When Selecting an antenna for a wireless system, designers need
to consider the future needs of the cell site.
One of the first considerations in a communications system is antennas.
Although, for example, the base station antenna rep-resents only
2% to 4% of the overall cost of a communications site, its performance
impact is enormous. These antennas represent the critical piece
of the puzzle that either initiates the transition of RF energy
into free space to communicate with remote users or pulls the remote
user's signal out of the air and allows it to be passed on to the
communications system. Using the wrong antenna for the job will
degrade the overall performance of an otherwise well engineered
system, resulting in customer dissatisfaction.
Another consideration is future needs. Cell site antenna products
that initially perform well when communications sites are not tightly
spaced or heavily loaded may have significant problems as site density
increases and traffic loading peaks. It can cost upwards of $2,000
to rent a crane and $3,000 a day for a crew of tower climbers and
riggers to go to a communications site and change or adjust installed
antennas. The site may have to be taken off the air or traffic rerouted
during this process. Obviously, if the system owner is to maximize
both returns and customer satisfaction, base station antenna selection
deserves the consideration any far-reaching decision should command.
Similar consideration is needed when choosing a point-to-point microwave,
earth station, or in-building antenna system.
There are now new choices of technology available and the most
sophisticated applications are switching from the traditional space
diversity to polarization diversity, which uses only one antenna
with two transmission lines. In this configuration, slant 45-degree
polarization diversity is used. One polarization diversity antenna
will replace two vertically polarized antennas spaced several feet
apart. The polarization diversity antenna, typically no larger than
a standard vertically polarized antenna, has one slant polarization
duplexed for Tx and Rx and the other slant polarization for Rx only.
The structure loading is reduced to only one antenna and two transmission
lines with this system (see
Figure 1c). In situations where the monthly rent is paid
by the number of antennas used, the use of polarization diversity
can save a substantial amount of money over the life of the system.
Figure 2a is a representative polar plot of the elevation pattern
of a directional PCS/wireless/cellular antenna that illustrates
the relationship between these factors. The plot shows a 40 dB range
of relative power. This is used to accurately portray the characteristics
of the antenna. Use of a lesser scale can mask the true character
of an antenna.
Gain vs. Aperture Size. Antenna directivity is a measure
of how an antenna focuses energy. This is determined by the antenna's
radiation pattern and is a function of the frequency of operation
and the three-dimensional area of the antenna's focus (azimuth and
elevation beamwidth).
The antenna's gain can be shown by the following formula:
Gain = Directivity
- loss from resistance of antenna/feed line conducting elements
- dielectric losses of radome
- impedance mismatch to external feed line
- radiation outside of intended polarization
Antenna gain increases as its aperture size increases. For PCS/wireless/cellular
panel or omni antennas, this can be equated to the antenna length.
As a general rule, gain doubles (3 dB increase) when the antenna's
length doubles or when the beamwidth is decreased by one half.
As the gain increases, the internal feed network increases in
internal losses start to increase faster than the increase in
gain. At this point, substantial increases in antenna length are
required for relatively small increases in gain. To make matters
worse, the elevation beamwidth will become very small, which makes
it difficult to achieve uniform cell illumination and makes the
antenna very sensitive to movement of the tower it is mounted
on. These factors establish practical limits on gain of approximately
18 dBi for 2 GHz panel antennas and 11 dBi for 2 GHz omni antennas.
Mechanical vs. Electrical Beamtilt. The footprint
of an antenna pattern behaves quite differently with mechanical
versus electrical tilting. When a panel-type antenna is mechanically
tilted, only the peak of the main beam is at the specified angle.
A way to visualize this is to cut a flat piece of paper into a
circular disk to represent the energy from the antenna. The antenna
would be at the center of the paper, and a line representing the
peak of the beam is drawn from the center to the edge of the paper.
When the paper is tilted to tilt the beam, it can be seen that
there is no tilt at the angle 90 degrees from the peak, but at
the opposite peak at 180 degrees, the beam is pointed upwards.
Thus, a mechanically tilted panel antenna gives a reduced coverage
footprint at the peak of the beam, but as the angle increases
from this point, the amount of beamtilt decreases. The effect
produces a pattern for a smaller tilt or no tilt at all.
A reduction in energy is seen at the horizon at cell sites off
boresight, and the front-to-back ratio of the mechanically beamtilted
antenna may actually degrade with the tilt angle.
With electrical beamtilt, the energy is phased between elements
of the antenna, such that the radiation from the top of the antenna
is slightly ahead of that from lower elements. This effectively
tilts the beam down. This can be visualized by taking a conical
coffee filter and placing it upside down to invert the cone. The
angle of tilt is now equal at all horizontal tilt angles.
Clearing Obstacles. When an antenna's energy is reflected
from an obstacle, such as the edge of a rooftop, there is a 180-degree
phase shift. This reflected signal adds to the direct beam in
a destructive manner, reducing received signal levels. There are
two guidelines for compensating for this effect. The first is
a field-derived rule of thumb that states that the -10 dB angle
of the antenna's elevation pattern must clear any obstructions.
Another more scientific approach is to calculate the Fresnel Zone
clearance and position the antenna such that the main beam has
at least 0.6 first Fresnel clearance. The first Fresnel Zone is
a locus of points along the main beam that will produce cancellation
of the main and reflected signals to a user. The object is to
make sure that there are no obstacles that intrude into this first
Fresnel Zone for optimum performance.
RF Exposure Guidelines. When adding equipment
to existing communications sites and developing new sites, a user
in the United States must determine compliance to the FCC's OET
(Office of Engineering and Technology) Bulletin No. 65 "Evaluating
Compliance with FCC-Specified Guidelines for Human Exposure to
Radio Frequency Radiation." This bulletin contains many tables
and figures to help an applicant make a fairly quick determination
that a facility is in compliance with the new limits. Note should
be taken of section 4 of this bulletin, dealing with controlling
exposure situations. In uncontrolled situations, people in the
general population are exposed to RF energy in their workplace
and are not fully aware of the potential for exposure and/or can
not exercise control over their exposure. There are no worldwide
guidelines or mandates regarding RF exposure so when working internationally,
the local PTT authorities need to be consulted.
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Figure 2c. This a polar coordinate
plot of an antenna with null fill. The graph shows the energy
on the surface of the earth from that antenna in reference
to its pattern and how null-fill improves the signal. |
Moving the Signal Between Base Stations
Supporting base station antennas are terrestrial microwave (TMW)
"dish" type point-to-point antennas or earth station antennas
(ESAs). Both of these choices can be more economical than local
copper or fiber connections. TMW antennas are typically used where
a link can be established to a switch or hub within a line-of-sight
radius. The ESA is used where line-of-sight links can not be established.
Terrestrial Microwave Connections
Terrestrial microwave point-to-point antennas are used primarily
for connecting a cell site to the switch (shown
in Figure la). This is known as backhaul. Microwave is
an excellent economical alternative to leased land-based telephone
lines because it often offers higher signal quality and less maintenance.
A microwave system can be deployed quickly and has much higher
reliability than land lines. Microwave antennas are matched to
the application, whether it is fixed short-haul base station-to-microcell
site links or long-haul repeater communications. These antennas
are offered in single and dual polarized as well as standard and
low profile versions to meet the needs of the system operator
and standards for local environmental impact.
As frequencies are increased, there are two factors that are
often overlooked in selecting an antenna for a given application.
The first is the twist and sway of the structure that the antenna
is mounted to. For example, if the frequency of an 8-ft diameter
antenna is increased from 2 GHz to 6.5 GHz, the maximum allowable
tower twist and sway decreases from 3.5 degrees to 1.0 degrees
- an increase in tower rigidity by 3.5 times. A 2-ft diameter
dish at 22 GHz will have a beamwidth of approximately 2 degrees;
therefore, the movement of the antenna must be limited to a fraction
of this value to assure optimal performance under windload conditions.
The second is rainfall. For microwave frequencies above about
12 GHz, the absorption of RF energy by rainfall becomes an important
factor in determining path length and antenna gain. The amount
of annual rainfall is not as important as the intensity of the
expected rain. Areas such as Washington State with its frequent
rains are not as much of a problem as areas such as Florida or
those along the Gulf Coast.
Earth Station Antenna Links
Earth station antennas can provide higher bandwidth and capacity
than their TMW counterparts. Their selection and design is more
difficult, and these systems are designed around the carrier-to-noise
density ratio required, government regulations, frequency band,
coordination requirements, satellite system utilization, site
location, and implementation parameters.
The satellite manufacturer will provide the effective isotropic
radiated power (EIRP) towards the earth that is known as the satellite's
"footprint." In multicarrier applications where there is a mix
of voice data and video, EIRP is replaced by EIRP/carrier, as
there is normally a back-off in power associated with multicarrier
transponders to avoid high levels of inter-modulation. This is
due to the limitation of the total power available as a composite
to all carriers. If this power limit is exceeded, unwanted spurious
intermodulation interference is generated, degrading signal quality.
The final carrier-to-noise ratio that is delivered to the demodulation
equipment includes degradation from the uplink and downlink paths
and terrestrial, adjacent satellite, cross polarized, and adjacent
transponder interferences. In this calculation, it is assumed
that these sources of interference are noncoherent and the downlink
path will normally dominate the calculation.
As can be seen in Figure
3, the earth station is the gateway to a large number
of communications services that are complimentary to PCS/wireless/cellular
communications systems.
 |
Figure 3. An ESA system in support
of PCS/wireless/cellular applications typically connects the
services shown. The ESA system is used for very long distance
interconnections and can concentrate large amounts of information. |
Transmission Line Basics
When communications systems are designed, the concept of a "link
energy budget" or just simply link budget is important. This link
budget takes into account the energy from the transmitter, the
path loss, antenna gains, and losses to the receiver at the opposite
end. To over-come losses from propagation by choosing the proper
antenna gain and pattern, it is important to reduce the losses
from the antenna to the base station or switch as well.
Meeting this calculated loss figure can be quite a challenge
and requires diligent selection of everything that configures
a wireless system. The first place to start is wherever there
are junctions or connections, and that means the transmission
line offers opportunities to keep system loss within acceptable
limits.
The transmission line system incorporates the main transmission
line cable, jumpers, connectors, and accessories that are part
of all site configurations. These components can be important
contributors of loss in the system if not properly selected and
matched. Foam-dielectric coaxial cable transmission line, most
commonly used in PCS/wireless/cellular installations, should have
copper inner conductors, low dielectric foam with closed cells,
and a solid copper outer conductor. Copper inner and outer conductors
provide low loss and high shielding efficiency, and the closed-cell
foam dielectric will prevent moisture accumulation and migration.
Moisture can rapidly increase attenuation and return loss.
The connector should electronically appear as an extension of
the transmission line. It should offer good shielding, low VSWR,
easy attachment, and low intermodulation levels. This is especially
true for the jumper assemblies used at the top of the tower between
the main feeder and the antenna and at the lower end of the feeder
to the RF equipment. These assemblies and the grounding and attachment
accessories associated with them are also critical items for proper
system performance.
Impedance. Impedance is a characteristic that has
been standardized in the industry at 50 ohms and represents
a compromise between optimum attenuation (about 75 ohms) and
optimum power handling (about 35 ohms). It is primarily determined
by a ratio of the inner and outer conductor dimensions and the
dielectric foam density, which determines the velocity of the
cable. For best system performance, impedance should be controlled
within close limits, typically to within 1 ohm from 50 ohms
nominal.
Shielding. Excellent shielding is one of the primary
reasons for using transmission line with a continuous outer
conductor. In practice, shielding is limited by the connectors
but should be at least 120 dB.
Attenuation. Attenuation is principally determined
by overall cable diameter and will vary approximately inversely
with cable diameter. In other words, the larger the cable the
lower the attenuation. A typical system will allow between 1 and
2 dB of loss due to the attenuation of the transmission line and
cable combination.
Power Ratings. Peak and average power ratings of transmission
lines are rarely a limitation for typical wireless communications
systems. A more important factor is VSWR. For a transmission line
system, measured VSWR consists of three components. First is the
reflection from the connector nearest the measuring equipment.
Second is the contribution from the other connectors, appropriately
reduced by the attenuation of the cable. Third is the contribution
from the cable itself and arises chiefly from very small dimensional
variations that are periodic at constant intervals of cable length.
The periodicity means that a large number of these very small
reflections are in phase and will combine at certain frequencies
where the periodic spacing is an integral number of half wave-lengths.
The result is that the cable con-tributes a number of narrow band
"spikes" of higher VSWR.
Intermodulation. Intermodulation (IM) is another critical
specification. IM is the mixing of two or more signals at a nonlinear
mechanical junction that will produce spurious signals at multiples
of the signals that are present at this junction. A principal
cause of the nonlinearity and consequent IM generation is the
use of ferromagnetic materials, such as stainless steel and nickel,
in the RF path. The second major cause of IM is low contact pressures
at connector inner conductor contacts on cable connections or
inside of the antennas. These generating mechanisms are minimized
by eliminating ferromagnetic materials in RF current paths, minimizing
the number of junctions, ensuring good metal-to-metal contacts
at all junctions, and soldering or high mechanical contact pressures.
This is true for all antenna types mentioned.
 |
Figure 4. An off-air building communication
extensions system accepts signals from an existing cell site
and extends it into a building. Conversely, this system will
take a signal from within the building that is normally blocked
and transmit it back to the outside existing PCS/wireless/cellular
site. |
In-Building Antennas
The last frontier in wireless communications systems is bringing
PCS/wireless/cellular capabilities into office buildings, parking
garages, hospitals, shopping malls, airports, convention centers,
amusement parks, hotels, and small underground areas such as train
or subway stations. To give wireless users the greatest mobility
would be to allow them to move seamlessly between outdoors and
indoors with no disruptions in call quality and service, but designing
an in-building communications system can be the most complex of
all sys-tem designs.
There are two basic types of in-building systems. The first extends
an existing cell site by borrowing its signal and bringing it
indoors. This is an off-air type of site. The second locates a
full cell site or a portion of a cell site in the building. This
is a service extension type of site.
The off-air site is used where the additional capacity from in-building
communications will not load the existing cell site beyond capacity,
and the in-building traffic is not expected to grow to the point
where it will decrease availability for the normal external subscribers.
The service extension type of site is used where there is enough
subscriber traffic to justify on-site cellular PCS/wireless/cellular
equipment. For the off-an type site (see
Figure 4), the cell site signal is captured by an antenna
located with a clear view of the closest cell site and coaxial
cable is used to route the signal to the in-building system. The
in-building system will often include one or more amplifiers to
overcome path loss to the building and the internal cable losses.
The signal is then distributed through a combination of radiating
coaxial cable and point source antennas. Point source antennas
work best in large unobstructed areas, such as the interiors of
shopping malls and convention centers, and the radiating coaxial
cable works best where coverage is restricted or needs to be contained
such as hospitals, elevator shafts, corridors, subway tunnels,
and office spaces. Four primary factors that must be taken into
account when designing in-building communications system are 1)
insertion loss of the cable, 2) coupling loss of the antenna or
radiating coaxial cable, 3) operating margin of the system, and
4) fire rating of the cable.
Insertion loss is a measure of the RF attenuation per unit length
of the cable and increases with frequency. Coupling loss is a
measure of the RF path attenuation between the antenna or radiating
coaxial cable and the intended coverage area.
Operating margin is the additional margin needed to assure a high
probability of coverage and allow for system growth. When selecting
components for in-building use, it is very important to know the
building codes for the municipality and the specific building
that will receive the installation. A "riser" fire rated cable
can usually be used in vertical shafts and open wall mounting,
but "plenum" fire rated cable should be used above suspended ceilings
or compartments where air ducts are connected to form part of
the building.
This overview of what makes up a typical PCS/wireless/cellular
antenna system is the first in a series of three articles. The
objective has been to show how the components interact with each
other and give the reader an understanding of what factors are
important to the application of these products. The second article
in this series will give specific technical criteria for evaluating
and selecting each of these types of antennas.
A
communications system has many component parts, each of which contributes
to the overall performance and ultimately affects operator revenues.
Well-designed components that are complementary with each other
will keep overall performance within acceptable limits (
