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Eight-Port Magic-T and Its Applications

Eight-Port Magic-T and Its Applications

 I. Several Typical Eight-Port Magic-T Combinations and Their Scattering Matrices

1. Incoherent Combination of Two Magic-Ts or Two Bridges: Formed Eight-Port Device and Its [S] Parameters

Figure 1 shows the photograph of an incoherent combination of two Magic-Ts. Through ingenious and special structural modifications, they are fabricated into a compact eight-port device. Specifically, the heights of the two balanced-arm waveguides of each Magic-T are narrowed; the H-arms and E-arms are bent 90° in different directions and integrated into a single structure. This design aligns the H-arms of the two Magic-Ts in the same direction with their broad sides connected, while the E-arms are oriented opposite to each other with their narrow sides connected. The outputs of the four balanced arms are arranged on both sides. Its [S] parameters are given in Equation (1), and Equation (2) presents the [S] parameters of the eight-port device formed by the incoherent combination of two bridges.
2. Eight-Port Device Formed by Pairwise Combination of Isolation Ports and Through Ports of Bridges, and Its [S] Parameters
3. Eight-Port Device Formed by Connecting the Isolation Ports of Bridges, and Its [S] Parameters
4.Eight-Port Device Formed by Connecting the Symmetric Ports of Four Magic-Ts, and Its [S] Parameters
5. Eight-Port Device Formed by Pairwise Connecting the Symmetric Ports and HE Ports of Magic-Ts, and Its [S] Parameters
6. An Eight-Port Device Composed of Cross-Connected Symmetric Ports and HE Ports of a Magic-T and Its [S]-Parameters
7. An Eight-Port Device Composed of Pairwise Connected EH Ports of Four Magic-Ts and Its [S]-Parameters
 
8. An Eight-Port Device Composed of Pairwise Connected Symmetric Ports and HE Ports of a Magic-T and Its [S]-Parameters
 
9. An Eight-Port Device Composed of Pairwise Connected Symmetric Ports of a Magic-T and Through Ports of a Bridge and Its [S]-Parameters
 
10. An Eight-Port Device Composed of Pairwise Connected HE Ports of a Magic-T and Through Ports of a Bridge and Its [S]-Parameters
11. An Eight-Port Device Composed of Pairwise Connected Isolated Ports of a Bridge and Symmetric Ports of a Magic-T and Its [S]-Parameters
 
12. An Eight-Port Device Composed of Pairwise Connected Isolated Ports of a Bridge and EH Ports of a Magic-T and Its [S]-Parameters 
 
It can be seen that the combination results of Groups 11 and 12 are consistent, and they can have equivalent functions.
II. Applications of the Eight-Port Magic-T

1. In high-performance pulsed Doppler radars, to avoid the noise added to the transmitted signal by the TR tube, an all-solid-state absorptive receiver protector is adopted, thereby improving the overall performance of the radar. The system composition and working principle are shown in Figures 13 and 14.

This receiver protector consists of the eight-port Magic-T shown in Figure 12, a reflected-wave quadrature directional coupler, coaxial-to-waveguide converters, waveguide loads, a pair of PIN waveguide switches, and a 90-degree bent coaxial output connector (see Figure 14). It has been applied in China's new-generation meteorological radar 98DSA and can also be used in other high-performance pulsed Doppler radars. The key technical indicators achieved are as follows:

  • Operating frequency: 2700 - 3000 MHz
  • Maximum pulse power handling capability: 100 kW (pulse width 4.65 μs, duty cycle 0.002)
  • Low-power insertion loss: ≤ 0.65 dB (0 V, 10 dBm) (measured value < 0.45 dB)
  • High-power isolation: ≥ 27 dB (+5 V, 600 mA) (measured value > 32 dB)
  • VSWR requirement at low power: J1SWR ≤ 1.4, J2SWR ≤ 1.4
  • VSWR requirement at high-power isolation: J1SWR ≤ 1.35, J2SWR ≤ 1.5
  • Switching speed: ≤ 2 μs

 2. An eight-port Magic-T fabricated based on the principle illustrated in Figure 11 is shown in Figure 15.

The eight-port Magic-T operates within the frequency range of 9700–10500 MHz. Featuring a non-standard waveguide aperture and flange, it offers a compact structure, small size, and excellent performance, making it suitable for specialized applications such as monopulse comparators. 
 
 
 
3. Four-Horn Monopulse Sum-and-Difference Comparator
  The four-horn monopulse sum-and-difference comparator is formed by connecting horn antennas to ports ①-④ of the eight-port Magic-T shown in Figure 8. (These horns are typically used as feeds for reflector antennas but can also serve as direct-radiating antennas.)
  The following signals can be obtained at ports H1, E1, H2, and E2 respectively:
H1 = [(① + ②) + (③ + ④)]** is the sum signal of the signals received by the four horns.
E1 = [(① + ②) - (③ + ④)]** is the elevation difference signal of the signals received by the four horns.
H2 = [(① + ③) - (② + ④)]** is the azimuth difference signal of the signals received by the four horns.
E2 = [(① + ④) - (② + ③)]** is the 45° angular difference signal of the signals received by the four horns.
  The 45° angular difference signal is typically disregarded in practical applications. However, in specific use cases, this error signal can also be utilized.

4. High-Power Variable Attenuator and High-Power Variable Power Divider**

  By connecting fixed short circuits to ports ⑤ and ⑥, and sliding short circuits to ports ⑦ and ⑧ in Figure 3, varying the position of the sliding shorts allows for obtaining power division ratios from 1:1 to 1:1000 at ports ③ and ④. This implements a high-power variable power divider.

  Based on the same principle and structure, connecting a high-power load to port ③ or ④ enables achieving a variable attenuation of 0-30 dB between ports ①-④ or ①-③, respectively, by changing the position of the sliding short circuits. This realizes a high-power variable attenuator.

 
  Similarly, in Figure 11, connecting fixed short circuits to ports ① and ②, sliding short circuits to ports ③ and ④, and a high-power load to port E1 or E2 allows for achieving a variable attenuation of 0-30 dB between ports H1 and H2 by changing the position of the sliding shorts. This also implements a high-power variable attenuator.

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