# Design and Implementation of C-Band Large-Power Planar Butler Matrix in SRS
## 3 Results
### Measurement  
A vector network analyzer (VNA, Agilent N5230A, Agilent, Santa Clara, CA, USA) was applied to measure the S-parameter. Figure 19 shows the Butler matrix S-Parameters measurement system. For the first measurement, two ports of the Butler matrix are connected to two cables of the vector network analyzer, and the rest of the ports are connected to N-matched loads.  
Figure 18: Simulation results of Butler matrix phase difference: (**a**) phase difference of input port 1; (**b**) phase difference of input port 2; (**c**) phase difference of input port 3; (**d**) phase difference of input port 4.  
The test results are shown in Figure 20, Figure 21, Figure 22, respectively, and are consistent with the simulation results. Figure 20a shows that the reflection coefficients of ports are better than \(-18\) dB. Figure 20b presents measured transmission coefficients when the Butler matrix is fed at input port 1 and input port 2. The coupling coeffects are well equalized, around \(-7\) dB between 3.8 and 4.2 GHz.  
Figure 20: Measured results of Butler matrix S-parameters: (**a**) ports’ reflection coefficients; (**b**) input ports’ transmission coefficient.  
Figure 19: The Butler matrix measurement system.  
Figure 22 shows the measured phase differences for the four input ports. The phase differences between different output ports when the signal is fed at each input port are \(45^{\circ}\pm 6^{\circ}\), \(-135^{\circ}\pm 5^{\circ}\), \(135^{\circ}\pm 6^{\circ}\)and \(-45^{\circ}\pm 6^{\circ}\) over the frequency range. Good agreement between simulations and measurements is found, although the measurement results are slightly worse than the simulation. The machining error of the short-ended stubs impacts the phase shift and will also stress the test connector. The machining parameter error of 0.4 mm is out of the range of machining accuracy and causes a phase error of about 2\({}^{\circ}\).  
Figure 23 shows the experimental [29; 30; 31] setup of the multipactor measurement [18], which includes the data acquisition and processing system. A portion of the incident power applied to the device under test is extracted, fed into a coupling circuit, and nulled against a part of the extracted reflected power. Power detection is performed using average power meters (Agilent E14419B), and the data recording rate is 1000 times per second. A traveling wave tube amplifier (TWTA) is applied to provide large microwave power. We adopt nulling of the forward/reverse power detection method to conduct multipactor experiments in test samples. The test frequency is 4 GHz. Using a dual-directional coupler, the incident and reflected microwave powers are sampled.  
Figure 21: Port isolation of the Butler matrix (a) input ports; (b) output ports.  
Figure 22: The phase difference of the Butler matrix.  
After realizing power-level zeroing, the spectrum analyzer monitors the zeroed signal [32] in real time. The multiactor threshold can be tested by pulse modulation signal with a duty cycle of 10%. Thirty minutes should be allotted for each test power level till breakdown occurs. Nulling signal level of forward/reverse power is less than \(-60\) dBm when test samples work correctly. Nulling signal level will increase as soon as the multi-pactor takes place.  
In addition, these test samples must be cleaned with alcohol before they are mounted in a vacuum. The temperature of the test sample must be monitored during the experiment. The air pressure in the vacuum must be less than \(1\times 10^{-6}\) Torr and remain for more than 24 h until the experiment begins. Radioactive source Cesium 137 was used to produce sufficient electrons. The multiactor microwave power threshold was tested to be 9 kW.  
The bandwidth of the Butler matrix is significantly better than that of other research works listed in Table 1, reaching 10%. According to the product of frequency and distance, the measurement results of multipactor microwave power threshold based on the square coaxial line Butler matrix is also good.