Ship Radar Transmitter and Receiver Circuitry: Magnetron and Fixed-Tune Designs
Transmitter chain
- The magnetron is a crossed-field oscillator: a cylindrical cavity resonator, pulsed by a modulator that delivers several kilovolts from a pulse-forming network or a switched supply. It oscillates; it is not an amplifier and cannot be driven by a separate RF source.
- Typical marine figures: 3–25 kW peak at 9.4 GHz, higher peak powers at 3 GHz. Magnetron life is finite and output falls with age, with the operating frequency drifting.
- Tuning: on older sets a TUNE control mechanically shifts the magnetron or trims the local oscillator. On a fixed-tune (factory-tuned) design the operator tunes only the receiver, usually automatically through AFC; the magnetron itself has no operator adjustment.
- Duplexer: circulator plus TR cell and diode limiter protect the mixer from the transmit pulse; TR recovery time adds directly to minimum range.
Receiver chain
Superheterodyne: RF input → mixer with local oscillator (klystron or Gunn diode) → IF at 30 or 60 MHz → logarithmic amplifier → video. Receiver bandwidth should be about 1/τ (matched filter): 0.1 µs needs roughly 10 MHz. Too wide admits extra noise; too narrow stretches the pulse and spoils range resolution. STC (sea) reduces near-range gain, and FTC (rain, a differentiator) shortens echoes to break up clutter.
Solid-state designs
GaN or LDMOS amplifiers replace the magnetron: no kilovolt supply, low peak power, long coded pulses with pulse compression. A range fault on such a set no longer points to magnetron wear.
Worked example
A set runs 12 kW peak, 0.08 µs pulse, 2000 Hz. Duty cycle = 0.08 × 10⁻⁶ × 2000 = 1.6 × 10⁻⁴, so average power ≈ 1.9 W. Matched bandwidth ≈ 1/0.08 µs = 12.5 MHz. If echoes fade slowly, magnetron current is normal, and the tuning indicator peaks off-scale, suspect receiver detuning or a failing front-end device rather than the magnetron.
Traps
- Adjusting a fixed-tune magnetron with a screwdriver.
- Trying to measure magnetron output with an ordinary multimeter.
- Assuming a bright performance-monitor return proves the antenna radiates into space.
- Touching the transmitter before discharging the pulse-forming network.