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Antennas. Hertzian dipole antenna. Heinrich Hertz (1857-1894). Schematic diagram of Hertz’ experiment. Propagation of electromagnetic wave. Electric field : red Magnetic field : blue. Reception of EM wave. current. V. Transmitting antenna. Receiving antenna.

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Presentation Transcript
slide2

Hertziandipole antenna

Heinrich Hertz (1857-1894)

slide4

Propagation of electromagnetic wave

Electric field : red

Magnetic field : blue

slide5

Reception of EM wave

current

V

Transmitting antenna

Receiving antenna

The charges on the receiving antenna move toward the antenna terminal, which causes voltage drop across them.

advantage of a resonant circuit
Advantage of a resonant circuit

Generate higher voltage than input voltage.

At resonance

Maximum current level depends on internal resistance.

slide14

How to generate time varying currents

Alternating currents accelerate electrons which emit electromagnetic waves propagating in perpendicular direction

Electronic circuit generate oscillating voltages

Output voltage

slide17

Far field approximation

Near field approximation

Electrostatic solution

Biot-Savart’s law

Coulomb’s law

slide19

Gain and directivityof an antenna

Isotropic pattern

Omnidirectional pattern

Directional pattern

Directivity 정의 :

(Efficiency)

: Gain takes into account losses and reflections of the antenna.

slide20

Friis equation

transmitter

receiver

slide24

Array factor

Array factor :

z-directed array

slide25

x-directed array

Array factor

Top view

slide27

How to change currents on elementary antennas?

Magnitudes and phases of currents on elementary antennas can be changed by amplifiers and phase shifters.

slide29

Pattern synthesis

Equi-phase surface

Equi-phase surface

slide30

Examples

(1) Two element array

(2) Two element array

slide31

(3) Five element array

3dB Beamwidth

Beam direction

(4) Five element array

(5) Five element array

slide32

Sample MATLAB codes

phi=0:0.01:2*pi; %0<phi<2*pi

k=2*pi;

d=0.5;% 0.5 lambda spacing.

shi=k*d*cos(phi);

alpha = pi*0.0;

beta = exp(i*alpha);

%Currents=[1,2*beta, 3*beta^2,2*beta^3,1*beta^4]; %Current excitations

Currents=[1, 1*beta, 1*beta^2, 1*beta^3,1*beta^4]; %Current excitations

E=freqz(Currents,1,shi); %E for different shi values

E = DB(E)+30; % 최대값에서 30dB 범위까지 그림.

E = (E>0.).*E;

polar(phi,E); %Generating the radiation pattern

slide33

N-element linear array antenna

Uniform Array : Magnitudes of all currents are equal. Phases increase monotonically.

slide34

Difference :

  • Universal Pattern is symmetric about y= p.
  • Width of main lobe decrease with N
  • Number of sidelobes = (N-2)
  • Widths of sidelobes = (2π/N)
  • Side lobe levels decrease with increasing N.
slide35

Visible and invisible regions

1

visible region

Array Factor의 특성

  • Array factor has a period of 2p with respect to ψ.
  • Of universal pattern, the range covered by a circle with radius “kd” become visible range.
  • The rest region become invisible range

Visible range of the linear array

slide36

Grating Lobes Phenomenon

1

grating lobes

major lobe

They have the same strength !

visible region

  • If the visible range includes more than one peak levels of universal pattern, unwanted peaks are called grating lobes.
  • To avoid grating lobes, the following condition should be met.

Example :

, no grating lobe occurs

, no grating lobe occurs

slide37

Example : array antenna (77GHz)

표면 전류 분포

두께 0.127mm

비유전율 2.2

17mm

10mm

77GHz에서 array element들이 모두 동위상을 갖도록 설계함.

slide38

Radiation pattern (77GHz)

elementarypattern

Radiation pattern of 8-element array

slide41

Beamforming Approaches : Digital Beamformer (DBF)

Interference or

multipath

signal direction

Digital

Signal

Processing

(Amplitude

&

Phase)

LPF

A/D

LPF

A/D

LPF

A/D

Desired signal

direction

LPF

A/D

~