1 antenna basics a
Post on 10-Apr-2015
278 Views
Preview:
TRANSCRIPT
1
©Kathrein/Scholz 07/01
Antenna Basics
Theory
Basic terms
Basic antenna types
2
©Kathrein/Scholz 07/01
An antenna is the converter between two kinds of electromagnetic waves :
cable bounded waves ⇔ free space waves
Antenna Basics / Theory
What is an antenna ?
3
©Kathrein/Scholz 07/01
Categorizing RF components into dual-pole (one termination) or quad-pole (two terminations) devices
example for a dual-pole device :50 Ohm load
examples for a quad-pole device :amplifier, filter
the antenna is a quad-pole device with the second termination connected to free space
Antenna Basics / Theory
AntennaCoaxial cable
quad-pole
dual-pole
4
©Kathrein/Scholz 07/01
antenna principle shown by bending a coax cable open
the pulsing electrical field, created by the transmitter‘s high frequency power, cannot leave the cable
Antenna Basics / Theory
the field lines become longer and are orthogonal to the wires
the field lines have reached the maximum length and allow a wave to free itself from the cable⇒ basic radiating element : λ/2 dipole
1
2
3
5
©Kathrein/Scholz 07/01
Antenna Basics / Theory
[ ] [ ]MHzfm 300
=λ
The resonance frequency of the dipole is determined by its
mechanical length, which is half of the corresponding wave length
t [sec]
wavelength
Zero
Max.
Relation between frequency and wave length :
Example : f =935 MHz ⇒ λ = 0.32 m ⇒ dipole length ~ 160 mm
Frequency :Number of cycles per second
Wavelength :Length of one cycle
6
©Kathrein/Scholz 07/01
Antenna Basics / Theory
electric field (E) magnetic Field (H)
voltage (U) current (I)
Electrical and magnetical field on a dipole
maximum voltage is between the ends of the dipole; the electrical field lines occur between these two charge centers
the current on the dipole causes a magnetical field with an opposite amplitude distribution (max. at the feeding point, min. at the dipole ends
7
©Kathrein/Scholz 07/01
Antenna Basics / Reality
Electrical and magnetical field on a dipole (Simulation of the original dipole)
Voltage (e-plane) Current (h-plane)
8
©Kathrein/Scholz 07/01
Wave propagation :
Permanent conversion from electrical into magnetical energy
and vice versa
Antenna Basics / Theory
9
©Kathrein/Scholz 07/01
For an optimized system performance, all components have to be matched
professional applications use a nominal impedance of 50 Ohms
exact value only for one frequency; over the operating band deviations from 50 Ohms are specified by the VSWR
Antenna Basics / Impedance
10
©Kathrein/Scholz 07/01
Input signal
Reflected signal
Termination
A generator will generate a frequency and send it to a termination.
Generator
The termination may not accept the entire input power (green line), and therefore will reflect some of the input power (red line)
back to the generator.
Antenna Basics / VSWR
11
©Kathrein/Scholz 07/01
Antenna Basics / VSWR
The forward running signal together with the return running signal create a standing wave (VSWR = voltage standing wave ratio)
minmax)(
UUsVSWR =
(range 1 to ∞)
}log20log20{][ VRr UUdBa −−=
Return loss attenuation
12
©Kathrein/Scholz 07/01
Antenna Basics / VSWR
13
©Kathrein/Scholz 07/01
Standard values for mobile communication networksVSWR < 1.5return loss < 14 dB
Antenna Basics / VSWR
VSWR 1.5 1.3 1.2
Missmatch loss (dB) 0.18 0.08 0.04
mismatch lossThe loss which is effecting the system performance due to the reflected/ returned power
14
©Kathrein/Scholz 07/01
Example :VSWR measurement GSM 1800 antenna739 494(65° 18 dBi 1710-1880 MHz)
Antenna Basics / VSWR
CH1 S11 SWR 100 m / REF 1
START 1 690 . 000 000 MHz STOP 1 900 . 000 000 MHz
Cor
PRm
7 Sep 2000 15:38:18
1
2
3
4
5
1 : 1 . 5450 1 690 . 000 000 MHz
CH1 Markers
2 : 1. 28691. 71000 GHz
3 : 1. 24551. 79500 GHz
4 : 1. 08481. 88000 GHz
5 : 1. 19221. 90000 GHz
VSWR 1.4
Spezified frequency range
15
©Kathrein/Scholz 07/01
Antenna Basics / VSWR
comparison of measurements directly at the antenna and at the end of the feeder cable
theoretically the VSWR and return loss is improved by the feeder cable attenuation(providing an ideally matched cable with VSWR = 1)
in reality this improvement is compensated by mismatches due to bad connector installations, bending of the cable and other reflection points
16
©Kathrein/Scholz 07/01
The polarization is defined as the direction of oscillation of the electrical field vector
dipole orientation vertical :vertical polarization ⇒ mainly used for mobile communicationdipole orientation horizontal :horizontal polarization ⇒ mainly used for broadcastingdipole orientation +/-45° slanted :cross polarization ⇒ used for polarization diversity with digital cellular networks
Antenna Basics / Polarization
17
©Kathrein/Scholz 07/01
for symmetrical antennas the 3-dimensional pattern can be described by a vertical and horizontal cutvertical polarization : horizontal pattern = H-plane (magnetic field)
vertical pattern = E-plane (electric field)half power beam widthopening angle of the beam determined by the half power points (reduction by 3 dB)
Antenna Basics / Radiation Pattern
Vertical pattern
Horizontal pattern
18
©Kathrein/Scholz 07/01
Antenna Basics / Antenna Gain
to concentrate the radiated power into the area around the horizon, half wave dipoles are arranged vertically and combined in phase
with every doubling of the dipoles number- the half power beam width approx. halves - the gain increases by 3 dB in the main direction
19
©Kathrein/Scholz 07/01
gain references
half wave dipole (dBd)
isotropic radiator (dBi)
Antenna Basics / Antenna Gain
relation : dBi = dBd + 2.15
Vertical pattern Horizontal pattern
20
©Kathrein/Scholz 07/01
Antenna Basics / Antenna Gain
Standard omni gain antenna for cellular application
(gain 11dBi / 9 dBd)
Horizontal pattern Vertical pattern
21
©Kathrein/Scholz 07/01
Antenna Basics / Antenna Gain
accordingly also in the horizontal plane a beam can be createdwith each halving of the beam width the gain is increased by 3 dB (the shown patterns are theoretically)
the resulting gain of an antenna is the sum of the „vertical“ and „horizontal“ gain
22
©Kathrein/Scholz 07/01
Antenna Basics / Panel Antenna
Standard directional panel antenna
for cellular networks
65° / 15.5 dBi
gain benefit from both planes
Horizontal pattern Vertical pattern
23
©Kathrein/Scholz 07/01
Antenna Basics / Panel Antenna
Three-dimensional radiation pattern
of a directional antenna
24
©Kathrein/Scholz 07/01
Antenna Basics / Panel Antenna
Horizontal cut :
horizontal pattern (magn. field)
Vertical cut :
vertical pattern(electr. field)
25
©Kathrein/Scholz 07/01
Antenna Basics / Panel Construction
26
©Kathrein/Scholz 07/01
Antenna Basics / Yagi Antenna
Yagi antenna
only one active dipole, low side lobe
suppression, low front-to-back-ratio,
mainly used for inexpensive receiving
applications
Horizontal pattern Vertical pattern
Side lobes
F/B ratio
27
©Kathrein/Scholz 07/01
Antenna Basics / Log.per. Antenna
Log. Per. Antenna (logarithmic periodic)
all the dipole structures are active,
excellent side lobes, specific application
in cellular networks
Horizontal pattern Vertical pattern
28
©Kathrein/Scholz 07/01
Antenna Basics / Patch Antenna
Patch antenna
printed board technology, instead
of a dipole a patch above a ground
plane creates the electrical field lines
29
©Kathrein/Scholz 07/01
Antenna Basics / Near-Far Field
spezified patterns and gain are only provided in the far field of the antenna
far-field (F) : plane wave front at the antenna antenna
small antennas (dimensions below one wave length) :
bigger antennas :
Example : 900 MHz Omni
L = 2,8m
λ = 0,325m
F = 48,25m
Near field
position λ
²2][ LmF >
λ10][ >mF
30
©Kathrein/Scholz 07/01
Isolation (decoupling) between 2 antennas 1800 MHz 65° 18 dBisignal level difference between and
Antenna Basics / Isolation
1 2
1 2
top related