== Working with Coupled Transmission Line Devices ==
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[[File:tline3.png|thumb|300px| The schematic symbol of the Generic Coupled T-Lines device.]]
Many passive RF devices such as directional couplers, hybrids and some filter designs involve segments of parallel coupled transmission lines. According to the coupled mode theory, one can define even and odd mode impedances (Z0e and Z0o) for such transmission lines. The resulting RF structure can be modeled as a four-port network device as shown in the opposite figure. Note that the four-port device has eight pins. Ports 1 and 2 correspond to the input and output of the first transmission line segment, while Ports 3 and 4 correspond to the input and output of the second (coupled) line segment. It is very important to connect and ground the negative pins at the input and output of the two transmission line segments.
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[[File:tline3.png|thumb|left|300px| The schematic symbol of the Generic Coupled T-Lines device.]]
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[[RF.Spice A/D]] provides three coupled line devices, all of which assumes lossless transmission lines:
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[[File:tline8.png|thumb|left|320px| Coupled Microstrips Calculator.]]
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[[File:tline14.png|thumb|left|320px| Coupled Striplines Calculator.]]
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[[File:tline9.png|thumb|left|320px| Coupled Microstrips Designer.]]
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[[File:tline15.png|thumb|left|320px| Coupled Striplines Designer.]]
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