1 Antenna Basics a.pdf

1

©Kathrein/Scholz 07/01

Antenna Basics

Theory

Basic terms

Basic antenna types

2


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


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


AntennaCoaxial cable

quad-pole

dual-pole

4


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


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



[ ] [ ]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



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


Antenna Basics / Reality

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

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

8


Wave propagation :

Permanent conversion from electrical into magnetical energy

and vice versa


9


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


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



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



13


Standard values for mobile communication networksVSWR < 1.5return loss < 14 dB


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


Example :VSWR measurement GSM 1800 antenna739 494(65° 18 dBi 1710-1880 MHz)


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



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


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


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


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


gain references

half wave dipole (dBd)

isotropic radiator (dBi)


relation : dBi = dBd + 2.15

Vertical pattern Horizontal pattern

20



Standard omni gain antenna for cellular application

(gain 11dBi / 9 dBd)

Horizontal pattern Vertical pattern

21



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


Antenna Basics / Panel Antenna

Standard directional panel antenna

for cellular networks

65° / 15.5 dBi

gain benefit from both planes


23



Three-dimensional radiation pattern

of a directional antenna

24



Horizontal cut :

horizontal pattern (magn. field)

Vertical cut :

vertical pattern(electr. field)

25


Antenna Basics / Panel Construction

26


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


Side lobes

F/B ratio

27


Antenna Basics / Log.per. Antenna

Log. Per. Antenna (logarithmic periodic)

all the dipole structures are active,

excellent side lobes, specific application

in cellular networks


28


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


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


Isolation (decoupling) between 2 antennas 1800 MHz 65° 18 dBisignal level difference between and

Antenna Basics / Isolation

1 2

1 2

1 Antenna Basics a.pdf

Documents

Transcript of 1 Antenna Basics a.pdf