summary of technical information - kathrein · pdf filesummary of technical information . ......

Summary of Technical Information

I. UMTS - the Future Mobile Communication System page 31. Network Planning page 32. WCDMA Technology and RET Benefits page 43. Conclusion page 6

II. Antenna Isolation with Site Sharing page 71. Factors Influencing the Isolation Value page 72. Save Distance between two Panel Antennas page 103. Optimised Minimum Distance between two Antennas page 10

III. Advanced Dipole Technology page 111. Introduction page 112. Kathrein´s dipole based Xpol-antenna design page 113. Typical measurements page 164. CPR against azimuth page 16

III. Indoor Environment page 191. Typical room page 192. Indoor antennas just convert the RF power page 193. Unobtrusive design page 194. Flexible signal distribution with Splitters and Tappers page 205. Advanced Indoor System Dual Band page 21

IV. Downtilting of antennas page 221. Downtilting the vertical pattern page 222. Optimum downtilt angles page 243. Consequences regarding the electrical parameters page 27

VI. Passive Intermodulation at Base Station Antennas page 281. Introduction page 282. What is Intermodulation? page 293. Where do intermodulation products come from? page 304. Why is intermodulation a problem? page 315. What solutions are there? page 33

3

The four letters UMTS, the abbreviation for

Universal Mobile Telecommunication System, are

already well known among the general public.

They stand for high data transmission rates and

multi media applications. The start of this new

system has been postponed many times due to

general delays in the technology as well as

scepticism in carrying out such huge investments.

But the thumbs are now up and many licensees

have to fulfil regulations regarding a minimum

coverage before the end of 2003.

While the end users do not care so much about

the used technology, UMTS means a big step

forward compared to GSM.

The main technology for implementing the

3. Generation of mobile systems will be WCDMA

(Wideband Code Division Multiple Access). The

applied frequency range is 1920–2170 MHz,

which contains two paired blocks of 60 MHz each.

UMTS – the Future Mobile Communication System

The technologies used with GSM and UMTS have

a big influence on network planning and the

required network optimisation due to some

essential differences.

In both cases, the vertical pattern downtilt plays a

major role concentrating the radiated power into

the cell to be covered and controlling the

interference from adjacent cells.

Network Planning

Traditionally with GSM, the downtilt angle has to

be altered only when the network structure

changes e.g. by adding new sites, which happens

may be once or twice a year. In this case it is

acceptable to send out installation teams to sites

to change the mechanical or electrical adjustable

downtilt angles of the antennas.

STMUMSGnosirapmoCAMDCWAMDTsseccA

(Time Division Multiple Access) (Wideband Code Division Multiple Access)separation of the subscribers separation of the subscribers

sedoc ybstols emit ybFrequency plan certain frequencies per cell the same full bandwidth in each cellHand over registration only in one cell registration in two or more cells

(hard hand over) (soft hand over)elbairavdexifezis lleC

4

With UMTS, there is a complex relationship

between capacity, coverage and interference. It is

expected that the electrical downtilt of the

antennas has to be modified several times a day!

It is clear that the previous technologies cannot

provide the fast and permanent access to vary the

downtilt angle of the antennas. This led to the

concept of a remote electrical downtilt (RET)

controlled from a central location within the

network e.g. the operational and maintenance

center (OMC).

In essence, CDMA uses the same frequency

band in each cell with the unpleasant

disadvantage for a specific subscriber that all the

other subscribers are ‘noise’ and cause

interference. Consequently, power levels in

CDMA networks are kept to a minimum in order to

reduce this interference. The power levels might

even be below the noise level, and a certain

subscriber can only be identified by using codes.

Power adjustment and cell breathing

To keep the noise low within a cell, the transmit

power of the downlink (base station) is also

altered. For each subscriber, the base station has

to provide exactly the right minimum power. This

requires an extensive and fast power adjustment.

If the load in the cell rises, either by an increased

number of subscribers, or by higher transfer data

rates, the power and with it the noise level will

grow and finally hinder communication. The base

station gets at its limit concerning power adjust-

ment and responses by turning down the power,

consequently reducing the coverage area and

with it the number of subscribers. This process

will continue until the power control is recovered.

The effect of a variable coverage area due to an

increased load and noise is called ‘cell breathing’.

The graph below describes the relationship

between number of users, noise increase and

cell range.

With RET it is possible to partly compensate

this effect and to optimise the power distribution in

critical areas.

WCDMA Technology and RET Benefits

Cell breathing and noise increase in UMTS voice

source: HU Berlin

number of users

nois

e in

crea

se

cell

rang

e [k

m]

1.61.41.2

10.80.60.40.2

0

20181614121086420

0 10 20 30 40 50

5

areas. For example, during rush hours the

network can concentrate on train stations or

airports. Furthermore, the network can be

adopted to meet the temporary requirements of

special events like music festivals, exhibitions or

major sporting events.

Network expansions

If a network grows due to an increased number of

subscribers, additional sites are generally added

in between the already existing ones. To avoid

interference between the sites, downtilt angles

have to be reset by installation teams who have to

visit each individual site.

With RET this adaptation could be carried out

centrally from the OMC without any work at the

site.

Soft hand over

To improve the low power uplink situation, sub-

scribers may be registered in more than one cell.

That means the weak signals are received in two

or three cells and added up by the system. It is

estimated that approximately 30 %– 40 % of the

subscribers will be in such a ‘soft-handover’

condition. This technology provides some benefits

regarding the uplink levels but on the other hand

it eats up capacity.

The amount of soft handover can be adjusted by

RET changing the overlapping areas in the

network.

High traffic area

Skilful planning, adding RET features to your

network, may increase capacity in high traffic

Antennacoverage

BTS

BTS

BTS

BTS

BTS

BTS

High traffic orsoft handover area

Decreasing the downtilt

Increasing the coverage

6

Conclusion

According to equipment suppliers and OEM’s,

remote electrical tilt will become a major feature of

UMTS networks.

For the operators, the decisive question with

respect to the implementation of RET systems is

the level of investment required. The correspond-

ing tenor forecast is that RET will be payed off

quite quickly:

– due to the achieved network optimisation, up to

20 % of WCDMA equipment can be saved

– the network will show lower bit failure rates and

a smaller amount of drop calls

The network operators, especially those in

Europe, have more experience with GSM than

with CDMA and now face the problem with

various new sites of how to decide in advance,

whether or not to use RET.

The Kathrein concept to upgrade the RET func-

tion with already installed antennas considers this

dilemma and allows the operators to postpone the

decision until tests have been performed.

7

Antenna Isolation with Site Sharing

Due to the environmental restrictions and growing

shortage of available sites, site-sharing has

become more and more regular. Apart from static

aspects, isolation between the antennas on the

same site is the biggest problem.

To get different systems with two separate

antennas working properly, an isolation of at

least 70–80 dB between both networks is

necessary. This isolation cannot be achieved

by the antennas alone. It must be generated

with the combination of filter isolation together

with the isolation of the antennas. The required

isolation offered, from the antennas should be

at least 30 dB.

Factors Influencing the Isolation Value

For the isolation values, different influencing

factors have to be considered:

Electrical specifications:With the same mechanical settings at a site,

variations of the electrical specifications impact

the isolation:

Frequency: Antennas are not filters! They also

receive frequencies out of the band they are

specified for. However, for these frequencies

they show worse VSWR values. The resulting

mismatch creates an attenuation called mismatch

loss, that increases the isolation between two

antennas.

Therefore, antennas operated in different

frequencies have higher isolation values than

antennas operated in the same frequency band.

Polarisation: The lowest isolation figures

apply, when two antennas have the same

polarisation. If the polarisation is different, the

isolation values increase. Taking one antenna

with vertical and one with slanted polarisation,

mainly the vertical component of the slanted

polarisation is responsible for the isolation. Due to

the fact that the amplitude of this vertical

component is 3 dB smaller compared to the

complete signal, the isolation is approx. 3 dB

higher.

Half-power beam width: With two antennas side

by side and facing into the same direction,

radiation against each other (orthogonal to the

main beam) finally determines the isolation.

The broader the half-power beam width, the hig-

her the radiation level at +/–90°. Consequently

the isolation decreases with a growing half-power

beam width of the two antennas. (see picture 1,

next page)

Electrical tilt: The electrical tilt is achieved by

feeding the dipoles with unequal phases of a

signal. The different phases lower the coupling

between two antennas, resulting in higher

isolation values for antennas equipped with fixed

or adjustable electrical tilt, rather than for anten-

nas without electrical tilt. (see picture 2, next

page)

Mechanical settings:Keeping the electrical specifications of two

antennas constant at a site, also variations of the

mechanical settings influence the isolation:

8

Vertical or horizontal separation: Antennas

have very dedicated radiation patterns with nulls

above and below the antennas main beam. This

results in a very small radiation level towards an

antenna that is directly above or below. Therefore,

two vertical separated antennas show higher

isolation values than two horizontally separated

antennas at the same distance. (see picture 3,

next page)

Angle: The signal level behind the antenna is

much smaller than the one in front or even at

+/–90°. If now two antennas do not point into the

same direction, but are separated through an

angle (e.g. 120°) between them, the mutual level

of radiation becomes less. For this reason, the

isolation grows with the azimuth angle between

the two antennas.

Pole-/Wall-Mounting: Despite the relatively

high front-to-back ratio of panel antennas,

the influence of a large plane (e.g. building

fascade) behind the antenna cannot be

completely neglected. The reflections from the

surface usually result in a slightly smaller

radiation pattern than normal, decreasing the

level of radiation towards the neighbouring

antenna.

Picture 1: Isolation values for different half-power beam width’s

0 0.25 0.5 0.75 1 1.25

55

50

45

40

35

30

Isol

atio

n dB

Distance a/m

65°

741 622: XPol A-Panel 824–960 65° 17dBi 9°T742 212: XPol F-Panel 1710–2170 65° 18dBi 0°–8°T

Picture 2: Isolation values for different downtilt angles

0

55

50

45

40

35

30

Isol

atio

n dB

Distance a/m

742 212: XPol F-Panel 1710–2170 65° 18dBi 0°–8°T742 212: XPol F-Panel 1710–2170 65° 18dBi 0°–8°T

0.5 1 1.5 2 2.5 3 3.5 4 4.5

0°T

2°T

4°T

6°T

8°T

0 0.25 0.5 0.75 1 1.25

50

48

46

44

42

40

38

36

34

32

30

Isol

atio

n dB

Distance a/m

90°

739 661: XPol A-Panel 806–960 90° 15dBi 8°T742 212: XPol F-Panel 1710–2170 65° 18dBi 0°–8°T

a

9

0

50

45

40

35

30

Isol

atio

n dB

Distance a/m, Distance b/m

739 707: XPol F-Panel 1710–1880 90° 16.5dBi 2°T

0.5 1 1.5 2 2.5 3 3.5 4

Horizontal separation

Vertical separation

a

Horizontal separation Vertical separation

bPicture 3: Vertical / horizontal separation

0

50

45

40

35

30

Isol

atio

n dB

Distance a/m

739 707: XPol F-Panel 1710–1880 90° 16.5dBi 2°T

0.5 1 1.5 2 2.5 3 3.5 4

Antennas on pipe masts

Antennas close to reflective structure

a

Antennas on pipe masts Antennas close toreflective structure

a

Picture 4: Pole / wall mounting

Therefore, two same antennas mounted on a wall

show higher isolation values than if being

mounted on a pole. (see picture 4, below)

Design: One of the biggest influencing factors is

the design of the antennas, since the current at

the edges of the reflector significantly influences

the isolation between two antennas. These

currents depend on the construction and the

kind of the radiating elements used (e.g. dipole,

patch).

Therefore, isolation values of one manufacturer

may not be used for antennas from another.

Kathrein antennas, with their proven dipole

construction, are designed for high isolation

values.

10

However, the stated save distance (see above)

is only a save distance and not the optimised

minimal possible distance. This distance may only

be found with measurements.

Kathrein has done a number of isolation

measurements for typical site configurations, that

are available for our customers.

In these measurements we have measured

values up to 50 dB. Values of more than 50 dB

also depend on the special site due to reflections

from buildings or parts of the pole. Therefore,

these values can no longer be seen as typical.

Optimised Minimum Distance between two Antennas

Save Distance between two Panel Antennas

There is a standard question of network planners

about the required minimum save distance for two

panel antennas in order to achieve isolation

values of more than 30 dB.

Vertical separation: The isolation values for

vertical separation are always quite good, there-

fore typically only the minimum possible distance

is needed.

Horizontal separation: The minimum save

distance depends on the wavelength and on the

horizontal half-power beamwidth:

Save distance a for an isolation value of 30 dB:

Angle separation: Taking a 120° angle, 30 dB of

isolation are already reached with the minimum

mechanical distance.

2 λ 65°

2.5 λ 90°

3 λ 105°

4.5 λ 120°

a

Minimum distance a Horizontal half-power beam width

(λ = wavelength)

The dipole is the oldest and most approved radia-

ting element in the field of mobile communication.

It is the basis for nearly every professional anten-

na type such as the yagi antenna, the log. peri-

odic antenna, and particular the panel antenna.

The latest development of panel antennas leads

to the sophisticated technology of slanted dual

polarization (Xpol).

Is the dipole technology suitable to fulfil thegrowing and stringent requirements ? Can

these odd metal structures still compete withnewer solutions like the patch radiator on aprinted board ?

The answer is definitely yes ! This article will

show that the dipole technology more than other

concepts provide the flexibility to perfect certain

characteristics without the effect of destroying

others. This feature is specific important for the

design of dual band cross-polarized antennas.

2.1 General description

Electrical :

Xpol antennas consist of two independently wor-

king slanted dipole systems, one for

+45° polarization and the other for -45° polariza-

tion.

The dipoles are symmetrically positioned in front

of a reflector screen. Both the power distribution

and the impedance transformation are carried out

by a low loss cable harness. Additional elements

for beam-shaping and isolation perfect the

design.

Mechanical:

The radome consists of a completely closed self-

supporting fiber-glass profile, into which the metal

parts are inserted. There are no drill-holes at all in

the profile, which is closed by two end caps with

short sealing rings. This concept offers ideal

permanent protection against environmental

influences and increases the

mechanical stability.

The improved separation of the electrical and the

mechanical function facilitates the optimization of

particular performances.

Advanced Dipole Technology

Antennen . Electronic

2. Kathreins’s dipole based Xpol-antenna design

1. Introduction

11


2.2. Outstanding characteristics2.2.1 Symmetrical construction

Xpol antennas are available with horizontal half

power beam widths of 65° and 90°. Starting from

a standard vertical polarized antenna, the requi-

red dipole-pair for 65° and the single dipole for

90° are rotated by +45° and -45°, resulting in

orthogonal polarizations (see fig. 1).

While the dipoles of the 90° type form an „X“ on

which the expression Xpol antenna is based, the

basic 65° dipole system is a rhomb.

Both designs are fully symmetrical referred to

the center line of the reflector screen, which is

the basic condition for symmetrical horizontal

radiation patterns .

65° Half-power Beam Width 90° Half-power Beam Width

Fig. 1: General construction of Xpol-antennas

Reflector

Dipolesystem

Feedingharness

-45 +45˚-45˚ +45˚

12

2.2.2 Beam-shaping

The dipole technology offers a high flexibility in

modeling the radiation patterns.

Beam width and shape are defined by the dipole

position to the reflector and the reflector dimen-

sions. Particular the vertical edges of the reflector

screen have a decisive influence on vertically

polarized components.

For slanted polarizations, consisting of vertical

and horizontal components, parasitic elements in

the reflector screen as further beam-shaping ele-

ments are added, which mainly have an effect on

horizontally polarized components.

Thus already the patterns of the basic dipole

system are optimized, which means a great

benefit in combining them. The quality of the

resulting pattern is improved regarding sidelobes

and gain, and the required number of single ele-

ments is minimized (see item 2.2.4.).

In addition, with the separate adjustability of the

vertical and the horizontal components, the resul-

ting polarizations are controllable.

Orthogonal polarizations provide the best pola-

rization diversity gain results, therefore the hori-

zontal radiation patterns for the vertical and the

horizontal component are standard measure-

ments for Xpol antennas.

If the patterns half power beam widths and there-

by the gain values resp. the amplitudes are iden-

tical, the polarizations are orientated +/- 45° and

consequently orthogonal (fig. 2).

Fig. 2: Vertical (V) and horizontal (H) components and resulting polarizations:

a) Equal amplitudes (V=1/H=1) ? orthogonal polarizations

b) Different amplitudes (V=1/H=0.7) ? non-orthogonal polarizations

90˚˚54-˚54+ V V

H-H

70˚

V -35˚

-H

V+35˚

H


13

The polarization diversity technology assigns

both systems of an Xpol-antenna to work in the

Rx- and Tx-mode simultaneously. Therefore a

minimum isolation of 30 dB between the antenna

inputs is required.

Kathrein’s dipole design guarantees a min. isola-

tion of 32 dB. Measurements of each antenna

during the production show a typical value of 35

dB !

Within the basic dipole system („X“ and „rhomb“),

the symmetrical construction provides high isola-

tion, while the isolation from one bay to the next

is improved by patented decoupling measures.

A perfect polarization orthogonality results in a

high cross-polar ratio (CPR), which is determined

by measuring the horizontal radiation patterns

with the operating polarizations +45° and -45°.

The CPR compares the level difference between

the similar polarized signals (co-polar) and the

dissimilar polarized signals (cross-polar) of the

radiated wave. A high CPR stands for a high

uncorrelation of the two signals and consequent-

ly for a good polarization diversity performance.

The dipole design provides excellent values also

apart from the main direction (coverage sector

width +/- 60°) and even at +/- 90° (see item 4)!


2.2.3 High isolation between the two antenna systems

Low-loss flexible semi-rigid coax cables distribu-

te the power to each dipole and take care of the

impedance transformation. The diameter of the

cables (and the corresponding attenuation)

varies with the application, diameters of 0.250?,

0.141? and 0.085? are in operation.

This system produces only a minimal attenuation,

which will become apparent by comparing it with

a printed circuit solution. As a standard the corre-

sponding cross-section of the conductive lines is

between the 0.085? and the 0.141? cable.

In addition these lines are open and radiate a part

of the power, which causes further losses.

That means, to reach the same gain values,

antennas using a printed board power distribution

have to compensate the higher losses by additio-

nal bays of radiating elements! This results in a

roughly 20% higher vertical antenna length and a

smaller vertical beam width.

Another advantage of the cable harness is the

flexibility regarding versions with electrical down-

tilt. The required variation of the phase relations

between the radiating elements is carried out

easily by changing the length of the cables. It is

not necessary to redesign the entire antenna.

2.2.4 Low-loss power distribution by cables

14


Since more than 15 years Kathrein is doing rese-

arch on the reduction of intermodulation (IM) pro-

ducts. There was already a self-designed measu-

ring device for IM products at 450 MHz with a

dynamic range of 160 dB in operation, when such

a device was not available on the market.

The extremely valuable experiences flowed into

the antenna design and determine for example

the applied material, the possible material combi-

nations and how a contact between two parts

should look like.

Kathrein antennas provide a typical 3rd order

IM-products attenuation of -150 dBc using two

transmitters with an output power of 20 W

(43 dBm) each.

2.2.5 Low intermodulation products

Antennas are confronted with all the environ-

mental influences such as cold and hot tempera-

tures, rain, ice, snow, lightning and high wind

velocities.

Kathrein antennas are well prepared, the mecha-

nical design is based on the environmental con-

ditions according to ETS 300 019-1-4.

Regarding the deviation of the electrical parame-

ters, especially rain, ice and snow on the radome

may cause problem because of their dielectric

parameters. Due to the fact that the antenna dep-

ths became smaller and smaller, this dielectric

load is very close to the radiating elements, wor-

king as an additional capacity. Consequently the

operational frequency range is shifted, which

goes together with the deterioration of electrical

parameters like VSWR, isolation and CPR.

The Kathrein dipole technology is highly resistant

against rain, ice and snow. Dipoles are very slim

structures with a small surface and therefore the

occurring additional capacity is relatively low.

Due to their larger surface, the capacity in-

fluence on patches is much higher. For example,

a wet radome can change the isolation of a patch

antenna significantly, while a dipole antenna

reacts much more good natured.

2.2.6 Continuance of the electrical parameters against enviromental influences

15


The following antenna parameters have a decisive

influence on the network and are important for the

judgement of antennas :

1. Half power beam width for co-polar polarization

2. Half power beam width for vertical / horizontal polarization

3. Front-to-back ratio - co-polar

4. Front-to-back ratio - total power

5. Cross-polar ratio

For a high cross-polar attenuation the half power

beam widths of the three polarization components

co-polar, vertical and horizontal are similar. This fea-

ture is perfectly performed by Kathrein´s Xpol-

antennas and consequently there is no need for net-

work planning reasons to differentiate between the

above polarization components.

These measurements also provide the front-to-back

ratio, which is an important feature for the network

planning. The front-to-back ratio can be determi-

ned as the worst case of either the vertical or the

horizontal polarized components. It is only requi-

red to calculate the total power, if the two compo-

nents have similar levels. In case of identical

levels, the total power value is 3 dB less compa-

red to the individual components.

Xpol dipole antennas provide typical front-to-back

ratios of 24 – 30 dB total power.

The following figures show the co-polar and

cross-polar as well as the vertical and horizontal

polarized patterns of 65° and 90° antennas.

Beside the symmetry of the patterns, the scalar

printout with a linear scale in dB shows clearly the

cross-polar ratio in each azimuth direction. The

dipole design provides excellent values also

apart from the main direction and even at +/- 90°!

Please note as well the high front-to-back ratio for

the co-polar and the cross-polar signal.

3. Typical measurements

Fig. 3: Typical horizontal co-polar and cross-polar pattern for 65° beam width (measurement)

0

-5

-10

-15

-20

-25

-30

-35

-40-180 -120 -60 0 60 120 180

XPol A-Panel 800/900 65˚ 17dBihorizontal radiation pattern

120˚-sector

azimuth [deg]

rela

tive

gain

[dB

]

co-polcross-pol

16


Fig. 4: Typical 65° horizontal pattern of vertical and horizontal polarized component (measurement)

0

-5

-10

-15

-20

-25

-30

-35

-40-180 -120 -60 0 60 120 180


120˚-sector

azimuth [deg]

rela

tive

gain

[dB

]

hor. polarizedvert. polarized

Fig. 5: Typical horizontal co-polar and cross-polar pattern for 90° beam width (measurement)

0

-5

-10

-15

-20

-25

-30

-35

-40-180 -120 -60 0 60 120 180


120˚-sector

azimuth [deg]

rela

tive

gain

[dB

]

co-polcross-pol

17


Fig. 6: Typical 90° horizontal pattern of vertical and horizontal polarized components (measurement)

0

-5

-10

-15

-20

-25

-30

-35

-40-180 -120 -60 0 60 120 180


120˚-sector

azimuth [deg]

rela

tive

gain

[dB

]

hor. polarizedvert. polarized

Fig. 7: CPR values against azimuth (according patterns fig. 3 and 4)

30

25

20

15

10

5

0-90 -60 -30 0 30 60 90

XPol A-Panel 800/900 65˚ 17dBiCross Polar Ratio

120˚-sector

azimuth [deg]

CP

R [d

B]

As already mentioned, the dipole design provides

excellent CPR values not only in main direction

but even at +/- 90°.

It is important for the coverage of a standard sec-

tor, to rely on high CPR values and consequently

on high diversity gains also at the sector edges,

where the antenna gain is already considerably

reduced.

4. CPR against azimuth

18

1. Lots of reflections from the walls, ceiling,

floor, furniture and persons (see sketch

above), destroy the free space radiation

patterns and the corresponding antenna gain.

2. The dimensions of normal rooms do not fulfil

the far field conditions (distance to the

antennas more than 3 m for GSM 900,

respectively 1.5 m for GSM 1800).

3. Therefore the measured far field patterns do

not apply; specific radiation patterns and

gains provide no benefit within closed rooms.

Kathrein refers to this physical facts with its pre-

sent indoor program with mainly two omni versi-

ons for ceiling mounting and one directional

antenna for wall mounting.

Apart from single band antennas, also multiband

versions are available.

Indoor Environment

Typical room

Indoor antennas just convert the RF power

Most clients prefer unobtrusive antenna appea-

rance in indoor applications. Kathrein reacted on

this demand by redesigning the most sold indoor

antennas 737 602 and 738 749.

The shape of the new models 741 571 and

741 572 adapts perfectly to the requirements of

modern buildings. In addition these antennas are

multiband types operating from 824 – 2170 MHz

and suitable for UMTS as well.

Unobtrusive design

2,5

m


19

Especially for the signal distribution within bigger

buildings with lots of indoor antennas, it is neces-

sary to design an indoor network with more or

less similar signal levels in all floors. Therefore

Kathrein provides 2-, 3- and 4-way splitters and

splitters with unequal power splitting (“Tappers“).

The resulting distribution attenuation (valid for

both directions), are given with the following

survey:

Flexible signal distribution with Splitters and Tappers


Splittersequal power ratio

Tappers2-way

2- way Power splitting 4/11

1

0 dB

-3 dB

-3 dB

4

1

0 dB

-1 dB

-7 dB



1

1

1

-4,7 dB

-4,7 dB

-4,7 dB

0 dB

1

1

1

1

-6 dB-6 dB-6 dB-6 dB

0 dB

10

1

-0,4 dB

-10,4 dB

0 dB

32

1

-0,1 dB

-15,1 dB

0 dB

20

Similar signal levels in all floors (without cable losses)

Advanced Indoor System Dual Band


4-way Splitter K63 22 64 1






directional antenna forwall mounting

738573

omni antenna forceiling mounting

741572 -13,4 dB

-13,4 dB

-13,1 dB

-12,4 dB

-12,4 dB

-12,4 dB

-6,4 dB

-6,4 dB

-3,4 dB

-2,4 dB

-8,4 dB

-10,4 dB

-7,4 dB

-1,4 dB

-0,4 dB

41

41

10

1

41

Combiner 792 902

GSM 900Base Station

GSM 1800Base Station

2-way SplitterK 63 22 62 1

2-way Tapper 4/1K 63 23 60 61

2-way Tapper 4/1K 63 23 60 61

2-way Tapper 4/1K 63 23 60 61

2-way Tapper 10/1K 63 23 61 01

5. Floor

4. Floor

6. Floor

3. Floor

2. Floor

1. Floor

relative signal strength 0 dB

-9,4 dB

21

Network planners often have the problem that the

base station antenna provides an overcoverage.

If the overlapping area between two cells is too

large, increased switching between the base sta-

tion (handover) occurs, which strains the system.

There may even be disturbances of a neighbou-

ring cell with the same frequency.

In general, the vertical pattern of an antenna

radiates the main energy towards the horizon.

Only that part of the energy which is radiated

below the horizon can be used for the coverage

of the sector. Downtilting the antenna limits the

range by reducing the field strength in the horiz-

on and increases the radiated power in the cell

that is actually to be covered.

The simplest method of downtilting the vertical

diagram of a directional antenna is a mechanical

tipping to achieve a certain angle while using an

adjustable joint. (see Figure 1) But the required

downtilt is only valid for the main direction of the

horizontal radiation pattern. In the tilt axis direc-

tion (+/-90° from main beam) there is no downtilt

at all. Between the angles of 0° and 90° the

downtilt angle varies according to the azimuth

direction.

This results in a horizontal half-power beam

width, which gets bigger with increasing downtilt

angles. The resulting gain reduction depends on

the azimuth direction. This effect can rarely be

taken into consideration in the network planning

(see Figure 2).

Downtilting of antennas


1.1 Mechanical downtilt

1. Downtilting the vertical pattern

22

3 dB

10

0

90°

0°

+90

Fig. 2:Changes in the horizontal radiation pattern when various downtilt angels are used (compared to the horizon)

Fig. 1:Mechanically downtilted A-Panel

MECHANICAL DOWNTILT

0°6°8°10°

036

91215

20


1.2 Electrical downtilt

In general, the dipols of an antenna are fed with

the same phase via the distribution system. By

altering the phases, the main direction of the ver-

tical radiation pattern can be adjusted. Figure 3,

shows dipols that are fed from top to bottom with

a rising phase of 70°. The different phases are

achieved by using feeder cables of different

lengths for each dipole.

The electrical downtilt has the advantage, that the

adjusted downtilt angle is constant over the whole

azimuth range. The horizontal half-power beam

width remains unaltered (see Figure 4). However,

the downtilt angle is fixed and cannot be chan-

ged.

23

3 dB

10

0

-90°

0°

+90

0°6°8°10°

ELECTRICAL

Figure 4:Changes in the radiation pattern using various downtilt angles

Figure 3:Phase variations for a fixed el. downtilt

1.3 Adjustable electrical downtilt

With this technique it is possible to combine the

advantages of the mechanical downtilt (i. e.

adjustment possibility) with those of electrical

downtilt (horizontal half-power beam independent

of downtilt angle). Instead of using different fixed

cables to achieve the various phases for the dipo-

les, mechanical phase-shifters are used.

P = 1

P = 2

P = 3.5

P = 2

P = 1

Phase-shifter

+ +?

+?

- -?

-?

Figure 5:Phase diagram of an adjustable phase-shifter

? = 0˚

? = 70˚

? = 140˚

? = 210˚

? = 280˚

Figure 6: Downtilt adjusting mechanism (with scale) for A-Panels

In standard applications the purpose of using a

downtilt is to limit the field strength in the horizon.

Considerable limitation is achieved if the radiated

power in the horizon is limited by 6 dB. This

means that one can easily predict the smallest

efficient tilt angle by simply tilting the vertical

radation pattern until the field strength in the hori-

zon is reduced by 6 dB.

But there is also a second important point when

calculating the optimum downtilt angle. Apart

from the main beam, vertical radiation patterns

also have two or more side lobes depending on

the number of dipoles within the antenna (see

Figure 7).

Maximum field strength reduction in the horizon is

achieved if the minimum between the main beam

and the first side-lobe is orientated towards the

horizon.


24

The adjustment mechanisms can be positioned

either on the rearside (Eurocell panels) or on the

bottom (F-Panels, A-Panels) of the antenna.

These phase-shifters can be used to set various

downtilt angles which remain constant over the

whole azimuth range.

2. Optimum downtilt angles

The optimum tilt angle for a particular antenna

depends on the vertical radiation pattern, especi-

ally on the half-power beam width, and therefore

also on the actual length of the antenna.

2.1 How to calculate the optimum downtilt angle


As the Figure 8 shows, the minimum tilt angle

that would be efficient lies at around 50° (power

in the horizon reduced by 6 dB). Using such an

angle, the antenna would beam more or less

directly into the ground. Therefore the use of a

downtilt with very small antennas (i.e. length up

to 500 mm) can not be recommended.

2.2 Small antennas – vertical half-power beam width 70°

25

Main beam

First upper side-lobe

Figure 7: Typical vertical radiation pattern

If the tilt angle is set too high, the field strength is

not reduced, but is increased again by the first

side-lobe.

Figure 8: Minimum efficient tilt angle for small antennas

10

3

0


The minimum efficient tilt angle for these anten-

nas (length 1.3 m) lies at 8°. At an angle of 19°

the first side-lobe lies on the horizon. This provi-

2.3 Standard antennas – vertical half-power beam width 13°

The minimum efficient tilt angle for these anten-

nas (length 2.6 m) lies at around 3°–4°. At an

angle of 8°–9° the first side-lobe lies on the hori-

zon. This provides a good range of angles for the

efficient tilting of long antennas.

2.4 Long antennas – vertical half-power beam width 6.5°

26

10

3

0

des a good range of angles for the efficient tilting

of standard antennas.

Figure 9: Minimum efficient tilt angle for standard antennas

Figure 11: Minimum efficient tilt angle for long antennas

10

3

0

10

3

0

Figure 10: First side-lobe lies on the horizon


For some special locations (e.g. on the tops of

high mountains, on the roof-tops of tall buildings

or for coverage in the street below etc.) a very

high downtilt angle might be necessary. To achie-

ve such high downtilt angles, a combination of

mechanically and electrically downtilted antennas

is also possible.

2.5 High downtilt angles for special locations

27

Taking all the above into account, it is easy to

imagine, how very sophisticated the development

of electrically adjustable downtilt antennas is,

since intensive measurements have to be carried

out.

All the electrical parameters must fulfil the speci-

fications with every single downtilt angle.

Electrical values such as those for side-lobe sup-

pression, isola-tion, cross-polar ratio, intermodu-

lation or beam tracking are especially critical.

Kathrein´s lengthy and outstanding experience

with vertical polarized electrical adjustable anten-

nas has enabled us to fully optimize the charac-

teristics of the new X-polarized and dual-band

X-polarized antenna models.

3. Consequences regarding the electrical parameters


28

Passive Intermodulation at Base Station Antennas

If a base station antenna transmits two or more

signals at a time, non-linearities can cause inter-

ferences, which may block one or more receiving

channels of the base station antenna. This can

result in a connection breakdown to a mobile.

1. Introduction

Figure 1: Base station communicating with two mobiles

The risk for this problem to occur increases with

the number of transmitting (Tx) frequencies

connected to one base station antenna.

With the standard XPol-antennas 2 Tx-antennas

are combined (see Figure 2).

Figure 2: XPol antenna with two duplexers

The latest technology using dual-band, dual-pola-

rised (XXPol) antennas, now again doubles the

number of antennas and hence also the number

of carriers in one radome, to combine both the

900 and 1800 MHz systems. But this also means

a further possible increase in interferences pro-

blems.

These interference problems are called

“Intermodulation”.

Tx1 Rxa Tx2 Rxb


Intermodulation products of even orders

Intermodulation (IM) is an undesirable modula-

tion which leads to unwelcome alterations to the

high frequency carrier output.

An input signal put into a linear passive device at

a certain frequency f1 will produce an output sig-

nal with no modification to the frequency.

Here only the amplitude and the phase can be

modified.

However, if the same signal is put into a passive

device with non-linear transmission characte-

ristics, then this will result in distortions to the

time-scale, leading to changes in the frequency.

This means that, in addition to the carrier fre-

quency f1, several harmonics are produced: 2 f1,

3 f1, 4 f1, ..., n f1.

Moreover, if the input signal contains two or more

frequency components, f1 and f2, the output

signal will generate a spectral composition. In

addition to the harmonics, this new spectral com-

position also includes all possible frequency com-

binations. These frequency combinations can be

expressed by the equation:

2. What is Intermodulation?

IMP = nf1± mf2IMP: Inter Modulation Products

n,m = 1, 2, 3, ...

Only the IMP > 0 are physically relevant.

The order of the IMP can be equated as: O = n + m

There are IMP of even and odd orders. The pro-

ducts of even orders have a large spacing to the

original Tx frequencies and therefore cause no

problems with single band antennas. The most

troublesome IMP are those of the odd orders:

Intermodulation products of odd orders

Since the IMP frequencies of the odd orders lie

very close to the original frequencies, they can

appear within the received signal band-width

and thereby degrade the overall communication

system.

29

2nd Order f1 + f2 / f2 – f1 3rd Order 2f1 – f2

4th Order 2 f1 + 2 f2 / 2 f2 – 2 f1 5th Order 3f1 – 2f2

7th Order 4f1 – 3f2

Large spacing compared to the original frequencies Close to the original frequencies


If high-power signals of different frequencies

exist, any device with non-linear voltage-current

characteristics will generate intermodulation

products. The level will depend on the degree of

the non-linearity and on the power-ratings of the

incident frequencies.

3. Where do intermodulation products come from?

Level

Frequency

Rx Tx

f f f f f f f f f

2f2 – f1

3f2 – f1

4f2 – f1

2f1 – f2

3f1 – f2

4f1 – f2

f1 f2

Level

Frequency

f1 f2

f

Figure 3: Input signals

Figure 4: IM spectrum of odd orders

30


Contact non-linearities at metal/metal joins

Contact non-linearities arise where discontinui-

ties exist in the current path of the contact. They

may have various causes and are not normally

visible to the naked eye. The following are poten-

tial causes:

Current mobile telephone systems are designed to

operate with a transmitting frequency range Tx and

a slightly shifted receiving frequency range Rx.

Problems arise when intermodulation products

occur in the receiving Rx frequency range (see

also Figure 4) which degrade the reception per-

formance. The following example for GSM 900

shows that, under certain conditions, the intermo-

dulation products of 3rd, 5th and even 7th or higher

orders may fall in the receiving band.

Material and surface-plating non-linearities

There are two main categories of non-linearities:

• Surface condition of the join, e.g. dirt, surface textures, ...

• Electron tunnelling effect in metal insulator metal joins

• Contact mating: Poor contact spring force or poor contact quality

• Non-linear conductive materials or treated surfaces (e.g. the treatment of

copper foils on printed circuit boards (PCB´s) – patch antennas on PCB)

• Magneto-resistance effect in non-magnetic materials

• Non-linearity due to non-linear dielectric

• Non-linearity due to variations of permeability into ferromagnetic materials

Material non-linearity is an important source of

intermodulation products if two or more signals

pass through ferro-magnetic material.

But the result of a poor contact join is of far more

significance!

4. Why is intermodulation a problem?

GSM 900Tx Band Rx Band

935 – 960 MHz 890 – 915 MHzIntermodulation Products f1 f2 fIM

3rd Order 2f1 - f2 936 MHz 958 MHz 914 MHz5th Order 3f1 - 2f2 938 MHz 956 MHz 902 MHz7th Order 4f1 - 3f2 941 MHz 952 MHz 908 MHz

31


The most disturbing intermodulation products in

the GSM 900 and 1800 systems are those of the

3rd order. These are the products with the highest

power level and also the ones that lie closest to

the original transmitting frequencies. These pro-

ducts may block the equivalent Rx channels. It is

therefore absolutely essential to keep the IMP´s

to a minimum level below the sensitivity of the

receiving equipment.

IM = 10 log PIMP3 [dBm]

These products are measured as Intermodulation Levels in either dBm or dBc.

The total intermodulation level compared to a power-rating of 1 mW is expressed in dBm:

IM = 10 log(PIMP3/PTx [dBc]

On the other hand, dBc is defined as the ratio of the third order intermodulation product

to the incident Tx carrier signal power:

The levels of intermodulation products according to the GSM standard are shown

in the following table:

A comparison of the carrier level and the level of the IMP expressed in distances

clearely illustrates this fact:

Level of IM products accord. GSM Standard < – 103 dBm(3rd order)

Referred to two carriers of 20 W each < – 146 dBc(43 dBm)

IM attenuation of Kathrein antennas Typically < –150 dBc

Comparison Carrier IM Product0 dBm — 150 dBm

Average distance earth – sun 150 Mill. kilometermm 51,0ecnatsid tnelaviuqE

32

In view of all the facts mentioned, the following

points must be taken into consideration when

designing passive devices such as antennas,

cables and connectors:


5. What solutions are there?

• All components such as feeder cables, jumpers, connectors etc. must fulfil the IM standards.

• All connectors must have good points of contact.

• Particular materials such as copper, brass or aluminium are recommended. Other materials

like steel and nickel should to be avoided in the signal path.

• Material combinations with a high chemical electrical potential should not be used as any

thin corrosion layer between the materials will act as a semi-conductor.

• All points of contact should be well-defined and fixed.

• All cable connections should be soldered.

Engineers at KATHREIN have been researching

ways of reducing intermodulation (IM) products for

more than 15 years now. Long before other such

devices became available on the market, Kathrein

developed a company-designed IM product measu-

ring device for the 450 MHz frequency with an ope-

rating sensitivity of –160 dBc.

Kathrein´s long-standing and extremely valuable

experience is incorporated into all our antenna

designs and helps to determine for example the

best material to use, all possible material combina-

tions and also what a point of contact between two

antenna parts should look like.

Kathrein antennas typically show a 3rd order

intermodulation product attenuation of –150 dBc,

where two transmitters each with an output

power-rating of 20 W (43 dBm) are used.

As explained earlier, there is an increased risk of

intermodulation with XX-pol. antennas since four

Tx antennas are used. IMP´s of the 2nd order

may also cause problems with XX-pol. antennas

due to the combination of the 900 and the

1800 MHz frequencies. Kathrein has therefore

introduced a 100% final test rate for intermodula-

tion products in their serial production of all

XX-pol. antennas.

33

internet: http://www.kathrein.de KATHREIN-Werke KG Phone +49 8031 184-0 Fax +49 8031 184-306 Anton-Kathrein-Straße 1 - 3 P.O. Box 10 04 44 83004 Rosenheim Germany

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