signs of end of life fm antennas and transmission line systems · antenna or transmission line if...
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Copyright © 2017 Electronics Research, Inc. All rights reserved.
Signs of End of Life
FM Antennas and
Transmission Line Systems
Thomas B. Silliman, PE
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Introduction
• Factors that influence the life of
antennas and feed lines
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Factors that influence the life
of Antennas and Feed lines
• Storm damage
– Wind
• Tower movement
– Lightning
• Salt on insulators
– Sea coast
• Time in use
– Bullet failure if station is not on air 24
hours a day2
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Factors that influence the life
of Antennas and Feed Lines
• Age of Antenna
– Time between failures
• Vandals
– Bullet holes in transmission line near
lights
• Tower or pole vibration
– Guy wire galloping
• Snubbers
• Vortex shedding
– Not likely with side mount antennas3
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Factors that influence the life
of Antennas and Feed lines
• Antenna and tower inspections
– Many FM multiplex antenna systems
are inspected annually.
• Broken hangers
• Pressure leaks
• Worn out insulators or insulators that need
to be coated
– Most single station side mount antennas
are only inspected if a problem occurs
• VSWR
• Pressure loss 4
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Inspection of feed line
showing missing button
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Inspection of Multiplex
Painted Antenna Insulator
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Antenna Inspection of Side
Mounted FM Antenna
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Antenna Inspection
Measuring All Components
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Storm Damage:
Wind - Tower Movement
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Sample Calculation of
Lambda Deflection
• Assume a 60 foot tall Lambda Tower
• Specification of radius of curvature
for the Lambda Tower is R = 3000
feet at full wind design under G
• R(Ɵ) = 60 feet where ϴ is radians
• Ө = 0.02 radians = 1.1459 degrees
• X = (3000) cos1.1459 = 2999.4 feet
• Deflection = 3000 – 2999.4 = .6 feet
• .6 feet = 7.2 inches 10
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One Solution to
Tower movement
• First Lambda Tower installed in 1990
• Replaced a top mounted pole on a
tower in Pittsburgh
• Antenna on pole had failed every
year since it was installed
• Has never failed since mounted on
Lambda tower
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Analysis of Tower Deflection
• Tower analysis today can be done
with finite element analysis
• Finite element analysis will predict
tower movement and deflection
• If antenna failures occur in less than
three years after installation, the
tower should be analyzed to
determine if excessive movement is
causing the premature antenna
failure.12
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Causes of excessive Tower
Movement
• Guy wires not tensioned properly
• Tower design
– Ice on wires
– Lack of snubbers
– Improper reinforcement design
– Antenna installed without analysis of
tower prior to additional loading
• Working on problems in the field, I
have seen all of these tower design
issues.13
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Lightning Damage
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Lightning Damage is
Repairable
• Symptoms
– Loss of pressure
– High VSWR
• Can allow moisture to enter antenna
and transmission line.
• Does not necessarily have an
adverse affect on the life time of the
antenna or transmission line if repairs
are done quickly.15
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Salt deposits on antenna and
insulators and tower steel
• Will affect antennas located near
ocean
• Radomes can be added to the
antenna to protect insulator surface
• Towers near sea coast should be
inspected every five years
– Guy wire deterioration
– Leg steel
• If hollow legs, can deteriorate leg steel.
• Inner members can rust if not protected.16
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Example of Guy Wire
Deterioration
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Example of
Tower leg Deterioration
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Time in use• FM antennas have inner connector
connectors, bullets.
– As temperature of inner conductor
changes, the inner conductor will
expand and contract differently than the
outer conductor.
• Temperature varies due to outside air
temperature
• Temperature varies when antenna input
power is turned off and then turned on at a
later time.
• Excessive movement of inner causes bullet
failure.19
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Example of Bullet Failure
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Age of Antenna
Time Between Failures
• Antennas can be repaired or rebuilt to
extend the life of the antenna.
– New bullets
– New insulators
– New inner conductors
• Rigid coax can also be rebuilt to
extend its life.
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Antenna being Rebuilt in the
Field
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Vandals
• Often, we find that someone has shot
an antenna. We usually find the hole
near a tower light. Can be patched.
• If maintained, antenna life is
unchanged.
• Symptoms:
– Loss of pressure
– Change in VSWR
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Guy Wire Galloping
• Can result when wires are not
tensioned properly
• Often will occur with ice on the wires
– Wind causes ice to form a trailing edge
– Trailing edge of ice on wire causes lift
with stiff wind.
– Wire will jump up and down shaking the
tower which can result in violent
antenna movement resulting in antenna
damage. 24
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Guy wire Snubbers
• Significantly reduce guy wire
galloping.
• Cables ride up and down on the guy
wire near the anchor on pullies.
• These cables are attached to shock
absorbers that are anchored to the
ground near the guy wire anchor.
• Are often used on tall towers 1000
feet tall and taller.
• Can be added to existing towers.25
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Guy wire Snubbers
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Guy wire Snubbers
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Guy wire Galloping
• Can cause cracks in the antenna
components and limit the useable life
time of the antenna.
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Voltage Standing Wave Ratio
• VSWR = (1 + p)/(1- p)
• Forward power = p+
• Reflected power = p-
• p = Square Root (p-/p+)
• Return loss = 10 log(p)
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Antenna VSWR
• Typical VSWR of installed antenna
without field matching will be in the
order 1.15
• Field matched FM antennas can have a
VSWR of less than 1.02.
• Return Loss for 1.02 VSWR = 40.09 dB
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Signs of End of Life
• Improvements in Technology
• Excessive loss of pressure
• Periodic VSWR trips
• Finding antenna parts on the ground
around the tower
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Example:
IDS Multiple user Antenna• Antenna had multiple dipoles that
were fed with pressurized flex coax
• Due to excessive air leaks in system,
George Werle, the site managing
engineer, had drilled drain holes in
the flex lines to drain out water.
• Antenna components often broke off
and fell to the roof of the IDS Building
• Decision was made to replace the
antenna. 32
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Example:
One Shell Plaza Antenna
• This nine station FM multiplex facility
operated for years in down town
Houston, Texas.
• The Shell Building is a 50 story
building.
• As Houston grew, other buildings
were built causing excessive
shadowing.
• Senior Road Tower was built with
new nine station FM Antenna.33
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Definition of End of Life of an
FM Antenna• My definition of end of life is simple.
• The end of life of an FM antenna
occurs when the cost of repairing the
antenna exceeds the cost of
replacing it.
• If properly maintained, there is no
reason that a side mounted ERI FM
antenna should remain in service for
less than 30 years assuming that
none of the issues presented earlier
exist.34
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Questions?
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Guy Wire Galloping Video
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