Antenna final

gistfile1.txt

1 what is antenna

way of converting guided waves in waveguide, feeder cable or transmission line into radiating waves

art of antenna design is to ensure this process efficiently as possible

2 conditions for radiation

charges + not uniform motion = reversing direction = direction changing or oscillating in periodic motion

3 near field far field

near field = radiating and reactive energy
far field = only radiating

4 far field radation from wires

ANTENNA PARAMETER

1 radiation pattern
plot of far field radiation = plot of power radiated per unit solid angle = radiation density

power density = P / 4 pi r

main lobe, backlobe, side lobes, Half power beamwidth

two special cases: isotropic => equally in all directions => cannot achieve in practice

omnidirectional: constant, horizontal plane but vary vertically

2 directivity

directivity = radiation intensity of anttenna in direction / radiation intensity of isotropic anntenna radiating the same total power

3 radiation resistance and efficiency

Rr = radiation resistance => power radiated = useful purpose / e = efficientcy = Rr / Rr + Rl
Rl = loss resistance => power loss in either conducting and dielectric part

4 Power gain =  ratio of its radiation intensity to radiation intensity of isotropic antenna
Gain = efficiency multi directivity

5 Bandwidth ability to poerate over wide frequency range

6 Reciprocity gain must be same for transmitting and receiveing => appliciable in receive mode

7 Receiving antenna aperture 
receive wave with power density S => produce power in its terminating impedance
Ae = effective aperture reduce when efficiency decrease
P = Ae multi S
8 Beamwidth and directivity
beamwidht smaller => directivity increase
9 Friis fomula antenna in freespace
10 Polarisation matching 

Antenna type
Log preiodic: VHF UHF, unidirectional, frequency independant, wideband
helix: omni, satelite, VHF UHF, space 
dipole: 2 identical conductive elements => television antenna
uda yagi: multiple parallel elements in a line, usually half-wave dipoles 300MHz - 3G UNiDIRECTIONAL, high gain, narrow band , fix frequency
monopole: straight rod-shaped conductor => omnidirectional da huong -> resonance
loop -> directional, long distance ptp, radio reception

parabolic reflector, a curved surface => high directivity=> direct in narrow beam => high gain 3G-30G, rada, satelite
waveguide shaped like a HORN -> direct beam-> ultra high frequency -> feeder for larger anten like parabolic-> nhin nhu cai loa-> no resonance tieng vang

array -> increase directivity and gain, eliminate undesire radiation
broadsie = perpencular => same magnitude and pahse, bidirectional 
endfire = paralle => same magnitude opposite phase, bidirectional