Scott Baxter 100_C5

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Chapter 5

Antennas forWireless Systems

Antennas forWireless Systems

Dipole

Typical WirelessOmni Antenna

Isotropic

July, 1998 5 - 1RF100 (c) 1998 Scott Baxter



Chapter 5 Section A

Introduction toAntennas for Wireless

Introduction toAntennas for Wireless




Understanding Antenna Radiation

The Principle Of Current Moments An antenna is just a passive

conductor carrying RF current

• RF power causes the current

flow• Current flowing radiates

electromagnetic fields

• Electromagnetic fields causecurrent in receiving antennas

The effect of the total antenna is thesum of what every tiny “slice” of theantenna is doing

• Radiation of a tiny “slice” isproportional to its length timesthe current in it

• remember, the current has a

magnitude and a phase!

TX RX

Width of band denotes current

magnitude

Zero current at each end

Maximum current at the middle

Current induced in receiving antenna is vector sum of

contribution of every tiny “slice” of

radiating antenna

each tiny imaginary “slice” of the antenna does its share

of radiating




Different Radiation In Different Directions


Each “slice” of the antenna producesa definite amount of radiation at aspecific phase angle

Strength of signal received varies,

depending on direction of departurefrom radiating antenna

• In some directions, thecomponents add up in phase

to a strong signal level• In other directions, due to the

different distances the variouscomponents must travel to

reach the receiver, they areout of phase and cancel,leaving a much weaker signal

An antenna’s directivity is the samefor transmission & reception

TX

Maximum Radiation:

contributions in phase,reinforce

Minimum Radiation: contributions out of phase,

cancel

Minimum Radiation: contributions out of phase,

cancel



Antenna Polarization

TX

Electromagnetic

Field

currentalmost

nocurrent

Antenna 1VerticallyPolarized

Antenna 2Horizontally

Polarized

RX

RF current in a conductor causeselectromagnetic fields that seek toinduce current flowing in the same direction in other conductors.

The orientation of the antenna iscalled its polarization.Coupling between two antennas is

proportional to the cosine of theangle of their relative orientation

To intercept significant energy, a receiving antenna must be orientedparallel to the transmitting antenna

• A receiving antenna oriented at right angles to the transmittingantenna is “cross-polarized”; will have very little current induced

• Vertical polarization is the default convention in wireless telephony

• In the cluttered urban environment, energy becomes scattered and“de-polarized” during propagation, so polarization is not as critical

• Handset users hold the antennas at seemingly random angles…..




Antenna Gain


Antennas are passive devices: they do not producepower

• Can only receive power in one form and passit on in another, minus incidental losses

• Cannot generate power or “amplify” However, an antenna can appear to have “gain”

compared against another antenna or condition. Thisgain can be expressed in dB or as a power ratio. It

applies both to radiating and receiving A directional antenna, in its direction of maximum

radiation, appears to have “gain” compared against anon-directional antenna

Gain in one direction comes at the expense of lessradiation in other directions

Antenna Gain is RELATIVE, not ABSOLUTE

• When describing antenna “gain”, thecomparison condition must be stated orimplied

Omni-directionalAntenna

DirectionalAntenna



Reference Antennas

Isotropic Radiator

• Truly non-directional -- in 3 dimensions• Difficult to build or approximate physically,

but mathematically very simple to describe• A popular reference: 1000 MHz and above

– PCS, microwave, etc.

Dipole Antenna

• Non-directional in 2-dimensional plane only• Can be easily constructed, physicallypractical

• A popular reference: below 1000 MHz – 800 MHz. cellular, land mobile, TV & FM

IsotropicAntenna

(watts or dBm) ERPEffective Radiated Power Vs. DipoleEffective Radiated Power Vs. Isotropic

Gain above Dipole reference

Gain above Isotropic radiator

(watts or dBm) EIRP

dBd

dBi

Quantity Units Dipole Antenna

Notice that a dipolehas 2.15 dB gain

compared to anisotropic antenna.




Effective Radiated Power


An antenna radiates all power fed to it from thetransmitter, minus any incidental losses.Every direction gets some amount of power

Effective Radiated Power (ERP) is the apparent

power in a particular direction• Equal to actual transmitter power times

antenna gain in that direction

Effective Radiated Power is expressed in

comparison to a standard radiator• ERP: compared with dipole antenna

• EIRP: compared with Isotropic antenna

A

B

ERP B A (ref)

100w275w

ReferenceAntenna

TX100 W

A

Directional

Antenna TX100 W

B

Example : Antennas A and B each radiate 100 watts fromtheir own transmitters. Antenna A is our reference, ithappens to be isotropic.Antenna B is directional. In its maximum direction, its

signal seems 2.75 stronger than the signal from antennaA. Antenna B’s EIRP in this case is 275 watts.



Antenna Gain And ERPExamples

Many wireless systems at 1900 & 800 MHz use omniantennas like the one shown in this figure

These patterns are drawn to scale in E-field radiationunits, based on equal power to each antenna

Notice the typical wireless omni antenna concentratesmost of its radiation toward the horizon, where usersare, at the expense of sending less radiation sharplyupward or downward

The wireless antenna’s maximum radiation is 12.1 dB

stronger than the isotropic (thus 12.1 dBi gain), and10 dB stronger than the dipole (so 10 dBd gain).

Isotropic

Dipole

Omni

12.1 dBi

10dBd

Gain Comparison

Isotropic

Dipole

Typical WirelessOmni Antenna

Gain 12.1 dBi or 10 dBd




Radiation PatternsKey Features And Terminology


An antenna’s directivity isexpressed as a series of patterns

The Horizontal Plane Pattern graphs

the radiation as a function of azimuth(i.e..,direction N-E-S-W)

The Vertical Plane Pattern graphs theradiation as a function of elevation (i.e..,

up, down, horizontal) Antennas are often compared by noting

specific landmark points on theirpatterns:

• -3 dB (“HPBW”), -6 dB, -10 dBpoints

• Front-to-back ratio

• Angles of nulls, minor lobes, etc.

Typical Example

Horizontal Plane Pattern

0 (N)

90

(E)

180 (S)

270

(W)

0

-10

-20-30 dB

Notice -3 dB points

Front-to-back Ratio

10 dBpoints

MainLobe

a Minor

Lobe

nulls or

minima



How Antennas Achieve Their Gain

Quasi-Optical Techniques (reflection, focusing)

• Reflectors can be used to concentrateradiation

– technique works best at microwave frequencies,

where reflectors are small

• Examples: – corner reflector used at cellular or higher

frequencies

– parabolic reflector used at microwavefrequencies

– grid or single pipe reflector for cellular

Array techniques (discrete elements)

• Power is fed or coupled to multipleantenna elements; each element radiates

• Elements’ radiation in phase in somedirections

• In other directions, a phase delay for eachelement creates pattern lobes and nulls

In phase

Out ofphase




Types Of Arrays

Collinear vertical arrays

• Essentially omnidirectional inhorizontal plane

• Power gain approximately equal

to the number of elements• Nulls exist in vertical pattern,

unless deliberately filled Arrays in horizontal plane

• Directional in horizontal plane:useful for sectorization

• Yagi – one driven element, parasitic

coupling to others

• Log-periodic – all elements driven – wide bandwidth

All of these types of antennas are used inwireless

RFpower

RFpower

CollinearVerticalArray

Yagi

Log-Periodic




Omni AntennasCollinear Vertical Arrays

The family of omni-directional wirelessantennas:

Number of elements determines

• Physical size• Gain• Beamwidth, first null angle

Models with many elements have

very narrow beamwidths• Require stable mounting and

careful alignment• Watch out: be sure nulls do

not fall in important coverageareas Rod and grid reflectors are

sometimes added for mild directivity

Examples: 800 MHz.: dB803, PD10017,

BCR-10O, Kathrein 740-1981900 MHz.: dB-910, ASPP2933

beamwidth

Angleoffirstnull

θ

-3dB

Vertical Plane Pattern

Number ofElements

PowerGain

Gain,dB

Angleθ

0.00 n/a3.01 26.57°4.77 18.43°6.02 14.04°6.99 11.31°7.78 9.46°8.45 8.13°

9.03 7.13°9.54 6.34°10.00 5.71°10.41 5.19°10.79 4.76°11.14 4.40°

1234567

891011121314

1234567

891011121314 11.46 4.09°

Typical Collinear Arrays




Sector AntennasReflectors And Vertical Arrays


Up

Down

Horizontal Plane Pattern

N

E

S

W

Typical commercial sectorantennas are vertical combinationsof dipoles, yagis, or log-periodicelements with reflector (panel or

grid) backing• Vertical plane pattern is

determined by number ofvertically-separated

elements – varies from 1 to 8, affectingmainly gain and vertical planebeamwidth

• Horizontal plane pattern is

determined by: – number of horizontally-spacedelements

– shape of reflectors (is reflectorfolded?)




Example Of Antenna Catalog Specifications

Frequency Range, MHz.

Gain - dBd/dBiVSWR

Beamwidth (3 dB from maximum)Polarization

Maximum power input - WattsInput Impedance - OhmsLightning ProtectionTermination - Standard

Jumper Cable

Electrical DataAntenna Model ASPP2933 ASPP2936 dB910C-M

1850-1990 1850-1990 1850-1970

3/5.1<1.5:1

32°Vertical

40050

Direct GroundN-Female

Order Sep.

6/8.1<1.5:1

15°Vertical

40050


Order Sep.

10/12.1<1.5:1

5°Vertical

40050


Order Sep.

Mechanical DataAntenna Model

Overall length - in (mm)Radome OD - in (mm)

Wind area - ft2 (m2)Wind load @ 125 mph/201 kph lb-f (n)Maximum wind speed - mph (kph)

Weight - lbs (kg)Shipping Weight - lbs (kg)

Clamps (steel)

ASPP2933

24 (610)1.1 (25.4)

.17 (.0155)4 (17)

140 (225)

4 (1.8)11 (4.9)

ASPA320

ASPP2936

36 (915)1.0 (25.4)

.25 (.0233)6 (26)

140 (225)

6 (2.7)13 (5.9)

ASPA320

dB910C-M

77 (1955)1.5 (38)

.54 (.05)14 (61)

125 (201)

5.2 (2.4)9 (4.1)

Integral




Example Of Antenna Catalog Radiation Pattern



• E-Plane (elevation plane)

• Gain: 10 dBd• Dipole pattern is superimposed at

scale for comparison (not oftenshown in commercial catalogs)

• Frequency is shown

• Pattern values shown in dBd

• Note 1-degree indices through

region of main lobe for mostaccurate reading

• Notice minor lobe and null detail!



Chapter 5 Section B

Other RF ElementsOther RF Elements




Antenna Systems

F R

D

uplexer

Combiner

BPF

TX

RX

TXTransmission LineJumper

Jumpers

DirectionalCoupler

Antenna

Antenna systems include more than just antennas Transmission Lines

• Necessary to connect transmitting and receiving equipment

Other Components necessary to achieve desired system function• Filters, Combiners, Duplexers - to achieve desired connections• Directional Couplers, wattmeters - for measurement of performance

Manufacturer’s system may include some or all of these items

• Remaining items are added individually as needed by system operator




Characteristics Of Transmission Lines


FoamDielectric

AirDielectric

Typical coaxial cablesUsed as feeders in wireless applications

Physical Characteristics Type of line

• Coaxial, stripline, open-

wire• Balanced, unbalanced

Physical configuration

• Dielectric:

– air – foam

• Outside surface – unjacketed – jacketed

Size (nominal outer diameter)• 1/4”,1/2”, 7/8”, 1-1/4”,

1-5/8”, 2-1/4”, 3”

Transmission Lines



Transmission LinesSome Practical Considerations


FoamDielectric

AirDielectric

Transmission lines practical considerations

• Periodicity of inner conductorsupporting structure can cause

VSWR peaks at some frequencies,so specify the frequency bandwhen ordering

• Air dielectric lines

– lower loss than foam-dielectric; dry airis excellent insulator – shipped pressurized; do not accept

delivery if pressure leak

• Foam dielectric lines

– simple, low maintenance; despiteslightly higher loss – small pinholes and leaks can allow

water penetration and gradualattenuation increases

Ch t i ti Of T i i Li C ti d



Characteristics Of Transmission Lines, Continued


dD

Characteristic Impedanceof a Coaxial Line

Zo = ( 138 / ( ε 1/2 ) ) Log10 ( D / d )

ε = Dielectric Constant= 1 for vacuum or dry air

Electrical Characteristics Attenuation

• Varies with frequency, size, dielectriccharacteristics of insulation

• Usually specified in dB/100 ft and/ordB/100 m Characteristic impedance Z0 (50 ohms is the

usual standard; 75 ohms is sometimes used)• Value set by inner/outer diameter ratio

and dielectric characteristics ofinsulation• Connectors must preserve constant

impedance (see figure at right) Velocity factor

• Determined by dielectric characteristicsof insulation.

Power-handling capability• Varies with size, conductor materials,

dielectric characteristics

Transmission Lines



Transmission LinesSpecial Electrical Properties

Zo=50ΩZLOAD=

50ΩZIN = 50Ω

Matched condition

Zo=50Ω

ZLOAD

=

83-j22Ω

ZIN = ?

Mismatched condition

Zo=50ΩZLOAD=

100ΩZIN=25Ω

λ/4

ZIN= ZO2

/ ZLOAD

Deliberate mismatch

for impedance transformation

Transmission lines have impedance-transforming properties

• When terminated with same

impedance as Zo, input to lineappears as impedance Zo

• When terminated withimpedance different from Zo,

input to line is a complexfunction of frequency and linelength. Use Smith Chart orformulae to compute

Special case of interest: Line sectionone-quarter wavelength long hasconvenient properties useful inmatching networks

• ZIN = (Zo

2

)/(ZLOAD)


Transmission Lines



Transmission LinesImportant Installation Practices


ObserveMinimumBending

Radius!

Respect specified minimumbending radius!

• Inner conductor must

remain concentric,otherwise Zo changes

• Dents, kinks in outerconductor change Zo

Don’t bend large, stiff lines (1-5/8” or larger) to make directconnection with antennas

Use appropriate jumpers,

weatherproofed properly. Secure jumpers against wind

vibration.

Transmission Lines



Transmission LinesImportant Installation Practices, Continued


200 ft~60 m

Max.

3-6 ft

During hoisting

• Allow line to support its ownweight only for distances

approved by manufacturer• Deformation and stretching

may result, changing the Zo

• Use hoisting grips,messenger cable

After mounting

• Support the line with propermounting clamps atmanufacturer’srecommended spacingintervals

• Strong winds will set updamaging metal-fatigue-inducing vibrations

RF Filters



RF FiltersBasic Characteristics And Specifications

Types of Filters

• Single-pole:

– pass – reject (notch)

• Multi-pole: – band-pass

– band-reject Key electrical characteristics

• Insertion loss

• Passband ripple

• Passband width – upper, lower cutoff frequencies

• Attenuation slope at band edge

• Ultimate out-of-band attenuation

Typical bandpass filters haveinsertion loss of 1-3 dB. andpassband ripple of 2-6 dB.

Bandwidth is typically 1-20% ofcenter frequency, depending onapplication. Attenuation slopeand out-of-band attenuationdepend on # of poles & design

Typical RF bandpass filter

0

A

t t e n u a t i o n ,

d B

Frequency, megaHertz

passband ripple insertion loss

-3 dB passband

width


RF Filters



RF FiltersTypes And Applications

Filters are the basic buildingblocks of duplexers and morecomplex devices

Most manufacturers’ networkequipment includes internalbandpass filters at receiver inputand transmitter output

Filters are also available forspecial applications

Number of poles (filter elements)and other design variablesdetermine filter’s electricalcharacteristics

• Bandwidth rejection

• Insertion loss• Slopes

• Ripple, etc.

Notice construction: RF inputexcites one quarter-wave

element and electromagnetfields propagate from elementto element, finally exciting thelast element which is directlycoupled to the output.

Each element is individually set

and forms a pole in the filter’soverall response curve.

Typical RF Bandpass Filter

∼λ/4


Basics Of Transmitting Combiners



Basics Of Transmitting Combiners


Typical tuned combinerapplication

TX TX TX TX TX TX TX TX

Antenna

Typical hybrid combinerapplication

TX TX TX TX TX TX TX TX

Antenna

~-3 dB

~-3 dB

~-3 dB

Allows multiple transmitters to feed singleantenna, providing

• Minimum power loss fromtransmitter to antenna

• Maximum isolation betweentransmitters

Combiner types

• Tuned

– low insertion loss ~1-3 dB – transmitter frequencies must be

significantly separated

• Hybrid – insertion loss -3 dB per stage

– no restriction on transmitterfrequencies

• Linear amplifier – linearity and intermodulation are

major design and operation issues

Duplexer Basics



Duplexer Basics

fR fT

RX TX

Antenna

Duplexer

Principle of operationDuplexer is composed of individualbandpass filters to isolate TX fromRX while allowing access to antenna

for both. Filter design determinesactual isolation between TX and RX,and insertion loss TX-to-Antennaand RX-to-Antenna.

Duplexer allows simultaneoustransmitting and receiving on oneantenna

• Nortel 1900 MHz BTS RFFEsinclude internal duplexer

• Nortel 800 MHz BTS does notinclude duplexer but commercialunits can be used if desired

Important duplexer specifications

• TX pass-through insertion loss• RX pass-through insertion loss

• TX-to-RX isolation at TXfrequency (RX intermodulationissue)

• TX-to-RX isolation at RXfrequency (TX noise floor issue)

• Internally-generated IMP limitspecification


Directional Couplers



Directional Couplers

Couplers are used to measureforward and reflected energy in atransmission line; it has 4 ports:

• Input (from TX),Output (to load)

• Forward and Reverse Samples Sensing loops probe E& I in line

• Equal sensitivity to E & H fields• Terminations absorb induced

current in one direction,leaving only sample of otherdirection

Typical performance specifications• Coupling factor ~20, ~30,

~40 dB., order as appropriatefor application

• Directivity ~30-~40 dB., f($) – defined as relative

attenuation of unwanted

direction in each sample

Principle of operation

ZLOAD=50Ω

Input

Reverse Sample

Forward Sample

RT

RT

Typical directional coupler

Main line’s E & I induce equal signals in

sense loops. E is direction-independent,but I’s polarity depends on direction andcancels sample induced in one direction.Thus sense loop signals are directional.One end is used, the other terminated.


Return Loss And VSWR Measurement



Return Loss And VSWR Measurement

Transmission

line

AntennaDirectionalcoupler Fwd

Refl

RFPower

A perfect antenna will absorb and radiate all the power fed to it

Real antennas absorb most of the power, but reflect a portionback down the line

A Directional Coupler or Directional Wattmeter can be used tomeasure the magnitude of the energy in both forward and

reflected directions Antenna specs give maximum reflection over a specific frequency

range Reflection magnitude can be expressed in the forms VSWR ,

Return Loss , or reflection coefficient • VSWR = Voltage Standing Wave Ratio


Ret rn Loss and VSWR



Return Loss and VSWR


Forward Power, Reflected Power,Return Loss, and VSWR can

be related by these equationsand the graph.

• Typical antenna VSWRspecifications are 1.5:1

maximum over a specifiedband.

• VSWR 1.5 : 1

= 14 db return loss

= 4.0% reflected power

VSWR vs. Return Loss

VSWR

0

10

20

30

40

50

1 1.5 2 2.5 3

VSWR =

Reflected PowerForward Power

Reflected PowerForward Power

1 +

1 -

Reflected Power

Forward Power

Return

Loss, dB = 10 x Log10 ( )

Swept Return Loss Measurements



Swept Return Loss Measurements


It’s a good idea to take swept or TDRreturn loss measurements of a newantenna at installation and torecheck periodically

• maintain a printed orelectronically stored copy of theanalyzer output for comparison

• most types of antenna ortransmission line failures are

easily detectable by comparisonwith stored data

Transmission

Line

AntennaDirectionalCoupler Fwd

Refl

Network Analyzer-10

-20

-30f1 f2

A Network Analyzer can alsodisplay polar plots, SmithCharts, phase response

A Spectrum Analyzer andtracking generator can beused if Network Analyzer notavailable

What is the maximum acceptable value of return loss as seen in sketch above?

Given: Antenna VSWR max spec is 1.5 : 1 between f1 and f2 Transmission line loss = 3 dB.Consideration & Solution: From chart, VSWR of 1.5 : 1 is a return loss of -14 dB, measured at the antenna

Power goes through the line loss of -3 db to reach the antenna, and -3 db to return Therefore, maximum acceptable observation on the ground is -14 -3 -3 = - 20 dB.



Chapter 5 Section C

Some AntennaApplication Considerations

Some AntennaApplication Considerations


Near-Field/Far-Field Considerations



Near Field/Far Field Considerations


Antenna behavior is very different close-in and far out

Near-field region: the area within about 10 times thespacing between antenna’s internal elements

• Inside this region, the signal behaves asindependent fields from each element of theantenna, with their individual directivity

Far-field region: the area beyond roughly 10 times thespacing between the antenna’s internal elements

• In this region, the antenna seems to be apoint-source and the contributions of theindividual elements are indistinguishable

• The pattern is the composite of the array

Obstructions in the near-field can dramatically alter theantenna performance

Near-field

Far-field

Local Obstruction at a Site



Local Obstruction at a Site


Diffractionoverobstructing

edge

Local obstruction example Obstructions near the site are

sometimes unavoidable

Near-field obstructions can

seriously alter pattern shape

More distant local obstructions can causesevere blockage, as for

example roof edge in thefigure at right

• Knife-edge diffractionanalysis can help

estimate diffraction loss inthese situations

• Explore other antennamounting positions

Estimating Isolation Between Antennas



Estimating Isolation Between Antennas


Often multiple antennas are needed at asite and interaction is troublesome

Electrical isolation between antennas

• Coupling loss between isotropicantennas one wavelength apart is22 dB

• 6 dB additional coupling loss witheach doubling of separation

• Add gain or loss referenced fromhorizontal plane patterns

• Measure vertical separationbetween centers of the antennas

– vertical separation usually is veryeffective

One antenna should not be mounted inmain lobe and near-field of another

• Typically within 10 feet @ 800 MHz• Typically 5-10 feet @ 1900 MHz

Vi ll E ti ti D i A l



Visually Estimating Depression Anglesin the field


Before considering downtilt,beamwidths, and depressionangles, do some personal

experimentation at a high siteto gain a sense of the anglesinvolved

Visible width of fingers, etc. can

be useful approximatebenchmark for visualevaluation

Measure and remember width

of your own chosen references Standing at a site, correlate

your sightings of objects youwant to cover with angles in

degrees and the antennapattern

distancewidth

angle = arctangent (width / distance)

Visually estimating angleswith tools always at hand

Typical Angles

Thumb width

Nail of forefinger

All knuckles

~2 degrees

~1 degree

~10 degrees“Calibrate” yourself using the formula!

Antenna Downtilt



What’s the goal?

Downtilt is commonly used for tworeasons

1. Reduce Interference

• Reduce radiation toward adistant co-channel cell

• Concentrate radiation withinthe serving cell

2. Prevent “Overshoot”• Improve coverage of

nearby targets far below theantenna

– otherwise within “null” ofantenna pattern

Are these good strategies?

How is downtilt applied?

Scenario 2

Cell A

Scenario 1

Cell B


Consider Vertical Depression Angles




Basic principle: important to matchvertical pattern against intendedcoverage targets

• Compare the angles towardobjects against the antennavertical pattern -- what’s radiatingtoward the target?

• Don’t position a null of theantenna toward an important

coverage target! Sketch and formula

• Notice the height and horizontaldistance must be expressed in

the same units before dividing(both in feet, both in miles, etc.)

Horizontaldistance

Verticaldistance

θ Depression

angle

θ = ArcTAN ( Vertical distance / Horizontal distance )

Types Of Downtilt



Types Of Downtilt


Mechanical downtilt

• Physically tilt the antenna

• The pattern in front goesdown, and behind goes up

• Popular for sectorizationand special omniapplications

Electrical downtilt

• Incremental phase shift isapplied in the feed network

• The pattern “droops” allaround, like an inverted

saucer• Common technique when

downtilting omni cells

Reduce Interference



d tScenario 1

The Concept: Radiate a strong signal toward

everything within the servingcell, but significantly reduce

the radiation toward the areaof Cell B

The Reality:

When actually calculated, it’ssurprising how small thedifference in angle is betweenthe far edge of cell A and thenear edge of Cell B

• Delta in the example isonly 0.3 degrees!!

• Let’s look at antennapatterns

Cell AConcept

Cell B

weakstrong

θ1 = ArcTAN ( 150 / ( 4 * 5280 ) )= -0.4 degrees

θ2 = ArcTAN ( 150 / ( 12 * 5280 ) )= -0.1 degrees

Reality

12 miles

4

heightdifference150 ft

θ21


Reduce Interference



Reduce InterferenceScenario 1 , Continued

It’s an attractive idea, but usually theangle between edge of serving celland nearest edge of distant cell is just too small to exploit

• Downtilt or not, can’t get muchdifference in antenna radiationbetween θ1 and θ2

• Even if the pattern were sharpenough, alignment accuracy andwind-flexing would be problems

– delta θ in this exampleis less than one degree!

• Also, if downtilting -- watch out

for excessive RSSI and IMinvolving mobiles near cell! Soft handoff and good CDMA power

control is more important

-0.4-0.1

θ1 = -0.4 degrees

θ2 = -0.1 degrees


Avoid Overshoot



Avoid OvershootScenario 2

Application concern: too little radiationtoward low, close-in coverage targets

The solution is common-sense matching

of the antenna vertical pattern to theangles where radiation is needed

• Calculate vertical angles to targets!!

• Watch the pattern nulls -- where do

they fall on the ground?• Choose a low-gain antenna with a

fat vertical pattern if you have awide range of vertical angles to “hit”

• Downtilt if appropriate

• If needed, investigate special “null-filled” antennas with smoothpatterns

Scenario 2


Other Antenna Selection Considerations



Before choosing an antenna for widespread deployment, investigate:

Manufacturer’s measured patterns

• Observe pattern at low end of band, mid-band, and high end of band

• Any troublesome back lobes or minor lobes in H or V patterns?• Watch out for nulls which would fall toward populated areas

• Be suspicious of extremely symmetrical, “clean” measured patterns

• Obtain Intermod Specifications and test results (-130 or better)

• Inspect return loss measurements across the band Inspect a sample unit

• Physical integrity? weatherproof?

• Dissimilar metals in contact anywhere?

• Collinear vertical antennas: feed method?• End (compromise) or center-fed (best)?

• Complete your own return loss measurements, if possible

• Ideally, do your own limited pattern verification

Check with other users for their experiences


Scott Baxter 100_C5

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