A millimetre wave-folded magic-T junction compensated with metal cone is designed using a particle swarm optimization (PSO) algorithm. An off-centred metallic frustum was used to enhance the bandwidth and a metallic post is used to compensate the mismatched E-arm. The geometrical parameters of the frustum and the post are optimized by PSO. The optimized magic-T for W-band application is designed and tested. The design features are simple in structure and easy to fabricate. The 2% bandwidth with centre frequency of 94 GHz and return loss less than −20 dB is achieved.

In communication and radar systems, waveguide junction components, such as couplers, magic-Ts, and orthogonal mode transformers (OMTs) have been used widely. Magic-T features good isolation and power dividing performance, thus plays a key role in monopulse radar systems [

This paper presents the design and optimization of a millimetre folded waveguide magic-T junction compensated with an off-centred metallic frustum in the cavity region and a metallic post in the E-arm. A three-dimensional finite element method (FEM) is used to compute the scattering parameters of each port of the proposed magic-T, and the calculated results were then sent to PSO code to evaluate the fitness. The PSO algorithm adjusts and updates the parameters accordingly until all parameters were optimized. The optimized folded magic-T junction was simulated and tested at

The configuration of the proposed folded magic-T is shown in Figures

E-face-folded magic-T (a) geometry (b) parameters configuration.

H-face-folded magic-T (a) geometry (b) parameters configuration.

As magic-T junction itself is inherently mismatched; it requires compensation units. Here, a metallic frustum was employed to achieve complete impedance matching in the H-arm and half-impedance matching in the E-arm, while a metallic post is employed to provide complete impedance matching of the E-arm.The frustum structure can effectively enhance the bandwidth performance of all the four ports. A detailed profile of the frustum and the post is shown in Figures

As for E-plane-folded magic-T, the main waveguide is placed directly under the E-plane branch, and a 45 degree cutoff is used to match the impedance at the mitred corner, as shown in Figure

E-plane corner and H-plane taper configuration.

According to PSO algorithm, the parameters were updated using the following rule:

The optimized parameters were obtained after 87 iterations for E-plane-folded magic-T and 102 iterations for H-plane-folded magic-T. The total simulation time on an Intel Core 2 2.1 GHz computer is 12.5 hours (frequency sweep step set to 0.1 GHz). However, a gradient method of the Quasi Newton optimizer is adopted in the HFSS; it suffers from the possible presence of local minima and numerical noise. It is nothing but

Optimized value of all parameters.

Parameter | Range (mm) | Optimized (mm) |
---|---|---|

Re | [0.3, 1.3] | 0.36 |

re | [0.1, 0.3] | 0.14 |

Le | [−2, 2] | 1.73 |

He | [0.2, 1.2] | 1.04 |

re2 | [0.1, 0.4] | 0.23 |

Ye | [−2.4, −0.1] | −2.06 |

Ze | [1.3, 3.3] | 1.78 |

l | [2, 6] | 3.02 |

Rh | [0.5, 2] | 1.50 |

rh | [0.1, 0.3] | 0.28 |

Lh | [0.5, 6.5] | 2.70 |

Hh | [0.2, 1.2] | 0.70 |

rh2 | [0.1, 0.4] | 0.11 |

Yh | [1.2, 4] | 1.41 |

Zh | [1.3, 4.3] | 2.01 |

Figure

Half compensated E-FMT

Fully compensated E-FMT refection coefficient of all ports.

E-FMT isolation and transmission coefficients.

Phase characteristics of transmission coefficients.

Fully compensated H-FMT

H-FMT isolation and transmission coefficients.

Measured return loss of the sum-difference device with horn.

An inverse Cassegrain antenna at millimeter waveband with the sum-difference network comprised of the proposed folded magic-T were also fabricated and tested. Figure

Photo of sum-difference device with horn.

Antenna gain of sum signal and standard horn at 94 GHz.

Normalized Direction of sum-difference signal at 94 GHz.

A HFSS (Ansoft) and PSO (Matlab) cooperating simulation was utilized to optimize waveguide magic-T junctions. The simulation time has been reduced effectively due to the fast convergence feature of PSO. A novel