radio serv- ice provider to take advantage of a rooftop an- tenna site, a base station had 24
SITE MANAGEMENT & TECHNOLOGY
Summer 1999
y
y
y
y
y
z
z
z
z
z
For a radio serv-
ice provider to
take advantage
of a rooftop an-
tenna site, a
base station had
to be located in the basement of a 14-story building and
connected to the roof by a run of plenum cables that
meet the appropriate codes. Bottom to top: (1) Radio
equipment located in a basement cabinet is connected
by LMR-400-LLPL cable (orange) to a series of lightning
protectors (2), from which runs of LMR-600-LLPL cable
(also orange) begin their ascent, crossing the ceiling at
the second floor (3) to enter a riser shaft (4) for a straight
shot at the roof. The cables exit a hatch at the top of the
riser shaft on the 14th floor (5) and are run along an
outside wall to the antenna mount (6).
x
x
x
x
x
{
{
{
{
{
|
|
|
|
|
}
}
}
}
} 25
Summer 1999
SITE MANAGEMENT & TECHNOLOGY
Demand for wireless communications
services in urban areas is increasing as
the work environment becomes more mo-
bile and the need for constant communi-
cations increases. This demand has cre-
ated the necessity for more antenna in-
stallations on building roofs. In
addition to the more traditional
two-way radio, paging and cellu-
lar applications, new services
such as local multipoint distribu-
tion systems (LMDS), wireless
Internet and unlicensed spread-
spectrum radio are all being used.
Well take Manhattan
Antenna siting issues in an
urban environment are a major
challenge. Clearly, sites with
good elevation give the best
coverage for omnidirectional
systems and clear the most ob-
stacles for point-to-point sys-
tems. But real estate at the top
of a building is expensive. It is
generally not practical to lease
space for radio equipment in the
penthouse. Instead, a boiler
room, garage or other relatively
inexpensive area is most likely
to be used for radio equipment.
Now the challenge for the
implementation engineer is to
get the signal from the radio
room in the basement to the
antenna on the roof. As we saw
in a recent survey of a site in
lower Manhattan, this can be a
major challenge, indeed.
The lower Manhattan building where
the system was installed is a 14-story,
By Robert Perelman
Perelman is vice president, commercial sales and
marketing, for Times Microwave Systems,
Wallingford, CT.
His email is
perelman@timesmicrowave.com.
50-year-old structure, with a riser shaft
from the second floor to the 14th floor.
The shaft goes up to a secondary struc-
ture on the roof where the antenna is
mounted. There is no easy access to this
shaft, so the cable must survive being
pulled through the shaft from the roof
to the second floor. This work must be
performed by union electricians who
are not always familiar with the care
required to successfully handle a frag-
ile coaxial cable.
The system we surveyed requires a
maximum 13dB of loss at 900MHz,
regardless of run length. Cable needs to
be low-VSWR and UV-resistant for the
portions of the run that are installed
outdoors. On top of this, the cable needs
to be plenum-rated in order to meet the
requirements of the National Electrical
Code (NEC) for indoor installation in
an air-handling space in the building.
In other installations, riser-rated or
general-purpose outdoor cable
may be used, depending on the
interpretation of the electrical
codes in the municipality
where the installation is being
performed. This combination
of requirements limits the
choice of cables that can be
used for this installation.
Testing
To meet the NEC, these
cables need to be listed by
Underwriters Laboratory. To
be listed as a plenum cable,
the cables must pass the
Steiner Tunnel Test that re-
quires several runs of cable be
placed in a horizontal cham-
ber. Air is flowed over the
cables at a controlled rate. A
precisely controlled flame is
then introduced. To pass the
test, there are limits for both
flame spread and smoke gen-
eration. This is intended to
simulate the situation of a fire
in a building. Cables that are
installed in air-handling
spaces, such as the space
above a false ceiling, are es-
pecially dangerous in a fire.
If they generate smoke, this
can make it impossible for occupants
of the building to be able to see well
enough to exit the building. To pass
this test, it is necessary to use special
jackets and dielectrics, which are more
fire-resistant than polyethylene. Gen-
erally fluorocarbon resin-based
As the demand for wireless services increases, more antennas must be installed
in less traditional installation environments, such as large buildings.
The antenna feeder considerations are different in this type of environment.
In-building cable installations
cable
Antenna sites are at a premium in urbanized areas such
as New York City. This 14-story rooftop installation is
cabled to a base station in the basement. 26
SITE MANAGEMENT & TECHNOLOGY
Summer 1999
materials, such as Teflon, are the best
choice for dielectrics because in addi-
tion to being fire-retardant, they have
excellent electrical properties and re-
sult in cable with low attenuation.
For cables going
between floors in a
building, there is
the somewhat less
severe NEC riser
category. To get
this rating, the
cables must pass a
vertical tray flame
test. This simulates
resistance to cables
spreading the fire
from floor to floor
in a building fire.
To meet this re-
quirement, special
jackets can be ap-
plied to cables
with polyethylene
foam dielectrics.
These ratings
form a hierarchy, meaning that ple-
num cables can be installed anywhere
in a building, including risers and air
handling spaces. Riser cables can be
installed anywhere in a building ex-
cept air-handling spaces. For this rea-
son, in large metropolitan areas, such
as New York City, the use of plenum
cables is mandated for all in-building
installations.
Cable construction
Two constructions of
low-loss plenum cables are
available: corrugated cop-
per cables with a helically
wrapped Teflon spacer di-
electric and flexible cables
with an expanded Teflon di-
electric. The corrugated
copper cables are stiff and
difficult to install without
damage. The spirally
wound spacer does not give
full support to the corru-
gated copper outer conduc-
tor. This allows the outer
conductor to kink during in-
stallation in tight spaces in
buildings. Connector instal-
lation on corrugated copper
cables is also difficult.
Flexible, low-loss plenum cables
use expanded Teflon as the dielectric.
Similar constructions have been used
in high-quality microwave cables for
aerospace, testing and other demand-
ing applications for many years. This
construction has many advantages.
The expanded Teflon has good me-
chanical strength and fully supports
the outer conductor of the cable, re-
sisting kinking and damage to the
cable during the installation process.
Because the Teflon is expanded, it has
both a low-dielectric constant and a
low dissipation factor, so the result-
ing cables have low attenuation. The
cable shown in the accompanying
photos on page 24 and 25 is Times
Microwave Systems LMR-400-LLPL
and LMR-600-LLPL. This cable has
an orange UV-resistant copolymer
jacket, selected to have the smoke and
flame properties necessary to allow
the cable to pass the UL plenum tests.
It is the lowest loss, flexible coaxial
cable available and is rugged enough
to be installed in difficult building en-
vironments without damage.
Installation
In the building that was surveyed,
the antennas are located on the top of
the 14-story structure. On the roof
level, there is a secondary structure
that provides the best elevation and is
Riser cables can
be installed
anywhere in a
building except
air-handling
spaces. In large
metropolitan
areas, the use of
plenum cables is
mandated for
in-building
installations. 27
Summer 1999
SITE MANAGEMENT & TECHNOLOGY
line-of-site to the transmitting antenna,
on a building near the World Trade
Center. The installation of the cable
was accomplished by bringing the reels
of cable to the roof and laying out the
lengths needed. The cable was pulled
down to the second floor. From that
point, it had to go about 50 feet down
a corridor and then down into the base-
ment. The radio equipment is located
in a basement boiler room, not exactly
the high-rent district, as can be seen
from the photographs.
The entire run of cable is 280 feet
long. The LMR-600-LLPL is used for
most of the run. It has a loss of 2.7dB/
100 feet at 900MHz, so the loss of the
280 foot run is 7.5dB at 900MHz, well
within the requirement for this system.
A short run of LMR-400-LLPL is used
from the radio to the lightning protector
and then to the main feeder run, which
adds about 0.5dB, for a total of 8dB,
still well within the 13dB specification.
Although a slightly larger,
corrugated-copper, plenum rated cable is
available with a slightly lower loss
(2.5dB/100 ft at 900MHz), the stiffness
and fragility of this cable would have
made it impossible to install in this build-
ing. Only a rugged, flexible cable was
possible to use in this installation. Many
buildings are similar to this one in the
difficult routes through which the cable
must be run. The importance of cable
flexibility to facilitate installation cannot
b