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3D153 V2 - U1 SS
Antenna Theory
101
Communication
Professional
03/12/2012

Additional Communication Flashcards

 


 

Cards

Term
201-1. Define communications system.
Definition
A group of related components designed to transfer information from one point to another.
Term

201-2. Match the communications component in column B with the characteristic it is associated with in column A. Items in column B may be used once, more than once, or not at all.
Column A
____ (1) Converts audio in RF.
____ (2) Radiates the energy into space.
____ (3) Converts modulated RF into audio.
____ (4) Carry the RF power from antenna to receiver.
____ (5) Carry the RF power from transmitter to antenna.

 

Column B
a. Transmission lines.
b. Transmitter.
c. Receiver.
d. Antenna.

Definition
(1) b.
(2) d.
(3) c.
(4) a.
(5) a.
Term
201-3. Name the transmitter requirements for successful communications.
Definition
Relatively stable in frequency and amplitude and free from excessive noise, harmonics, and spurious outputs.
Term

201-4. Match the modulation component in column B with the characteristic it is associated with in column A. Items in column B may be used once, more than once, or not at all.
Column A
____ (1) Consists of only one sideband.
____ (2) Use for systems incorporated time-division multiplexing.
____ (3) Phase of the frequency is shifted.
____ (4) Amplitude of the RF waves varies at an audio rate.
____ (5) Commonly used in radio teletype applications.
____ (6) Frequency of carrier wave is varied.

Column B
a. AM.
b. FM.
c. FSK.
d. PSK.
e. Pulse.
f. SSB

Definition

(1) f.
(2) e.

(3) d.
(4) a.
(5) c.
(6) b.

Term
201-5. Name the five basic functions of a communications receiver.
Definition
Reception, selection, detection, amplification, and reproduction.
Term
201-6. What is the difference between a receiver’s selectivity and its sensitivity?
Definition
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.
Term
202-1. What is a transceiver?
Definition
A transceiver is a combination of a transmitter and a receiver built as a single unit and sharing common tuned circuits.
Term
202-2. What has also allowed our transceivers to become lighter, smaller, and easier to operate?
Definition
Computerization.
Term
203-1. Define "transmission line" and give an example of one.
Definition
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.
Term

203-2. Match the type of line loss in column B with the characteristic it is associated with in column A. Items in column B may be used once, more than once, or not at all.
Column A
____ (1) Also called dielectric loss.
____ (2) Loss when line acts like a transmitting antenna.
____ (3) HF currents flow near the outer conductor surface.
____ (4) Loss because no insulation is perfect.
____ (5) Conductor resistance to transfer energy.
____ (6) Dissipates power into neighboring wires.

Column B
a. I2R.
b. Skin-effect.
c. Radiation.
d. Induction.
e. Capacitive
f. Leakage

Definition
(1) e.
(2) c.
(3) b.
(4) f.
(5) a.
(6) d.
Term
203-3. What are the major power losses in a flexible coaxial cable?
Definition
The dielectric and skin effect.
Term
203-4. Why is there very little radiation loss in a rigid coaxial cable?
Definition
Because the energy is confined between the two conductors.
Term
203-5. At what frequencies are waveguides used?
Definition
For frequencies so high that their wavelength is miniscule.
Term
203-6. What physical properties determine the characteristics of a rectangular waveguide?
Definition
Internal height and width.
Term
203-7. What effect does internal moisture have on a waveguide?
Definition
It can cause serious arching.
Term
203-8. Identify the primary determinants of a line’s capacitance.
Definition
The amount of capacitance is primarily determined by the size of the conductors, the space between them, plus the dielectric material.
Term
203-9. What determines Zo of a transmission line?
Definition
Its series inductance and shunt capacitance.
Term
203-10. Describe how a transmission line’s characteristic impedance is affected by changing its physical length.
Definition
Changing the physical length of a transmission line has no effect on characteristic impedance.
Term
203-11. What does "cutoff frequency" refer to when talking about transmission lines?
Definition
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.
Term
203-12. Define "wavelength."
Definition
The distance in space occupied by one cycle of a radio wave at any given instant.
Term
203-13. As signal frequency decreases, what happens to wavelength?
Definition
Wavelength increases.
Term
203-14. What is the unit of measure for electrical length?
Definition
Number of wavelengths.
Term
203-15. If the transmission line’s physical length remains constant, what happens to electrical length as frequency decreases?
Definition
Electrical length decreases.
Term
203-16. Describe a nonresonant transmission line.
Definition
It is a line having no reflected waves.
Term
203-17. What is an incident wave?
Definition
Voltage and current waves as they move from source to load.
Term
203-18. What is a standing wave?
Definition
It is the vector sum of the forward and reflected waves.
Term
203-19. Define "VSWR."
Definition
This is a comparison (expressed as a ratio) of the maximum and minimum voltages found along the lines.
Term
203-20. If a transmission line has a perfect impedance match, what would the VSWR be?
Definition
1:1
Term
203-21. What will likely happen with a high VSWR?
Definition
A high VSWR not only causes communications to fail, it can also damage the equipment or transmission line
Term
203-22. At what level will voltage be at an open termination?
Definition
Voltage will be at maximum across an open.
Term
203-23. What is the phase relationship between forward and reflected voltage waves when the line is terminated in a short?
Definition
Voltage is reflected 180° out of phase
Term
204-1. What is the purpose of the antenna?
Definition
It is a transducer; it converts RF energy as current oscillations into electric and magnetic fields of force.
Term
204-2. In radio communications, which major force fields are we concerned with?
Definition
Electric and magnetic
Term
204-3. What is the relationship between the electric and magnetic fields?
Definition
If an electric field is changing, a magnetic field is created; if a magnetic field is changing, an electric field is created.
Term
204-4. Which type of electric current continually changes in size and direction?
Definition
Alternate current is continually changing in size and periodically changing in direction
Term
204-5. Define radio waves.
Definition
EM fields of force; that is, magnetic fields generated by continually changing electric fields
Term
204-6. What happens to magnetic fields above approximately 10,000 cps?
Definition
They are radiated from the conductor in the form of radio waves
Term
204-7. Describe the induction and radiation fields.
Definition
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
Term
204-8. At what distance from the antenna are the induction field and the radiation field the same
strength?
Definition
One-sixth of a wavelength from the antenna
Term
204-9. Explain the rate of decrease in the two fields’ strengths as the distance from the antenna
increases.
Definition
The induction field decreases as the square of the distance increases. The radiation field decreases linearly with distance
Term
204-10. What are the determining factors that affect the pattern of radiation?
Definition
Antenna design, the earth, and surrounding objects affect the direction of radiation
Term
205-1. What is the polarization if the E field component travels in a plane parallel to the Earth’s surface?
Definition
Horizontal
Term
205-2. How can an antenna receive a maximum transfer of energy from the EM fields?
Definition
An antenna needs to be in the proper plane of polarization for maximum transfer of energy from the EM fields
Term
205-3. Describe circular polarization.
Definition
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.
Term
206-1. Differentiate between a resonant and nonresonant antenna?
Definition
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.
Term
206-2. What is the frequency range of a resonant antenna?
Definition
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.
Term
206-3. What is a frequency independent antenna?
Definition
A nonresonant antenna designed to operate over a wide range of frequencies using maximum power is known as a frequency independent antenna
Term
206-4. In the real world, what is the ideal standing wave ratio?
Definition
1.1 to 1.
Term
206-5. What is the fundamental frequency of an antenna?
Definition
The lowest frequency at which it resonates.
Term
206-6. What is the concept of antenna reciprocity?
Definition
The more efficient it is for transmitting, the more efficient it is for receiving
Term
206-7. What is an isotropic antenna?
Definition
A theoretical antenna that radiates equally well in all directions
Term
206-8. How can an antenna deliver twice as much power to the receiving antenna?
Definition
The antenna does this by redirecting the energy from other directions
Term
206-9. What allows all available power to be absorbed and radiated by the antenna without reflections back down the line?
Definition
Proper impedance match.
Term
206-10. Describe an antenna coupler.
Definition
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
Term
206-11. What can you use to match a 50-ohm cable to a 600-ohm antenna?
Definition
A balun can be used to match the impedance
Term
207-1. Describe each classification of antenna in accordance with its radiation pattern.
Definition
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
Term
207-2. Why would you use an omnidirectional antenna?
Definition
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.
Term
207-3. Give examples of a deployable bidirectional antenna.
Definition
Deployable bi-directional antennas are usually the inverted-V or center-fed, half-wave dipole.
Term
207-4. What is the biggest factor about setting up unidirectional antennas?
Definition
Since a unidirectional antenna concentrates almost all the radio signal in one specific direction, it must be carefully oriented.
Term
207-5. How can you make a long-wire antenna directional?
Definition
A long-wire antenna can also be made directional by placing a terminating resistor at the distant end of the antenna.
Term
207-6. Define beamwidth.
Definition
The beamwidth of a directive antenna is the width in degrees, of the major lobe between the two directions at which the relative radiated power is equal to one-half its value at the peak of the lobe.
Term
208-1. What are the two basic types of antenna?
Definition
Hertz and Marconi
Term
208-2. What are Hertz antenna specifically designed for?
Definition
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.
Term
208-3. How did Mr. Marconi overcome great antenna heights at lower RF ranges?
Definition
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.
Term
209-1. What is the most commonly used HF antenna for covering short distances in a tactical environment?
Definition
Vertical monopole.
Term
209-2. Which HF antenna is a complicated multidipole array that’s often used in high-powered fixed radio stations?
Definition
HF discone.
Term
209-3. When an LPA receives a certain frequency, which parts of the log periodic antenna are actually being used?
Definition
Only the parts of the LPA that are resonant to the operating frequency are actually being used.
Term
209-4. What reduces the gain and efficiency of half-wave dipole antenna?
Definition
Excessive antenna sag can actually reduce the gain and efficiency of a half-wave dipole antenna.
Term
209-5. Describe multiband half-wave dipole antenna?
Definition
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 half-wavelength 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.
Term
209-6. Define inverted-V antenna?
Definition
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.
Term
209-7. What is the minimum length of a long-wire antenna?
Definition
A half -wavelength.
Term
209-8. How many wavelengths are required to produce the best results on a sloping long-wire?
Definition
The sloping wire produces best results when it is more than two wavelengths long.
Term
209-9. What improvements does the sloping V offer over the standard long-wire antenna?
Definition
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.
Term
209-10. Which elements on the inverted L antenna provide omnidirectional radiation for ground-wave propagation and high-angle radiation for short-range sky wave propagation?
Definition
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.
Term
209-11. What is a NVIS antenna?
Definition
The NVIS omnidirectional antenna is a HF high-angle radiation and low operating frequencies antenna.
Term
209-12. What are all satellite communications antennas designed to be?
Definition
Highly directional and high-gain in order to overcome free-space loss affecting satellite systems.
Term
209-13. What is the basic principle of all parabolic antennas?
Definition
Reflection of a radio signal for controlled directivity.
Term
209-14. Which type of polarization does a helical satellite antenna uses to radiate a signal?
Definition
Circular polarization.
Term
209-15. What is the horn antenna used for?
Definition
In the transmission and reception of RF microwave signals.
Term
210-1. What are the typical applications for airborne antennas?
Definition
Wideband and satellite communications; telemetry data collection, tracking, and signal relay; timing signal reception; and satellite navigation signal reception.
Term
210-2. What angles should the signal arrive at the aircraft antenna for frequencies in the lower portions of the HF band?
Definition
Vertical angles between 20 and 60 degrees above the horizon
Term
210-3. Where on the aircraft is the vertical whip antenna typically mounted?
Definition
Typically, on top of the aircraft’s fuselage.
Term
210-4. What are two problems associated with airborne antenna operating above HF?
Definition
(1) A serious problem at these frequencies involves the interference between the direct ray and ground-reflected 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.
(2) 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.
Term
210-5. What types of application use the fixed-blade and other low-profile antenna styles?
Definition
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.
Term
210-6. Where on the aircraft would satellite communications antennas be mounted?
Definition
On the top of the aircraft.
Term
210-7. Which type of antenna is mounted on the nose of the aircraft?
Definition
A steerable parabolic dish antenna used for telemetry tracking is mounted in an aircraft’s nose.
Term
210-8. What antenna has a unique design where the antenna becomes part of the skin of the vehicle?
Definition
A unique antenna design where the antenna becomes part of the skin of the vehicle is the conformal antenna.
Term
211-1. What is the first thing you look at when selecting an antenna for an HF circuit?
Definition
The type of propagation.
Term
211-2. What is the first step in selecting an antenna for HF sky wave propagation?
Definition
Find the distance of the circuit so that you can find the required takeoff angle.
Term
211-3. Describe the ideal antenna site selection setting.
Definition
A clear, flat area with no trees, buildings, fences, power lines, or mountains.
Term
211-4. Name the 5 steps in the HF sky wave antenna selection process?
Definition
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.
Term
211-5. Differentiate the one-half wavelength above the ground and the one-quarter wavelength above the ground radiation pattern of a dipole antenna.
Definition
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.
Term
211-6. What is the gain of an 8-meter vertical dipole antenna at 18 MHz and a radiation angle of 30°?
Definition
7.5 dBi.
Term
211-7. For a LOS system (microwave), what determines if a repeater is necessary?
Definition
The distance to be spanned and the terrain
Term
212-1. What kind of location should you pick for a tactical antenna that uses frequencies above 30 MHz?
Definition
One that allows LOS communications
Term
212-2. Why does dry ground limit a radio set’s range?
Definition
Dry ground has high resistance and limits the range of the radio set
Term
212-3. What object has the greatest effect on an antenna?
Definition
The earth
Term
212-4. Why shouldn’t you use sawed-off branches and foliage for camouflage?
Definition
The leaves soon wilt, change color, and become conspicuous against the natural growth
Term
212-5. To what level can the poor electrical ground caused by the dry, sandy soil of a desert reduce the effectiveness of a whip antenna?
Definition
As little as one-third of its normal efficiency
Term
212-6. In mountainous areas with high winds, what should you do to the antenna masts?
Definition
Secure the masts with extra guy ropes in anticipation of high winds.
Term
212-7. What causes sensitive components to either malfunction or get out of alignment?
Definition
Rough handling
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