CDC 3D153 v2 Flashcards
Define communications system.
A group of related components designed to transfer information from one point to another.
Name the transmitter requirements for successful communications.
Relatively stable in frequency and amplitude and free from excessive noise, harmonics, and spurious outputs.
Name the five basic functions of a communications receiver.
Reception, selection, detection, amplification, and reproduction.
What is the difference between a receiver’s selectivity and its sensitivity?
Sensitivity—the ability of a receiver to reproduce the signal of a very weak station.
Selectivity—the ability of a receiver to select and reproduce a desired signal from several closely spaced stations or from interfering frequencies.
What is a transceiver?
A transceiver is a combination of a transmitter and a receiver built as a single unit and sharing common tuned circuits.
What has also allowed our transceivers to become lighter, smaller, and easier to operate?
Computerization.
Define “transmission line” and give an example of one.
A conductor or series of conductors used to carry energy from a source to a load. Examples include the flexible coaxial cable, the rigid coaxial cable, an AC power cord on a stereo, a cable television wire, and a telephone cord.
What are the major power losses in a flexible coaxial cable?
The dielectric and skin effect.
Why is there very little radiation loss in a rigid coaxial cable?
Because the energy is confined between the two conductors.
At what frequencies are waveguides used?
For frequencies so high that their wavelength is miniscule.
What physical properties determine the characteristics of a rectangular waveguide?
Internal height and width.
What effect does internal moisture have on a waveguide?
It can cause serious arching.
Identify the primary determinants of a line’s capacitance.
The amount of capacitance is primarily determined by the size of the conductors, the space between them, plus the dielectric material.
What determines ZO of a transmission line?
Its series inductance and shunt capacitance.
Describe how a transmission line’s characteristic impedance is affected by changing its physical length.
Changing the physical length of a transmission line has no effect on characteristic impedance.
What does “cutoff frequency” refer to when talking about transmission lines?
The frequency at which the value of XL and XC are such that the signals will be developed across the series inductance and shunted by the capacitance and, thus, not pass along the line.
Define “wavelength.”
The distance in space occupied by one cycle of a radio wave at any given instant.
As signal frequency decreases, what happens to wavelength?
Wavelength increases.
What is the unit of measure for electrical length?
Number of wavelengths.
If the transmission line’s physical length remains constant, what happens to electrical length as frequency decreases?
Electrical length decreases.
Describe a nonresonant transmission line.
It is a line having no reflected waves.
What is an incident wave?
Voltage and current waves as they move from source to load.
What is a standing wave?
It is the vector sum of the forward and reflected waves.
Define “VSWR.”
This is a comparison (expressed as a ratio) of the maximum and minimum voltages found along the lines.
If a transmission line has a perfect impedance match, what would the VSWR be?
1:1.
What will likely happen with a high VSWR?
A high VSWR not only causes communications to fail, it can also damage the equipment or transmission line.
At what level will voltage be at an open termination?
Voltage will be at maximum across an open.
What is the phase relationship between forward and reflected voltage waves when the line is terminated in a short?
Voltage is reflected 180° out of phase.
What is the purpose of the antenna?
It is a transducer; it converts RF energy as current oscillations into electric and magnetic fields of force.
In radio communications, which major force fields are we concerned with?
Electric and magnetic.
What is the relationship between the electric and magnetic fields?
If an electric field is changing, a magnetic field is created; if a magnetic field is changing, an electric field is created.
Which type of electric current continually changes in size and direction?
Alternate current is continually changing in size and periodically changing in direction.
Define radio waves.
EM fields of force; that is, magnetic fields generated by continually changing electric fields.
What happens to magnetic fields above approximately 10,000 cps?
They are radiated from the conductor in the form of radio waves.
Describe the induction and radiation fields.
The radiation field detaches from the antenna and travels through space. The induction field is the portion of the EM field that immediately surrounds the antenna and collapses completely when the antenna voltage and current reverse.
At what distance from the antenna are the induction field and the radiation field the same strength?
One-sixth of a wavelength from the antenna.
Explain the rate of decrease in the two fields’ strengths as the distance from the antenna increases.
The induction field decreases as the square of the distance increases. The radiation field decreases linearly with distance.
What are the determining factors that affect the pattern of radiation?
Antenna design, the earth, and surrounding objects affect the direction of radiation.
What is the polarization if the E field component travels in a plane parallel to the Earth’s surface?
Horizontal.
How can an antenna receive a maximum transfer of energy from the EM fields?
An antenna needs to be in the proper plane of polarization for maximum transfer of energy from the EM fields.
Describe circular polarization.
In a circularly polarized antenna, the plane of polarization rotates in a corkscrew pattern, making one complete revolution during each wavelength. A circular polarized wave radiates energy in both the
horizontal and vertical planes and all planes in between.
Differentiate between a resonant and nonresonant antenna?
In a resonant antenna, almost the entire radio signal fed to the antenna is radiated. If the antenna is fed with a frequency other than its designed frequency, much of the signal is lost and is not radiated.
What is the frequency range of a resonant antenna?
The frequency of a resonant antenna effectively radiates a radio signal for frequencies close to its designed frequency, usually within a range of plus or minus 2 percent.
What is a frequency independent antenna?
A nonresonant antenna designed to operate over a wide range of frequencies using maximum power is known as a frequency independent antenna.
In the real world, what is the ideal standing wave ratio?
1.1 to 1.
What is the fundamental frequency of an antenna?
The lowest frequency at which it resonates.
What is the concept of antenna reciprocity?
The more efficient it is for transmitting, the more efficient it is for receiving.
What is an isotropic antenna?
A theoretical antenna that radiates equally well in all directions.
How can an antenna deliver twice as much power to the receiving antenna?
The antenna does this by redirecting the energy from other directions.
What allows all available power to be absorbed and radiated by the antenna without reflections back down the line?
Proper impedance match.
Describe an antenna coupler.
A coupler is a matching device inserted between a transmitter and its antenna to make a transmitter “think” it is connected to a low-SWR antenna.
What can you use to match a 50-ohm cable to a 600-ohm antenna?
A balun can be used to match the impedance.
Describe each classification of antenna in accordance with its radiation pattern.
An omnidirectional antenna radiates radio energy in a circular pattern. A bidirectional antenna has two main lobes, with nulls between them. A unidirectional antenna has a single large lobe in one direction and
greatly reduced lobes or nulls in other directions.
Why would you use an omnidirectional antenna?
The omnidirectional antenna is used when it is necessary to communicate in several different directions at once, such as an ATC tower or operating in a multi-station net.
Give examples of a deployable bidirectional antenna.
Deployable bi-directional antennas are usually the inverted-V or center-fed, half-wave dipole.
What is the biggest factor about setting up unidirectional antennas?
Since a unidirectional antenna concentrates almost all the radio signal in one specific direction, it must be carefully oriented.
How can you make a long-wire antenna directional?
A long-wire antenna can also be made directional by placing a terminating resistor at the distant end of the antenna.
Define beamwidth.
The beamwidth of a directive antenna is the width in degrees, of the major lobe between the two direction at which the relative radiated power is equal to one-half its value at the peak of the lobe.
What are the two basic types of antenna?
Hertz and Marconi.
What are Hertz antenna specifically designed for?
Hertz antennas are ungrounded lengths of wire specifically designed to be either a half-wavelength long, such as a dipole or doublet antenna or more than a full wavelength long, such as a long-wire antenna.
How did Mr. Marconi overcome great antenna heights at lower RF ranges?
Mr. Marconi found that when a quarter-wave vertical antenna has its base on the ground, the earth below the antenna acts like a large reflector (or mirror) and supplies another quarter-wavelength. In effect, the quarter-wave vertical antenna acts like a half-wave antenna.
What is the most commonly used HF antenna for covering short distances in a tactical environment?
Vertical monopole.
Which HF antenna is a complicated multidipole array that’s often used in high-powered fixed radio stations?
HF discone.
When an LPA receives a certain frequency, which parts of the log periodic antenna are actually being used?
Only the parts of the LPA that are resonant to the operating frequency are actually being used.
What reduces the gain and efficiency of half-wave dipole antenna?
Excessive antenna sag can actually reduce the gain and efficiency of a half-wave dipole antenna.
Describe multiband half-wave dipole antenna?
When space or other resources aren’t adequate to erect separate dipoles, you can combine three or four dipoles to occupy the space normally required for one. Each wire in a multiband dipole is cut to a halfwavelength of an assigned frequency. All the separate antennas are connected to the same antenna feed block or connector and are fed by a single transmission line.
Define inverted-V antenna?
The inverted-V is a half-wave dipole, supported in the center by a single mast, with both antenna legs anchored near the ground. It is designed and cut for a specific frequency and has a bandwidth of plus or
minus 2 percent.
What is the minimum length of a long-wire antenna?
A half -wavelength.
How many wavelengths are required to produce the best results on a sloping long-wire?
The sloping wire produces best results when it is more than two wavelengths long.
What improvements does the sloping V offer over the standard long-wire antenna?
Sloping V long-wire version is an improvement over the standard long wire and the sloping long wire, in that the two legs tend to reinforce each other for improved performance and increased effective radiated power.
Which elements on the inverted L antenna provide omnidirectional radiation for ground-wave propagation and high-angle radiation for short-range sky wave propagation?
Inverted L antenna provides omnidirectional radiation for ground-wave propagation from the vertical element and high-angle radiation from the horizontal element for short-range sky wave propagation.
What is a NVIS antenna?
The NVIS omnidirectional antenna is a HF high-angle radiation and low operating frequencies antenna.
What are all satellite communications antennas designed to be?
Highly directional and high-gain in order to overcome free-space loss affecting satellite systems.
What is the basic principle of all parabolic antennas?
Reflection of a radio signal for controlled directivity.
Which type of polarization does a helical satellite antenna uses to radiate a signal?
Circular polarization.
What is the horn antenna used for?
In the transmission and reception of RF microwave signals.
What are the typical applications for airborne antennas?
Wideband and satellite communications; telemetry data collection, tracking, and signal relay; timing signal reception; and satellite navigation signal reception.
What angles should the signal arrive at the aircraft antenna for frequencies in the lower portions of the HF band?
Vertical angles between 20 and 60 degrees above the horizon.
Where on the aircraft is the vertical whip antenna typically mounted?
Typically, on top of the aircraft’s fuselage.
What are two problems associated with airborne antenna operating above HF?
- A serious problem at these frequencies involves the interference between the direct ray and groundreflected ray. As a result, a receiver in an aircraft which approaches a ground station from a considerable distance at a constant altitude will experience a marked rise and fall in signal strength as the aircraft moves in.
- Another difficult problem is the aircraft antenna design. Obstructions forming part of the airframe produces undesired shadows and diffraction effects. In addition, reflections from these obstructions produce interference effects that result in many lobes.
What types of application use the fixed-blade and other low-profile antenna styles?
Fixed-blade and other low-profile antenna styles are used for such things as UHF/SHF satellite communications, telemetry data relay, transponder tracking, and global positioning system navigation.
Where on the aircraft would satellite communications antennas be mounted?
On the top of the aircraft.
Which type of antenna is mounted on the nose of the aircraft?
A steerable parabolic dish antenna used for telemetry tracking is mounted in an aircraft’s nose.
What antenna has a unique design where the antenna becomes part of the skin of the vehicle?
A unique antenna design where the antenna becomes part of the skin of the vehicle is the conformal antenna.
What is the first thing you look at when selecting an antenna for an HF circuit?
The type of propagation.
What is the first step in selecting an antenna for HF sky wave propagation?
Find the distance of the circuit so that you can find the required takeoff angle.
Describe the ideal antenna site selection setting.
A clear, flat area with no trees, buildings, fences, power lines, or mountains.
Name the 5 steps in the HF sky wave antenna selection process?
HF sky wave antenna selection involves:
1. Determining the range.
2. Determining the type of coverage (omnidirectional, bidirectional, directional).
3. Determining the operating frequency.
4. Determining the takeoff angle required to clear site obstacles.
5. Selecting the antenna with the highest gain at the required takeoff angle that can be erected on the
available site with the available materials.
Differentiate the one-half wavelength above the ground and the one-quarter wavelength above the ground radiation pattern of a dipole antenna.
When a half-wave dipole antenna is placed one-half wavelength above the ground, the radiation pattern has two main lobes. When it’s lowered to a quarter-wavelength above the ground, the radiation is directed
upwards in one large lobe.
What is the gain of an 8-meter vertical dipole antenna at 18 MHz and a radiation angle of 30°?
7.5 dBi.
For a LOS system (microwave), what determines if a repeater is necessary?
The distance to be spanned and the terrain.
What kind of location should you pick for a tactical antenna that uses frequencies above 30 MHz?
One that allows LOS communications.
Why does dry ground limit a radio set’s range?
Dry ground has high resistance and limits the range of the radio set.
What object has the greatest effect on an antenna?
The earth.
Why shouldn’t you use sawed-off branches and foliage for camouflage?
The leaves soon wilt, change color, and become conspicuous against the natural growth.
To what level can the poor electrical ground caused by the dry, sandy soil of a desert reduce the effectiveness of a whip antenna?
As little as one-third of its normal efficiency.
In mountainous areas with high winds, what should you do to the antenna masts?
Secure the masts with extra guy ropes in anticipation of high winds.
What causes sensitive components to either malfunction or get out of alignment?
Rough handling.
Which radio wave travels near the earth’s surface of the earth and are greatly affected by the earth’s conductivity?
Since ground waves travel near the surface of the earth, they’re greatly affected by the earth’s conductivity and by any obstruction (such as mountains or buildings) on its surface.
What are the limiting factors for direct wave communications?
Direct waves continue to travel in a straight line until they are interrupted by an object or weaken over a great distance. The average distance of direct wave communications is therefore limited by the height of the transmit or receive antenna.
What determines the earth’s conductivity?
The type of soil and water in the propagation path.
Name the radio wave used in long-distance communications.
Sky wave transmissions are very effective for long-distance communications in the HF range (3 – 30 MHz).
What frequency is used for ground wave propagation?
Low and very low frequencies.
How we extend LOS distance?
Increasing the height of the transmitting antenna, the receiving antenna, or both.
What is the best type of surface for surface wave transmission?
Seawater.
What gives sky wave propagation its ability to communicate beyond the optical LOS?
Refraction.
Describe the skip zone.
The area between the most distant point reached by the ground waves of a particular signal and the point at which the ionospheric wave first returns to the earth. In this zone, you would experience a zone of silence because no radio signals are received.
Define skip distance.
The distance from the transmitter to the point at which the refracted sky-wave first returns to earth.
What is the primary loss from multihop transmission?
Each time a hop is made, considerable signal strength loss occurs. This loss results primarily from absorption.
What causes the cancellation and summation effects of the received signal to occur at the receiver?
Because of the relative amplitude and phase differences of these various signals.
