antenna basics installation2009_kethrein

8/8/2019 Antenna Basics Installation2009_Kethrein

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Antenna Basics / Theory

What is an antenna ?

Kathrein/Scholz 07/04

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

cable bounded waves free space waves


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antenna principle shown by bending acoax cable open

the pulsing electrical field, created by the



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transmitters high frequency power,cannot leave the cable

the field lines become longer andare orthogonal to the wires

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the field lines have reached the maximumlength and allow a wave tofree itself from the cable basic radiating element : /2 dipole

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


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The resonance frequency of the dipole is determined by its mechanical

length, which is half of the corresponding wave length

Frequency :

Number of cycles per second

Max.


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[ ][ ] MHz f

m 300= Relation between frequency and wave length :

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

ave eng :

Length of one cycle

t [sec]

wavelength

Zero + _


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volta e U current I

Electrical and magnetical field on a dipole

maximum voltage is between the endsof the dipole; the electrical field linesoccur between these two charge


6electric field (E) magnetic Field (H)

centers

the current on the dipole causes amagnetical field with an oppositeamplitude 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)


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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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For an optimized system performance, allcomponents have to be matched

professional applications use a nominalimpedance of 50 Ohms

Antenna Basics / Impedance


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


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Input signal

A generator will generate a frequency and send it to a termination.

Antenna Basics / VSWR


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Reflected signal

The termination may not accept the entire input power (green line),and therefore will reflect some of the input power (red line) back to thegenerator.


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The forward running signal together with the

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


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Reflection Factor

VSWR

r

r

U

U s

+==

1

1

min

max (range 1 to )

r dBa r log20][ =

Return loss attenuation

Uvr =


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Standard values for mobile communication networksVSWR < 1.5

return loss < 14 dB


mismatch loss


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VSWR 1.5 1.3 1.2

Missmatch loss (dB) 0.18 0.08 0.04

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


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Example :VSWR measurement

GSM 1800 antenna739 494(6518 dBi 1710-1880 MHz)


CH1 S11 SWR 100 m / REF 1

Cor

PRm

7 Sep 2000 15 :38:18

1

1 : 1 . 5450 1 690 . 000 000 MHz

CH1 Marker s

2 : 1 . 28691 . 71000 GHz

3 : 1 . 24551 . 79500 GHz

4 : 1 . 08481 . 88000 GHz

5 : 1 . 19221 . 90000 GHz


14START 1 690 . 000 000 MHz STOP 1 900 . 000 000 MHz

2

3

4

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VSWR 1.4

Spezified frequency range


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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

Jumper cable

0.5 dB loss

r1 = 0.2(VSWR 1.5)

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

Cable :

Only losses/

no VSWR

Cable :

Only VSWR/

no losses

Only antenna


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Feeder line

1.5 dB loss

0 dB -18 dB

(VSWR 1.3)

r2 = 0.09(VSWR 1.2)

rtotal = 0.29(VSWR 1.82)

.increases the total VSWRright calculation:

VSWR = 1.82

return loss = 10.8 dB+ 4 dB loss14.8 dB

VSWR = 1.46

29,0rrr 21total =+=

VSWR limit at the BTS should not be setlower 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

Antenna Basics / Polarization


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po e or entat on or zonta :horizontal polarization mainly used forbroadcastingdipole orientation +/-45slanted :cross polarization used for polarizationdiversity with digital cellular networks


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

oints reduction b 3 dB

Antenna Basics / Radiation Pattern


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Vertical pattern

Horizontal pattern

Half power beam width


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gain references

half wave dipole (dBd)

Antenna Basics / Antenna Gain

Vertical pattern Horizontal pattern


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isotropic radiator (dBi)

relation : dBi = dBd + 2.15


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Standard omni gain antenna for cellular application

(gain 11dBi / 9 dBd)


19Horizontal pattern Vertical pattern


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accordingly also in thehorizontal plane a beamcan be createdwith each halving of thebeam width the gain isincreased by 3 dB (the


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theoretically)

the resulting gain of anantenna is the sum of thevertical and horizontalgain


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Antenna Basics / Panel Antenna

Standard directional panel antenna

for cellular networks

65/ 15.5 dBi

gain benefit from both planes


21Horizontal pattern Vertical pattern


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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 / Panel Construction


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