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Naval Weapons Systems. Energy Fundamentals. Energy Fundamentals. RA dio D etection A nd R anging Radar is an electromagnetic wave that acts like any other electromagnetic wave (i.e. - radio, light, etc.) Characteristics of a radio wave assuming a frequency of 2 Hertz:. Cycle. Amplitude.
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Naval Weapons Systems Energy Fundamentals
Energy Fundamentals • RAdio Detection And Ranging • Radar is an electromagnetic wave that acts like any other electromagnetic wave (i.e. - radio, light, etc.) • Characteristics of a radio wave assuming a frequency of 2 Hertz: Cycle Amplitude 1 second
Traveling Wave Characteristics • Frequency • Period • Wavelength • Velocity • Amplitude • ...examples....
Maxwell’s Theory • An accelerating electric field will generate a time-varying magnetic field. • A time-varying magnetic field will generate a time-varying electric field. • ...and so on...and so on...and so on...
Generation of Electromagnetic Radiation + + - - - + a b c d = Alternating Current Source
Formation of Electric and Magnetic Fields around an Antenna E-line Mag field e- e- e- Electric field | Magnetic Field | Direction of Propagation.
What is Polarization? • The directionof polarization of an antenna is defined as the electric field vector. • 3 Kinds: • Horizontal • Vertical • Circular
Propagation Paths of E-M Waves • Reflection • Refraction • Diffraction
θ1 Medium 1 Medium 2 θ2 Snell’s Law n1*Sin θ1=n2*Sin θ2
...Reflection... Phase shift = 180 degrees. Angle of incidence = Angle of reflected wave. Reflected Wave IncidentWave
...Refraction... • Incident wave passes through two transparent media in which the velocity of light differs... • Incident wave divides into a reflected wave and a refracted wave. • The result is that the energy ray will bend toward the area of higher density.
island not detected ...Diffraction... ...plane waves traveling in a straight path bend around a boundary or obstruction. detected
Wave Propagation – Distance and Frequency • Ground Waves • Sky Waves • Space Waves
Ground Wave... • Very low frequencies, 5-10Khz • Vertical polarization • Waves travel along earth’s surface. • Very long wavelengths - unsuitable for ships & aircraft, except comms • Shore-based installations (HF-DF)
Sky Wave... • E-M energy refracts back towards the earth’s surface in upper ionosphere layer. • E-M energy then reflects back toward upper layer again. • Frequencies used up to 550 KHz effectively • Wavelengths still too long for anything but comms by aircraft and ships.
Space Wave... • Above 30 MHz, ionosphere will not refract E-M waves back toward earth. • Energy tends to travel in straight line.
Transmission Range Factors • Antenna Height • Target Height • Ducting • Losses • Spreading • Absorption • Constructive / Destructive Interference
Transmission Losses • Spreading: Energy per unit area proportional to: • R is distance from xmitter • Absorption: Molecules of medium absorb some of the energy as it passes through. 1 R2
Basic Comms Path p. 40 in textbook
Transmitter • Transducers – Devices that change energy form • i.e. – acoustic waves to EM waves • Often boost the power of the signal to increase distance.
Transmission Channel • Air • Water • Wire • Co-axial • Fiber Optics • Beer
Transmit/Receive Capability • Simplex – one or the other • i.e. – car radio • Half-Duplex – both, but not at the same time. • i.e. – “walkie-talkie” or BTB • Full-Duplex – both and at the same time • i.e. – telephone system and most shipboard communications.
Receiver • Tuner – this is what allows you to listen in on a particular frequency.
Modulation • The process of encoding information on the “Carrier Wave”. • A simple Sine wave. • The Sine wave has 3 independent parameters: • Amplitude • Frequency • Phase
Noise • Noise is bad on a communications circuit. • Two types: • Broadband Noise – “White Noise” • Narrowband Noise – “Interference”
Signal-to-Noise Ratio • Can be expressed in a pure number: • Signal power / Noise power • More commonly expressed in Decibels. • Signal level is on a relative scale compared to the noise. • The more positive the dB number, the clearer the signal. • Unless you want to hide it!!!
