July, 1998 5 - 1RF100 (c) 1998 Scott Baxter
Antennas for Wireless Systems
Antennas for Wireless Systems
Chapter 5
Dipole
Typical WirelessOmni Antenna
Isotropic
July, 1998 5 - 2RF100 (c) 1998 Scott Baxter
Introduction toAntennas for Wireless
Introduction toAntennas for Wireless
Chapter 5 Section A
July, 1998 5 - 3RF100 (c) 1998 Scott Baxter
Understanding Antenna RadiationThe 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 cause
current in receiving antennas The effect of the total antenna is the sum
of what every tiny “slice” of the antenna is doing
• Radiation of a tiny “slice” is proportional to its length times the current in it
• remember, the current has a magnitude and a phase!
TX RX
Width of banddenotes current
magnitude
Zero currentat each end
Maximum currentat the middle
Current induced inreceiving antennais vector sum of
contribution of everytiny “slice” of
radiating antenna
each tiny imaginary “slice”of the antennadoes its share
of radiating
July, 1998 5 - 4RF100 (c) 1998 Scott Baxter
Different Radiation In Different Directions
Each “slice” of the antenna produces a definite amount of radiation at a specific phase angle
Strength of signal received varies, depending on direction of departure from radiating antenna
• In some directions, the components add up in phase to a strong signal level
• In other directions, due to the different distances the various components must travel to reach the receiver, they are out of phase and cancel, leaving a much weaker signal
An antenna’s directivity is the same for transmission & reception
TX
MaximumRadiation:contributions
in phase, reinforce
MinimumRadiation:contributionsout of phase,
cancel
MinimumRadiation:contributionsout of phase,
cancel
July, 1998 5 - 5RF100 (c) 1998 Scott Baxter
Antenna Polarization
To intercept significant energy, a receiving antenna must be oriented parallel to the transmitting antenna
• A receiving antenna oriented at right angles to the transmitting antenna 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…..
TX
ElectromagneticField
currentalmost
nocurrent
Antenna 1VerticallyPolarized
Antenna 2Horizontally
Polarized
RX
RF current in a conductor causes electromagnetic fields that seek to induce current flowing in the same direction in other conductors.
The orientation of the antenna is called its polarization.
Coupling between two antennas is proportional to the cosine of the angle of their relative orientation
July, 1998 5 - 6RF100 (c) 1998 Scott Baxter
Antenna Gain
Antennas are passive devices: they do not produce power
• Can only receive power in one form and pass it 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. This gain 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 a non-directional antenna
Gain in one direction comes at the expense of less radiation in other directions
Antenna Gain is RELATIVE, not ABSOLUTE
• When describing antenna “gain”, the comparison condition must be stated or implied
Omni-directionalAntenna
DirectionalAntenna
July, 1998 5 - 7RF100 (c) 1998 Scott Baxter
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, physically practical• A popular reference: below 1000 MHz
– 800 MHz. cellular, land mobile, TV & FM
IsotropicAntenna
(watts or dBm) ERP Effective Radiated Power Vs. Dipole
Effective 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 gaincompared to an isotropic antenna.
July, 1998 5 - 8RF100 (c) 1998 Scott Baxter
Effective Radiated Power
An antenna radiates all power fed to it from the transmitter, 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 antennaAB
ERP B A (ref)
100w275w
ReferenceAntenna
TX100 W
A
DirectionalAntenna
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, itssignal seems 2.75 stronger than the signal from antennaA. Antenna B’s EIRP in this case is 275 watts.
July, 1998 5 - 9RF100 (c) 1998 Scott Baxter
Antenna Gain And ERPExamples
Many wireless systems at 1900 & 800 MHz use omni antennas like the one shown in this figure
These patterns are drawn to scale in E-field radiation units, based on equal power to each antenna
Notice the typical wireless omni antenna concentrates most of its radiation toward the horizon, where users are, at the expense of sending less radiation sharply upward 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
July, 1998 5 - 10RF100 (c) 1998 Scott Baxter
Radiation PatternsKey Features And Terminology
An antenna’s directivity is expressed 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 the radiation as a function of elevation (i.e.., up, down, horizontal)
Antennas are often compared by noting specific landmark points on their patterns:
• -3 dB (“HPBW”), -6 dB, -10 dB points
• 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 MinorLobe
nulls orminima
July, 1998 5 - 11RF100 (c) 1998 Scott Baxter
In phase
Out of phase
How Antennas Achieve Their Gain
Quasi-Optical Techniques (reflection, focusing)• Reflectors can be used to concentrate
radiation– technique works best at microwave frequencies,
where reflectors are small
• Examples:– corner reflector used at cellular or higher
frequencies– parabolic reflector used at microwave frequencies– grid or single pipe reflector for cellular
Array techniques (discrete elements)• Power is fed or coupled to multiple antenna
elements; each element radiates• Elements’ radiation in phase in some
directions• In other directions, a phase delay for each
element creates pattern lobes and nulls
July, 1998 5 - 12RF100 (c) 1998 Scott Baxter
Types Of Arrays
Collinear vertical arrays• Essentially omnidirectional in
horizontal 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 in wireless
RF power
RF power
CollinearVerticalArray
Yagi
Log-Periodic
July, 1998 5 - 13RF100 (c) 1998 Scott Baxter
Omni AntennasCollinear Vertical Arrays
The family of omni-directional wireless antennas:
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 coverage areas
Rod and grid reflectors are sometimes added for mild directivity
Examples: 800 MHz.: dB803, PD10017, BCR-10O, Kathrein 740-198
1900 MHz.: dB-910, ASPP2933
beamwidth
Angleof
firstnull
-3 dB
Vertical Plane Pattern
Number of Elements
PowerGain
Gain, dB
Angle
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°
1234567891011121314
1234567891011121314 11.46 4.09°
Typical Collinear Arrays
July, 1998 5 - 14RF100 (c) 1998 Scott Baxter
Sector AntennasReflectors And Vertical Arrays
Typical commercial sector antennas are vertical combinations of dipoles, yagis, or log-periodic elements with reflector (panel or grid) backing
• Vertical plane pattern is determined by number of vertically-separated elements
– varies from 1 to 8, affecting mainly gain and vertical plane beamwidth
• Horizontal plane pattern is determined by:
– number of horizontally-spaced elements
– shape of reflectors (is reflector folded?)
Vertical Plane PatternUp
Down
Horizontal Plane PatternN
E
S
W
July, 1998 5 - 15RF100 (c) 1998 Scott Baxter
Example Of Antenna Catalog Specifications
Frequency Range, MHz.Gain - dBd/dBiVSWR
Beamwidth (3 dB from maximum)Polarization
Maximum power input - WattsInput Impedance - OhmsLightning ProtectionTermination - StandardJumper Cable
Electrical DataAntenna Model ASPP2933 ASPP2936 dB910C-M
1850-1990 1850-1990 1850-19703/5.1
<1.5:1
32Vertical
40050
Direct GroundN-Female
Order Sep.
6/8.1<1.5:1
15Vertical
40050
Direct GroundN-Female
Order Sep.
10/12.1<1.5:1
5Vertical
40050
Direct GroundN-Female
Order Sep.
Mechanical DataAntenna ModelOverall 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)
ASPP293324 (610)
1.1 (25.4)
.17 (.0155)4 (17)
140 (225)
4 (1.8)11 (4.9)
ASPA320
ASPP293636 (915)
1.0 (25.4)
.25 (.0233)6 (26)
140 (225)
6 (2.7)13 (5.9)
ASPA320
dB910C-M77 (1955)
1.5 (38)
.54 (.05)14 (61)
125 (201)
5.2 (2.4)9 (4.1)
Integral
July, 1998 5 - 16RF100 (c) 1998 Scott Baxter
Example Of Antenna Catalog Radiation Pattern
Vertical Plane Pattern
• E-Plane (elevation plane)
• Gain: 10 dBd
• Dipole pattern is superimposed at scale for comparison (not often shown in commercial catalogs)
• Frequency is shown
• Pattern values shown in dBd
• Note 1-degree indices through region of main lobe for most accurate reading
• Notice minor lobe and null detail!
July, 1998 5 - 17RF100 (c) 1998 Scott Baxter
Other RF ElementsOther RF Elements
Chapter 5 Section B
July, 1998 5 - 18RF100 (c) 1998 Scott Baxter
Antenna Systems
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
F R
Duplexer
Combiner
BPF
TX
RX
TXTransmission LineJumper
Jumpers
DirectionalCoupler
Antenna
July, 1998 5 - 19RF100 (c) 1998 Scott Baxter
Characteristics Of Transmission Lines
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”Foam
DielectricAir
Dielectric
Typical coaxial cablesUsed as feeders in wireless applications
July, 1998 5 - 20RF100 (c) 1998 Scott Baxter
Transmission LinesSome Practical Considerations
Transmission lines practical considerations• Periodicity of inner conductor
supporting structure can cause VSWR peaks at some frequencies, so specify the frequency band when ordering
• Air dielectric lines– lower loss than foam-dielectric; dry air
is excellent insulator – shipped pressurized; do not accept
delivery if pressure leak• Foam dielectric lines
– simple, low maintenance; despite slightly higher loss
– small pinholes and leaks can allow water penetration and gradual attenuation increases
FoamDielectric
AirDielectric
July, 1998 5 - 21RF100 (c) 1998 Scott Baxter
Characteristics Of Transmission Lines, Continued
Electrical Characteristics Attenuation
• Varies with frequency, size, dielectric characteristics of insulation
• Usually specified in dB/100 ft and/or dB/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 of insulation
• Connectors must preserve constant impedance (see figure at right)
Velocity factor• Determined by dielectric characteristics
of insulation. Power-handling capability
• Varies with size, conductor materials, dielectric characteristics
dD
Characteristic Impedance of a Coaxial Line
Zo = ( 138 / ( 1/2 ) ) Log10 ( D / d )
= Dielectric Constant = 1 for vacuum or dry air
July, 1998 5 - 22RF100 (c) 1998 Scott Baxter
Transmission LinesSpecial Electrical Properties
Transmission lines have impedance-transforming properties
• When terminated with same impedance as Zo, input to line appears as impedance Zo
• When terminated with impedance different from Zo, input to line is a complex function of frequency and line length. Use Smith Chart or formulae to compute
Special case of interest: Line section one-quarter wavelength long has convenient properties useful in matching networks
• ZIN = (Zo2)/(ZLOAD)
Zo=50ZLOAD=
50ZIN = 50
Matched condition
Zo=50ZLOAD=
83-j22
ZIN = ?
Mismatched condition
Zo=50ZLOAD=
100ZIN=25
/4
ZIN= ZO2
/ ZLOAD
Deliberate mismatchfor impedance transformation
July, 1998 5 - 23RF100 (c) 1998 Scott Baxter
Transmission LinesImportant Installation Practices
Respect specified minimum bending radius!
• Inner conductor must remain concentric, otherwise Zo changes
• Dents, kinks in outer conductor change Zo
Don’t bend large, stiff lines (1-5/8” or larger) to make direct connection with antennas
Use appropriate jumpers, weatherproofed properly.
Secure jumpers against wind vibration.
ObserveMinimumBendingRadius!
July, 1998 5 - 24RF100 (c) 1998 Scott Baxter
Transmission LinesImportant Installation Practices, Continued
During hoisting
• Allow line to support its own weight 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 proper mounting clamps at manufacturer’s recommended spacing intervals
• Strong winds will set up damaging metal-fatigue-inducing vibrations
200 ft~60 mMax.
3-6 ft
July, 1998 5 - 25RF100 (c) 1998 Scott Baxter
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 have insertion loss of 1-3 dB. and passband ripple of 2-6 dB.
Bandwidth is typically 1-20% of center frequency, depending on application. Attenuation slope and out-of-band attenuation depend on # of poles & design
Typical RF bandpass filter
0
Att
enu
atio
n,
dB
Frequency, megaHertz
passband rippleinsertion loss
-3 dB passbandwidth
July, 1998 5 - 26RF100 (c) 1998 Scott Baxter
RF FiltersTypes And Applications
Filters are the basic building blocks of duplexers and more complex devices
Most manufacturers’ network equipment includes internal bandpass filters at receiver input and transmitter output
Filters are also available for special applications
Number of poles (filter elements) and other design variables determine filter’s electrical characteristics
• Bandwidth rejection
• Insertion loss
• Slopes
• Ripple, etc.
Notice construction: RF input excites one quarter-wave element and electromagnet fields propagate from element to element, finally exciting the last element which is directly coupled to the output.
Each element is individually set and forms a pole in the filter’s overall response curve.
Typical RF Bandpass Filter
/4
July, 1998 5 - 27RF100 (c) 1998 Scott Baxter
Basics Of Transmitting Combiners
Allows multiple transmitters to feed single antenna, providing
• Minimum power loss from transmitter to antenna
• Maximum isolation between transmitters
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 transmitter frequencies
• Linear amplifier– linearity and intermodulation are major
design and operation issues
Typical tuned combiner application
TX TX TX TX TX TX TX TX
Antenna
Typical hybrid combiner application
TX TX TX TX TX TX TX TX
Antenna
~-3 dB
~-3 dB
~-3 dB
July, 1998 5 - 28RF100 (c) 1998 Scott Baxter
Duplexer Basics
Duplexer allows simultaneous transmitting and receiving on one antenna
• Nortel 1900 MHz BTS RFFEs include internal duplexer
• Nortel 800 MHz BTS does not include duplexer but commercial units can be used if desired
Important duplexer specifications
• TX pass-through insertion loss
• RX pass-through insertion loss
• TX-to-RX isolation at TX frequency (RX intermodulation issue)
• TX-to-RX isolation at RX frequency (TX noise floor issue)
• Internally-generated IMP limit specification
fR fT
RX TX
Antenna
Duplexer
Principle of operation
Duplexer is composed of individualbandpass filters to isolate TX fromRX while allowing access to antennafor both. Filter design determinesactual isolation between TX and RX,and insertion loss TX-to-Antenna and RX-to-Antenna.
July, 1998 5 - 29RF100 (c) 1998 Scott Baxter
Directional Couplers
Couplers are used to measure forward and reflected energy in a transmission 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 other direction
Typical performance specifications• Coupling factor ~20, ~30,
~40 dB., order as appropriate for application
• Directivity ~30-~40 dB., f($)– defined as relative
attenuation of unwanted direction in each sample
Principle of operation
ZLOAD= 50
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.
July, 1998 5 - 30RF100 (c) 1998 Scott Baxter
Return Loss And VSWR Measurement
A perfect antenna will absorb and radiate all the power fed to it
Real antennas absorb most of the power, but reflect a portion back down the line
A Directional Coupler or Directional Wattmeter can be used to measure 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
Transmission line
AntennaDirectionalcoupler Fwd
Refl
RFPower
July, 1998 5 - 31RF100 (c) 1998 Scott Baxter
Return Loss and VSWR
Forward Power, Reflected Power, Return Loss, and VSWR can be related by these equations and the graph.
• Typical antenna VSWR specifications are 1.5:1 maximum over a specified band.
• 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 PowerForward Power
ReturnLoss, dB = 10 x Log10 ( )
July, 1998 5 - 32RF100 (c) 1998 Scott Baxter
Swept Return Loss Measurements
It’s a good idea to take swept or TDR return loss measurements of a new antenna at installation and to recheck periodically
• maintain a printed or electronically stored copy of the analyzer output for comparison
• most types of antenna or transmission line failures are easily detectable by comparison with stored data
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.
Transmission Line
AntennaDirectionalCoupler Fwd
Refl
Network Analyzer-10
-20
-30f1 f2
A Network Analyzer can also display polar plots, Smith Charts, phase response
A Spectrum Analyzer and tracking generator can be used if Network Analyzer not available
July, 1998 5 - 33RF100 (c) 1998 Scott Baxter
Some Antenna Application Considerations
Some Antenna Application Considerations
Chapter 5 Section C
July, 1998 5 - 34RF100 (c) 1998 Scott Baxter
Near-Field/Far-Field Considerations
Antenna behavior is very different close-in and far out Near-field region: the area within about 10 times the
spacing between antenna’s internal elements • Inside this region, the signal behaves as
independent fields from each element of the antenna, with their individual directivity
Far-field region: the area beyond roughly 10 times the spacing between the antenna’s internal elements
• In this region, the antenna seems to be a point-source and the contributions of the individual elements are indistinguishable
• The pattern is the composite of the array Obstructions in the near-field can dramatically alter the
antenna performance
Near-field
Far-field
July, 1998 5 - 35RF100 (c) 1998 Scott Baxter
Local Obstruction at a Site
Obstructions near the site are sometimes unavoidable
Near-field obstructions can seriously alter pattern shape
More distant local obstructions can cause severe blockage, as for example roof edge in the figure at right
• Knife-edge diffraction analysis can help estimate diffraction loss in these situations
• Explore other antenna mounting positions
Diffractionover
obstructing edge
Local obstruction example
July, 1998 5 - 36RF100 (c) 1998 Scott Baxter
Estimating Isolation Between Antennas
Often multiple antennas are needed at a site and interaction is troublesome
Electrical isolation between antennas• Coupling loss between isotropic
antennas one wavelength apart is 22 dB
• 6 dB additional coupling loss with each doubling of separation
• Add gain or loss referenced from horizontal plane patterns
• Measure vertical separation between centers of the antennas
– vertical separation usually is very effective
One antenna should not be mounted in main lobe and near-field of another
• Typically within 10 feet @ 800 MHz• Typically 5-10 feet @ 1900 MHz
July, 1998 5 - 37RF100 (c) 1998 Scott Baxter
Visually Estimating Depression Anglesin the field
Before considering downtilt, beamwidths, and depression angles, do some personal experimentation at a high site to gain a sense of the angles involved
Visible width of fingers, etc. can be useful approximate benchmark for visual evaluation
Measure and remember width of your own chosen references
Standing at a site, correlate your sightings of objects you want to cover with angles in degrees and the antenna pattern
distance
width
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!
July, 1998 5 - 38RF100 (c) 1998 Scott Baxter
Antenna DowntiltWhat’s the goal?
Downtilt is commonly used for two reasons
1. Reduce Interference• Reduce radiation toward a
distant co-channel cell• Concentrate radiation within
the serving cell 2. Prevent “Overshoot”
• Improve coverage of nearby targets far below the antenna
– otherwise within “null” of antenna pattern
Are these good strategies? How is downtilt applied?
Scenario 2
Cell A
Scenario 1
Cell B
July, 1998 5 - 39RF100 (c) 1998 Scott Baxter
Consider Vertical Depression Angles
Basic principle: important to match vertical pattern against intended coverage targets
• Compare the angles toward objects against the antenna vertical pattern -- what’s radiating toward the target?
• Don’t position a null of the antenna toward an important coverage target!
Sketch and formula
• Notice the height and horizontal distance must be expressed in the same units before dividing (both in feet, both in miles, etc.)
= ArcTAN ( Vertical distance / Horizontal distance )
Horizontaldistance
Verticaldistance
Depression angle
July, 1998 5 - 40RF100 (c) 1998 Scott Baxter
Types Of Downtilt
Mechanical downtilt
• Physically tilt the antenna
• The pattern in front goes down, and behind goes up
• Popular for sectorization and special omni applications
Electrical downtilt
• Incremental phase shift is applied in the feed network
• The pattern “droops” all around, like an inverted saucer
• Common technique when downtilting omni cells
July, 1998 5 - 41RF100 (c) 1998 Scott Baxter
Reduce Interference Scenario 1
The Concept: Radiate a strong signal toward
everything within the serving cell, but significantly reduce the radiation toward the area of Cell B
The Reality: When actually calculated, it’s
surprising how small the difference in angle is between the far edge of cell A and the near edge of Cell B
• Delta in the example is only 0.3 degrees!!
• Let’s look at antenna patterns
Cell AConcept
Cell B
weakstrong
1 = ArcTAN ( 150 / ( 4 * 5280 ) ) = -0.4 degrees
2 = ArcTAN ( 150 / ( 12 * 5280 ) ) = -0.1 degrees
Reality
12 miles4
height difference
150 ft 21
July, 1998 5 - 42RF100 (c) 1998 Scott Baxter
Reduce Interference Scenario 1 , Continued
It’s an attractive idea, but usually the angle between edge of serving cell and nearest edge of distant cell is just too small to exploit
• Downtilt or not, can’t get much difference in antenna radiation between 1 and 2
• Even if the pattern were sharp enough, alignment accuracy and wind-flexing would be problems
– delta in this example is less than one degree!
• Also, if downtilting -- watch out for excessive RSSI and IM involving mobiles near cell!
Soft handoff and good CDMA power control is more important
-0.4-0.1
1 = -0.4 degrees
2 = -0.1 degrees
July, 1998 5 - 43RF100 (c) 1998 Scott Baxter
Avoid Overshoot Scenario 2
Application concern: too little radiation toward low, close-in coverage targets
The solution is common-sense matching of the antenna vertical pattern to the angles 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 a wide range of vertical angles to “hit”
• Downtilt if appropriate
• If needed, investigate special “null-filled” antennas with smooth patterns
Scenario 2
July, 1998 5 - 44RF100 (c) 1998 Scott Baxter
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