- Stub (electronics)
In electronic signaling, a

**stub**is a section oftransmission line with a calculated length. It is used forimpedance matching or to obtain an exactcapacitance orinductance .Because stubs take on reactive properties as a function of their

electrical length , stubs are most common inUHF ormicrowave circuits where the line lengths are more manageable. Stubs are often used for antenna matching.Smith chart s can also be used to determine what length line to use to obtain a desired reactance.**hort circuited stubs**The general equation for the input impedance of a short circuited line is ::$Z\_\{in\}^\{SC\}\; =\; j\; cdot\; Z\_\{0\}\; cdot\; tan(eta\; cdot\; l\_\{line\})$where j is the

imaginary unit , $Z\_0$ is thecharacteristic impedance of the line, β is thephase constant of the line, and "l" is the physical length of the line.Thus, depending on whether $an(eta\; cdot\; l\_\{line\})$ is positive or negative, the stub will be inductive or capacitive, respectively.

The Length of a stub to act as a capacitor is then given by:

::$d\; =\; \{\; 1\; over\; \{\; eta\; imes\; (\; \{\; pi\; -\; tan^\{-1\}(\{1\; over\; \{w\_C\; imes\; Z\_o)\; -\; \{\; n\; imes\; pi\; )$

The Length of a stub to act as an inductor is given by:

::$d\; =\; \{\; 1\; over\; \{\; eta\; imes\; (\; \{\; tan^\{-1\}(\{w\_L\; over\; Z\_o\})\; +\; \{\; n\; imes\; pi\; \}\; \}\; )\; \}\; \}$

**Open circuited stubs**For an open circuit, the general equation for input impedance is ::$Z\_\{in\}^\{OC\}\; =\; -j\; cdot\; Z\_\{0\}\; cdot\; cot\; (eta\; cdot\; l\_\{line\})$where j is the

imaginary unit , $Z\_0$ is thecharacteristic impedance of the line, β is thephase constant of the line, and "l" is the physical length of the line.It follows that whether $cot(eta\; cdot\; l\_\{line\})$ is positive or negative, the stub will be capacitive or inductive, respectively.

The length of an open circuit stub to act as an Inductor:

::$d\; =\; \{\; 1\; over\; \{\; eta\; imes\; (\; \{\; pi\; -\; cot^\{-1\}(\{w\_L\; over\; Z\_o\; \}\; )\; +\; \{\; n\; imes\; pi\; \}\; \}\; )\; \}\; \}$

The length of an open circuit stub to act as a capacitor:

::$d\; =\; \{\; 1\; over\; \{\; eta\; imes\; (\; \{\; cot^\{-1\}(\{\; 1\; over\; \{\; w\_C\; imes\; Z\_o\; \}\; \}\; )\; +\; \{\; n\; imes\; pi\; \}\; \}\; )\; \}\; \}$

**Computer network stub**Electrical length = (Rise time) * (Speed of light) * (Cable propagation relative to light speed)

Maximum stub length = Electrical length * (1/6) [

*cite web|title=Application Bulletin AB-6 RS-485 Systems--Making Them Work|url=http://www.nve.com/Downloads/ab6.pdf 071115 nve.com*]This is relates to buses like SCSI, RS485, CAN-bus, Ethernet etc..

**References**

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