This is the Review Notes in Microwave Communications as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.
Summary of Important Microwave Communications Review Notes
Attenuation is the progressive decrease of signal strength with increasing distance
If K-factor is greater than 1, the array beam is bent towards the earth
The antenna separations (in meters) required for optimum operation of a space diversity system can be calculated from: S = 3λR/L
Rainfall is an important factor for fading of radio waves at frequencies above 10 GHz
Theoretically electromagnetic radiation field strength varies in inverse proportion to the square of the distance, but when atmospheric attenuation effects and the absorption of the terrain are taken into account the attenuation can be as high as the inverse sixth power of the distance
Slow fading – an attenuation that occurs over many different wavelengths of the carrier
Slow fading is not due to multipath
Large reflector causes multipath or frequency-selective fading
In microwave transmission using digital radio, delayed spreading causes most intersymbol interference
SONAR – a shipboard equipment which measures the distance between the ship’s bottom and the ocean floor
The cavity resonator is equivalent to an LC resonant circuit
The input signal of a traveling-wave tube is inserted at the cathode end of the helix
Coupling into and out of a traveling-wave tube can be accompanied by a waveguide match, cavity match and direct coax-helix match
A high-power microwave pulse of the order of megawatts can be generated by a magnetron
A traveling-wave tube (TWT) amplifies by virtue of the absorption of energy by the signal from an electron stream
The purpose of the electromagnetic field which surrounds a traveling-wave tube is to keep the electrons form spreading out
Tunnel diode and klystron tube are used as an oscillator device in the SHF band
Microwave frequencies are normally regarded as those in the range of 1 to 100 GHz
The highest frequency which a conventional vacuum-tube oscillator can generate is not limited by the degree of emission from the cathode
As the electron beam moves through a klystron’s intercavity drift space the velocity modulation at the input cavity creates density modulation at the output cavity
The frequency of the oscillation generated by a magnetron, is mainly determined by the dimension of each cavity resonator
If the instantaneous RF potentials on the two sides of a magnetron cavity are of opposite polarity, the operation is in the π mode
The Gunn diode oscillator depends on the formation of charge domain
Circular ferrite device can be used instead of duplexer to isolate a microwave transmitter and receiver when both are connected to the same antenna
To achieve good bearing resolution when using a pulsed-radar set, an important requirement is a narrow, antenna-beam width in the horizontal plane
When used in conjunction with a radar set, the purpose of an echo box is to provide an artificial target which may be used to tune the radar receiver
In a radar-set receiver, the usual mixer stage is a silicon crystal
Klystron oscillators are most often used in the frequency range of 3000 to 30000 MHz
Oscillations of a klystron tube are maintained by bunches of electrons passing the cavity grids
Ferrite emitter allows microwave to pass in only one direction
In an SHF pulsed radar set, a reflex klystron can be used as a local oscillator
A PPI cathode-ray tube as used on a radar set indicates both the range and azimuth of a target
The resonant frequency of a cavity resonator depends upon its physical dimensions
The maximum usable range of the usual radar set (on any particular range setting) is determined by the interval between transmitted pulses
A reflex klystron is oscillating at the frequency of its resonant cavity. If the reflector voltage is made slightly less negative, the frequency will decrease
The coarse frequency adjustment of a reflex klystron is accomplished by adjusting the flexible wall of the resonant cavity
In a pulsed radar set, the STC circuit is used to reduce interference from the effects of sea return
In a pulsed radar set, the function of the duplexer is to allow the transmitter and the receiver to operate from a common antenna
The aquadag coating on the inside of a PPI tube is used as a second anode and to prevent the build-up of secondary field
If the duration of the radar transmitted pulse, on a particular range of operation is increased, the required bandwidth of the receiver’s IF amplifiers may be decreased
The main frequency determining element of a klystron is its resonant cavity
A thin layer of dirt and grime covers the reflecting surface of the parabolic dish of a radar set. The practical effect on the performance of the radar will have no noticeable effect
Isolator permits a microwave signals to travel in one direction with virtually no loss, but severely attenuates any signal attempting to travel in the reverse direction
It is possible to increase the maximum range of a radar equipment by lower the pulse frequency, raising the peak power of the transmitter, and narrowing the beam width and increasing the pulse duration
When it is desired that short-range targets be clearly seen on a pulsed-radar set, it is important that the receiver and display system have the shortest possible time
Magnetron is used as a high-power microwave oscillator
Modern loran navigational system operates at loran C: 100 kHz
Circulator ferrite device can be used instead of a duplexer to isolate a microwave transmitter and receiver when both are connected to the same antenna
The pulse frequency is equal to duty cycle/pulse width
In a radar set, a blocking oscillator can be used to produce a trigger pulse for the transmitter
The intensity of the echoes (target definition) on a PPI display is determined by the pulse frequency and antenna rotation rate
A duplexer circuit allows a transmitter and a receiver to operate from the same antenna with virtually no interaction. This circuit may be replaced by a circulator
156.76 MHz and 156.8 MHz frequencies are used by a class-C Emergency Position-Indicating Radio Beacon
The repetition rate of a pulsed radar system indicates the frequencies of the duty cycle
The radar set, sensitivity-time control circuit can reduce sea-return response
Isolator is a ferrite device that buffers a microwave source from the effects of a varying load, and thereby prevents the formation of standing waves
Pulsed radar sets are primarily used to find the target’s range and bearing
Radio frequency is mainly concerned in the design of an antenna system for pulsed radar set
In a radar set receivers, an ac AFC system may be used to maintain the desired klystron frequency
The input signal is introduced into the traveling-wave tube at the cathode end of the helix
The display on the PPI scope of a radar set will have greater intensity at lower antenna rotation speeds and higher pulse repetition
The operating frequency of loran C lies within the LF band
The PPI tubes have an aquadag coating on the inside of the tube. The purpose of this coating is to act as the second anode
A traveling-wave tube is used at frequencies in the order of 8000 MHz
The main benefit of using microwave is more spectrum space for signals
Radio communications are regulated in the Philippines by the NTC
Mobile radio is not a common microwave application
Coaxial cable is not widely used for long microwave transmission lines because of its high loss
Stripline and microstrip transmission lines are usually made with PCBs
The most common cross section of a waveguide is a rectangle
Signal propagation in a waveguide is by electric and magnetic fields
When the electric field in a waveguide is perpendicular to the direction of wave propagation, the mode is said to be transverse electric
The dominant mode in most rectangular waveguide is TE1,2
A magnetic field is introduced into waveguide by a probe
A half-wavelength, close section of a waveguide that acts as a parallel resonant circuit is known as cavity resonator
Decreasing the volume of a cavity causes its resonant frequency to decrease
Hot carrier is a popular microwave mixer diode
Varactor and step-recovery diodes are widely used in frequency multiplier
Gunn diode is a popular microwave oscillator
Tunnel diode does not ordinarily operate with reverse bias
Low-power Gunn diodes are replacing reflex klystrons
Cathode-ray tube is not a microwave tube
In a klystron amplifier, velocity modulation of the electron beam is produced by the catcher cavity
A reflex klystron is used as an oscillator
For proper operation, a magnetron must be accompanies by a permanent magnet
The operating frequency of klystrons and magnetrons is set by the cavity resonators
A magnetron is used only as an oscillator
A common application for magnetron is in radar
In a TWT, the electron beam is density-modulated by a helix
The main advantage of a TWT over a klystron for microwave amplification is wider bandwidth
High-power TWTs are replacing Klystrons
Horn – the most widely used microwave antenna
MTBF is a measure of reliability expressed as the average number of hours between successive failures
Baseband repeater – an active microwave radio repeater that can provide drops and inserts
If the correction factor k equals 4/3 of the earth’s curvature, the microwave beam would have a curvature that is more than that of the earth
The optimum clearance from an obstacle in a microwave system is accepted as 0.6 of the first Fresnel zone radius
The microwave beam curves the same than that of the earth when the value of the correction factor k equals infinity
The ability of a radar to determine the bearing to a target depends upon the antenna directivity
The Doppler effect allows speed of a target to be measured
The Doppler effect is a change in frequency produced by relative motion between the radar set and a target
The most widely used radar transmitter component is a magnetron
Low-power radar transmitters and receivers use Gunn diode
Spark gap in a duplexer protects the receiver from the higher transmitter output
Most radar antennas use a horn and parabolic reflector
The most common radar display is the PPI
A radar antenna using multiple dipoles or slot antennas in a matrix with variable phase shifters is called a phased array
10 GHz is a typical radar operating frequency
A microwave system requires the use of repeaters when the distances involve are greater
Waveguides are transmission line which convey electromagnetic waves in highest frequencies
A microwave band of 10.9 to 36 GHz is considered as K-band
92.4 + 20 log F + 20 log D – a microwave communications system space loss calculation formula
A waveguide is also a high pass filter
Frequency diversity – a method of diversity reception where the signal is transmitted on two different frequencies over the same path
Litz wire – a wire used to reduce the skin effect
Space diversity transmission means transmitting and receiving on two or more antennas operating on the same frequencies
Frequency diversity – the best system configuration to overcome multipath fading of microwave system over the water
When the value of k increases, the effective result is flattening of the equivalent curvature
A traveling wave tube consists of electron gun, helix and collector
Advantage of periscope antenna in microwave: shorten waveguide length
The cut-off frequency of a waveguide is the lowest frequency the waveguide operates
When the clearance above the obstruction is equal to the radii of even Fresnel zones at the point of reflection, the RSL is decreased
The wavelength of a wave in a waveguide is greater than in free space
Water causes more attenuation particularly on 183 GHz frequency
Oxygen cause excessive attenuation at 60 GHz frequency
Terrestrial is a microwave link between the down-town terminal and another out of town terminal
Attenuator is used in the traveling wave tube to prevent oscillation
Space diversity is a method of diversity reception applied to reflective path to reduce fading
K X 4000 miles is the effective earth’s radius used in communications design
IF bandwidth of a radar system is inversely proportional to pulse width
EADI stands for Electronic Altitude and Director Indicator
DME aircraft navigational system determines the time to station (TTS) or time to go (TTG)
A radio altimeter operate at 43500 MHz frequency
Terrestrial Flight Telephone System allows passengers to make telephone calls, send faxes, and computer data shop and play computer games, etc
ADF stands for Automatic Direction Finder
RF carrier of the middle marker is modulated at 1300 Hz
The major advantage of using a helix traveling wave tube is its wide bandwidth
RADAR stands for Radio Detection and Ranging
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