EW Wk1 Flashcards
Explain the main military uses of the EM spectrum.
Radio communications, primary and secondary radar, IR detection and ultraviolet seekers.
Speed of light?
300 000 000m/s
162 000nm/s
186 000sm/s
Relationship between wavelength and frequency.
Inversely proportional.
Higher freq = shorter wavelength.
Polarisation of an EM wave is determined by which field?
E field.
Relationship between power density and range?
Inversely proportional.
Power density decreases as range increases.
2 forms of atmospheric attenuation?
Absorption and scattering.
Explain atmospheric absorption.
When energy dissipates as it interacts with gas molecules.
Explain atmospheric scattering.
When EM waves hit molecules and change their direction.
Explain diffraction in relation to propagation.
Bending waves, longer wavelengths diffracted more.
Different waves?
Ground wave (surface wave and space wave), sky wave.
Factors affecting a space wave?
Attenuation -(absorption,scattering).
Avge path length around 25-40km.
For longer distances relay stations are required.
Which layers of the earths atmosphere have a major effect on the performance of EM waves?
Ionosphere and troposphere.
3 types of radar transmitter?
Magnetron.
Travelling wave tube.
Field effect transistor amplifier.
What target information can we get from radar?
Range. Bearing. Height. Course. Speed. Shape/size/outline.
Different types of radar display.
PPI -Plan Position Indicator.
A-scope.
B-scope.
An ES receiver should:
Intercept.
Measure.
Identify.
Display.
Describe amplitude comparison.
Technique that involves measurement of the relative amplitudes of a signal at different angles with respect to the target.
Draw and label block schematic for an ES Rx.
Antenna. Amplifier. Receiver. Processor. DOA resolver. Output.
Describe unintentional pulse modulation and how it can be used.
Caused by system noise/ different operation/ instillation of components and helps with specific emitter identification SEI.
Explain SNR.
The ratio that describes a radar’s ability to see targets in the presence of noise.
Explain bandwidth.
The difference between the upper and lower frequencies in a continuous set of frequencies and is typically measured in Hz.
What external factors will effect signal quality (SNR)?
Distance from source.
Atmospheric conditions, sun spot activity.
No line of sight, multi path interference.
Jamming.
How can height information be obtained from a circular scanning radar?
Use 2 beams (one angled).
How does a search radar calculate height using a V-beam?
By measuring the time difference between illumination in each beam.
2 types of sector scan?
Bi-directional.
Uni-directional.
2 means of transporting RF energy.
Co-axial cable.
Waveguides.
What is a sidelobe?
Energy reflected near to the parabola edges produce extra lobes of RF energy radiating out from the antenna.
Describe the two methods of beam steering.
Mechanical- beams are steered by the rotation of the antenna.
Electronic- Phase and frequency and is much quicker and agile than mechanical.
What is the function of a Cosecant squared radar?
More power at longer range to give similar sized returns for same sized contacts at different ranges.
What does a cassegrain antenna achieve and how?
Avoids the problem of a blindspot by using a sub-reflector that changes polarisation.
Define beamwidth.
The width of the beam measured in degrees from the -3dB power points.
State the main components in an antenna system.
Feed.
Reflector.
Transmitting medium (coaxial cable, waveguide).
State the relationship between antenna size and wavelength.
Proportional.
Smaller antenna, smaller wavelength.
Define beamwidth and how it is measured.
1/2 power points on main beam. -3dB points.
State main 2 types of array.
Linear.
Planar.
2 methods of beam steering.
Mechanical.
Electrical.
Different ways of measuring DOA.
Amplitude.
Phase.
Explain position fixing.
Multiple LOBs combined to create AOP.
What does frequency effect?
Range (atmospheric attenuation), antenna size.
What does PRF effect?
Data rate and MUR.
What does PW effect?
Min detection range, max detection range, power, range resolution.
What does ARP effect?
Max range, data rate.
What does BW effect?
Data rate, angular resolution.
What is considered a low/med/high frequency?
<4GHz 4-7GHz 7-18GHz
What is a low/med/high PRF?
<400Hz, 400-1000Hz, 1000Hz+
What is a low/med/high PW?
<2us, 2-10us, 10us+
What is a low/med/high ARP?
<5s, 5-15s, 15s+
