this document contains copyrighted information of © 2006 waterford consultants, llc and others...
TRANSCRIPT
This document contains copyrighted information of © 2006 Waterford Consultants, LLC and others
Richard P. Biby, P.E. Waterford Consultants, LLC
Waterford, VA 20197 (540) 882-4290
Tower Designs – the Good, the Bad and the
Ugly(and a few geeky topics)
Who Am I BS & MS Electrical Engineering & Computer Engineering,
George Washington University, Washington, DC Registered Professional Engineer (VA) Former CTO of Crown Castle International, Inc. Owner of 20ish towers in the DC / VA area Owner of Fryers Tower Source Publisher, AGL Magazine Founder and Chief Technology Officer of Waterford Consultants,
LLC Active in analysis of Non-Ionizing Radiation for approximately 10
years Founder of Sitesafe, Inc.
What RF Engineers Want
20’ Tip to Tail Vertical Separation Interleave 800 MHz and 1900 MHz To be at the top of the tower No visual impairment between antenna and
cell phone To be the only carrier on the tower Three antennas (Cellular), Two antennas
(PCS)
What RF Engineers Usually Get
20’ Tip to Tail Vertical Separation Interleave 800 MHz and 1900 MHz To be at the top of the tower No visual impairment between antenna and
cell phone To be the only carrier on the tower Three antennas (Cellular), Two antennas
(PCS)
What RF Engineers Will Accept (If management, the attorneys, regulatory affairs, operations, construction and real estate insists)
20’ Tip to Tail Vertical Separation Interleave 800 MHz and 1900 MHz To be at the top of the tower No visual impairment between antenna and
cell phone To be the only carrier on the tower Three antennas (Cellular), Two antennas
(PCS)
Why 20’ Tip to Tail?
Interference Physically increasing spacing between antennas
reduces amount of energy from one carrier’s TX antennas into another carrier’s RX antennas
Industry “standard” which could use some additional research
Performance A system can receive weaker signals if there is no
large source of background noise Could result in less sites ($$ savings)
Why 20’ Tip to Tail?
Few documented Interference issues when spacing closer
Why Interleave 800 & 1900 MHz? Reduction in interference
800 MHz antennas receive 1900 MHz signals less efficiently then a 1900 MHz receive antenna, and vice versa. Vertically interleaving 800 & 1900 MHz carriers essentially doubles the separation spacing
To be at the Top of the Tower
Best position for Coverage, transmit and receive
Will, typically, cost the most to be on top Carriers willing to be anchor tenant will often
end up paying more to secure top spot Best position for reduced interference May have increased lightining exposure
No Visual Impairment Between Antenna and Cell Phone
Any visual impairment will have an effect (negatively) on the effective radiated power (ERP) of the site, reducing coverage, increasing costs, and will also reduce the mobile units ability to talk back – either reducing coverage or requiring phone to transmit at higher power, reducing battery life.
Not as big an issue in dense, highly populated areas where coverage will be limited by capacity constraints rather than coverage.
Three antennas (Cellular), Two antennas (PCS)
Cellular systems typically have two receive antennas (for space receive diversity) and one transmit (no diversity needed on transmit)
PCS systems typically have one antenna which is receive only and one that transmits and receives
Some systems may have a forth antenna for transmission if the site is very heavily loaded.
A Method For Combating Rayleigh Fading Polarization Diversity
Use where Space Diversity Isn’t convenient Sometimes zoning considerations or
aesthetics preclude using separate diversity receive antennas
Dual-polarized antenna pairs within a single radome are becoming popular
Antenna pair within one radome can be V-H polarized, or diagonally polarized Each individual array has its own
independent feedline
Antenna A
Antenna B
Combined
A B A B
V+Hor\+/
Cross Polarization
Antenna Mounting Considerations 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
A Method For Combating Rayleigh Fading Space Diversity
Fortunately, Rayleigh fades are very short and last a small percentage of the time
Two antennas separated by several wavelengths will not generally experience fades at the same time
“Space Diversity” can be obtained by using two receiving antennas and switching instant-by-instant to whichever is best
Required separation D for good decorrelation is (10-20) D = (12-24) ft. @ 800 MHz. D = (5-10) ft. @ 1900 MHz.
D
Signal received by Antenna 1
Signal received by Antenna 2
Combined Signal
Summary of Alternative Towers
Pros Reduced Visual Impact
Cons Increased costs (2, 3 or 4
times) Reduced Number of
carriers per site Higher operational costs Increased Engineering
Complexity & Reduced performance
Special Thanks To…
Much of the material in this presentation has been developed by Mr. Scott Baxter, P.E. and is used with his permission.
Stealth Technologies
Invisible Towers, Inc.
Fun with Antennas(Time Permitting)
Basic Antenna Characteristics 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
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
Basic Antenna Characteristics 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
Reference Antennas Antenna Gain And ERP - Examples
Many wireless systems 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 (typical) wireless antenna’s maximum radiation is 12.1 dB stronger than the isotropic (thus 12.1 dBi gain), and 10 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 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 orminim
Antennas used in Wireless Omni Antennas - Collinear 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
Transmission Line Characteristics Some 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
Antenna Downtilt 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
Antenna Selection/Installation ScenarioReduce radiation interference to another cell
The Vision 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
User AVision
User B
weak
strong
1 = ArcTAN ( 150 / ( 4 * 5280 ) ) = -0.4 degrees
2 = ArcTAN ( 150 / ( 12 * 5280 ) ) = -0.1 degrees
Reality
12 miles
4
height difference
150 ft21