02 antenna basics

8/6/2019 02 Antenna Basics

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Antenna Basics

Theory

Basic terms

Basic antenna types


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An antenna is the converter between two kinds of electromagnetic waves :

cable bounded waves free space waves

Antenna Basics / Theory

What is an antenna ?


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antenna principle shown by bending a

coax cable open

the pulsing electrical field, created by the

transmitters high frequency power,cannot leave the cable


1

the field lines become longer andare orthogonal to the wires

2


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the field lines have reached the maximumlength and allow a wave to

free itself from the cable

basic radiating element : /2 dipole

3

4m band dipole (68-87 MHz) side-mounted on apipe mast


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[ ][ ]MHzf

m 300=

The resonance frequency of the dipole is determined by its mechanical

length, which is half of the corresponding wave length

Relation between frequency and wave length :

Example : f =935 MHz = 0.32 m dipole length ~ 160 mm

Frequency :

Number of cycles per second

Wavelength :

Length of one cycle

t [sec]

wavelength

Zero

Max.

+_


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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 linesoccur between these two chargecenters

the current on the dipole causes amagnetical field with an opposite

amplitude distribution (max. at thefeeding point, min. at the dipole ends


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Antenna Basics / Reality

Electrical and magnetical field on a dipole (Simulation of the original dipole)

Voltage (e-plane) Current (h-plane)


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Wave propagation :

Permanent conversion from electrical into magnetical energy

and vice versa



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Antenna Basics / Impedance

VSWR / Impedance will be treated in a separate presentation


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For an optimized system performance, allcomponents have to be matched

professional applications use a nominal

impedance of 50 Ohms

exact value only for one frequency; overthe operating band deviations from 50Ohms are specified by the VSWR

Antenna Basics / Impedance


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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


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The forward running signal together with the

return running signal create a standing wave(VSWR = voltage standing wave ratio)

Reflection Factor

VSWRr

r

U

Us

+==1

1

min

max(range 1 to )

rdBar log20][ =

Return loss attenuation

Ur

Uv

r=


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Standard values for mobile communication networks

VSWR < 1.5

return loss < 14 dB


VSWR 1.5 1.3 1.2

Missmatch loss (dB) 0.18 0.08 0.04

mismatch loss

The loss which is effecting the system performance due to the reflected/returned power


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Example :

VSWR measurement

1800 / UMTS antenna742 215

(6518 dBi 1710-2170 MHz)


VSWR 1.5

Spezified frequency range

Full measurements


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comparison of measurements directlyat the antenna and at the end of thefeeder cable


The attenuation of the feeder cableseem to improve the VSWR

0 dB -14 dB(VSWR 1.5)

-2 dB -16 dB

Feeder line

1.5 dB loss

Jumper cable

0.5 dB loss

0 dB -18 dB

(VSWR 1.3)

r1 = 0.2(VSWR 1.5)

r2 = 0.09

(VSWR 1.2)

rtotal = 0.29

(VSWR 1.82)

On the other hand the VSWR of the cable andconnectors is not 1 ! In reality a VSWR of

roughly 1.2 has to be considered whichincreases the total VSWR

VSWR = 1.82

return loss = 10.8 dB

+ 4 dB loss

14.8 dB

VSWR = 1.46

29,0rrr 21total =+=

VSWR limit at the BTS should not be set

lower than the antenna spec.


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The polarization is defined as the direction ofoscillation of the electrical field vector

dipole orientation vertical :

vertical polarization mainly used for mobilecommunication

dipole orientation horizontal :

horizontal polarization mainly used forbroadcasting

dipole orientation +/-45slanted :

cross polarization used for polarization

diversity with digital cellular networks

Antenna Basics / Polarization


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for symmetrical antennas the 3-dimensional pattern can be described by

a vertical and horizontal cut

vertical polarization : horizontal pattern = H-plane (magnetic field)

vertical pattern = E-plane (electric field) half power beam width

opening angle of the beam determined by the half power

points (reduction by 3 dB)

Antenna Basics / Radiation Pattern

Vertical pattern

Horizontal pattern

Half power beam width


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Antenna Basics / Antenna Gain

to concentrate the radiated power into

the area around the horizon, half wavedipoles are arranged vertically and

combined in phase (vertical gain) What happens if dipoles are not

combined in phase??

with every doubling of the dipolesnumber

- the half power beam width approx.halves

- the gain increases by 3 dB in the maindirection

Power increase factor

3 dB 2

6 dB 4

9 dB 8

10 dB 10

20 dB 100


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gain references

half wave dipole (dBd)

isotropic radiator (dBi)


relation : dBi = dBd + 2.15

Vertical pattern Horizontal pattern


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Standard omni gain antenna for cellular application

(gain 11dBi / 9 dBd)

Horizontal pattern Vertical pattern


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accordingly also in thehorizontal plane a beamcan be created (horizontal

gain)

with each halving of the

beam width the gain is

increased by 3 dB (theshown patterns aretheoretically)

the resulting gain of an

antenna is the sum of thevertical and horizontal

gain


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Antenna Basics / Panel Antenna

Standard directional panel antenna

for cellular networks 900 MHz

65/ 15.5 dBi

gain benefit from both planes



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Three-dimensional radiation pattern

of a directional antenna


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Horizontal cut :

horizontal pattern (magn. field)

Vertical cut :

vertical pattern(electr. field)


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Antenna Basics / Near-Far Field

specified patterns and gain are only provided in the far field of the antenna

far-field (F) : plane wave front at the antenna

small antennas (dimensions below one wave length) :

bigger antennas :

Example : 900 MHz Omni

L = 2,8m

= 0,325m

F= 48,25m

Near field

position

2][

LmF >

10][ >mF


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Antenna Basics / Panel Construction


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Antenna Basics / Log.per. Antenna

Log. Per. Antenna (logarithmic periodic)

all the dipole structures are active,

excellent side lobes, specific application

in cellular networks



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Antenna Basics / Patch Antenna

Patch antenna

mainly printed board technology;

instead of a dipole, a patch above a groundplane creates the electrical field lines

02 antenna basics

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