses the importance of phase alignment in the combining process. It highlights that while the polarizers introduce a predictable phase shift, adjustments can be made using phase shifters to ensure optimal performance. This adaptability is crucial for maintaining efficiency across different stages of power combining.
Additionally, the document introduces a waveguide analog of the quasi-optical combiner, featuring an orthomode junction and a quarterwave polarizer. This waveguide implementation serves similar functions as the wire grid and vane polarizer but is tailored for different operational environments.
Overall, the document outlines a comprehensive approach to microwave power combining, emphasizing the potential for combining signals of arbitrary amplitude. This technology could lead to more versatile and efficient systems in various applications, including transmitters and other microwave technologies. The insights provided in this technical support package reflect NASA's commitment to advancing aerospace-related developments with broader technological implications.
