21.5 RADAR Components: Magnetron, Duplexer, ATR & Synchro

21.5 RADAR Components: Magnetron, Duplexer, ATR & Synchro

Quick Answer: Magnetron = transmitter output power. Duplexer/circulator = electronic switch letting one antenna transmit and receive. ATR box = prevents the received signal from entering the transmitter. Synchro transmitter and receiver = carries the antenna's angular position to the indicator. Echo box = checks radar performance at sea. DSP = improved weak signal or target enhancement.

Sub-topic 3-O-091 (Components) is a naming exercise, but the components make sense as a set once you see the problem they collectively solve: one antenna must transmit kilowatts and then receive microwatts, milliseconds later, without destroying the receiver.

The magnetron

The component or circuit providing the transmitter output power for a RADAR system is the magnetron.

A magnetron is a cavity oscillator — a vacuum tube in which electrons spiralling in crossed electric and magnetic fields excite resonant cavities machined into a copper anode block. It is not an amplifier; it generates the pulse itself. Marine navigational radars use magnetrons because they deliver kilowatts of peak pulse power at 9 GHz cheaply, compactly and reliably.

Two properties worth knowing:

The duplexer, the TR and the ATR

What is the purpose or function of the RADAR duplexer/circulator? It is an electronic switch that allows the use of one antenna for both transmission and reception.

The magnetron produces kilowatts. The receiver front end is destroyed by milliwatts. They share one antenna. The duplexer resolves this by switching, at microsecond speed, between two paths:

Two gas-filled or solid-state switching elements do the work, and the pool tests them by name:

• TR (transmit-receive) box — fires during transmit to short the receiver path, protecting the receiver from the outgoing pulse.
• ATR (anti-transmit-receive) box — prevents the received signal from entering the transmitter. During receive, the ATR blocks the magnetron branch so the faint echo goes to the receiver instead of being wasted down a dead transmitter path.

Learn the ATR by its keyed sentence. TR protects the receiver from the transmitter; ATR keeps the received signal out of the transmitter. The pool asks only the ATR direction, and the distractors invert it.

A circulator is the modern solid-state equivalent: a ferrite device that routes energy one way around three ports, so power entering port 1 leaves at port 2 (the antenna) and energy entering port 2 leaves at port 3 (the receiver) — never back toward the transmitter.

The synchro pair

What is the purpose of a synchro transmitter and receiver? Transmits the angular position of the antenna to the indicator unit.

A radar display is useless without bearing. The scanner rotates on the mast; the display is on the bridge. A synchro pair solves the problem electromechanically: a synchro transmitter geared to the antenna shaft produces a set of three-phase-like stator voltages encoding shaft angle, and a synchro receiver in the indicator reproduces that angle exactly.

The elegance is that no counting, no reference pulse and no calibration is needed — the receiver's rotor simply follows the transmitter's rotor. Modern radars replace synchros with optical encoders and digital bearing data, but the function is unchanged and the pool asks the classic name.

The echo box

What device can be used to determine the performance of a RADAR system at sea? An echo box.

An echo box is a resonant cavity coupled to the radar. The transmitted pulse rings it; the cavity re-radiates a decaying signal that the radar receives as an artificial target. The duration of the ringing — measured as the length of the "sun-burst" or plume on the display — indicates overall system performance.

Why it is the right tool at sea:

• It exercises the whole chain — transmitter power, duplexer, antenna, receiver sensitivity — in one measurement
• It gives a repeatable number for trend monitoring; a shortening plume means the system is degrading, usually a weakening magnetron
• It needs no external target, no shore reference and no test range

That last point is decisive. You cannot take a vessel to a calibrated range mid-ocean, but you can key the radar into an echo box on the mast in any conditions.

Digital signal processing

Digital signal processing (DSP) of RADAR signals (compared with analog) causes improved weak signal or target enhancement.

DSP integrates successive returns from the same bearing and range cell. A genuine target returns consistently scan after scan; sea clutter and receiver noise are random. Averaging therefore builds the target and suppresses the noise, pulling a small vessel or a buoy out of clutter that would bury it on an analogue display. The same processing enables scan-to-scan correlation, target trails, and the automatic tracking that underpins ARPA (section 21.6).