Day 1 Flashcards by Ryan Heir

How do you convert a real number into a dB?

LOG the number, x 10

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How do you convert a dB into a real number?

Divide by 10, put number to power of 10

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What dB value would equate to half the power?

-3

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What dB power would equate to double?

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What is a decibel?

Logarithmic scale for a huge numerical range

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What is Bandwidth?

The difference between upper and lower frequencies of the signal. Too small will cut off pulse, too large will allow more noise.

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What is Bandwidth?

The difference between upper and lower frequencies in a continuous set of frequencies typically measured in Hz. Too small will cut off pulse, too large will allow more noise.

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What is signal to noise ratio? SNR

Shown in dB, it is a measure of a radars ability to see targets in the presence of noise.

SNR > 1 = detectable signal
SNR

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What are the sources of noise in a system?

Internal

External

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What are the internal sources of noise in a system?

Equipment, transmission and antenna

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What are the external sources of noise?

Interference, Jamming, Weather and Ground clutter

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What is EW broken into?

EA - Jamming, deception & neutralisation
ES - SIGINT - COMINT/ELINT & Warning
EP - EMCON, masking, hardening, WRM

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What is an electronic wave?

Is a sinusoidal wave that consists of electric (E) and magnetic (H) fields always at 90 degrees to each other. No need for a medium, obey same laws of wave motion.

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What is the speed of light?

3 x 10^8 metres per second
162,000 nm per second
186,000 miles per second

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What does radar stand for?

Radio detection and ranging

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What is frequency?

The number of cycles that occur in 1 second in Hz

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What is wavelength?

The physical distance one complete cycle of the wave occupies

What is the relationship between wavelength and frequency?

Inversely proportional

What is the formula for calculating wavelength and frequency?

C = 300,000,000 microseconds (ms)
λ = C / F
F = C / λ

What is phase?

A wavelength rotates through 360 degrees. 
1st quarter = 90 degrees
Halfway = 180 degrees
3rd quarter = 270 degrees
Full wavelength = 360 degrees

What is the difference between an analogue and digital signal?

Analogue - variable quantity which changes continuously with time
Digital - Discrete permitted values with time

What are the military’s main uses of the EM spectrum?

Radio communications
Primary and Secondary Radar
IR Detection
Ultraviolet Seekers

What is polarisation?

The orientation of the radio wave in respect to the earth. EM energy is free to vibrate in any of the infinite planes at 90 degrees to the direction of propagation.

Polarisation is determined by the orientation of which field?

The Electric Field

What are the types of Polarisation?

Random (Static, Light) Linear: Horizontal, Vertical, Slant Circular and Elliptical: Left hand and Right hand Rotating: Clockwise and Anti-Clockwise

Explain Circular & Elliptical Polarisation

RHCP = Right Hand Circular Polarisation From the point of origin, away from you. i.e. RHCP = Clockwise LHCP = Anti-clockwise

What is propagation?

The process of how EM waves travel through the different layers of the atmosphere. All EM waves, regardless of frequency, propagate in the same way. In a vacuum they travel in straight lines, indefinitely, at the speed of light, however in the earth's atmosphere they are subject to losses.

What is Isotropic Distribution?

The spread of EM waves equally in all directions, as if coming from a point at the centre of a sphere

What is Power Density?

The amount of energy per unit area in an EM wave Decreases as range from the source increases Measured in Watts per square metre

What is the power density formula?

Pt / 4 π R^2 ``` Pt = transmitted power R = range from transmitter ```

What is the reflected power density formula?

Reflected Pd = Pt / 16 π ^2 R^4

What is the ESM Range Advantage?

Probability of Interception (POI) For intercepting equipment, the advantage occurs because signals only have to travel one way from the emitter to the intercept platform. Power entering an airborne ELINT antenna is about 4 times greater than the reflected power returning to a Radar antenna.

What is Atmospheric Attenuation?

The weaking of signal as it propagates through atmosphere. Caused mainly by: Absorption Scattering

Explain Atmospheric Absorption

Occurs when the energy from the EM wave dissipates as it interacts with gase molecules which absorb the radiation Most of it occurs due to oxygen and water molecules. As altitude increases, attentuation decreases as lower level of water molecules At altitude the RF windows are more pronounced allowing better propagation

Explain Atmospheric Scattering

Occurs when EM waves hit molecules in the atmosphere and change their direction as a result If the circumference of the sphere is smaller than the wavelength, little energy is scattered. If larger, the sphere will scatter a constant high-level of energy If similar, the sphere will scatter large amounts of energy that varies over large values

What is diffraction?

The bending of waves around edges of objects such as building or mountains. The longer the wavelength, the greater the amount of diffraction Can generate surface waves, increasing operational range At higher frequencies, diffraction is negligible

What is reflection?

State the different wave types

``` Ground Wave (Surface and Space) Sky Waves ```

State the factors affecting a Space Wave

Attenuation = Absorption and Scattering Average Path length is restricted to around 25-40km For longer distances relay stations are required

Which layers of the Earth's atmosphere have a major effect on the performance of EM waves?

Ionosphere | Troposphere