RADAR Questions - CSUM, Saarheim
8-2 A3 RADAR range is measured by the constant: A. 150 meters per microsecond. B. 150 yards per microsecond. C. 300 yards per microsecond. D. 18.6 miles per microsecond.
150 meters is the round-trip distance that a microwave travels in one microsecond. ANSWER A
8-1384 In a pulse modulated magnetron what device determines the shape and width of the pulse? A. Pulse Forming Network. B. Thyratron. C. LC parallel circuit D. Dimensions of the magnetron cavity.
A pulse-forming network is a rectangular wave generator based on an LC network that alternately stores, and then releases, the energy. ANSWER A
8-1681 The ATR box: A. Protects the receiver from strong RADAR signals. B. Prevents the received signal from entering the transmitter. C. Turns off the receiver when the transmitter is on. D. All of the above.
ATR stands for Anti-transmit-Receive Tube. The ATR tube disconnects the RADAR transmitter during the period of echo receive. The ATR "box" keeps the weak echo from going the wrong way into the transmitter, rather than where it should go, to the receiver. Thus, the ATR "box" is a transmitter isolator to the received signals coming from the antenna unit. If this isolation does not occur, a signal entering the transmitter port will be re-reflected, causing a misleading reflection to occur in the receiver. Conversely, the TR tube has the primary job of disconnecting the receiver during pulse transmit. ANSWER B
8-27 C1 One of the best methods of reducing noise in a RADAR receiver is? A. Changing the frequency. B. Isolation. C. Replacing the resonant cavity. D. Changing the IF strip.
All microwave receivers must incorporate copper shielding, preventing stray noise from leaking into the circuit This will also help isolate one stage from another. At microwave wavelengths, it takes a total copper enclosure to provide adequate isolation. ANSWER B
8-3 A6 An X band RADAR operates in which frequency band? A. 1 - 2 GHz. B. 2-4GHz. C. 4 - 8 GHz. D. 8-12GHz.
An X-band RADAR operates from 9,300 to 9,500 MHz. ANSWER D
3-9103 What device can be used to determine the performance of a RADAR system at sea? A. Echo box. B. Klystron. C. Circulator. D. Digital signal processor.
An echo box is a good way to test RADAR performance when far out to sea with no local targets to return an echo. The echo box is a resonant cavity constructed in such a way as to slightly delay the return echo when it is reflected back to the receiver. ANSWER A
3-9304 A common shipboard RADAR antenna is the: A. Slotted array. B. Dipole. C. Stacked Yagi. D. Vertical Marconi.
An interesting RADAR antenna is the slotted array, which uses a number of in-phase radiating openings. This acts much like a co-linear array, but the slotted array is horizontal, allowing for the horizontal lobe to be sharpened. A parabolic trough to the rear of the slotted array also allows unwanted vertical beamwidth to be reduced, too. ANSWER A
27C3 Noise can appear on the LCD as: A. Erratic video and sharp changes in intensity. B. Black spots on the screen. C. Changes in bearings. D. None of the above.
Anytime a RADAR operator spots erratic video and big changes in display intensity, they can assume that a component within the receiver "front end" may be failing, creating noise. ANSWER A
3-9203 We are looking at a target 25 miles away. When a RADAR is being operated on the 25 mile range setting what is the most appropriate pulse width and pulse repetition rate? A. 1.0 μS PW and 500 pps. B. 0.25 μS PW and 1,000 pps. C. 0.01 μS PW and 500 pps. D. 0.05 μS PW and 2,000 pps.
As in question 3-9201, the maximum PRR is non-negotiable. We MUST have a period between pulses that is greater than the round trip time. Remember, period is the inverse of the PRR. First, let's figure out the round triptime of a target 25 miles away. It is 2 x (25 + 186.000) = 0.000269 seconds. This is equal to a PRR of 1 + 0.000269, or 3717 pulses per second (PPS), our maximum acceptable PRR. All four answers fall comfortably within that limit, so all of them would "work," but not all of them would work equally well. 25 miles is still a pretty distant target, so we want a lot of energy to get a strong return. Greater pulse width (PW) means more energy, so we have only one BEST answer. ANSWER A
8-6 A6 What is the relationship between pulse repetition rate and pulse width? A. Higher PRR with wider pulse width. B. The pulse repetition rate does not change with the pulse width. c. The pulse width does not change with the pulse repetition rate. D. Lower PRR with wider pulse width.
As the operator switches to a longer range scale, the modern RADAR will automatically select a lower pulse repetition rate along with a wider pulse width. This will allow the detection of distant small targets. To give you an idea of range and pulse width variables, ponder this: 2,400 pulses, each pulse 0.12 microseconds in length, will yield 288 microseconds of energy transmitted during each second. If we had 800 pulses, each 0.4 microseconds in length, this would amount to 320 microseconds of energy in 1 second. But each second of time has 1,000,000 ·microseconds. Too long a pulse on short range would not allow enough time for the receiver to respond to the echo. Too short a pulse at long range may not illuminate the target as a received echo. When the operator selects the desired range, the modern RADAR uses automatic circuits that yield the best combination of all transmitting and receiving RADAR parameters. ANSWER D
8-2 A5 How long would it take for a RADAR pulse to travel to a target 10 nautical miles away and return to the RADAR receiver? A. 12.34 microseconds. B. 1.234 microseconds. C. 123.4 microseconds. D. 10 microseconds.
Be sure to read these types of questions carefully to see whether or not they want the range to the target, or the range to the target and back again as a time delay. It takes 6.17 microseconds for the RADAR wave to travel 1 nautical mile, or 10 nautical miles to the target in 61.7 microseconds, and the return echo would take another 61 .7 microseconds, or a total time delay of 123.4 microseconds. Remember, up and back, a RADAR mile, means times two of the target distance. ANSWER C
8-21 C5 The klystron local oscillator is constantly kept on frequency by: A. Constant manual adjustments.detector. B. The Automatic Frequency Control circuit. C. A feedback loop from the crystal D. A feedback loop from the TR box.
Because the magnetron's frequency can drift, the automatic frequency control circuit causes the klystron to track any changes in magnetron frequency and uses the difference between the two frequencies to maintain a constant intermediate frequency. ANSWER B
3-9202 When a RADAR is being operated on the 6 mile range setting what is the most appropriate pulse width and pulse repetition rate? A. 1.0 μS PW and 500 pps. B. 2.0 μS PW and 3,000 pps. C. 0.25 μS PW and 1,000 pps. D. 0.01 μS PW and 500 pps.
Borrowing from problem 3-9201 above, we can see that our maximum PRF for unambiguous returns is around 16,000 PPS. (If the maximum PRF for 48 miles is 1,942 we can increase our PRF by a factor of eight for an object only 6 miles away). We are in no danger of "overclocking" even at 3,000 PPS, as in Answer B. However, answer B suggests a 2.0 μS pulse width, which is WAY overkill for such a short distance. We can give our radar a bit of a rest by using a tiny 0.25 μS pulse width, which will still give us a comfortably strong signal with good resolution for fast moving boats, or even your odd low-flying airplane! We still want to use a moderately fast PRR, however, so 1000 PPS is a good compromise. ANSWER C
3-9105 Digital signal processing (DSP) of RADAR signals (compared with analog) causes: A. Improved display graphics. B. Improved weak signal or target enhancement. C. Less interference with SONAR systems. D. Less interference with other radio communications equipment.
Digital signal processing (DSP) allows for the subtraction of both internal and external noise, enhancing weak echo response. ANSWER B
8-2 A6 What is the distance in nautical miles to a target if it takes 308.5 microseconds for the RADAR pulse to travel from the RADAR antenna to the target and back. A. 12.5 nautical miles. B. 25 nautical miles. C. 50 nautical miles. D. 2.5 nautical miles.
Divide 308.5 microseconds by 6.17 to find the total distance the signal travels (to the target and back to the RADAR receiver). Then, divide the answer by 2 to find the one-way distance to the target. 308.5 + 6.17 = 50 + 2 = 25. ANSWER B
8-41 E3 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.
Early RADARs used synchro systems that were geared at a ratio of around 10:1 and required a cam and microswitch system at both the antenna and the indicator to insure that the antenna was in step with the indicator. Today, modern RADARs rely on a resolver circuit, which constantly keeps antenna alignment in sync with the indicator readout. ANSWER A
8-1183 Which of the following statements about most modern RADAR transmitter power supplies is false? A. High voltage supplies may produce voltages in excess of 5,000 volts AC. B. There are usually separate low voltage and high voltage supplies. C. Low voltage supplies use switching circuits to deliver multiple voltages. D. Low voltage supplies may supply both AC and DC voltages.
Here they ask which statement is FALSE. Most RADARs may feed the modulator with a high-voltage, DC (NOT AC) power supply, using half wave, full wave, or bridge rectification. These power supplies MUST be cooled by an airflow system. ANSWER A
8•1281 High voltage is applied to what element of the magnetron? A. The waveguide. B. The anode. C. The plate cap. D. The cathode.
High voltage is found within the heart of the magnetron on the sealed cathode. Cathode current is several amps, and the cathode is large so that it can withstand the tremendous amount of heat that develops. Keep metal tools well away from the magnetron because of its large permanent magnet. The exposed anode is at ground potential, and a large negative voltage of several kilovolts is applied to the cathode during the magnetron's oscillating state. ANSWER D
8-2A4 If a target is 5 miles away, how long does it take for the RADAR echo to be received back at the antenna? A. 51.4 microseconds. B. 123 microseconds. C. 30.75 microseconds. D. 61.7 microseconds.
If the target is 5 miles away, the RADAR wave must travel a total of 10 miles - 5 miles to the target, and 5 miles return as an echo. Ten nautical miles times 6.17 microseconds per mile gives us about 61.7 microseconds. Watch out for C; this is not a one-way trip! ANSWER D
8-1683 The TR box: A. Prevents the received signal from entering the transmitter. B. Protects the receiver from the strong RADAR pulses. C. Turns off the receiver when the transmitter is on. D. Protects the receiver from the strong RADAR pulses and turns off the receiver when the transmitter is on.
In the TR (transmit/receive) box, a solid-state circuit acts as a mechanical TR relay. The transmit/receive box protects the receiver from the high-powered signals produced during transmit. ANSWER D
8-24 C3 In the AFC system, the discriminator compares the frequencies of the: A. Magnetron and klystron. B. PRR generator and magnetron. C. Magnetron and crystal detector. D. Magnetron and video amplifier.
In the automatic frequency control system, the discriminator compares the frequencies of the magnetron and the klystron. ANSWER A
8-27C4 RADAR interference on a communications receiver appears as: A A varying tone. B. Static. C. A hissing tone. D. A steady tone.
Interference from an energized RADAR heard over a communications receiver is a steady tone at the pulse repetition rate frequency. It sounds like a musical tone. ANSWER D
8-27C6 Noise can: A Mask larger targets. B. Change bearings . C. Mask small targets. D. Increase RADAR transmitter interference.
Internal circuits creating noise will often mask distant faint echo returns. This will cause the operator to miss small targets that do not exhibit sufficient echo strength to be resolved by the receiver, and appear on the display. ANSWER C
8-39 E5 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.
It is desirable to sharpen the horizontal beamwidth by providing the longest possible antenna array, improving resolution to individual targets. ANSWER B
8-40 E1 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.
It is the job of the synchro system to keep the RADAR sweep and antenna in alignment. After several years of operation, mechanical slippage could require you to get into the antenna unit and readjust the synchro settings and heading marker microswitch. ANSWER A
8-2A2 ,One RADAR mile is how many microseconds? A. 6.2. B. 528.0. C. 12.34. D. 0.186.
It takes a RADAR pulse 6.17 microseconds to travel out one mile. One "RADAR mile" considers the echo return, so 2 X 6.. 17 = 12.34 microseconds. Be careful of A, it is only the time out, not out and back. ANSWER C
3-9006 If the elapsed time for a RADAR echo is 62 microseconds, what is the distance in nautical miles to the object? A. 5 nautical miles. B. 87 nautical miles. C. 37 nautical miles. D. 11.5 nautical miles.
It takes a radar signal 6.2 microseconds to travel 1 nautical mile. In 62 microseconds, the radar signal will travel 10 total miles - 5 miles to the target and 5 miles back. 62 + (6.2 x 2) = 5. ANSWER A
8-21 C4 What device(s) could be used as the local oscillator in a RADAR receiver? A. Thyratron. B. Klystron. C. Klystron and a Gunn diode. D. Gunn diode.
Klystrons and Gunn diodes are used as oscillators in RADAR receivers. ANSWER C
8-3A5 The major advantage of an S-band RADAR over an X-band RADAR is: A It is less affected by weather conditions. B. It has greater bearing resolution. C. It is mechanically less complex. D. It has greater power output.
Lower-frequency RADARs on the S-band exhibit poor reflectivity from nearby thunderstorms, and thus have the ability to "see" better through light snow and rain. ANSWER A
3-9004 Shipboard RADAR is most commonly operated in what band? A. VHF. B. UHF. C. SHF. D. EHF.
Marine RADARs operate on super high frequency bands - 3000 MHz to 30,000MHz. ANSWER C
8-1 A2 Which of the following has NO effect on the maximum range capability? A. Carrier frequency B. Recovery time. C. Pulse repetition frequency. D. Receiver sensitivity.
Maximum range of a RADAR system IS influenced by peak power output, receiver sensitivity, pulse repetition frequency, and the RADAR band frequency of operation. But the question asks what has NO effect on maximum range, and that would be RECOVERY TIME. Recovery time is associated with MINIMUM range calculations as a period of time where the receiver is unable to receive the echo, a function of the TR tube to respond to within 6 dB of normal sensitivity at the conclusion of the transmitter pulse. Recovery time would not factor in maximum range capability. ANSWER B
3-9405 Exposure to microwave energy from RADAR or other electronics devices is limited by U.S. Health Department regulations to mW/centimeter. A. 0.005 B. 5.0 c. 0.05 D. 0.5
Microwave energy from a RADAR unit to your body may not exceed 5 mW per centimeter. The higher we go in frequency, the more critical the consideration to stay out of the main lobe of a marine RADAR. ANSWER B
8-27 C2 The primary cause of noise in a RADAR receiver can be attributed to: A Electrical causes. B. Atmospheric changes. C. Poor grounding. D. Thermal noise caused by RADAR receiver components.
Noise in a RADAR receiver can be attributed to thermal noise caused by sensitive RADAR receiver components. Erratic video "blotches" with sharp changes in intensity on the screen may illustrate noise, and the imminent failure of a sensitive receiver component ANSWER D
What is the normal range of pulse repetition rates? A. 2,000 to 4,000 pps. B. 1,000 to 3,000 pps C. 500 to 1,000 pps. D. 500 to 2,000 pps.
Okay, RADAR techs, here are a few common letters to remember: PPS = pulses per second; PRR = pulse repetition rate, and PRF = pulse repetition frequency. They all mean the same thing! The normal range of pulse repetition rates on most marine RADARs is between 500 to 2,000 pulses per second. The time in between each pulse is called the pulse repetition interval, and this is equal to 1 divided by the pulse repetition rate, with an answer in micro-seconds. Pulse repetition rate may also be called pulse repetition frequency. Pulse repetition interval is sometimes called pulse repetition time. Pulse repetition rate is the number of pulses per second sent to the target It also is the number of pulses per second sent to the magnetron. Small targets many miles away from the operating radar are best detected with a lower pulse repetition rate, which is usually an automatic function as the operator switches to a longer range that will generate a longer pulse width. ANSWER D
3-9204 What pulse width and repetition rate should you use at long ranges? A. Narrow pulse width and slow repetition rate. B. Narrow pulse width and fast repetition rate. C. Wide pulse width and fast repetition rate. D. Wide pulse width and slow repetition rate.
On long-range radar detection, the RADAR will automatically switch to a wider pulse width at a slower repetition rate. ANSWER D
3-9205 What pulse width and repetition rate should you use at short ranges? A. Wide pulse width and fast repetition rate. B. Narrow pulse width and slow repetition rate. C. Narrow pulse width and fast repetition rates. D. Wide pulse width and slow repetition rates.
On short-range target detection, the RADAR will automatically select narrow pulse width and a faster repetition rate. ANSWER C
8-1 A1 A radio wave will travel a distance of three nautical miles in: A. 6.17 microseconds. B. 37.0 microseconds. C. 22.76 microseconds. D. 18.51 microseconds.
RADAR and radio waves travel at approximately the speed of light; it takes 6.17 s for a RADAR wave to travel one nautical mile from the transmitter. To travel three nautical miles, it would take 18.51 (6.17 x 3) microseconds to travel this distance, and your correct answer is 18.51 microseconds. The way you can remember the number 6 in microseconds per nautical mile is never to get "seasix." ANSWER D
8-27 C5 In a RADAR receiver the most common types of interference are? A. Weather and sea return. B.Sea return and thermal. C.Weather and electrical. D. Jamming and electrical.
RADAR operating at peak efficiency aboard a ship will many times display distant thunderstorms, close-in sea return, and flying birds several miles away. ANSWER A
3-9402 Prior to testing any RADAR system, the operator should first: A. Check the system grounds. B. Assure the display unit is operating normally. C. Inform the airport control tower or ship's master. D. Assure no personnel are in front of the antenna.
RADAR systems use microwave signals to detect targets. These are the same frequencies used to heat food in microwave ovens. Remember the Amana Radar Range? You don't want to cook your fellow sailors when testing RADAR, so make sure no one is standing in front of the antenna! ANSWER D
8-24 C1 The STC circuit is used to: A. Increase receiver stability. B. Increase receiver sensitivity. C. Increase receiver selectivity. D. Decrease sea return on a RADAR receiver.
STC stands for Sensitivity Time Control, a circuit to adjust amplifier gain with TIME, during a single pulse repetition period. A bias voltage, that varies with time, is applied to the IF amplifier. Close-in targets are decreased in strength, with more distant targets maintained at their normal level. This will help mitigate false echoes from wind waves, dubbed "popcorn," close-in to your RADAR station. ANSWER D
8-25 C4 Sea clutter on the RADAR scope cannot be effectively reduced using front panel controls. What circuit would you suspect is faulty? A. Sensitivity Time Control (STC) circuit. B. False Target Eliminator (FTE) circuit. C. Fast Time Constant (FTC) circuit. D. Intermediate Frequency (IF) circuit.
Sea clutter is actually a positive indication that a marine RADAR is operating properly in the short-range setting. However, in heavy weather, sea clutter may sometimes cover up close-in targets, such as small boats and buoys. Switching in the sensitivity time control (STC) circuit attenuates close-in echoes so you can better identify real close-in targets. Think of "STC" as short timing control. ANSWER A
8-25 C5 What circuit controls the suppression of sea clutter? A. EBL circuit. B. STC circuit. C. Local oscillator. D. Audio amplifier.
Sea clutter- called "popcorn"- is always a problem on windy days, and the STC adjustable on/off circuit will help minimize ocean wave return. ANSWER B
8-25 C3 Sea return is: A. Sea water that gets into the antenna system. B. The return echo from a target at sea. C. The reflection of RADAR signals from nearby waves. D. None of the above.
Sea return is echoes reflected off the face of nearby waves. You can even see the wake of certain ships as sea return. ANSWER C
8-6 A5 Small targets are best detected by: A. Short pulses transmitted at a fast rate. B. Using J band frequencies. C. Using a long pulse width with high output power. D. All of these answers are correct.
Small targets many miles away from the operating RADAR are best detected with a lower pulse repetition rate, which should automatically generate a longer pulse width. Higher power RADARs will many times display small target echoes that otherwise might be missed with lower power output RADARs. ANSWER C
3-9403 In the term "ARPA RADAR," ARPA is the acronym for which of the following? A. Automatic RADAR Plotting Aid. B. Automatic RADAR Positioning Angle. C. American RADAR Programmers Association. D. Authorized RADAR Programmer and Administrator.
The ARPA RADAR takes the mystery out of distant target movements. Unlike a conventional display showing only the position of target echoes, the ARPA RADAR shows target history, target direction, and closest point of approach. There are even RADAR alarms that sound if the ARPA RADAR detects that you may be on a collision course. ANSWER A
3-9101 The ATR Box: A. Prevents the received signal from entering the transmitter. B. Protects the receiver from strong RADAR signals. C. Turns off the receiver when the transmitter is on. D. All of the above.
The ATR (anti-transmit/receive) circuit keeps received echoes out of the transmitter. - ANSWER A
8-1684 What device is located between the magnetron and the mixer and prevents received signals from entering the magnetron? A. The ATR tube. B. The TR tube. C. The RF Attenuator. D. A resonant cavity.
The ATR circuit (tube) keeps received echoes out of the transmitter. ANSWER A
8-1A5 Which of the following components allows the use of a single antenna for both transmitting and receiving? A. Mixer. B. Duplexer. C. Synchronizer. D. Modulator.
The RADAR duplexer allows one antenna to toggle between transmit and receive. The duplexer also provides isolation to any transmitter pulses working back into the RADAR receiver, with fast receiver time recovery for close-in target detection. Further, the duplexer must not load the receiver in any fashion, as the receiver must receive extremely weak target echoes. ANSWER B
3-9404 Which of the following is NOT a precaution that should be taken to ensure the magnetron is not weakened: A. Keep metal tools away from the magnet. B. Do not subject it to excessive heat. C. Keep the TR properly tuned. D. Do not subject it to shocks and blows.
The RADAR magnetron is a delicate network and may be damaged by a metal tool flying into the magnet. This question asks which of the following is NOT a precaution to insure the magnetron is not damaged. Keeping the TR properly tuned may have little influence on damage to the magnetron. ANSWER C
3-9104 What is the purpose of a synchro transmitter and receiver? A. Synchronizes the transmitted and received pulse trains. B. Prevents the receiver from operating during the period of the transmitted pulse. C. Transmits the angular position of the antenna to the indicator unit. D. Keeps the speed of the motor generator constant.
The RADAR synchro keeps the PPI sweep line in the same angular position as the rotating antenna. When servicing a marine RADAR, it is advised to go out for a sea trial. Point the bow of the ship to a distant target, such as a buoy and make sure that the bow, buoy, and heading marker are in perfect angular alignment, dead ahead. Next, repeat this test off the stern. ANSWER C
8-1782 The triggering section is also known as the: A. PFN. B. Timer circuit. C. Blocking oscillator. D. Synchronizer.
The RADAR synchronizer is sometimes referred to as a timer or keyer, supplying synchronizing signals that time transmit pulses, indictor display, and many other circuits in the receiver. ANSWER D
8-1783 Operation of any RADAR system begins in the: A. Triggering section. B. Magnetron. C. AFC. D. PFN.
The RADAR system MUST operate in a specific timed relationship, including 'the interval between transmitted pulses. Timing pulses relate to pulse repetition frequency, with additional RADAR components timed by the output of the synchronizer or by timing signals from the transmitter as it is turned on. This is the heart of a RADAR system, the triggering section synchronizer. ANSWER A
8-25 C2 The STC circuit: A. Increases the sensitivity of the receiver for close targets. B. Decreases sea return on the PPI scope. C. Helps to increase the bearing resolution of targets. D. Increases sea return on the PPI scope.
The STC (sensitivity time control) decreases return echoes from the sea in the direction of the incoming wind. It is the front side of the ocean waves that creates the greatest number of false-echo sea returns. ANSWER B
3-9201 When a RADAR is being operated on the 48 mile range setting, what is the most appropriate pulse width (PW) and pulse repetition rate (pps)? A. 1.0 μS PW and 2,000 pps. B. 0.05 μS PW and 2,000 pps. C. 2.5 μS PW and 2,500 pps. D. 1.0 μS PW and 500 pps.
The absolute upper limit for a PRR is determined by the fact that we must never send out more than one pulse for every one that returns. Otherwise, we don't know which pulse we're looking at. At 48 miles, the one-way trip time for the pulse is 48 x (1 + 186.292) = 0.000258 seconds. We double that to get our round trip time of 0.000515 seconds. The time between our pulses must be greater than 0.000515 seconds. To find the PPS, we simply take the reciprocal of our period, or 1 + 0.000515, which gives us 1942 PPS. Our PRR must be LESS than 1942 PPS. This eliminates answers A, B, and C. ANSWER D
8-1286 The anode of a magnetron is normally maintained at ground potential: A. Because it operates more efficiently that way. B. For safety purposes. C. Never. It must be highly positive to attract the electrons. D. Because greater peak-power ratings can be achieved.
The anode is at ground potential to protect operations personnel from shocks and to eliminate the problem of insulating the magnetron from the chassis. ANSWER B
8-24 C1 The AFC system is used to: A. Control the frequency of the magnetron. B. Control the frequency of the klystron. C. Control the receiver gain. D. Control the frequency of the incoming pulses.
The automatic frequency control (AFC) system controls the frequency of the klystron. ANSWER B
8-24 C4 An AFC system keeps the receiver tuned to the transmitted signal by varying the frequency of the: A. Magnetron. B. IF amplifier stage. C. Local oscillator. D. Cavity duplexer.
The automatic frequency control function takes place in the local oscillator. ANSWER C
8-19 C3 RADAR receivers are similar to: A. FM receivers. B. HF receivers. C. T.V. receivers. D. Microwave receivers.
The characteristics of a microwave receiver apply precisely to RADAR equipment- the demand for an extremely low noise floor, mechanical shielding between different receiver stages, receiver protection against strong transmitter overloads, and microsecond switching time between transmit and receive operations. ANSWER D
8-22 C2 The usual intermediate frequency of a shipboard RADAR unit is: A. 455kHz. B. 10.7 MHz. C. 30 or 60 MHz. D. 120 MHz.
The common intermediate frequency of shipboard RADAR units is 30 MHz or 60 MHz between the oscillator and the transmitted frequency. The RADAR may employ automatic frequency control (AFC), or a manual tune control, to keep the output of the local oscillator in a closed loop circuit. The output of the discriminator is a DC error voltage, and indicates the degree of mistuning between the transmitter and the local oscillator. ANSWER C
8-24 C2 A circuit used to develop AFC voltage in a RADAR receiver is called the: A. Peak detector. B. Crystal mixer. C. Second detector. D. Discriminator.
The discriminator is tuned to the IF frequency. When operating properly, the difference in magnetron and klystron frequency is the IF frequency. As long as the difference remains the same as the IF frequency, no error voltage is produced. If there is a difference, an error voltage is produced and is applied to the klystron to change the frequency difference back to the IF frequency. This corrective action is called automatic frequency control (AFC). ANSWER D
3-9102 What is the purpose or function of the RADAR duplexer/circulator? It is a/an: A. Coupling device that is used in the transition from a rectangular waveguide to a circular waveguide. B. Electronic switch that allows the use of one antenna for both transmission and reception. C. Modified length of waveguide that is used to sample a portion of the transmitted energy for testing purposes. D. Dual section coupling device that allows the use of a magnetron as a transmitter.
The duplexer/circulator is like an electronic transmit/receive switch, toggling outward bound pulses from the magnetron to the antenna waveguide, while also passing incoming echo pulses from the antenna's same waveguide to the receiver. This allows one RADAR antenna to both transmit energy and receive the weak echoes. The duplexer/circulator prevents transmit pulses from entering the receiver directly. The rare failure of the duplexer/circulator could easily damage the RADAR receiver circuitry. ANSWER B
8-1186 Which of the following is not part of the transmitting system? A. Magnetron. B. Modulator. C. Pulse Forming Network. D. Klystron.
The klystron may be found in the RADAR receiver local oscillator, using 30 MHz or 60 MHz intermediate frequency. ANSWER D
3-9303 What happens to the beamwidth of an antenna as the gain is increased? The beamwidth: A. Increases geometrically as the gain is increased. B. Increases arithmetically as the gain is increased. C. Is essentially unaffected by the gain of the antenna. D. Decreases as the gain is increased.
The larger the RADAR parabola, the greater will be the energy concentration, leading to an increase in gain. The decreased beamwidth allows more energy to be concentrated into a narrow plane for better target discrimination. ANSWER D
8-21 C2 In a RADAR unit, the local oscillator is a: A. Hydrogen Thyratron. B. Klystron. C. Pentagrid converter tube. D. Reactance tube modulator.
The local oscillator is the klystron (LOK). ANSWER B
8-1285 A negative voltage is commonly applied to the magnetron cathode rather than a positive voltage to the magnetron anode because: A. The cathode must be made negative to force electronics into the drift area. B. A positive voltage would tend to nullify or weaken the magnetic field. C. The anode can be operated at ground potential for safety reasons. D. The cavities might not be shock-excited into oscillation by a positive voltage.
The magnetron anode is normally operated at ground potential to reduce the likelihood of an accidental shock when technicians are working on a turned-on RADAR TR unit. ANSWER C
8-1284 The magnetron is: A. A type of diode that requires an internal magnetic field. B. A triode that requires an external magnetic field. C. Used as the local oscillator in the RADAR unit. D. A type of diode that requires an external magnetic field.
The magnetron is a diode with a magnetic field between the cathode and anode; the magnetic field controls the tube. When the magnet gets weak, the magnetron must be replaced. ANSWER D
8-1181 The magnetron is used to: A. Generate the output signal at the proper operating frequency. B. Determine the shape and width of the transmitted pulses. C. Modulate the pulse signal. D. Determine the pulse repetition rate.
The magnetron is a thermionic vacuum tube with a large, permanent magnet surrounding the tube, and is found in the power output stage of a RADAR. When properly assembled, the magnetron will generate a stable output signal at the proper operating frequency. ANSWER A
8-1283 What device is used as a transmitter in a marine RADAR system? A. Magnetron. B. Klystron. C. Beam-powered pentode. D. Thyratron.
The magnetron produces high-power outputs for marine RADARs. The klystron (incorrect Answer B) is generally used as the oscillator for the system. ANSWER A
8-5 A3 The minimum range of a RADAR is determined by: A. The frequency of the RADAR transmitter. B. The pulse repetition rate. C. The transmitted pulse width. D. The pulse repetition frequency.
The minimum range at which a RADAR can detect a target is determined by the pulse duration, sometimes called pulse width or pulse length. As the RADAR operator switches down to a close-in scale, the RADAR automatically shortens its pulse width to as little as 0.1 microsecond. A 0.1 microsecond pulse covers approximately 160 yards of range on the display. Small-boat marine RADARs use even shorter pulses for target detection as close as 75 feet. ANSWER C
3-9206 When a RADAR is being operated on the 1.5 mile range setting, what is the most appropriate pulse width and pulse repetition rate? A. 0.25 μS PW and 1,000 pps. B. 0.05 μS PW and 2,000 pps. C. 1.0 μS PW and 500 pps. D. 2.5 μS PW and 2,500 pps.
The modern ship RADAR has no operator functions to manually adjust pulse width or pulse repetition rates- this is accomplished automatically when the operator changes range scales. The two functions are interrelated, and are strictly controlled by the range selected on the RADAR controls. Veteran RADAR operators indeed had more options with older RADAR gear, but now range adjustments automatically select the optimum pulse width and pulse repetition rate. ANSWER B
8-1386 'fhe purpose of a modulator in the transmitter section of a RADAR is to: A. Improve bearing resolution. B. Provide the correct wave form to the transmitter. C. Prevent sea return. D. Control magnetron power output.
The modulator is found in the RADAR transmitter, and controls RADAR transmit pulse width by means of a rectangular DC pulse of the required duration and amplitude. ANSWER B
8-1182 The purpose of the modulator is to: A. Transmit the high voltage pulses to the antenna. B. Provide high voltage pulses of the proper shape and width to the magnetron. C. Adjust the pulse repetition rate. D. Tune the Magnetron to the proper frequency.
The modulator is found in the transmitter to produce properly-timed, high-amplitude, rectangular pulses which are then fed to the magnetron. ANSWER B
3-9003 What is the normal range of pulse widths? A. .05 microseconds to 0.1 microseconds . B. .05 microseconds to 1.0 microseconds . C. 1.0 microseconds to 3.5 microseconds . D. 2.5 microseconds to 5.0 microseconds.
The normal range of RADAR pulse widths is .05 iJS to 1.0 iJS (micro-seconds). If we were transmitting 2,400 pulses-per-second, with each pulse 0.12 micro-seconds in LENGTH, this would yield 288 micro-seconds of energy transmitted in that second. If we had 800 pulses, each 0.4 micro-seconds in length, this would amount to 320 micro-seconds of energy within one second, but each second of time has 1,000,000 micro-seconds- too long a pulse on short range would not allow enough time for the receiver to respond to the echo. Too short a pulse at long range may not illuminate the target adequately. The modern RADAR will usually develop the correct algorithm of pulses-per-second and pulse length. ANSWER B
In a solid-state RADAR modulator, the duration of the transmitted pulse is determined by: A. The thyratron. B. The magnetron voltage. C. The pulse forming network. D. The trigger pulse.
The pulse forming network determines the pulse width, which in turn determines the duration of the transmitted pulse. ANSWER C
3-9005 The pulse repetition rate (prr) of a RADAR refers to the: A. Reciprocal of the duty cycle. B. Pulse rate of the local oscillator. C. Pulse rate of the klystron. D. Pulse rate of the magnetron.
The pulse repetition rate (PRR) is the number of pulses per second sent to the target. It also is the number of pulses per second sent from the magnetron. ANSWER D
8-6 A3 The pulse repetition rate (PRR) refers to: A. The reciprocal of the duty cycle. B. The pulse rate of the local oscillator tube. C. The pulse rate of the klystron. D. The pulse rate of the magnetron.
The pulse repetition rate (PRR) is the number of pulses per second sent to the target. It is also the number of pulses per second sent by the magnetron. ANSWER D
8-1781 What RADAR circuit determines the pulse repetition rate (PRR)? A. Discriminator. B. Timer (synchronizer circuit). C. Artificial transmission line. D. Pulse-rate-indicator circuit.
The pulse repetition rate is determined by the RADAR timer synchronizer circuit. ANSWER B
8-1184 The purpose of the Pulse Forming Network is to: A. Act as a low pass filter. B. Act as a high pass filter. C. Produce a pulse of the correct width. D. Regulate the pulse repetition rate.
The pulse-forming network is found in the RADAR modulator, and produces a pulse with a specific correct width. ANSWER C
8-39 E3 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.
The resolution of the RADAR beam is proportional to the length of the rotating antenna. The narrower the beam in the horizontal plane, the better the ability to differentiate multiple targets. ANSWER D
8-1282 The characteristic of the magnetron output pulse that relates to accurate range measurement is its: A. Amplitude. B. Decay time. C. Rise time. D. Duration.
The rise time of the magnetron output must be as short as possible to produce the desired rapid pulse rates. If the rise time is too long, the measured target distance is inaccurate. ANSWER C
8-25 C6 The sensitivity time control (STC) circuit: A. Decreases the sensitivity of the receiver for close objects. B. Increases the sensitivity of the receiver for close objects. C. Increases the sensitivity of the receiver for distant objects. D. Decreases the sensitivity of the transmitter for close objects.
The sensitivity time control is used to minimize false echoes from sea return during heavy weather. It minimizes the pick up of echoes from nearby targets. ANSWER A
3-9406 RADAR collision avoidance systems utilize inputs from each of the following except your ship's: A. Gyrocompass. B. Navigation position receiver. C. Anemometer. D. Speed indicator.
The ship's anemometer, which measures wind speed, will not add to your RADAR collision avoidance input. However, your ship speed will, and same with navigation position from a GPS, as well as your gyro compass. But the anemometer will have no input. ANSWER C
8-1 A1 Choose the most correct statement containing the parameters which control the size of the target echo. A. Transmitted power, antenna effective area, transmit and receive losses, RADAR cross section of the target, range to target. B. Height of antenna, power radiated, size of target, receiver gain, pulse width. C. Power radiated, antenna gain, size of target, shape of target, pulse width, receiver gain. D. Magnetron gain, antenna gain, size of target, range to target, wave-guide loss.
The size and signal strength of a RADAR target echo may be determined by the target cross sectional area, range to the target, transmit and receive path losses, RADAR antenna effective area, and your transmitter power output. Some of the other answers look pretty good, but go with answer A, and look at the drawings to see why distant small targets appear and disappear on the display! Antenna effective area are the key words in the correct answer! ANSWER A
8-39 E6 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.
The slotted array is commonly used on many small boat X-band RADARs. It has a number of in-phase radiating slots that act as a stacked array, narrowing the horizontal beamwidth for increased resolution of individual targets. Slotted waveguide antennas are always encased in a fiberglass radome. ANSWER C
8-39 E4 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.
The slotted waveguide array eliminates the requirement of a large reflector parabola to concentrate the microwave energy in a tight horizontal pattern. While the slotted waveguide array may have a pair of "wings," these are usually relatively small, and will further increase the f-gain of the array as they provide a ground plane for the individual slots. Depending on how the slotted waveguide array is fed, the actual main lobe may be several degrees offset from a line perpendicular to the antenna. ANSWER A
8-39 E2 A typical shipboard RADAR antenna is a: A. Rotary parabolic transducer. B. Slotted waveguide array. C. Phased planar array. D. Dipole.
The slotted waveguide array is the most popular type of marine RADAR antenna. Although we occasionally will see a parabolic reflector or a printed circuit board antenna system, the slotted waveguide is preferred because of its low loss and predictable signal dispersion. ANSWER B
8-1185 The purpose of the Synchronizer is to: A. Generate the modulating pulse to the magnetron. B. Generate a timing signal that establishes the pulse repetition rate. C. Insure that the TR tube conducts at the proper time. D. Control the pulse width.
The synchronizer is a timer circuit to calculate the interval between transmitted pulses and to insure the interval is of the proper length. These timing pulses insure synchronized operation as related to the pulse repetition rate. ANSWER B
8-1 A4 What RADAR component controls timing throughout the system? A. Power supply. B. Indicator. C. Synchronizer. D. Receiver.
The synchronizer is a timing circuit which feeds multiple RADAR circuits, such as transmitter, receiver, duplexer, indicator, and any other RADAR circuit needing timing pulses. If the synchronizer circuit should fail, the entire RADAR will cease to operate. ANSWER C
8-1383 A shipboard RADAR uses a PFN driving a magnetron cathode through a step-up transformer. This results in which type of modulation? A. Frequency modulation. B. Amplitude modulation. C. Continuous Wave (CW) modulation. D. Pulse modulation.
The term "PFN" stands for Pulse-Forming Network, which is a type of pulse modulation. ANSWER D
8-1784 The timer circuit: A. Determines the pulse repetition rate (PRR). B. Determines range markers. C. Provides blanking and unblanking signals for the CRT. D. All of the above.
The timer circuit determines pulse repetition rate and range markers, and provides blanking and unblanking signals for the cathode-ray tube in a raster scan display. ANSWER D
8-5A4 Short range RADARs would most likely transmit: A. Narrow pulses at a fast rate. B. Narrow pulses at a slow rate. C. Wide pulses at a fast rate. D. Wide pulses at a slow rate.
To detect an echo when the target is close by, you need a narrow pulse with a fast pulse repetition rate. A wide, slow pulse would actually cover up the return echo. ANSWER A
8-20 C3 An RF mixer has what purpose in a RADAR system? A. Mixes the CW transmitter output to form pulsed waves. B. Converts a low-level signal to a different frequency. C. Prevents microwave oscillations from reaching the antenna. D. Combines audio tones with RF to produce the RADAR signal.
To obtain the required gain and bandwidth of a signal, a crystal mixer is employed to lower the frequency of the received signal to the 30-60 MHz band. ANSWER B
3-9106 The component or circuit providing the transmitter output power for a RADAR system is the: A. Thyratron. B. SCR. C. Klystron. D. Magnetron.
Transmitter power output is delivered by the magnetron. ANSWER D
8-41 E2 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.
Usually, 3 stator lines carry the angular information from the antenna to the indicator. ANSWER B
8-41 E1 Waveguides can be constructed from: A. Brass. B. Aluminum. C. Copper. D. All of the above.
Waveguide construction requires tighter tolerances, and must be protected from any forces that could damage the integrity of the waveguide run. Sturdy brass may be good for long runs, but aluminum and copper will keep weight down aloft. ANSWER D
8-42 E4 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.
Waveguide is used exclusively at RADAR microwave frequencies because losses are lower than coaxial feedline. Waveguide analysis is similar to open line; that is, two parallel lines. At microwave frequencies it is not practical to have open line because it must be insulated and high-quality insulators are hard to make for microwave frequencies. ANSWER D
8-3 A4 A RADAR operating at a frequency of 3 GHz has a wavelength of approximately: A. 1 centimeter. B. 10 centimeters. C. 3 centimeters. D. 30 centimeters.
Wavelength in em = 30,000/frequency in MHz (30,000 + 3,000 = 10.0 em). Remember, for this formula the frequency is in MHz. ANSWER B
3-9002 The RADAR range in nautical miles to an object can be found by measuring the elapsed time during a RADAR pulse and dividing this quantity by: A. 0.87 seconds. B. 1.1 microseconds C. 12.346 microseconds . D. 1.073 microseconds .
We know that it takes 6.173 (previously rounded to 6.2) microseconds for a RADAR wave to travel 1 nautical mile. Therefore, it will take twice that, or 12.35 microseconds, to make the trip from the RADAR antenna to the target 1 nautical mile away and then back to the antenna. ANSWER C
8-40 E6 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.
While newer RADAR transmitters and receivers might be built directly into the bottom side of the rotating antenna assembly, there are still many RADAR installations with the TR equipment many feet away from the rotating antenna. This requires waveguide to interconnect the TR to the antenna array, and long horizontal runs should be sloped slightly downward with a small drain hole drilled in the elbow to allow condensation either to escape or dry out. ANSWER A
8-1586 What is the purpose or function of the RADAR duplexer/circulator? A. An electronic switch that allows the use of one antenna for both transmission and reception. B. A coupling device that is used in the transition from a rectangular waveguide to a circular waveguide. C. A modified length of waveguide used to sample a portion of the transmitted energy for testing purposes. D. A dual section coupling device that allows the use of a magnetron as a transmitter.
You can consider the duplexer and circulator as an electronic switch that allows the outgoing energy and incoming echo to switch between the antenna and the receiver without outgoing energy directly coupling into the receiver causing burnout. ANSWER A