Transmission Line Stub
Understanding Transmission Line Stubs
Stubs are the basic building blocks for matching and filtering in microstrip and stripline circuits. They replace lumped inductors and capacitors at microwave frequencies where lumped components are impractical.
Stub Behavior
- Open stub, l < lambda/4: Capacitive reactance (equivalent to a capacitor to ground).
- Open stub, l = lambda/4: Short circuit (series resonance). Used for DC shorts and bypassing.
- Shorted stub, l < lambda/4: Inductive reactance (equivalent to an inductor to ground).
- Shorted stub, l = lambda/4: Open circuit (parallel resonance). Used for DC blocking.
Stub Applications
- Matching: Shunt stub cancels reactive part of load impedance.
- Filtering: Stub-based low-pass and bandstop filters.
- Bias insertion: Quarter-wave shorted stub provides DC path while blocking RF.
Frequently Asked Questions
What is a transmission line stub?
A stub is a short transmission line section connected to the main line, terminated in open or short circuit. It provides frequency-dependent reactance for matching, filtering, and biasing. Stubs replace lumped components at microwave frequencies.
How long should a stub be?
Length depends on the desired reactance. A lambda/4 open stub appears as a short circuit at its resonant frequency (used for bypassing). A lambda/4 shorted stub appears as an open circuit (used for bias insertion). Other lengths provide specific reactance values.
What is the difference between open and shorted stubs?
Open stubs are capacitive for l < lambda/4 and switch to inductive at lambda/4. Shorted stubs are inductive for l < lambda/4, switching at lambda/4. Shorted stubs require via holes (microstrip) but handle DC bias. Open stubs are easier to fabricate.