Wideband radial power combiner/divider fed by a mode transducer

A radial power combiner/divider capable of a higher order (for example, N=24) of power combining/dividing and a 15% bandwidth (31 to 36 GHz). The radial power combiner/divider generally comprises an axially-oriented mode transducer coupled to a radial base. The mode transducer transduces circular TE01 waveguide into rectangular TE10 waveguide, and the unique radial base combines/divides a plurality of peripheral rectangular waveguide ports into a single circular TE01 waveguide end of the transducer. The radial base incorporates full-height waveguides that are stepped down to reduced-height waveguides to form a stepped-impedance configuration, thereby reducing the height of the waveguides inside the base and increasing the order N of combining/dividing. The reduced-height waveguides in the base converge radially to a matching post at the bottom center of the radial base which matches the reduced height rectangular waveguides into the circular waveguide that feeds the mode transducer.

BACKGROUND

a. Field of invention

The invention relates to radial power divider/combiners and, in particular, to radial power divider/combiners that are suitable for use in solid-state power-amplifier (SSPA) devices.

b. Background of the invention

Solid State Power Amplifiers (SSPAs) are used in a variety of applications ranging from satellites, radar, and other RF applications requiring high output power. Typical SSPAs can achieve signal output levels of more than 10 watts using solid-state amplifiers such as Monolithic Microwave Integrated Circuits (MMICs), or individual tube amplifiers.

A fundamental problem with conventional SSPA technology is that individual MMICs produce less power and operate at lower efficiency compared to the individual tube devices. At Ka-band, for example, currently available MMIC chips have output power capability that is approximately an order of magnitude less compared to the Traveling Wave Tube Amplifier (TWTA). The efficiency is approximately half.

Although a single MMIC amplifier chip cannot achieve the requisite level of output power without excessive size and power consumption issues, MMIC technology is far more practical than tubes in space and other applications. MMIC technology offers a reduction in supply voltage, potential reduction in cost, improvement in linearity and reliability.

Consequently, efforts have been made to combine the outputs of several individual MMIC amplifiers to achieve the desired total transmitter output, and it has been found that a combination of a large number of MMICs is attractive for applications where these advantages outweigh the lost efficiency. Consequently, existing SSPA designs using MMIC chips typically use a radial splitting and combining architecture in which a signal is divided into a number of individual components. Each individual signal component is amplified by a respective amplifier, and the outputs of the amplifiers are combined into a single output that achieves the desired overall signal amplification.

However, to meet the output power requirements of space telecommunication systems, it is necessary to power combine a large number of individual MMICs in the SSPA, and yet this must be done in a highly efficient manner.

Existing power-combiners such as the in-phase Wilkinson combiner or the 90-degree branch-line hybrid combine a number of binary combiners in a cascaded manner, but this architecture becomes very lossy and cumbersome when the number of combined amplifiers becomes large. For example, to combine eight amplifiers using a conventional, binary microstrip branch-line hybrid at Ka-band (about 26.5 GHz), the combiner microstrip trace tends to be about six inches long and its loss tends to exceed 3 dB. A 3 dB insertion loss infers that half of the RF power output is lost, and this is unacceptable for most applications.

To overcome these loss and size problems, other approaches including the stripline radial combiner, oversized coaxial waveguide combiner, and quasi-optical combiner, have been investigated. The stripline radial combiner, using multi-section impedance transformers and isolation resistors, still suffers excessive loss at Ka-band, mainly because of the extremely thin substrate (<10 mil) required at Ka-band. The coaxial waveguide approach uses oversized coaxial cable, which introduces moding problems and, consequently, is useful only at low frequencies. The quasi-optical combiner uses hard waveguide feed horns at both the input and output to split and combine the power, and these are very large and cumbersome.

United States Patent Application 20050174194 by Wu, You-Sun et al. published Aug. 11, 2005 shows an N-Way Radial Power Divider/Combiner in which an input signal is provided to a transmission antenna that propagates into a divider. Within the divider, the input signal is divided into a plurality N of individual signals by waveguides disposed in a radial configuration around the transmitting antenna such that at least a portion of the input signal radiated by the antenna enters an input end of each waveguide. The individual signals are received by receiving antennas and provided to respective amplifiers. The amplifiers amplify the respective individual signals by a desired amplification factor. The amplified individual signals are provided to a plurality of transmitting antennas within the combiner. Inside the combiner, the amplified individual signals are combined to form an output signal that is received by a receiving antenna in the combiner. Though a ten-way divider/combiner is shown, N is said to be in the range of two to 100. The overall insertion loss of the 10-way power divider-combiner was measured using input signals from 20 to 30 GHz, and at 26.5 GHz, the loss for the combiner alone is 0.71 dB at 26.5 GHz.

It would be desirable to adapt a radial power-combiner architecture similar to the foregoing for a higher frequency bandwidth to power combine a larger number of amplifiers with better efficiency, using a smaller combining circuit that has minimum power loss. This is herein achieved by increasing the number of combining ports using reduced height waveguides in the radial base. The radial base has reduced-height waveguides with rectangular waveguide inputs leading a circular waveguide output, defining properly spaced and properly chosen waveguide steps having incremental height changes. The reflections from the walls of the reduced height waveguides are matched by a matching post coupled to a “Marie” mode transducer. The present invention provides a low-loss, compact radial power divider/combiner for use in high-frequency SSPAs that offers an unparalleled size, weight, and power combination, thereby offering a replacement for tube-based flight and ground amplifiers used in earth-orbiting defense missions and radar applications, as well as satellite secure communications systems requiring large bandwidths (secure satellite uplinks, downlinks, and cross-links), etc.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a radial power divider/combiner for dividing/combining large number of amplifier signals within a wide bandwidth using reduced height waveguides inside a radial base.

It is a more specific object to provide a low-loss, compact radial power divider/combiner for use in wideband high-frequency (15% bandwidth in the 30-36 GHz range) Solid State Power Amplifier (SSPA) applications that offers an unparalleled size, weight, and power combination.

It is another object to provide a radial power divider/combiner that facilitates replacement of tube-based flight and ground amplifiers with solid state MMIC-based amplifiers for use in earth-orbiting defense missions and radar applications, as well as satellite secure communications systems requiring large bandwidths (secure satellite uplinks, downlinks, and cross-links), etc.

According to the present invention, the above-described and other objects are accomplished by providing a novel radial power combiner/divider with a higher order of power combining/dividing within a wide high-frequency bandwidth. The radial power combiner/divider generally comprises an axially-oriented mode transducer coupled to a radial base. The unique mode transducer transduces circular TE01 waveguide into rectangular TE10 waveguide, and the unique radial base combines/divides a plurality of ports into/from the single circular TE01 waveguide end of the transducer. The radial base incorporates full-height waveguides at the plurality of ports that are stepped down to reduced-height waveguides using stepped impedance transformers. This presents a stepped-impedance configuration that allows for reduced height waveguides inside the radial base (the height of the waveguides otherwise limiting the order N of combining), and hence a higher order combiner/divider. The reduced-height waveguides in the base converge radially to a matching post at the bottom center of the radial base which matches the reduced height rectangular waveguides into the circular waveguide that feeds the mode transducer. The matching post allows for a better output match at the circular waveguide of the radial base, which in turn with the mode transducer allows for a good output match of the divider/combiner as a whole.

The combiner/divider is herein illustrated in detail in the context of an N=24 combiner for use in Ka-band over the band of 31-36 GHz, with an input match <−20 dB under equal excitation of all input ports, an output match <−24 dB coming out of the mode transducer, and an insertion loss <0.6 dB. Of course, those skilled in the art will understand that certain exemplary specifications described herein in regard to the preferred embodiment are not limiting, and that the invention may be modified for other frequency ranges (other than 30-36 GHz), to power combine a different number of amplifiers (other than N=24), and that standard waveguide notations such as WR sizes and the like are for illustrative purposes only with regard to the illustrated embodiment.

While for the purposes of this description the innovation has been described as a power combiner, it also functions as a power divider.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is a radial power divider and/or combiner for dividing/combining a increased number N of amplifier signals within a wide bandwidth using compact radial format. The radial power combiner/divider generally comprises an axially-oriented mode transducer coupled to a radial base. The mode transducer transduces circular TE01 waveguide into rectangular TE10 waveguide, and the radial base combines/divides a plurality of ports into/from the single circular TE01 waveguide end of the transducer. The radial base is formed with a plurality of internal waveguides leading from peripheral output ports and converging radially to the center, the internal waveguides incorporating a stepped impedance configuration that allows a reduction in their size and increase in the order N of combining. The base also includes a matching post at the bottom center which matches the reduced height rectangular waveguides into the circular waveguide that feeds the mode transducer.

The invention may be implemented as a power combiner or power divider, or may be combined in a power divider/combiner.

The combiner/divider is herein illustrated in detail in the context of an N=24 combiner for use in Ka-band over the band of 31-36 GHz, with an input match <−20 dB under equal excitation of all input ports, an output match <−24 dB coming out of the mode transducer, and an insertion loss <0.6 dB.

FIG. 1is a perspective view of an N-way power divider/combiner2according to a preferred embodiment of the present invention which, in this particular example, is tuned for N=24 ports and a 15% 31 to 36 GHz bandwidth.

The power divider/combiner2generally comprises a radial base20with a plurality N of internal waveguides (here N=24) running axially and internal to the base20from peripheral ports22(spaced evenly around the base20) and converging to a matching post (obscured) in the center of base20. For testing purposes, a plurality of matching loads30are shown mounted axially around the base20to balance the ports22not in use, and each load30is coupled to a non-use port22by machine-screw attachment to the periphery of the base20. The base20has a topside center output port (obscured) for mounting a mode transducer10. The mode transducer10is a three-section transducer with distal ports16,18that convert the TE01 circular waveguide mode at the center output port of base20back into standard rectangular TE01 waveguide mode at transducer port16.

FIG. 2is a composite drawing illustrating the radial base20(A), sectioned along its width at (B) and (C), with exploded illustrations at (D) & (E) showing the sectioned internal waveguides50. The radial base20(A) is preferably formed in the two sections as shown at (B) & (C) which are secured together by machine screws. The two sections of radial base20may be formed from Aluminum, Invar, Copper or other suitable waveguide material. The waveguides50are formed partially in the first section (B) of the base20and partially in the second section (C) and join when the sections (B) & (C) are joined to form full waveguides leading axially outward to ports22. The illustrated ports22are formed as standard size WR28 rectangular TE01 waveguide ports, though other port sizes may suffice.

As best seen at exploded illustrations (D) & (E) the sectioned internal waveguides50of the first section (D) are evenly spaced and radially converge toward a central cylindrical cavity52that is formed with a central cylindrical matching post54at the center. The matching post54protrudes upward to a plateau even with the inner surface of the first section. The sectioned internal waveguides50of the second section (E) likewise converge to the topside center output port which is formed as a central cylindrical aperture55that conforms to the cavity52. In accordance with the present invention, each axial waveguide50(along both sections) is formed with a rectangular cross-section that extends uniformly from ports22to one or more constricted steps56(three successive steps56A-C being here illustrated), the steps56effectively forming a rectangular stepped-impedance configuration with incremental height changes.

FIG. 3is a composite diagram showing identical cross-sections (from above) of an axial waveguide50with an exemplary set of dimensions indicated thereon suited for attaining the performance specifications of the illustrated embodiment (bandwidth 30-36 GHz, N=24 amplifiers). All dimensions are shown in mils ( 1/1000 inches). The waveguide50begins at 140 mils width to the first step56A which is constricted by a difference of 22 mils, then continues 113 mils along at 118 mils wide to the second step56B which is constricted by a difference of 34 mils, then continues 111.5 mils along at 70 mils wide to the third step56C which is constricted by a difference of 14 mils. Each step56A-C is rounded with a 10 mil radius.

In general operation when used as a combiner, rectangular TE01 waveguide signals input to ports22form reflections along the walls of the stepped-height waveguides50which must be combined properly into a TE01 circular waveguide mode, and this purpose is served by the matching post54, which provides a circular waveguide output through the topside center output port (aperture55) into the mode transducer10described below. Thus, the radial base20has standard rectangular TE10 mode waveguide input and a circular waveguide TE01 mode output at aperture55.

FIG. 4is a perspective view of the “Marie” mode transducer10ofFIG. 1with circular waveguide (CWG) port18including a distally attached coupling flange at one end of the transducer body11and rectangular waveguide port16(either WR28 or WR24) at the other end also including a coupling flange. The illustrated circular waveguide port18is a standard circular (CWG) port or the like, for example, input size WR28 (circular waveguides are not called out in standards like rectangular waveguides and so the designation “circular waveguide (CWG)” is herein used. In the preferred embodiment a circular waveguide was chosen to support the desired circular TE01 mode over the band of interest, and the size is sufficient to combine the 24 inputs/outputs.

A cross-section of the mode transducer body11is show atFIG. 5with exemplary dimensions (in inches). The flange of port18is secured to transducer body11as shown and is attached directly to the base20(via machine screws) for coupling the transducer body11thereto to aperture55in communication with the cavity52(and matching post54) of base20.

FIG. 6is a front view of port18with flange, andFIG. 7is a front view of port16with flange. As stated above, in the preferred (illustrated) embodiment port18may be a standard circular CWG input size WR28 waveguide port, though other standard port sizes are possible. Port16may be either of a WR28 or WR34 rectangular output, though again other standard port sizes are possible.

The transducer body11of the mode transducer10is designed to convert the radial base20circular TE01 waveguide output at aperture55back to rectangular TE10 waveguide mode. Generally, the transducer body11of the mode transducer10was designed based on the concept of S. S. Saad, J. B. Davies, and O. J. Davies, “Analysis and Design of a Circular TE01 Mode Transducer,” Microwave, Optics and Acoustics, vol. 1, pp. 58-62, Jan. 1977. Saad et al. therein disclose the concept of a “Marie Mode” transducer for transducing multiple rectangular TE10 modes to circular TE01 mode. Multiple TE01 modes are transitioned into an intermediate mode, which is transitioned into a circular TE01 mode and vice versa. The present transducer employs different symmetry considerations and dimensions.

As seen inFIG. 5, the transducer body11includes three distinct sections beginning at the TE01 end (left) with a tapered cylindrical waveguide section110running approximately one-third the length of transducer body10and tapering inward to transition the multiple TE01 modes from base20into an intermediate cylindrical mode. Next, an outwardly-tapered rectangular waveguide section112running approximately one-third the length of transducer body10and tapering outward to transition the intermediate cylindrical mode to an intermediate rectangular mode. Finally, a pyramidal section114running the last third the length of transducer body10to transition the intermediate rectangular mode to a rectangular TE01 mode.

The exact profile, contour and length of each section110-114must be precisely tuned in order to make it possible to combine 24 inputs, operating from 31 to 36 GHz. Consequently,FIGS. 8-10are each composite drawings illustrating the particular profile, contour and length of each of section110-114, respectively.

Beginning at the TE01 end,FIG. 8shows the tapered cylindrical waveguide section110, including a perspective view (A), and a side view (B) with dimensions (in inches), left end view (C) and right end view (D). The cylindrical waveguide section110begins at the left with a full cylindrical cross-section of constant radius, as seen at (C), running 0.4724 inches, then beginning a gradual taper to a cross-shaped section at right and as seen at (D). The dimensions (inches) and angular disposition of the cross-shaped section are indicated inFIG. 8(D). The cylindrical waveguide section110tapers inward to transition the multiple TE01 modes from base20into an intermediate cylindrical mode.

The cylindrical waveguide section110merges into an outwardly-tapered rectangular waveguide section112shown inFIG. 9, which likewise runs approximately one-third the length of transducer body10and tapers from the cross-shaped section ofFIG. 8(D)to a flat waveguide section.

FIG. 9shows the outwardly-tapered rectangular waveguide section112, including a perspective view (A), a side view (B) with dimensions (inches), top view (C), and two different cross-sections including section (D) taken along line AA ofFIG. 9(C), and section (E) taken along line BB ofFIG. 9(C). The outwardly-tapered rectangular waveguide section112begins at the left with the cross-shaped section conforming to that ofFIG. 8(D), the arms of the cross tapering away and graduating to the flat waveguide section at right, thereby converting the intermediate cylindrical mode to an intermediate rectangular mode.

Finally,FIG. 10illustrates the pyramidal section114that runs the last third of transducer body10to transition the intermediate rectangular mode to a fully rectangular TE01 mode.FIG. 10shows the pyramidal section114from various perspectives, including a perspective view (A), a right-end view (B) with both linear and angular dimensions (inches), side view (C), and top view (D). The pyramidal section114begins at left conforming to the flat horizontal rectangular waveguide section112at the right ofFIG. 9(A), and graduating to a flat orthogonal waveguide section at the right ofFIG. 10(A), thereby converting the intermediate rectangular mode to a fully rectangular TE01 mode at output port16ofFIGS. 5 and 7.

The three above-described sections110,112, and114are preferably integrally formed in a unitary transducer body11, which is then attached to ports16,18.

It is noteworthy that the above-described transducer10can easily be designed to provide two different rectangular waveguide outputs by modification of only section114, leading to an alternate design for multiple frequency ranges with a common circular waveguide input.

For operation of the power divider/combiner2as a divider. In this case a signal generator will provide an input signal to the divider2at the input flange16of mode transducer20via a coaxial cable attached to the flange16via a connector, which may be an SMA connector, for example. Once inside the mode transducer20, the signal propagates down through the transducer body11through the a pyramidal section114which transitions from rectangular TE01 mode to intermediate rectangular mode, then through tapered rectangular waveguide section112which transitions the intermediate rectangular mode to an intermediate cylindrical mode, and finally through the tapered cylindrical waveguide section110which transitions the intermediate rectangular mode to a single cylindrical TE10 modes which is propagated into base20. The matching post54provides a circular waveguide output from the transducer10into a rectangular mode within each axial waveguide50in the base20. The waveguides50maintain a rectangular cross-section to the constricted steps56which impart a rectangular stepped-impedance configuration as a result of their incremental height changes. Thus, inside the base20the signals are effectively divided to form N output signals (here N=24). One or more of these output signals may then be provided to a signal receiver coupled to ports22. The signal receiver may be a test device, such as a spectrum analyzer, or a multiple-amplifier module.

By reversing signal direction, the divider/combiner may function as a combiner. In this case, a plurality N of TE01 waveguide signals are input at ports22(via coaxial cables or the like) and propagate in through the waveguides50, which maintain a rectangular cross-section to the constricted steps56. The steps56impart a rectangular stepped-impedance configuration as a result of their incremental height changes. The N signals are combined and transitioned by matching post54from rectangular TE01 mode to circular TE10 mode, and the combined signal is output through port18into the mode transducer body11. Inside the transducer body the signal propagates through the tapered cylindrical waveguide section110, then through tapered rectangular waveguide section112, and finally through the rectangular waveguide section114which transitions the intermediate rectangular mode to a single rectangular TE01 mode which is output through port16. A prototype N=24 combiner has been constructed for Ka-band and demonstrated over the band of 31-36 GHz with an input match <−20 dB under equal excitation of all ports22, output match <−24 dB at the port in flange16of the mode transducer10, and an insertion loss <0.6 dB. The functional bandwidth of the combiner/divider2exceeds the initial design goal of 31-36 GHz.

Generally, for normal operation all ports22will be used. However, for testing purposes only one or two ports22will be used, and a plurality of matching loads30are shown mounted axially around the base20(FIG. 1) to balance the ports22not in use. The divider/combiner2may be tested using a conventional vector network analyzer (VNA) consisting of a sweep oscillator, a test set which includes two ports, a control panel, an information display, and coaxial cables to attach to the divider/combiner2.

FIGS. 11 and 12illustrate the requisite test connections for “Output Match” and “InsertionLoss” measurements, andFIGS. 13 and 14, respectively show the port matching results on the rectangular waveguide ports01-12.

InFIG. 11, measurements were competed with the VNA port1fixed on port25of the divider/combiner20, VNA port2on ports01through12, the transducer at S/N2, and Base S/N2.

FIG. 13illustrates the port match over the intended bandwidth 31-36 GHz, which shows the match <−24 dB over the bandwidth.

InFIG. 12, measurements were completed with the VNA port1fixed on port25of the divider/combiner20, VNA port2on ports01through14(keeping orientation same as above), the Transducer at S/N2, and Base at S/N2.

FIG. 14illustrates the port match over the intended bandwidth 31-36 GHz, which shows similarly good behavior. The mode transducer insertion loss is calculated by measuring the two transducers SN1and SN2back-to-back, and dividing the loss by half. The agreement with theory from the design is excellent. For the Mode transducer with the WR34 output port there is a good match <−27 dB, and low measured insertion loss. The input match of each of the other input ports2through ports24were likewise measured, and the measurements indicate the level of repeatability and error to be expected. This combiner has an input match <−20 dB under equal excitation of all input ports, an output match <−24 dB at the RWG port of the Marie Transducer, and an insertion loss <0.6 dB. The functional bandwidth of the combiner exceeds the initial design band of 31-36 GHz. This excellent performance demonstrates the potential for this power combiner2to enable a new class of high-power, high-efficiency solid-state amplifiers. It should now be apparent that the above-described radial power divider/combiner is capable of replacing tube-based flight and ground amplifiers with solid state MMIC-based amplifiers for use in earth-orbiting defense missions and radar applications, as well as satellite secure communications systems requiring large bandwidths (secure satellite uplinks, downlinks, and cross-links), etc.