Element 8 Sample Questions

Questions from each subelement of the FCC's Element 8 pool. Pick an answer to see whether you're right, how everyone else did, and why.

Element 8 exam guide

A Radar principles

For a range of 5 nautical miles, the RADAR pulse repetition frequency should be:

  1. 16.2 Hz or more.
  2. 16.2 kHz or less.
  3. 16.2 MHz or less.
  4. 1.62 kHz or more.

Answer: B, 16.2 kHz or less.. An echo needs about 12.36 µs to travel out and back for each nautical mile (one radar mile). Each pulse's echo must return before the next pulse is sent. For 5 nmi the round trip is 5 × 12.36 µs = 61.8 µs, so the maximum PRF = 1 / 61.8 µs ≈ 16.2 kHz. Any PRF at or below that avoids range ambiguity.

The primary operating frequency of a reflex klystron is controlled by the:

  1. Dimensions of the resonant cavity.
  2. Voltage applied to the cavity grids.
  3. Voltage applied to the repeller plate.
  4. Level of voltage on the control grid.

Answer: A, Dimensions of the resonant cavity.. In a reflex klystron, oscillation occurs at the resonant frequency of the cavity the beam passes through. That frequency depends on the cavity's physical dimensions, so they set the primary operating frequency, and mechanical tuning changes it over a wide range. Repeller voltage only fine-tunes the frequency within the narrow band of a repeller mode.

Blocking oscillators operate on the formula of:

  1. T = R x C.
  2. I = E/R.
  3. By using the receiver’s AGC.
  4. None of the above are correct.

Answer: A, T = R x C.. A blocking oscillator conducts in a brief burst, then is cut off while a capacitor discharges through a resistor. The RC time constant, T = R × C, sets the off time and so the repetition rate. For example, 100 kΩ × 0.01 µF = 1 ms. Radar uses blocking oscillators as timers and pulse generators for triggers and modulators.

B Transmitting systems

The magnetron is used to:

  1. Generate the output signal at the proper operating frequency.
  2. Modulate the pulse signal.
  3. Determine the shape and width of the transmitted pulses.
  4. Determine the pulse repetition rate.

Answer: A, Generate the output signal at the proper operating frequency.. The magnetron is the radar's microwave power oscillator. When the modulator pulses it, its resonant cavities oscillate and produce high-power RF at the operating frequency, such as about 9.4 GHz (X band) or 3 GHz (S band). The magnetron only generates the RF energy. Pulse timing, width and shape are decided by the circuits that drive it.

What device is used as a transmitter in a marine RADAR system?

  1. Beam-powered pentode.
  2. Klystron.
  3. Thyratron.
  4. Magnetron.

Answer: D, Magnetron.. Marine radars traditionally use a magnetron as the transmitter. It is a self-excited microwave power oscillator that produces short, high-peak-power pulses in the X band (about 9.4 GHz) or S band (about 3 GHz) each time the modulator fires it. It is compact, rugged and inexpensive, which suits shipboard use.

What RADAR circuit determines the pulse repetition rate (PRR)?

  1. Artificial transmission line.
  2. Timer (synchronizer circuit).
  3. Discriminator.
  4. Pulse-rate-indicator circuit.

Answer: B, Timer (synchronizer circuit).. The timer, or synchronizer, is the radar's master clock. It produces the trigger pulses that set the pulse repetition rate (PRR). Each trigger fires the modulator and transmitter and, at the same moment, starts the display sweep and range circuits. This keeps every echo correctly timed for range.

C Receiving systems

RADAR receivers are similar to:

  1. FM receivers.
  2. Microwave receivers.
  3. HF receivers.
  4. T.V. receivers.

Answer: B, Microwave receivers.. Marine radars operate at microwave frequencies, about 9.4 GHz (X-band) and 3 GHz (S-band). Their receivers use waveguide, a crystal mixer and a microwave local oscillator to convert echoes down to an IF. This is the same superheterodyne design used in other microwave receivers.

An RF mixer has what purpose in a RADAR system?

  1. Prevents microwave oscillations from reaching the antenna.
  2. Combines audio tones with RF to produce the RADAR signal.
  3. Mixes the CW transmitter output to form pulsed waves.
  4. Converts a low-level signal to a different frequency.

Answer: D, Converts a low-level signal to a different frequency.. The returning echo is very weak and at microwave frequency, which is hard to amplify directly. The mixer combines it with the local oscillator signal. Their difference, for example 9,460 MHz − 9,400 MHz = 60 MHz, is a much lower intermediate frequency. That IF can be amplified efficiently before detection, so the mixer is a frequency converter for low-level signals.

In a RADAR receiver AGC and IAGC can vary between:

  1. 5 and 30 db.
  2. 10 and 15 db.
  3. 30 and 60 db.
  4. 20 and 40 db.

Answer: D, 20 and 40 db.. AGC and instantaneous AGC (IAGC) reduce IF gain when strong returns arrive, so clutter and large targets do not saturate the receiver. In typical radar receivers this control range runs from about 20 dB to 40 dB of gain variation.

D Display & control systems

The synchronizer primarily affects the following circuit or function:

  1. Receiver.
  2. Modulator.
  3. I.F. Amplifier.
  4. Mixer.

Answer: B, Modulator.. The synchronizer's main output is the trigger that fires the modulator. The modulator then delivers a high-voltage pulse to the magnetron, so the synchronizer controls when and how often the radar transmits. The receiver chain processes echoes continuously and does not depend on the synchronizer to operate.

Range markers are determined by:

  1. The video amplifier.
  2. The CRT.
  3. The timer.
  4. The magnetron.

Answer: C, The timer.. Range markers must stay locked to the moment the transmitter fires, or they would not show true distance. For that reason they are generated from the timer (synchronizer) trigger: a gated marker oscillator starts with each trigger and counts off fixed time intervals. Each interval corresponds to a set number of nautical miles.

An important component of the VRM system is the:

  1. Interference rejection circuit.
  2. STC sensitivity control.
  3. Shift register.
  4. Resolver.

Answer: C, Shift register.. A digital VRM makes its marker by counting. Starting at each transmit trigger, clock pulses are counted, and when the count matches the stored number for the selected range, a marker pulse goes to the video and brightens one ring. Shift registers hold and move that range count and feed the digital readout, so they are central to the VRM circuit.

E Antenna systems

What is the most common type of RADAR antenna used aboard commercial maritime vessels?

  1. Parabolic.
  2. Slotted waveguide array.
  3. Multi-element Yagi array.
  4. Truncated parabolic.

Answer: B, Slotted waveguide array.. Most commercial marine radars use a slotted waveguide array. It is a length of waveguide with a row of radiating slots, sealed in a horizontal radome or open bar. It gives a very narrow horizontal beam for good bearing resolution and a wider vertical beam that keeps targets painted as the ship rolls. It is also rugged, light and low in wind load, which suits a rotating mast-top antenna.

On a basic synchro system, the angular information is carried on the:

  1. Stator lines.
  2. Deflection coils.
  3. Rotor lines.
  4. DC feedback signal.

Answer: A, Stator lines.. A synchro transmitter's rotor gets a fixed AC reference voltage on its rotor lines, R1 and R2. Its three stator windings sit 120° apart, and the voltages induced in them change in amplitude and phase with shaft angle. The angle therefore travels to the receiving synchro on the three stator lines, S1, S2 and S3.

To insert RF energy into or extract RF energy from a waveguide, which of the following would not be used?

  1. Current loop.
  2. Voltage probe.
  3. Coupling capacitance.
  4. Aperture window.

Answer: C, Coupling capacitance.. Energy enters or leaves a waveguide through its fields, by one of three standard methods. A voltage probe couples to the electric field where it is strongest. A current loop couples to the magnetic field. An aperture or slot window lets fields pass between a guide and a cavity or another guide. A discrete coupling capacitor is a lumped-circuit part and is not one of these field-coupling methods.

F Installation, maintenance & repair

Range rings on the PPI indicator are oval in shape. Which circuit would you suspect is faulty?

  1. Range marker circuit.
  2. Sweep generation circuit.
  3. Timing circuit.
  4. Video amplifier circuit.

Answer: B, Sweep generation circuit.. Range rings are drawn by the sweep, which moves the beam outward at the same rate in every direction as the trace rotates. If one deflection axis has the wrong amplitude or linearity, distance is scaled differently in different directions. Circles then become ovals. A fault in the sweep generation circuit distorts the shape of everything on the PPI, including the rings.

In a marine RADAR set, a high VSWR is indicated at the magnetron output. The waveguide and rotary joint appear to be functioning properly. What component may be malfunctioning?

  1. The waveguide array termination
  2. The waveform generator
  3. The STC circuit
  4. The magnetron

Answer: A, The waveguide array termination. A slotted waveguide array ends in a matched termination, a resistive load that absorbs the energy the slots don't radiate. If that load is burned, cracked or wet, energy reflects back toward the transmitter and VSWR at the magnetron rises. With the waveguide run and rotary joint cleared, the array termination is the remaining part of the RF path to suspect.

If a CRT is dropped:

  1. The phosphor might break loose.
  2. Most likely nothing will happen because they are built with durability in mind.
  3. It might implode, causing damage to workers and equipment.
  4. It might go out of calibration.

Answer: C, It might implode, causing damage to workers and equipment.. A CRT's evacuated glass envelope is under heavy atmospheric pressure over its whole surface. A sharp blow from a drop can crack it and cause an implosion. The glass collapses inward and then scatters fragments outward, injuring nearby workers and damaging equipment. Handle CRTs carefully and wear eye and hand protection.