21.3 Radar Antennas & Waveguides

21.3 Radar Antennas & Waveguides

Quick Answer: Ship radar antennas are usually rotating slotted waveguide arrays (narrow horizontal beam, wider vertical beam). Higher gain → narrower beamwidth. Microwaves ride waveguides, not lossy coax, from transceiver to scanner; couple with a probe/J-hook. Choke joints seal RF at the rotating joint. SARTs on 9 GHz X-band paint ~12 dots on the rescuing ship’s PPI.

Key topic 093 is physical: how the pulse leaves the magnetron and becomes a painted target—and how survival craft use your radar as a homing interrogator.

Slotted waveguide array — the shipboard standard

A common shipboard radar antenna is the slotted array (slotted waveguide). A length of waveguide with precisely cut slots radiates a controlled beam. Mechanically it looks like the familiar rotating “bar” on the mast.

Bearing (azimuth) resolution is set mainly by horizontal beamwidth—two targets at the same range need angular separation on the order of the beamwidth to paint separately. Lengthening the array at a fixed frequency narrows the horizontal beam and raises gain.

Gain vs beamwidth

Pool fact: as the gain of an antenna is increased, the beamwidth decreases. You cannot have a skinny high-gain lobe that is also wide; energy conservation in the pattern forces the trade-off. That is why ARPA tracking and small-target detection care about antenna size and frequency band, not only peak power.

Parabolic dish gain vs frequency (pool numeric)

For a parabolic dish, doubling the operating frequency increases gain by about 6 dB (pool answer). Physically, the aperture is more wavelengths across at the higher frequency, so directivity rises. Marine navigation radars still prefer slotted arrays for 360° mechanical scan, but Element 3 uses the dish fact to test aperture theory.

Waveguide vs coaxial line at microwave

At SHF radar frequencies and kilowatt peak powers, a hollow metal waveguide is the preferred feed:

Conductance in a waveguide takes place through electromagnetic and electrostatic fields associated with the walls—not by a center conductor carrying conduction current the way a coax does. Modes (e.g., TE₁₀ dominant in rectangular guide) describe field patterns; below cutoff frequency a mode will not propagate—one reason waveguide dimensions are fixed to the radar band.

Coupling energy in and out

To couple energy into and out of a waveguide, use a thin piece of wire as an antenna (probe) or a small loop—pool distractors like “wide copper sheeting,” “LC circuit,” or pure capacitive coupling miss the microwave probe concept.

To extract the radar signal from the waveguide for sampling or mixing, a J-hook pickup is the named device on Element 3.

Choke joints and rotating joints

The scanner rotates while the transceiver may be fixed below. A rotating joint in the waveguide path must pass microwave power with low SWR. Choke joints use a half-wave RF “trap” geometry so that a mechanical gap still looks like a continuous wall at the operating frequency—maintaining low leakage and stable match while allowing rotation and weather sealing. Poor joints show as high SWR, arcs, or lost sensitivity.

Installation hygiene:

1. Keep waveguide runs as short and straight as practical; minimize twists and crush damage.
2. Seal flanges dry—moisture in waveguide is a classic sensitivity killer.
3. Bond and ground per marine practice; observe compass-safe distances for displays and scanners.
4. Never service the scanner without lockout so it cannot radiate or rotate under you.

Primary radar vs secondary replies (SART & RACON)

Your navigational radar is a primary sensor: it paints passive echoes from hulls and coastlines. Two secondary devices intentionally answer your pulse:

SART — Search and Rescue Transponder

A SART is a survival-craft 9 GHz (X-band) radar transponder. When switched ON and interrogated by a compatible X-band marine radar:

• It transmits a distinctive series of responses.
• On the rescuing ship’s PPI, the signal appears as about twelve equally spaced dots along the bearing to the SART, extending outward in range.
• As the rescuer closes, dots widen into arcs then concentric circles, guiding the final approach.

Hold the SART as high as possible for range; its audible alarm tells survivors that an interrogating radar (possible assistance) is nearby. S-band-only radars will not trigger a standard X-band SART—know which band your set uses when discussing GMDSS carriage.

RACON — radar beacon

A RACON on a buoy, lighthouse, or bridge listens for radar pulses and replies with a coded (often Morse) paint so the aid is positively identified on the PPI—not merely a passive stronger echo like a corner reflector.

Radar equipment & safety cues (094 overlap)

• TWT permanent magnet field: intended to prevent the electron beam from spreading in traveling-wave tubes used in some coherent/amplifier radars.
• Before testing: no personnel in front of the antenna.
• Magnetron care: keep tools away from the magnet; avoid heat and mechanical shock.
• Microwave exposure limits appear on the pool (5.0 mW/cm² in the Element 3 wording)—treat as a hard operational constraint during dockside tests.

Exam-day checklist (3-O 093–094 antenna/waveguide)

1. Common ship antenna = slotted array.
2. Gain up → beamwidth down; dish ×2 frequency → +6 dB gain.
3. Waveguide energy via E/H fields in the guide; couple with thin wire probe; extract with J-hook.
4. Prefer waveguide over coax for high-power microwave radar feeds; choke/rotating joints keep match while scanning.
5. SART = X-band 9 GHz, ~12 dots on PPI; RACON = coded active reply.
6. Clear the beam before TX tests; respect magnetron magnet safety.

With radar timing and microwave plumbing fixed, the final section leaves the mast for space: LEO distress systems, INMARSAT maritime satcom, and GPS.