Technical Introduction to CDMA

1
Chapter 5
Antennas for Wireless Systems
2
Chapter 5 Section A
Introduction to Antennas for Wireless
3
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!

4
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 antennas directivity is the same for
  transmission reception

5
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..

6
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

7
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

Isotropic Antenna
8
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 antenna

Example Antennas A and B each radiate 100 watts
from their own transmitters. Antenna A is our
reference, it happens to be isotropic. Antenna B
is directional. In its maximum direction,
its signal seems 2.75 stronger than the signal
from antenna A. Antenna Bs EIRP in this case is
275 watts.
9
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 antennas 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).

10
Radiation PatternsKey Features And Terminology

• An antennas 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.

11
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

12
Types Of Arrays
Collinear Vertical Array

• 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

Yagi
Log-Periodic
13
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
14
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?)

15
Example Of Antenna Catalog Specifications
16
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!

17
Chapter 5 Section B
Other RF Elements
18
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
• Manufacturers system may include some or all of
  these items
• Remaining items are added individually as needed
  by system operator

19
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

20
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

21
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

22
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)

23
Transmission LinesImportant Installation
Practices

• Respect specified minimum bending radius!
• Inner conductor must remain concentric, otherwise
  Zo changes
• Dents, kinks in outer conductor change Zo
• Dont 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.

24
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
  manufacturers recommended spacing intervals
• Strong winds will set up damaging
  metal-fatigue-inducing vibrations

25
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

26
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 filters electrical
  characteristics
• Bandwidth rejection
• Insertion loss
• Slopes
• Ripple, etc.

27
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

28
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

29
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

30
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

31
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 1.51
  maximum over a specified band.
• VSWR 1.5 1
• 14 db return loss
• 4.0 reflected power

32
Swept Return Loss Measurements

• Its 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.

33
Chapter 5 Section C
Some Antenna Application Considerations
34
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 antennas 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 antennas 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

35
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

36
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 _at_ 800 MHz
• Typically 5-10 feet _at_ 1900 MHz

37
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

38
Antenna DowntiltWhats 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?

39
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 -- whats radiating
  toward the target?
• Dont 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.)

q ArcTAN ( Vertical distance / Horizontal
distance )
40
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

41
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, its 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!!
• Lets look at antenna patterns

42
Reduce Interference Scenario 1 , Continued

• Its 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, cant get much difference in
  antenna radiation between q1 and q2
• Even if the pattern were sharp enough, alignment
  accuracy and wind-flexing would be problems
• delta q 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
43
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

44
Other Antenna Selection Considerations

• Before choosing an antenna for widespread
  deployment, investigate
• Manufacturers 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