CMRT Signal Processing & Antenna Theory 2 — Questions and Answers
Question 1: What is the purpose of a matched filter in radar signal processing?
- To eliminate all Doppler shifts
- To maximize the signal-to-noise ratio at the output (Correct answer)
- To broaden the transmitted pulse width
- To attenuate strong target returns
Correct answer: To maximize the signal-to-noise ratio at the output
A matched filter maximizes SNR by correlating the received signal with a replica of the transmitted signal.
Question 2: Which antenna parameter describes how efficiently an antenna converts input power into radiated electromagnetic waves?
- Gain
- Beamwidth
- Radiation efficiency (Correct answer)
- VSWR
Correct answer: Radiation efficiency
Radiation efficiency is the ratio of radiated power to input power, accounting for ohmic losses in the antenna.
Question 3: In marine radar, what does STC (Sensitivity Time Control) primarily compensate for?
- Doppler frequency shifts from moving targets
- Strong returns from nearby sea clutter (Correct answer)
- Frequency drift in the magnetron oscillator
- Interference from other radar systems
Correct answer: Strong returns from nearby sea clutter
STC reduces receiver gain for returns arriving shortly after transmission to suppress strong sea clutter from nearby waves.
Question 4: What phenomenon causes the radar horizon to extend slightly beyond the geometric horizon?
- Ionospheric refraction
- Tropospheric refraction bending the radar beam downward (Correct answer)
- Diffraction around the Earth's curvature
- Antenna tilt compensation
Correct answer: Tropospheric refraction bending the radar beam downward
Tropospheric refraction causes radar waves to bend slightly downward, extending the radar horizon beyond the geometric horizon.
Question 5: A slotted waveguide antenna for marine radar primarily radiates energy through:
- A dipole element at the feed point
- Slots cut into the waveguide walls that act as individual radiators (Correct answer)
- A parabolic reflector focusing waveguide energy
- Leakage at the waveguide flanges
Correct answer: Slots cut into the waveguide walls that act as individual radiators
Slots cut into the broad wall of the waveguide interrupt surface currents, causing each slot to radiate as a small dipole.
Question 6: What is the Nyquist sampling theorem's implication for radar A/D converters?
- Sampling rate must equal the transmitted pulse repetition frequency
- Sampling rate must be at least twice the highest signal frequency of interest (Correct answer)
- Sampling rate must match the IF bandwidth
- Sampling must occur only during the transmit pulse
Correct answer: Sampling rate must be at least twice the highest signal frequency of interest
The Nyquist theorem requires sampling at least twice the maximum signal frequency to avoid aliasing.
Question 7: In radar antenna theory, the first sidelobe level relative to the main beam for a uniformly illuminated aperture is approximately:
- -13 dB (Correct answer)
- -20 dB
- -30 dB
- -6 dB
Correct answer: -13 dB
A uniformly illuminated aperture produces a sinc-function pattern with first sidelobes approximately 13.3 dB below the main beam.
CMRT Signal Processing & Antenna Theory 2
What is the purpose of a matched filter in radar signal processing?