An ultra-wideband (UWB) 16-way ring cavity power divider uses a symmetrically coaxial taper. To extend its operating bandwidth, stepped-impedance matching is employed. Measured return loss is better than 10.8 dB from 5.8 to 18.4 GHz. Average insertion loss is about 12.5 dB, including the 12 dB divider loss. Amplitude and phase imbalances are approximately ±0.7 dB and ±7 degrees, respectively, across the entire operating frequency range.
To address the rapidly developing demands of RF/microwave industrial and military applications, UWB radiating systems are attracting industry attention. In all radar and communications systems, the output power of the transmitter is a major determinant of operating range. To achieve higher output power over wide bandwidths, various multi-way power combiners/dividers have been described.1-20 These include substrate integrated waveguide power dividers,1 rectangular waveguide power dividers,3-5 radial waveguide power dividers,6-7 conical power dividers8-9 and coaxial waveguide power dividers.10-12 With an increasing number of ports, the radius of a radial waveguide6-7,13 or a conical line8 increases. This introduces higher-order modes that cannot be effectively suppressed. Bandwidth and insertion loss are also important considerations as the structure becomes larger and more complex.
In this article, a UWB 16-way ring cavity power divider, also suitable as a power combiner, is described. It consists of a coaxial taper transition, an oversized coaxial waveguide with a stepped inner conductor and a ring cavity with 16 coaxial probes. A coaxial taper feed port provides uniform excitation12 for excellent amplitude and phase balance. Measurements agree closely with simulation. Such a structure can be extended to large numbers of power-dividing ports.
POWER DIVIDER DESIGN
Figure 1 shows the hybrid, coaxial/ring cavity, 16-way power combiner/divider terminated by SMA connectors (coaxial ports). The input signal is fed to the left input connector and then divided into 16 equal output signals, where each output connector is fed in parallel from an oversized coaxial waveguide. A coaxial taper feed port provides axially symmetric electromagnetic field excitation for the ring cavity power divider and maintains good output port amplitude and phase balance. It also provides good impedance matching to the input port to the ring cavity. A stepped inner conductor further improves wideband impedance matching. In general, to provide proper impedance matching, the length of each coaxial probe should be about λg/4, where λg is the waveguide wavelength at the center frequency; however, the length of the capacitively-loaded coaxial probe is less than λg/4.12
As shown in Figure 1, the length of L0 is maintained at about λg/4 to make the outer side of the ring cavity the short wall. When the number of ports increases, the radii R and R0 increase, but the width of the ring cavity (R–R0) is kept constant. That is, when the number of probes increases, only the perimeter of the ring cavity increases; the section of the ring cavity is kept constant, preventing higher-order modes from propagating. This power-dividing structure is, therefore, suitable for large numbers of ports for high-power, active power-combining systems.
The approximate equivalent circuit of the 16-way ring cavity power divider is shown in Figure 2. The stepped discontinuity from the output port of the coaxial taper to the ring cavity is modeled as a capacitive reactance -jXs. ‐jXs can be altered easily by changing L2, L5, R1 and Rs; the adjustability of ‐jXs facilitates impedance matching. The characteristics of the proposed structure can be analyzed with microwave network theory. According to Hu et al.,12 for an N-port lossless reciprocal network
The ring cavity, 16-way power combiner/divider structure is axially symmetric. Port 1 is the input port and the remaining n ports are the output ports. Assuming that the input port is impedance matched, the S-parameter matrix is given as
If this structure is lossless, this is the unitary matrix. So that
For this ring cavity, 8-way power combiner/divider, n is equal to 16, so the average value of isolation and return loss of the output ports is
SIMULATION AND MEASUREMENT
Using this analysis, the 16-way ring cavity power divider was designed, simulated and optimized using the electromagnetic simulation tool Ansoft-HFSS. The optimized dimensions are listed in Table 1, and the fabricated power divider is shown in Figure 3. The power divider was measured using a Keysight network analyzer, and the measured S-parameters are compared with the simulated in Figure 4. The average insertion loss is around 12.5 dB, including the ideal 12.04 dB power-dividing loss. Figure 5 shows the measured transmission characteristics. A maximum amplitude imbalance of ±0.7 dB and a phase imbalance of ±7 degrees are achieved over the entire band. These imbalances are attributed to fabrication errors.
A UWB 16-way ring cavity power divider contains a large number of power-dividing ports while exhibiting UWB performance. Measurements demonstrate good amplitude and phase balance, low loss and agree with the simulation. Based on this performance, the design has the potential to be a useful building block in power-combining amplifier networks.
The work for this article was supported by National Natural Science Foundation of China (Grant No: 61271026).
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Yu Zhu received his B.S. degree in electrical information engineering from Southeast University Chengxian College in 2014 and is working toward his M.S. degree in electrical engineering at the University of Electronic Science and Technology of China (UESTC). His research interests include mmWave techniques with a focus on RF/microwave passive devices.
Kaijun Song received his M.S. degree in radio physics and a Ph.D. in electromagnetic field and microwave technology from UESTC in 2005 and 2007, respectively. Since 2007, he has been with the EHF Key Laboratory of Science in the School of Electronic Engineering at UESTC, where he is a full professor. His research fields include microwave and mmWave/THz power-combining technology, UWB circuits and technologies, microwave/mmWave devices, circuits and systems and microwave remote sensing technologies.
Shunyong Hu received his B.Sc. degree in applied physics from Shandong University of Science and Technology in 2010 and is working toward his Ph.D. in electromagnetic fields and microwave technology at UESTC. His research interests include microwave and mmWave power-combining technology and microwave passive component design.
Yong Fan received his B.E. degree from Nanjing University of Science and Technology in 1985 and his M.S. degree from UESTC in 1992. He is a senior member of the Chinese Institute of Electronics. His research interests include mmWave communication, electromagnetic theory, mmWave technology and mmWave systems.