10. Microwave Passive Components

Most of microwave engineering is passive: routing, dividing, combining, and matching power without amplification. The components below are the building blocks.

10.1 Terminations and matched loads

At the end of any waveguide or transmission line, you need somewhere for the power to go. A matched load absorbs incident power without reflection. In waveguide, a matched load is typically a tapered wedge of microwave-absorbing material (carbon-loaded foam or ferrite) placed at the closed end. In coax, a precision 50 Ω resistor with a coax body. VSWR less than 1.10 for a good lab load means less than 0.2% of power reflected.

10.2 Attenuators

Reduce signal power by a known amount.

• Fixed attenuators (pads): a network of resistors providing a known dB drop (3 dB, 10 dB, 20 dB), wideband, low VSWR.
• Variable attenuators: adjustable via a thumbwheel (mechanical) or a control voltage (electronic).
• Rotary-vane waveguide attenuators: a thin resistive vane rotates inside the waveguide; the angle to the field controls absorption. Highly accurate and stable.
• Step attenuators: a series of fixed pads switched in via PIN diodes or relays. Common in modern test equipment.

10.3 Phase shifters

Shift the signal's phase without changing amplitude.

• Mechanical waveguide phase shifters: a dielectric slab that slides into the guide changes $\beta$ along its length.
• Ferrite phase shifters: a magnetized ferrite in the waveguide shifts phase via Faraday rotation; non-reciprocal in some geometries.
• PIN-diode phase shifters: digital switching among different line lengths or loading reactances. The standard for phased-array antennas because they are fast (microseconds), small, and low-loss.

10.4 Directional couplers

A four-port device that samples a fraction of the forward-traveling wave on a main line into a coupled port, with most of the power continuing past.

                main line (ports 1–2)
   port 1 ──────────────────────── port 2
                  ║║║║║                ↑
                  ║║║║║              forward wave
                  coupling ↓↑↓       passes through
                  region ↓↑↓
   port 4 ──────────────────────── port 3
                coupled line (ports 3–4)

• Coupling factor (e.g., −20 dB): fraction of forward power into the coupled port.
• Directivity (e.g., 30 dB): how well the coupler distinguishes forward from backward power. Ideally infinite; in practice, finite directivity causes a small leak from reverse waves into the "forward sample" port.
• Isolation = coupling + directivity in dB: total isolation between input port and isolated port.

Used for: power monitoring without interrupting the line, reflectometry (separating incident and reflected waves), mixer LO injection without splitter losses, lab measurements.

10.5 Magic-T (hybrid junction)

A waveguide structure with four ports arranged so that a signal entering one port splits 50/50 into two output ports while the fourth port is isolated. Reciprocally, the fourth port couples 50/50 into the same outputs while port 1 is isolated.

                       port 4 (isolated when input at 1)
                          ║
                          ║
          E-arm           ║
                          ║
   port 1 ════════════════╬══════════════ port 2
                          ║
                          ║   H-arm
                          ║
                       port 3 (sum/difference port)

Useful for balanced mixers (signal and LO into ports 2 and 3, IF out of port 4 with image rejection), monopulse radar (sum and difference patterns from two antenna feeds), and rat-race ring equivalents in microstrip.

10.6 Circulators and isolators (ferrite components)

A ferrite is an oxide ceramic with strong magnetic properties at microwave frequencies. A magnetic bias field through a ferrite gives it a remarkable property: microwave waves polarized one way travel essentially unattenuated, while waves polarized the orthogonal way (or moving in the opposite direction) experience a different propagation constant. The ferrite is non-reciprocal: it behaves differently for forward and reverse propagation.

This single property enables two essential components:

• Isolator: passes signal forward, absorbs reverse. Insertion loss ~0.3 dB; isolation ~20 dB. Used to protect oscillators and amplifiers from reflections that could cause mode hopping or oscillation.
• Circulator: a 3-port (or 4-port) device. Power entering port 1 exits port 2; power entering port 2 exits port 3; power entering port 3 exits port 1. Port 1 → 2 → 3 → 1 in a ring; reverse direction is blocked.

Why circulators matter in radar. A radar wants to use the same antenna for transmitting and receiving (a single big dish, not two). Without a circulator, you would route the powerful transmit pulse straight into the sensitive receiver and destroy the LNA. With a circulator: the transmit pulse from port 1 exits port 2 into the antenna; reflected echoes from the antenna enter port 2 and exit port 3 into the receiver. The transmitter never sees the receive signal (port 1 to port 3 is blocked).

Modern radars also use PIN-diode T/R switches for the same purpose, but circulators add the elegance of being passive with no control signal needed.