A Case Study: The Replacement of the CN Tower Radome

A Case Study: The Replacement of the CN
Tower Radome
Katherine M Sahlin1, Alar Ruutopold1, Milton B. Punnett2
1
2
Saint-Gobain Performance Plastics
Saint-Gobain Performance Plastics -Retired
Abstract
In 1973, a PTFE coated fiberglass composite radome was designed and installed for
the Canadian Broadcasting Company for protecting their antennas at the top of the
CN Tower in Toronto, Canada. The radome membrane was a RF transmissive, air
supported torroidal structure and presents a distinctive, non-cluttered appearance to
Toronto’s skyline. This installation was one of the first of its kind in North America.
The flexible composite membrane used was substantially similar to the permanent
architectural membranes being introduced and gaining acceptance at that time. The
unusual torroidal shape and the air supported nature of the radome design required
unique fabrication and installation technology/procedures. The newness of the
technology and an involved customer provided a much needed opportunity to
provide maintenance and long term observation of the membrane and the blower
system.
In 2002, the membrane was replaced with a similar PTFE coated fiberglass
composite.
This paper will review the fabric design, radome design, installation, maintenance,
replacement and the physical properties of the composite after 27 years of real time
exposure.
Keywords: Radome, CN tower, PTFE coated fibreglass, air inflated, RF
performance, strength, exposure
1
1. Introduction
In June 1973, Birdair Structures was contacted by CN tower in Toronto regarding
the feasibility of enclosing the microwave level of the record height tower with an
air supported inflated radome. It was recognized that exposed antenna mounting on
the ‘skypod’ was not practical. Birdair’s proposal was a simple inflated torus section
completely encircling the section (photos 1 & 2). The unique 360º, RF transparent
enclosure was aesthetically pleasing and promised performance otherwise
unobtainable, including:
A. Complete protection for equipment and servicing;
B. Flexibility for future changes with uninterrupted viewing;
C. A smooth exterior to prevent ice and snow accumulation.
Photo 1: CN Tower by day
Photo 2: CN Tower by night
A brief summary of corporate history might be useful so the players become clear.
In 1973, Birdair Structures was a leading designer, fabricator and installer of fabric
structures and Chemical Fabrics Corporation, had just launched the SHEERFILL®
Architectural Fabric product line. In 1979, CHEMFAB bought Birdair Structures but
later divested all but the Fabricated Systems Group, which included the radome
business. In 2000, CHEMFAB was bought by Saint-Gobain.
2.
Site
The site is the 550m (1800 ft) tall CN Tower in downtown Toronto, Canada.
Specifically, the microwave transmission level is the lowest of the skypod (Figure 1)
and the 7.8m (25.5 ft) high, 42.1m (138 ft) diameter radome makes up the outside
skin. There is a room behind the antenna for the electronic gear. The entire level is
pressurized. The inflation system is under the equipment deck.
2
Figure 1: Schematic of the skypod configuration with the microwave level (1), the
open observation deck (2), enclosed observation decks (3&4), rotating restaurant
(5), FM radio transmitter levels (6&7).
3.
Wind Tunnel Test & Radome Design
Because of the unusual nature of the proposed design, wind tunnel tests were done at
the University of Western Ontario using a fixed conventional structure and an
inflatable radome modelled with a 0.025mm (0.001) inch thick urethane film (photo
3).
Photo 3: Wind Tunnel Model with conventional structure modelled in wood and
radome structure modelled with urethane film.
The wind load is counteracted by internal air pressure to produce a stable radome
geometry. The impact of gusts with low internal pressure showed no severe
dimpling. The 3 stage blower system (Figure 2) was copied from the 64m (210 ft)
3
Telstar radome built in France in 1962. In this design, wind speed is sensed by a
local anemometer and blowers are automatically turned on or off as wind speed
changes. Blower 1 is always on to maintain a minimum internal pressure of 63 mm
(2.5”) H2O.
Figure 2: Three stage blower system activation chart indicating blower operating
regions as well as lines of critical internal pressures needed to maintain radome
shape at specific wind speeds and levels of turbulence.
One revision to the design was made after the wind tunnel testing. It was found that
while the air intakes at the base of the skypod were sufficient for normal wind
conditions, the position did not capture sufficient air at higher wind speeds. The high
pressure blower intake was moved to the top of the skypod.
The maximum design stress, calculated via classical membrane analysis, was 18
kN/m (100 pli) at 150 degrees yaw angle at the maximum wind speed of 49 m/s (110
mph). Under normal conditions, one 5 hp blower is all that is required.
4.
Material
The material chosen for the structure was a new PTFE coated fibreglass material
made by CF and used twice previously for the Student Union at LeVerne college in
California and for a small window radome at Cape Canaveral.
The fiberglass substrate was a plain weave with an 18 x 19 yarns/in count, weighing
approximately 0.54 kg/m2 (16 osy). This was dip-coated with PTFE and FEP to a
finished weight of 1.59 kg/m2 (47 osy). Other properties are shown in Table 1. The
material was considered to have excellent resistance to weathering.
Property
(units)
Thickness
mm (inch)
Breaking Strength
kN/m (lbf/in)
Strength after Folding kN/m (lbf/in)
24 hr Water Absorption
%
Result (warp x fill)
0.96
(0.038)
164 x 151 (935 x 863)
123 x 143 (700 x 817)
0.1
Table 1: Initial material properties of G18T50
4
This fabric allowed for a safety factor of approximately 4.5. Critical to the
application, PTFE/fibreglass is almost invisible to RF waves across a wide spectrum
of wavelengths. Transmission losses on G18T50 were measured at less than 0.2 db
at 7.5 GHz increasing to 0.5 db at 15 GHz. The radome was considered satisfactory
for up to 20 GHz. The hydrophobic nature of the fluoropolymer surface ensures that
the RF transmission remains satisfactory even when the material is wet as
demonstrated by the microwave transmission results on a similar fabric (Table 2).
Specimen
Condition
Dry
Wet
Dry
Wet
Frequency (GHz)
10.036
10.033
8.654
8.646
Dielectric
Constant
1.96
2.00
1.95
2.05
Dissipation
Factor
0.0015
0.0046
0.0010
0.0038
Table 2: Measurements of microwave dielectric properties on PTFE coated
fibreglass structural fabric. Wet condition is after being soaked in water for 48
hours.
5.
Fabrication and Initial Installation
The radome is fabricated from 90 identical panels joined with 76 mm (3”) lap
welded joints with rope edges and bolt holes, top and bottom, for clamping to the
tower structure (Figure 3). The radome was pre-fabricated in three 120 degree
sections, each weighing 680 kg (1500 lbs). The radome sections were accordion
folded, shipped on pallets and delivered to the site via the elevator. The top edge of
the fabric sections were hoisted and temporarily attached to a series of rollers on the
circular I-beam at the upper lip pf the radome level (Figure 4 and Photo 4). This
allowed each segment to be spread open like a shower curtain. Each panel was
bolted under clamp plates at top and bottom around the exterior sills. Fiberglass
clamps were used to join the three sections together using a standard rope edge
overlap. After clamping, the radome was inflated to shape. The installation is
sensitive to weather but took less than one day.
5
Figures 3 (left): Panel layout – each panel has a rope edge top and bottom with bolt
holes for installation.
Figure 4 (right): Detail of sequences needed to install 120º prefabricated radome
sections – hoisting prefabricated sections to the I-beam trolley is shown in steps 1 &
2, Opening of the panels around the radome circumference in step 3, transferring the
radome to the bolt flange in step 4, final closures in step 5.
Photo 4: In progress on radome installation.
6.
Design Features
The 3.3m (10 ft) wide base deck of the radome includes a series of 1m x 2.5m (3 ft x
8 ft) hatch plates. By using an electric winch and removing the plates directly under
an antenna position, a reflector could be hoisted directly from the ground 366m
(1200 ft) below. Two high volume blowers help maintain radome inflation while the
hatch was open. While this method was used several times, managing the antenna
dish to keep it from being blown into the tower was deemed unsafe.
6
Photo 5: Hatchways in floor of radome level.
7.
Maintenance and Inspection
The maintenance of the radome was very basic. It consisted of changing belts and
lubricating the blowers at annual intervals. These regular visits kept us in
communication with the owner and the site management.
In 1995, the CN Tower owners contacted CHEMFAB regarding the remaining
useful life of the radome as it had been in use for over 20 years. Seeing the
opportunity for some ‘real time, real use’ data we requested to remove some
material for testing and provide a recommendation. We had a special contractor
remove small sections from two of the panels-one on the west face and another on
the south face of the radome. The removal process consisted of sealing new fabric to
the outside of the radome and then carefully cutting the older fabric from inside of
the patched area. This material was tested and the results are shown in Table 3.
Original Test Results on Material (average from 5 rolls)
Breaking Strength
Warp
164 ± 24 (935 ± 137)
kN/m (lbf/in)
Fill
151 ± 13 (863 ± 82)
Tear Strength, N(lbf)
Warp
329 ± 49 (74 ± 11)
Fill
472 ± 40 (106 ± 9)
Test Results for CN Tower Material - 21 Years Exposure (Avg. of 3 specimens)
South Exposure
West Exposure
Breaking Strength
Warp
114 (650 ± 56) [70%] 137 (780 ± 44) [83%]
kN/m (lbf/in) [% Ret. Orig]
Fill
110 (630 ± 17) [74%] 107 (613 ± 57) [72%]
Tear Strength, N(lbf)
Warp
231 (52 ± 1) [70%]
240 (54 ± 2) [73%]
[% Ret. Orig]
Fill
334 (75 ± 4) [71%]
338 (76 ± 3) [72%]
Original Specification
Breaking Strength
Warp
123 (700)
kN/m (lbf/in)
Fill
123 (700)
Tear Strength, N (lbf)
Warp
267 (60)
Fill
267 (60)
Table 3: Results of testing after 21 years on samples removed from radome
At that time our recommendation was that replacement was not immediately
necessary, however we recommended repeating the sampling/testing in 5 years time.
7
In 2002, in preparation for a change in ownership of the tower, Saint-Gobain was
contracted to replace the radome envelope with our more current RaydelTM R48
PTFE coated fiberglass.
8.
Replacement
The original panel drawings were replicated with minimal changes from the original.
Specifically the new radome would be delivered in six sections instead of the
original three to accommodate limited access to the work space working in a space
filled with antennas. Additionally these sections would be heat sealed together
during installation instead of using fibreglass clamps.
The technical challenge for replacing the existing radome was the requirement that
the new radome be installed inside of the existing radome prior to its removal. This
requirement was driven by the existing antennas on the radome deck and the risk of
untensioned fabric exposure with high winds.
The radome area was prepared by (1) welding bolts to the inside of the original
clamp flange (Photo 6). The original radome was bolted between the upper flange
and clamp bars on the outside of the flange; bolts were needed on the inside of the
flange for the new radome. The lower bolts allowed for interior access so a duplicate
set were not needed. (2) Window grating and three window bars at each of 12
locations were removed at the open observation level to allow access for the rigging
crew to the outside of the existing radome (Photo 7). (3) D-rings were sealed with
patches to each panel of the original radome and ropes were fixed for eventual panel
retrieval (photo 8).
Photo 6
Photo 7
Photo 8
The new radome sections were delivered to the radome level in their crates via a
maintenance elevator and then out an opening cut into the outside face of the
electrical equipment room (Photo 9). Each accordion folded section was hoisted,
maneuvered around the circumference (and antennas and scaffolding), raised and
then bolted into place along the top edge using clamp bars (Photo 10).
8
Photo 9
Photo 10
The sections were heat sealed to each other using a special tool consisting of a
backer board, a hand tacker and a clamping device which looked remarkably like a
very large pair of BBQ tongs (Photo 11). As each clamp bar was lifted and the new
radome bolted on top of the original at the bottom flange, slots were cut in the
bottom edge of the original radome at each bolt. As the last clamp bars was bolted
down the new radome started to inflate inside of the original.
Photo 11
Photo 12
The original radome was set free along the bottom edge by sequentially loosening
the clamp bars, removing the rope in the bottom edge of the original radome panel
(Photo12) and pushing the rope edge of the original radome out from under the new
radome rope edge.
Sections two or three panels wide of the original radome were cut longitudinally to
separate it from the rest and cut along the top to release it from the CN Tower
structure. It was pulled up and through one of the 12 openings onto the open
observation deck by means of the fixed ropes (Photo 13 &14). This last step of
freeing the old radome and removing the panels was done in one 18 hour day.
9
Photo 13
9.
Photo 14
Final Testing
Several panels from the old radome were returned to Saint-Gobain for testing. The
results of this testing are shown in Table 4.
Property (units)
Warp Result
Breaking Strength
kN/m (lbf/in)
Trapezoidal Tear
N (lbf)
40# Deadload
Elongation (%)
Coating Adhesion
kN/m (lbf/in)
132 ± 17 (753 ± 98)
n=20
229 ± 33 (51.5 ± 7.4)
n=19
0.37 ± 0.04
n=5
3.0 ± 0.4 (17.1 ± 2.3)
n=10
% of
original
81
69
Fill Result
124 ± 12 (710 ± 70)
n=19
340 ± 38 (76.4 ± 8.5)
n=20
4.4 ± 0.4
n=5
% of
original
84
72
Table 4: Physical properties of original G18T50 radome material after 28+ years of
installed life.
It may be observed that the Raydel membrane retained 70% of its original tear
strength and 80% of its original tensile strength. The retained tensile strengths are
still above the original specified values after 28 years of service. We consider these
to be excellent weathering results especially for a structure in such a demanding
environment.
References
[1]
Birdair Job File 7373
[2]
PUNNETT, MILTON B., CN Tower Microwave Radome, Presented at the
International Electrical Electronics Conference, Toronto, Ontario, Canada,
September 1975
[3]
Milton B. Punnett, personal interview
[4]
Saint-Gobain Job File J02015 CN Tower
[5]
Gerald Mirando, personal interview
[6]
Alar Ruutopold, personal interview
10