Beyond transmission lines and filters, a range of passive components are essential in microwave systems: couplers, dividers, circulators, and attenuators.

Directional Couplers

A directional coupler samples a fraction of the forward or reverse traveling wave on a transmission line without significantly disturbing it. Key parameters: coupling factor (C, dB), directivity (D, dB), and insertion loss. Used for power monitoring, signal sampling, and reflectometer circuits in VNAs.

Power Dividers / Combiners

A Wilkinson power divider splits input power equally into two output ports (−3 dB each) while maintaining isolation between the two outputs. The isolation resistor absorbs any power imbalance between ports. Used in antenna arrays, balanced amplifiers, and mixer feeds.

Circulator

A three-port non-reciprocal device: power entering port 1 exits port 2; power entering port 2 exits port 3; power entering port 3 exits port 1. Uses magnetised ferrite. Critical applications: isolating a PA from antenna reflections, separating transmit and receive paths (T/R switch replacement), and protecting LNAs from high-power pulses in radar.

Attenuators

Fixed attenuators (pads) reduce signal power by a known amount while maintaining Z₀ impedance. Pi and T resistive networks are common topologies. Used for impedance bridging, reducing reflections, and setting test signal levels. Accuracy and flatness with frequency are key specifications.

PIN Diode Switches

PIN diodes operate as variable resistors at RF: forward biased → low resistance (ON); reverse biased → high resistance (OFF). Used for fast antenna switching, T/R switching in radar, and variable attenuators. Switching speed: nanoseconds to microseconds depending on carrier lifetime.

Self-Resonant Frequency (SRF)

No real component is purely one thing. Every capacitor has a parasitic series inductance (its ESL, from the body and mounting), and every inductor has a parasitic parallel capacitance — so each behaves as a series or parallel LC that resonates at its self-resonant frequency, \(f_{SRF}=1/(2\pi\sqrt{LC})\). Below its SRF a capacitor is capacitive; at the SRF its reactances cancel and it looks purely resistive (just the ESR, the impedance minimum); above the SRF it turns inductive and stops decoupling. The practical rule is to operate components well below their SRF, and to parallel capacitors of different values so their SRFs stagger and keep the impedance low across a wide band. Smaller packages push the SRF higher, which is why RF bypass capacitors use tiny 0201/0402 parts.