Antennas
The proper use of antennas can improve the performance of a
WLAN dramatically. In fact, antennas are probably the single easiest way to
refine the performance of a WLAN. But it is important to have an understanding
of the basics of antenna theory, as well as the various types that are available
for use in WLANs.
All antennas have the three fundamental properties:
-
Gain A
measure of increase in power
-
Direction The shape of the transmission pattern
-
Polarization The angle at which the energy is emitted
into the air
All three of these properties are discussed in detail in the
following sections.
Gain
Gain is
the amount of increase in energy that an antenna appears to add to an RF signal. There are different
methods for measuring gain, depending on the reference point chosen.
Basic antenna gain is rated in comparison to isotropic or
dipole antennas. An isotropic antenna is a theoretical
antenna with a uniform three-dimensional radiation pattern (similar to a light
bulb with no reflector). The dBi rating is
used to compare the power level of a given antenna to the theoretical isotropic
antenna (hence the use of the i in dBi). The FCC,
as well as many other regulatory bodies, use dBi for defining power levels in
the rules and regulations covering WLAN antennas. Most mathematical calculations
that include antennas and path loss also use the dBi rating. An isotropic
antenna is said to have a power rating of 0 dBi (that is, zero gain/loss when
compared to itself).
Unlike isotropic antennas, dipole antennas are
physical antennas that are standard on many WLAN products. Dipole antennas have
a different radiation pattern when compared to an isotropic antenna. The dipole
radiation pattern is 360 degrees in the horizontal plane and usually about 75
degrees in the vertical plane (assuming the dipole antenna is standing vertically)
and resembles a bowtie in shape (see Figure
2-11). Because the beam is slightly
concentrated, dipole antennas have a gain over isotropic antennas in the
horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in
comparison to an isotropic antenna).
Some antennas are rated in comparison to dipole antennas. This
is denoted by the suffix dBd. Hence, dipole antennas have a gain of 0 dBd (0 dBd
= 2.14 dBi).
Note
Note that many WLAN vendors' documentation refers to dipole
antennas as having a gain of 2.2 dBi. The actual figure is 2.14 dBi, but is
often rounded up.
To ensure a common understanding, many WLAN vendors have
standardized on dBi (which is gain using a theoretical isotropic antenna as a
reference point) to specify gain measurements. However, some antenna vendors
still rate their products in dBd, instead of
dBi, as the
reference point. To convert any number from dBd to dBi, just add 2.14 to the dBd
number. For instance a 3-dBd antenna would have a rating of 5.14 dBi (or rounded
up to 5.2 dBi).
Directional Properties
Any antenna, except for an isotropic antenna (theoretical
perfect antenna that radiates equally in all directions), has some sort of
radiation pattern. That means that it radiates energy in certain directions more
than others. A good analogy for antenna directionality is that of a reflector in
a flashlight. The reflector concentrates and intensifies the light beam in a
particular direction. This is very similar to what a dish antenna does to an RF
signal.
In RF, you usually have to give up one thing to gain something
else. In antenna gain, this comes in the
form of coverage area or what is known as beamwidth. As the
gain of an antenna goes up, the beamwidth (usually) goes down.
An isotropic antenna's coverage can be thought of as a perfect
round balloon. It extends in all directions equally. The size of the balloon
represents the amount of RF energy that the transmitter is sending to the
antennas, and the antenna is converting the energy to radiated RF energy. As you
learn about other antenna types, you will see that the overall energy radiated
from the antenna is not increased, it is just redirected. As was the case with
the dipole antenna discussed earlier in this chapter, this perfect round balloon
of energy that an isotropic antenna provides becomes something totally different
in shape.
Omni-Directional Antennas
An omni antenna is designed to provide a 360-degree radiation
pattern (on one plane, usually the horizontal plane). This type of
antenna is used when coverage in all directions surrounding the antennas on that
one plane is required. The standard 2.14-dBi Rubber Duck is one of the most
common omni antennas. When an omni antenna is designed to have higher gain, it
results in loss of coverage in certain areas.
Imagine again, the balloon of energy for an isotropic antenna,
which extends from the antenna equally in all directions. Now imagine pressing
in on the top and bottom of the balloon. This causes the balloon to expand in an
outward direction, covering more area in the horizontal pattern, but reducing
the coverage area above and below the antenna. This yields a higher gain, as the
antenna appears to extend to a larger coverage
area. The higher the gain on an antenna means the smaller the vertical
beamwidth.
If you continue to push in on the ends of the balloon, it
results in a pancake effect with very narrow vertical beamwidth, but very large
horizontal coverage (see Figure 2-12).
This type of antenna design can deliver very long communications distances, but
has one drawback: poor coverage below the antenna.
In some cases, the gain of an antenna can be high enough and
the radiation patterns so small, that even small motions of the antenna (from
wind, for instance) can cause the signal to move away from the intended target
and lose communication. For this reason, extremely high-gain antennas are
typically mounted to a very strong and permanent structure and almost never used
in a mobile or portable environment.
With high-gain omni antennas, this problem can be partially
solved by designing in something called downtilt.
An antenna that uses downtilt is designed to radiate at a slight angle rather
that at 90 degrees from the vertical element. Downtilt helps for local coverage,
but reduces effectiveness of the long-range capability (see Figure 2-13). Cellular antennas use downtilt.
Directional Antennas
Directional
antennas can be used to provide farther range in certain directions and to
isolate the radios for other signals. You can choose from a wide assortment of
available directional antennas, from short-range wide-coverage areas to very
focused and narrow coverage areas. As stated earlier, an antenna does not add
any additional power to the signal; instead, it redirects energy from one
direction and focuses energy in a particular direction. This results in more
energy on certain directions and less energy radiating on other directions. As
the gain of a directional antenna increases,
the overall coverage area usually decreases. Common form factors for WLAN
directional antennas include dish antennas, patch antennas, and Yagi
antennas.
Consider the common Mag-Lite flashlight (one of the
adjustable-beam-focus flashlights). There are only two batteries, and the one
light bulb, but the intensity and width of the light beam can be changed. Moving
the back reflector and directing the light in tighter or wider angles
accomplishes this. As the beam gets wider, the intensity in the center
decreases, and it travels a shorter distance. The same is true of a directional
antenna. The same power is reaching the antenna, but by building it in certain
ways, the RF energy can be directed in tighter and stronger waves, or wider and
less-intense waves, just as with the flashlight.
Polarization
Two planes are used in RF radiations: the E and the H plane.
The E plane (electric field) defines the orientation of the radio waves as they
are radiated from the antenna. If the E is perpendicular to the Earth's surface,
it is referred to as vertically polarized. In
WLAN systems, for instance, an omni-directional antenna is usually a vertically
polarized antenna.
Horizontally polarized (linear) antennas have their
electric field parallel to the Earth's surface. WLANs seldom use horizontally
polarized antennas, except in certain outdoor, point-to-point systems.
Antenna Examples
You can choose from a wide variety of antennas for use with
WLAN equipment. The use of different antennas can simplify the installation of a
WLAN system, and in some cases reduce the overall cost of the system. A thorough
understanding of different antenna types available will enable the survey
engineer and installer to provide a WLAN that not only provides adequate
coverage but also helps to stay within budgetary constraints.
Appendix B,
"Antenna Radiation Patterns," provides an assortment of WLAN antennas and the
associated polar plots. The polar plot is the common method to define an
antenna's beamwidth, or radiation pattern, and gain factors.
Patch Antenna
A patch antenna is typically small and somewhat flat and is
usually designed to mount against a wall or on a small bracket. It has a
beamwidth that is less than 180 degrees, and is sometimes referred to as a hemispherical antenna.
Panel Antenna
A panel antenna (sometimes also referred to as a sectorized antenna) is similar to a patch antenna, but
is generally a higher gain and physically larger. Many times a panel antenna has
an adjustable back reflector that can be used to change the beamwidth as well as
mounting brackets that can be adjusted for downtilt.
Panel antennas are usually used outdoors and can have gains
ranging from as little as 5 dBi to more than 20 dBi. They can be used as a
single antenna or in multiples to cover a larger area.
Yagi Antenna
A Yagi antenna has a series of small elements, referred to as
reflectors or directors, and an active element. These are placed in a
straight line and direct the energy in a given direction. Generally Yagi
antennas have fairly high gain. The more reflectors and directors a Yagi has,
the higher the gain. Due to the short wavelength for frequencies used in WLAN
systems, the elements are fairly small, and most Yagis used for 2.4-GHz or 5-GHz
contain some type of cover to protect the antenna's components from the weather
and to provide more structural strength. Yagi antennas can range in gain from as
low as 5 dBi to as high as 17 dBi or more.
Dish Antennas
There are really two main types of dish antennas: the parabolic
and the grid dish. The parabolic dish contains a reflector that is solid in
construction, and a driven or active element supported in the center of the
reflector. These are similar to what you would find for a standard satellite TV
dish antenna, except the placement of the active element is typically
centralized on the WLAN antenna.
The grid dish antenna is very similar to the parabolic antenna, except the
reflector is not solid. It is made of a grid-type structure to permit wind and
rain to flow through it. This provides less wind resistance and therefore
requires a smaller mounting structure. Chapter 14, "Outdoor Bridge Deployments," provides
more information about outdoor mounting.
Diversity
Diversity antenna systems are
used to overcoming a phenomenon known as multipath distortion
or multipath fading. It uses two identical
antennas, located a small distance apart, to provide coverage to the same
physical area.
To understand diversity, it is important to give you an
overview of multipath distortion as well as an understanding of how this can
occur. Multipath distortion is a form of RF interference that can occur when a
radio signal has more then one path between the transmitting antenna and the
receiving antennas. Environments with a high probability of multipath
interference include such places as airport hangars, steel mills, manufacturing
areas, distribution centers, and other locations where the antenna is exposed to
metal walls, ceilings, racks, shelving, or other metallic items that reflect
radio signals and create this multipath condition (see Figure 2-14).
When an antenna transmits, it radiates RF energy in more than
one definite direction. This causes RF to move between the transmitting and
receiving antenna in the most direct (desired) path while reflecting or bouncing
off metallic and other RF-reflective surfaces. The process of reflecting the RF
waves causes several things to occur. First, the reflected RF waves traveled
farther than the desired direct RF wave. This means the reflected waves will get
to the receiving antenna later in time. Second, because of the longer
transmission route, the reflected signal loses more RF energy while traveling
than the direct route signal. Third, the signal will lose some energy as a
result of the reflection or bounce. In the end, the desired wave, along with
many reflective waves, are combined in the receiver. As these different
waveforms combine, they cause distortion to the desired waveform and can affect
receiver-decoding capability.
When these reflected signals are combined at the receiver,
although RF energy (signal strength) may be high, the data would be
unrecoverable. Changing the location of the antenna can change these reflections
and diminish the chance of multipath interference. You have likely encountered
multipath distortion with common products such as televisions and radios. For
example, when an indoor antenna is used on a television set, it is possible to
see images of the same picture slightly offset or distorted. This ghost, or
fuzzy picture, is the result of the transmitted television signal reflecting off
metal items in the home such as a refrigerator or a filing cabinet. You can
usually fix this multipath interference just by adjusting or moving the antenna.
Because an access point (AP) can't physically
move its antenna, many have been designed with two antenna ports. The radio
performs an assessment of each antenna port and selects to use the antenna with
the best reception.
Another example of multipath interference occurs while
listening to the radio when driving an automobile. As you pull up to a stop
sign, the radio station might appear distorted, or you may even lose the signal
altogether as a result of a radio null, which is also referred to as a dead spot. As you move the car forward a few inches or
feet, the radio reception starts to come in clearer. As you move the vehicle,
you are actually moving the antenna slightly, out of the point where the
multiple signals converge. In all probability, the radio signal was reflecting
off another vehicle or metal object nearby.
In some cases, if signals are received in equal strength, yet
delayed in such a manner that they are opposite in polarity, they will actually
cancel each other out completely, creating a total absence of received signal by
the receiver. This is known as a multipath
null.
Many do not understand the method of how a diversity antenna
system works, and this lack of understanding often leads to confusion and
improper installation. The diversity antenna system includes two antennas that
are connected to an RF switch, which in turn connects to the receiver (see Figure 2-15). The receiver actually switches
between antennas on a regular basis as it listens for a valid signal.
Note that this switching occurs extremely fast. The AP samples
part of the radio header and determines and utilizes the best antenna to receive
the client's data and then uses that very same antenna when transmitting back to
the client. If the client doesn't respond, the AP will then try sending the data
out the other antenna port.
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A golf course with an electronic scoring application used an AP
with an outdoor antenna to cover the front nine holes of the golf course.
Originally the AP was placed in the clubhouse, and one outdoor antenna was used
to cover the front nine of the course. Because there was little multipath
interference (few things outside to reflect the radio signal), one antenna was
sufficient and communication seemed to be fine. In this case, the customer had
used a directional Yagi antenna. This antenna was chosen for its distance
characteristics and ease of installation.
Later it was determined that coverage was needed on the back
nine of the golf course as well. Instead of adding another AP, the customer
decided just to connect another directional Yagi antenna to the other antenna
port and point it off in another direction (the back nine), as shown in Figure 2-16. While driving around in the
golf cart, performing a survey, the customer had no issues with coverage.
But as the tournament started and many
users were added, they encountered difficulty. When the first users (clients on
the front nine of the course) registered to the AP, the AP sampled both antennas
(one at a time) and selected the antenna pointing to the front section.
When users started migrating to the back nine, and more users
entered the front nine, problems started popping up. As the AP was communicating
to the users on the front section of the course users on the back section could
not hear that RF traffic because the back-nine antenna was being used at that
instant. Therefore, the back users tried to send their own traffic, which was
not heard by the AP.
In the case of the golf course, two methods could resolve this
problem. One method is to replace the directional Yagi antenna with a
similar-gain omni antenna. The AP's radio would then be able to work in all
directions rather than the limited directional pattern of the Yagi.
Another method is to add an AP to cover the other radio cell.
This way both APs could properly handle the RF traffic, and each AP could use
the higher-gain Yagi antenna to cover each
area. |
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