Topic 39
Antennas
- The slotted waveguide array is the most common RADAR antenna on commercial maritime vessels.
- Slotted waveguide arrays fed from one end exhibit frequency scan (beam direction shifts with frequency).
- The center of the transmitted lobe from a slotted waveguide is several degrees offset from a line perpendicular to the antenna.
- Good bearing resolution depends on a narrow antenna beam in the horizontal plane.
- A longer antenna produces a narrower horizontal beamwidth.
8-39E1 (modified — acronym expanded)
Slotted waveguide arrays, when fed from one end exhibit:
- A. Frequency scan.
- B. High Voltage Standing Wave Ratio (VSWR).
- C. Poor performance in rain.
- D. A narrow elevation beam.
Answer: A
8-39E2
A typical shipboard RADAR antenna is a:
- A. Rotary parabolic transducer.
- B. Slotted waveguide array.
- C. Phased planar array.
- D. Dipole.
Answer: B
8-39E3
Good bearing resolution largely depends upon:
- A. A high transmitter output reading.
- B. A high duty cycle.
- C. A narrow antenna beam in the vertical plane.
- D. A narrow antenna beam in the horizontal plane.
Answer: D
8-39E4
The center of the transmitted lobe from a slotted waveguide array is:
- A. Several degrees offset from a line perpendicular to the antenna.
- B. Perpendicular to the antenna.
- C. Maximum at the right hand end.
- D. Maximum at the left hand end.
Answer: A
8-39E5
How does antenna length affect the horizontal beamwidth of the transmitted signal?
- A. The longer the antenna the wider the horizontal beamwidth.
- B. The longer the antenna the narrower the horizontal beamwidth.
- C. The horizontal beamwidth is not affected by the antenna length.
- D. None of the above.
Answer: B
8-39E6
What is the most common type of RADAR antenna used aboard commercial maritime vessels?
- A. Parabolic.
- B. Truncated parabolic.
- C. Slotted waveguide array.
- D. Multi-element Yagi array.
Answer: C
Topic 40
Transmission Lines
- VSWR of a microwave line can be measured using a dual directional coupler, a power meter, and a network analyzer (all three).
- Characteristic impedance formula: ZO = √(L/C). With 250 μH and 1000 pF, ZO = 500 ohms.
- Improperly shielded or terminated long transmission lines cause communications receiver interference.
- Trap: cutting a transmission line in half does not change its characteristic impedance — each half retains the same ZO.
- Standing waves on a transmission line indicate some energy is not absorbed by the load.
- Horizontal waveguide runs must be sloped slightly downward at the elbow with a small drain hole to prevent moisture buildup.
8-40E1
The VSWR of a microwave transmission line device might be measured using:
- A. A dual directional coupler and a power meter.
- B. A network analyzer.
- C. A spectrum analyzer.
- D. A dual directional coupler, a power meter, and a network analyzer.
Answer: D
8-40E2
The impedance total (ZO) of a transmission line can be calculated by ZO = √L/C when L and C are known. When a section of transmission line contains 250 microhenries of L and 1000 picofarads of C, its impedance total (ZO) will be:
- A. 50 ohms.
- B. 250 ohms.
- C. 500 ohms.
- D. 1,000 ohms.
Answer: C
8-40E3
If long-length transmission lines are not properly shielded and terminated:
- A. The silicon crystals can be damaged.
- B. Communications receiver interference might result.
- C. Overmodulation might result.
- D. Minimal RF loss can result.
Answer: B
8-40E4
A certain length of transmission line has a characteristic impedance of 72 ohms. If the line is cut at its center, each half of the transmission line will have a ZO of:
- A. 36 ohms.
- B. 144 ohms.
- C. 72 ohms.
- D. The exact length must be known to determine Zo.
Answer: C
8-40E5
Standing waves on a transmission line may be an indication that:
- A. All energy is being delivered to the load.
- B. Source and surge impedances are equal to ZO and ZL.
- C. The line is terminated in impedance equal to ZO.
- D. Some of the energy is not absorbed by the load.
Answer: D
8-40E6
What precautions should be taken with horizontal waveguide runs?
- A. They should be sloped slightly downwards at the elbow and a small drain hole drilled in the elbow.
- B. They should be absolutely level.
- C. They should not exceed 10 feet in length.
- D. None of the above.
Answer: A
Topic 41
Synchros and Resolvers
- The PPI sweep is kept in sync with the antenna using synchro systems.
- In a synchro system, angular information is carried on the stator lines.
- Resolvers are the most common antenna position indicating device in modern RADARs.
- A basic resolver contains one rotor winding and two stator windings 90° apart.
- An antenna synchro transmitter has two rotor and three stator windings.
- Antenna direction can be sent to the display via 3-phase synchros, 2-phase resolvers, or optical encoders — any of these methods works.
8-41E1
The position of the PPI scope sweep must indicate the position of the antenna. The sweep and antenna positions are frequently kept in synchronization by the use of:
- A. Synchro systems.
- B. Servo systems.
- C. DC positioning motors.
- D. Differential amplifiers.
Answer: A
8-41E2
On a basic synchro system, the angular information is carried on the:
- A. DC feedback signal.
- B. Stator lines.
- C. Deflection coils.
- D. Rotor lines.
Answer: B
8-41E3
What is the most common type of antenna position indicating device used in modern RADARs?
- A. Resolvers.
- B. Servo systems.
- C. Synchro transmitters.
- D. Step motors.
Answer: A
8-41E4
Which of the following statements about antenna resolvers is correct?
- A. Most resolvers contain a rotor winding and a delta stator winding.
- B. Resolvers consist of a two rotor windings and two stator windings that are 90 degrees apart.
- C. The basic resolver contains a rotor winding and two stator windings that are 90 degrees apart.
- D. Resolvers consist of a "Y" connected rotor winding and a delta connected stator winding.
Answer: C
8-41E5
An antenna synchro transmitter is composed of the following:
- A. Three rotor and two stator windings.
- B. Two rotor and three stator windings.
- C. Three rotor and three stator windings.
- D. A single rotor and 3 stator windings.
Answer: B
8-41E6
RADAR antenna direction must be sent to the display in all ARPAs or RADAR systems. How is this accomplished?
- A. 3-phase synchros.
- B. 2-phase resolvers.
- C. Optical encoders.
- D. Any of the above.
Answer: D
Topic 42
Waveguide Theory
- Waveguides can be made from brass, aluminum, or copper (all are valid materials).
- A center conductor suspended between parallel conductive ground planes is called a stripline.
- Waveguide theory is based on the movement of an electromagnetic field (not current flow through wires).
- Waveguides are used at microwave frequencies because they have lower transmission losses than other feedline types.
- Waveguide theory also relies on skin effect and use of ¼ wave stubs.
- The signal is removed from a waveguide or magnetron with a thin wire called a J-Hook.
8-42E1
Waveguides can be constructed from:
- A. Brass.
- B. Aluminum.
- C. Copper.
- D. All of the above.
Answer: D
8-42E2
A microwave transmission line constructed of a center conductor suspended between parallel conductive ground planes is called:
- A. Microstrip.
- B. Coax.
- C. Stripline.
- D. Waveguide.
Answer: C
8-42E3
Waveguide theory is based upon:
- A. The movement of an electromagnetic field.
- B. Current flow through conductive wires.
- C. Inductance.
- D. Resonant charging.
Answer: A
8-42E4
A waveguide is used at RADAR microwave frequencies because:
- A. It is easier to install than other feedline types.
- B. It is more rugged than other feedline types.
- C. It is less expensive than other feedline types.
- D. It has lower transmission losses than other feedline types.
Answer: D
8-42E5
Waveguide theory is based on the principals of:
- A. Ohm’s Law.
- B. High standing waves.
- C. Skin effect and use of ¼ wave stubs.
- D. None of the above.
Answer: C
8-42E6
How is the signal removed from a waveguide or magnetron?
- A. With a thin wire called a T-hook.
- B. With a thin wire called a J-Hook.
- C. With a coaxial connector.
- D. With a waveguide flange joint.
Answer: B
Topic 43
Waveguide Components
- A rotary joint connects the stationary waveguide to the rotating antenna array.
- Resistive losses in a waveguide are very small because the inner surface area is large.
- A right-angle bend in an X-band waveguide must have a radius greater than 2 inches.
- Trap: coupling capacitance is NOT used to insert or extract RF energy from a waveguide — use a current loop, aperture window, or voltage probe instead.
- Inside a waveguide the magnetic field is strongest at the edges.
- At microwave frequencies waveguides have considerably less loss than conventional coaxial transmission lines.
8-43E1
A rotary joint is used to:
- A. Couple two waveguides together at right angles.
- B. Act as a switch between two waveguide runs.
- C. Connect a stationary waveguide to the antenna array.
- D. Maintain pressurization at the end of the waveguide.
Answer: C
8-43E2
Resistive losses in a waveguide are very small because:
- A. The inner surface of the waveguide is large.
- B. The inner surface of the waveguide is small.
- C. The waveguide does not require a ground connection.
- D. The heat remains in the waveguide and cannot dissipate.
Answer: A
8-43E3
A right-angle bend in an X-band waveguide must have a radius greater than:
- A. Three inches.
- B. Six inches.
- C. One inch.
- D. Two inches.
Answer: D
8-43E4
To insert RF energy into or extract RF energy from a waveguide, which of the following would not be used?
- A. Coupling capacitance.
- B. Current loop.
- C. Aperture window.
- D. Voltage probe.
Answer: A
8-43E5
The following is true concerning waveguides:
- A. Conduction is accomplished by the polarization of electromagnetic and electrostatic fields.
- B. Ancillary deflection is employed.
- C. The magnetic field is strongest at the center of the waveguide.
- D. The magnetic field is strongest at the edges of the waveguide.
Answer: D
8-43E6
At microwave frequencies, waveguides are used instead of conventional coaxial transmission lines because:
- A. They are smaller and easier to handle.
- B. They have considerably less loss.
- C. They are lighter since they have hollow centers.
- D. Moisture is never a problem with them.
Answer: B