BASIC RADIO PROPARGATION Flashcards
CONTINUOS POWER IN A PULSE MODULATION
~AVG POWER CONSUMED OVER TIME INCLUDIGN THE PERIODS OF NO TRANSNSMISSION
PULSE POWER
POWER CONTAINED WITHIN A PULSE
WHAT IS AMPLITUTDE MODULAITON
The info that is impressed onto the carrier wave by altering the ampitude f the carrier wave.
Is suceptibile to noise
What is frequncy modulation
~The info that is impressed onto the carrier wave by altering the frequency of the carrier of the wave.
~Less suceptible to noise (bc the noise changes the amplitude ,since in freq modulation the amplitude is constant ,it can detect the noise by escluding the different amplitude )
~More efficeint
~Used in commercial radio
PHASE modualtion
~a modulation used in GPS where the phase of the carrier wave is reversed
~requires complex demodulator
~used in gps
~reverses phase of the carrier
Single side band(SSB)
~Remove one sideband increases efficiency
~in aviation is use for HF and VOLMET
SUPPRESSED CARRIER WAVE(SCW)
~further energy saving by removing the transmitted carrier wave
~requires a BEAT FREQUENCY OSCILLATOR (BFO) at the receiver to re insert the carrier
SIGNAL CALSSIFICATION
~N0N: carrier without modulation (used by NDB)
~A1A: carrier with keyed modulation (used by NDB)
~A2A:carrier with amplitude modulated morse code(used by NDB)
~A3E:carrier with amplitude modulated speech (used by VHF comms)
Antenna
~Electrical device which converts electrical power into radio waves and vice versa
~
POLARISATION
~describes the plane of oscillation of the elctrica component of a wave with regard to its direction of propargation
~determined by the orientation of the antenna
~linear :vertical or horizontal
~circular: requires helical antenna
Elliptical :requires helical antenna
Received antenna must be aligned in the same way as the transmitter to properly receive the transmitted signal
Dipole antenna
~simplest type of antenna
~wire of length equal to 1/2 wavelentgh
~omnidirectional
~cone of confusion overhead
Loop antenna
~good at direction finding
~used in automatic direction finding (ADF)
~looks for the null position (nop hase difference either side of the loop)
~requuiers a second antenna to eliminate the one of the nulls
Parabolic antenna
~amplifies weak signals
~used in old weather radar
~highly directional
Slotted planar array antenna
~used in modern a/c
~highly directional
~sereis of slots which are scanned electronically
~more e3fficent than a parabolic antenna drastically reduces side lobes concentrating energy into the main beam
Helical antenna
~omnidirectional: normal mode (VHF and comms)
~directional, axial mode(GPS)
Produces a circular /elliptical plane of polarisation
Factors affecting propagation
~REFLECTION: abrupt change in directionwhen hits a surface
(Caused by: terrain,builidngs,a/c water droplets)
~REFRACTION:a change in direction of wave caused by a change in propagation speed,when moving from one medium to another.
~DIFFRACTION: bending of waves around objectes or through apertures
(Tendency of some waves to follow the earth curvature)
~ABSORPTION: the transfer of a waves energy to the matter it has encountered
~ATTENUATION:gradual loss of energy of a wave propagating through a medium
~MULTI PATH PROPAGATION:when one signal is reflected from surfaces between the transmitter and receiver and arrives at the receiver via more than path simultaneously.
~SUPERPOSITON: combination of two or more similar waves
~INTERFERENCE: the result of superpositon of two waves,can be constructive or distructive
~FADING: interfence of signal resulting in changes in signal strength and polarisation ,caused by multi path ,weather
Static interfence
~natural ,stops at VHF
~man made ,VHF and above
WAVE PROPAGATION
~SKY WAVES: EM waves refracted by the E and F layers of thee ionosphere
(VLF,LF,MF,HF ) possible sky wave at night therefore ,susceptible to fading
~GROUND WAVES : travelling along the surface of the earth
~SPACE WAVES :travelling directly trhough the air from the transmitter to receiver.
(VHF,UHF,SHF,EHF). VHF is limited din range bc is not reflected by the ionosphere and does not have surface wave.
SPACE WAVES
MAX range NM : 1,23 x ( sqroot tx (ft) + sqroot Rx (ft) )
IONOSPHERE
60/400km above the surface
Vetrically structered into 3 layers»_space;D,E,F
~UV,X RAYS RADIATION
~radiation waves passing through ionosphere layers are attenuated ,LOWER FREQ experience LESS attenuation
Layers
~D LAYER: Lowest layer,thickness reduces at night,MF and HF waves significantly attenuated,main cause of daytime absorption of HF waves.
~E LAYER: refracts EM waves back to the surface SKY WAVE,weakens at night base rises increasing range of refracted waves,during periods of intense solar activty SPORADIC E layers may form,capbale of refracting VHF
~F LAYER: refracts waves back to the surface SKY WAVE,comprises a single F2(higher) layer at night,sperate F1 (lower) layer forms during day
SKY WAVES
~DEAD SPACE/ SKIP ZONE: distance between the limit of the surface wave and the sky wave
~SKIP DISTANCE: distance bewteen transmitter and the first sky wave return arrives
