Tutorial Questions Flashcards
Which factors define a LREW, TI and Tracking radar?
LREW -
D Band
Low PRF
Long Pulse Duration
Low RPM
Range 200+ nm
Sacrifices range resolution and minimum range for long range
TI -
E/F band
Medium PRF
Short pulse duration
High RPM
Range 100 nm
High Range resolution
Height Indication
TR -
I/J/K band
High PRF
Very short pulse duration
Very high range resolution
High data rate
What is the difference between a primary and secondary radar?
Primary - sends a radar pulse out and detects reflections back
Secondary- sends a radar pulse out and waits for a transponder signal to be sent back from target
How does operating frequency affect radar performance?
- Shape and size of antenna
- Performance in adverse weather
- High frequencies are attenuated more than low frequencies
- HF radars for very long distance refracts through ionosphere for over the horizon radar
Draw and label generic radar system, explain each function and draw an example pulse train for each point
Label the typical radar pulse train leaving the oscillator (in the time domain) to
show PRI, Pulse Duration and Carrier Frequency. Draw a diagram showing the
relationship between Peak Power and Average Power.
Draw a typical radar pulse train in the frequency domain labelling Peak Power,
Centre Frequency, PRF and Bandwidth.
Why would you use a long pulse duration in radar?
A long pulse duration would give a longer range with low range resolution and a large minimum range.
What effect does a short pulse have on range resolution and minimum range?
A short pulse duration provides a shorter range but with high range resolution and a short minimum range.
Explain the operation of a Magnetron in bullet points.
- Cathode emits electrons
- Big magnet bends the electron stream
- instead of going to the anode the electrons are bent into the resonant cavities
What are the principal requirements of a radar receiver?
Good SNR → Noise generated in the receiver must be kept to a minimum.,
Electronic Protection Measures (EPM) → For military radars, the requirement to overcome Electronic Counter Measures (ECM),
Moving Target Indication / Pulse Doppler → The ability to distinguish between moving targets and static targets.,
High Gain → To be able to amplify the reflected signal to a suitable level for feeding to a PPI display.,
Sufficient Dynamic Range → In order for the receiver to detect weak signals without becoming saturated by stronger ones.,
Sufficiently Wide Bandwidth → Necessary to prevent distortion of the received pulse.
Explain how bandwidth affects the shape of our radar pulse and what effect this has
on performance.
Bandwidth affects the shape of a radar pulse by determining its duration and rise time, with higher bandwidth leading to shorter, sharper pulses, which in turn improves range resolution, shortens minimum range, and enhances overall radar performance, though it may introduce more noise.
With the use of a diagram explain the difference between linear and logarithmic
amplifiers and how they affect the output of the IF Amp chain.
Linear Amplifiers:
- Operation: Amplify signals in a straight proportional manner.
- Effect on Output: Maintain the relative amplitude differences between signals, making them suitable for accurately preserving the signal’s original waveform.
Logarithmic Amplifiers:
- Operation: Amplify signals based on their logarithmic values.
- Effect on Output: Compress a wide range of signal amplitudes into a smaller range, making weak signals more noticeable and managing strong signals to prevent saturation, which is useful for detecting a wide range of signal strengths.
Draw a typical radar beam pattern and label the beamwidth and maximum power
point.
Why are antenna sidelobes undesirable?
Waste of power
Produce false echoes
They are a path for EM interference and jamming
What happens to a radar beam’s mainlobe and sidelobes if shading/weighting is
applied?
As shading and weighting is applied the BW is increased and the beams mainlobe is decreased causing a reduction in sidelobes.
What effect do we need to compensate for in an end-fed slotted array?
Squint Angle
With the aid of a diagram explain the operation of a Klystron
A Klystron amplifies RF signals through the process of velocity modulation and the subsequent bunching of electrons. Here’s how it operates:
- Electron Gun: It generates and accelerates an electron beam.
- Buncher Cavity: The initial RF signal to be amplified is fed into this cavity. The RF signal creates an oscillating electric field in the buncher cavity.
- Velocity Modulation: As the electron beam passes through the buncher cavity, it experiences acceleration during the positive half of the RF cycle and deceleration during the negative half. At the null points of the RF signal, the beam is unaffected. This velocity modulation causes electrons to bunch together as faster electrons catch up with slower ones downstream.
- Drift Space: The bunched electrons travel through a drift space, where the separation between fast and slow electrons reduces, enhancing the bunching effect.
- Catcher Cavity: The bunched electrons enter this cavity, where they induce an RF signal that is much stronger than the initial one. This cavity extracts energy from the bunched electrons, amplifying the RF signal.
- Collector: After passing through the catcher cavity, the spent electron beam is collected and removed from the device.
By using these cavities and the principle of velocity modulation, the Klystron effectively amplifies the RF signal.
WTAD Explain the operation of a TWT
A Traveling Wave Tube (TWT) amplifies radio frequency (RF) signals using the interaction between an electron beam and a slow-wave structure.
- Electron Gun: It generates and accelerates an electron beam.
- Slow-Wave Structure: The RF signal to be amplified is introduced into this structure, which slows down the RF wave to match the velocity of the electron beam.
- Interaction: As the RF signal and the electron beam travel together, they interact. The varying electric field of the RF signal modulates the velocity of the electrons, causing them to bunch together and transfer energy to the RF wave, thereby amplifying it.
- Collector: After amplification, the electron beam is collected, and the amplified RF signal is extracted from the slow-wave structure.
- Helix: Often, a helical coil (helix) is used as the slow-wave structure to maintain continuous interaction between the RF signal and the electron beam over a longer distance.
This process allows the TWT to provide wide bandwidth and high power amplification for RF signals.
WTAD Explain the operation of a MTI Radar
Components and Flow:
1. Trigger: Initiates the radar’s operation by sending a pulse to the modulator.
2. Modulator: Generates a high-power pulse to drive the transmitter, starting the transmission cycle.
3. Power Amplifier (TX): Amplifies the modulated signal and sends it to the antenna for transmission.
4. Antenna: Transmits the RF pulse and receives the echoes reflected from targets.
5. Receiver (RX): Receives the reflected signals (echoes) and sends them to the RF amplifier.
6. RF Amplifier: Amplifies the received signal before sending it to the mixer.
7. STALO (Stable Local Oscillator): Generates a stable frequency (9.9 GHz) to mix with the received signal, aiding in frequency conversion and phase coherence.
8. Mixer (Receive): Mixes the amplified received signal with the STALO signal to produce an Intermediate Frequency (IF) signal.
9. IF Amplifier: Amplifies the IF signal for further processing.
10. COHO (Coherent Oscillator): Generates a reference signal at a fixed frequency (100 MHz) to maintain phase coherence between transmitted and received signals.
11. Mixer (Transmit): Mixes the COHO signal with the STALO signal to produce the transmitted frequency.
12. Phase Detector: Compares the phase of the received IF signal with the reference COHO signal, detecting any phase shift caused by moving targets.
13. Delay Line Canceller: Delays the received signal by one pulse repetition interval (PRI) and subtracts it from the next pulse, effectively canceling out the signals from stationary targets and highlighting moving targets.
14. Voltage Comparator: Compares the amplitude of the processed signal to a threshold to determine the presence of a target.
15. Rectifier: Converts the processed signal into a video signal representing target information.
16. Video Output: Displays the detected moving target information.
- The radar transmits a pulse using the modulator, power amplifier, and antenna.
- The antenna also receives the echoes, which are amplified and mixed with the STALO signal to create an IF signal.
- The IF signal is amplified and its phase is compared with the COHO reference in the phase detector.
- The delay line canceller processes the signal to cancel out stationary objects.
- The voltage comparator and rectifier convert the processed signal into a video output showing moving targets.
This phase-coherent method ensures that any phase shifts detected are due to the motion of targets, effectively distinguishing moving objects from stationary background.
How can we determine the velocity range of a PD and how does this affect velocity ambiguity?
Doppler filters are aligned between the central and first [upper side band] spectral lines. The system detects the if Doppler return is in one of these filters giving a velocity indication. Therefore the [PRF] will give us the Velocity Range of the System.
When the Doppler frequency of the target return exceeds the PRF of the radar then the central line of the spectrum will lie outside the [acceptance window] and the first upper or lower side-band will appear in one of the Doppler filters.
Which side-band appears in the filter bank depends on whether the target is [opening or closing] the radar which can give rise to [velocity ambiguity]
