This document gives basic antenna definitions and discusses antenna
concepts with a focus on the pros and cons of omni and directional
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
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An antenna gives the wireless system three fundamental properties:
gain, direction and polarization. Gain is a measure of increase in power. Gain
is the amount of increase in energy that an antenna adds to a radio frequency
(RF) signal. Direction is the shape of the transmission pattern. As the gain of
a directional antenna increases, the angle of radiation usually decreases. This
provides a greater coverage distance, but with a reduced coverage angle. The
coverage area or radiation pattern is measured in degrees. These angles are
measured in degrees and are called beamwidths.
An antenna is a passive device which does not offer any added power to
the signal. Instead, an antenna simply redirects the energy it receives from
the transmitter. The redirection of this energy has the effect of providing
more energy in one direction, and less energy in all other directions.
Beamwidths are defined in both horizontal and vertical plains.
Beamwidth is the angular separation between the half power points (3dB points)
in the radiation pattern of the antenna in any plane. Therefore, for an antenna
you have horizontal beamwidth and vertical beamwidth.
Figure 1: Beamwidth of Antenna
Antennas are 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). In other words,
a theoretical isotropic antenna has a perfect 360 degree vertical and
horizontal beamwidth or a spherical radiation pattern. It is an ideal antenna
which radiates in all directions and has a gain of 1 (0 dB), i.e. zero gain and
zero loss. It is used to compare the power level of a given antenna to the
theoretical isotropic antenna.
Figure 2: Radiation Pattern of an Isotropic
Antennas can be broadly classified as omnidirectional and directional
antennas, which depends on the directionality.
Unlike isotropic antennas, dipole antennas are real antennas. The
dipole radiation pattern is 360 degrees in the horizontal plane and
approximately 75 degrees in the vertical plane (this assumes the dipole antenna
is standing vertically) and resembles a donut in shape. Because the beam is
slightly concentrated, dipole antennas have a gain over isotropic antennas of
2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of
2.14 dBi, which is in comparison to an isotropic antenna. The higher the gain
of the antennas, the smaller the vertical beamwidth is.
Imagine the radiation pattern of an isotropic antenna as a balloon,
which extends from the antenna equally in all directions. Now imagine that you
press in on the top and bottom of the balloon. This causes the balloon to
expand in an outward direction, which covers more area in the horizontal
pattern, but reduces the coverage area above and below the antenna. This yields
a higher gain, as the antenna appears to extend to a larger coverage
Figure 3: Radiation Pattern of an Omni
Omnidirectional antennas have a similar radiation pattern. These
antennas provide a 360 degree horizontal radiation pattern. These are used when
coverage is required in all directions (horizontally) from the antenna with
varying degrees of vertical coverage. Polarization is the physical orientation
of the element on the antenna that actually emits the RF energy. An
omnidirectional antenna, for example, is usually a vertical polarized antenna.
Figure 4: Antenna Polarization
Directional antennas focus the RF energy in a particular direction. As
the gain of a directional antenna increases, the coverage distance increases,
but the effective coverage angle decreases. For directional antennas, the lobes
are pushed in a certain direction and little energy is there on the back side
of the antenna.
Figure 5: Radiation Pattern of a Directional
Another important aspect of the antenna is the front-to-back ratio. It
measures the directivity of the antenna. It is a ratio of energy which antenna
is directing in a particular direction, which depends on its radiation pattern
to the energy which is left behind the antenna or wasted. The higher the gain
of the antenna, the higher the front-to-back ratio is. A good antenna
front-to-back ratio is normally 20 dB.
Figure 6: Typical Radiation Pattern of a Directional Antenna with
An antenna can have a gain of 21 dBi, a front-to-back ratio of 20 dB or
a front-to-side ratio of 15 dB. This means the gain in the backward direction
is 1 dBi, and gain off the side is 6 dBi. In order to optimize the overall
performance of a wireless LAN, it is important to understand how to maximize
radio coverage with the appropriate antenna selection and placement.
Wireless propagation can be effected by reflection, refraction or
diffraction in a particular environment. Diffraction is the bending of waves
around the corners. RF waves can take multipaths between the transmitter and
receiver. A multipath is a combination of a primary signal and reflected,
refracted or diffracted signal. So on the receiver side, the reflected signals
combined with the direct signal can corrupt the signal or increase the
amplitude of the signal, which depends on the phases of these signals. Because
the distance traveled by the direct signal is shorter than the bounced signal,
the time differential causes two signals to be received.
These signals are overlapped and combined into a single one. In real
life, the time between the first received signal and the last echoed signal is
called the delay spread. Delay spread is the parameter used to signify
multipath. The delay of the reflected signals is measured in nano seconds. The
amount of delay spread depends upon the amount of obstacles or infrastructure
present between the transmitter and receiver. Therefore, delay spread has more
value for the manufacturing floor due to lot of metallic structure present as
compared to the home environment. Overall, multipath limits the data rate or
lowers the performance.
Figure 7: Multipath Effects in Indoor
Indoor RF propagation is not the same as it is outdoors. This is due to
the presence of solid obstructions, ceilings, and floors that contribute to
attenuation and multipath signal losses. Therefore, multipath or delay spread
is more in the indoor environment. If the delay spread is more, the
interference is more and will cause lower throughput at a particular data rate.
Indoor environment can also be classified as near line of sight (LOS)
and non LOS. In near LOS environments, where you can see access points (APs)
such as in the hallways, multipath is usually minor and can be overcome easily.
The amplitudes of the echoed signals are much smaller than the primary one.
However, in non LOS conditions, the echoed signals can have higher power
levels, because the primary signal might be partially or totally obstructed,
and generally more multipath is present.
Multipath has been a semi-fixed event. However, other factors such as
moving objects can enter into play. The particular multipath condition changes
from one sample period to the next. This is called time variation.
Multipath interference can cause the RF energy of an antenna to be very
high, but the data is unrecoverable. You should not limit the analysis only to
the power level. As low RF signal does not mean poor communication, but low
signal quality does mean poor communication. You must analyze signal quality
and Rx level side by side. High Rx level and low signal quality means there is
a lot of interference. You must analyze the channel frequency plan again in
such a scenario. Low Rx level and low signal quality means that there is a lot
Indoor wave propagation is also affected by the building material. The
density of the materials used in the construction of a building determines the
number of walls the RF signal can pass through and still maintain adequate
coverage. Paper and vinyl walls have little effect on signal penetration. Solid
walls, solid floors and pre-cast concrete walls can limit signal penetration to
one or two walls without degrading coverage. This can vary widely based any
steel reinforcing within the concrete. Concrete and concrete block walls can
limit signal penetration to three or four walls. Wood or drywall typically
allows for adequate penetration of five or six walls. A thick metal wall causes
signals to reflect off, which results in poor penetration. Steel reinforced
concrete flooring restricts coverage between floors to perhaps one or two
The higher the frequency, the shorter the wavelength is. Shorter
wavelengths have more probability to get absorbed and distorted by a building
material. Therefore, 802.11a, which operates in a higher frequency band, is
more prone to the building material effect.
The actual effect on the RF must be tested at the site. Therefore, a
site survey is necessary. You should do a site survey to see the signal level
you receive on the other side of the walls. A change in the type of antenna and
location of the antenna can eliminate multipath interference.
Omni antennas are very easy to install. Due to the 360 degrees
horizontal pattern, it can even be mounted upside down from a ceiling in the
indoor environment. Also, because of its shape it is very convenient to attach
these antennas to the product. For example, you might see Rubber Duck antennas
attached to the wireless APs. In order to obtain an omnidirectional gain from
an isotropic antenna, energy lobes are pushed in from the top and the bottom,
and forced out in a doughnut type pattern. If you continue to push in on the
ends of the balloon (isotropic antenna pattern), a pancake effect with very
narrow vertical beamwidth results, but with a large horizontal coverage. This
type of antenna design can deliver very long communications distances, but has
one drawback which is poor coverage below the antenna.
Figure 8: Omni Antenna with No Coverage Below the
If you try to cover an area from a high point, you see a big hole below
the antenna with no coverage.
This problem can be partially solved with the design of something
called downtilt. With downtilt, the beamwidths are manipulated to provide more
coverage below the antenna than above the antenna. This solution of downtilt is
not possible in an omni antenna because of the nature of its radiation
The omni antenna is usually a vertically polarized antenna, so you
cannot have advantages of using cross polarization here to fight interference.
A low gain omni antenna provides a perfect coverage for an indoor
environment. It covers more area near the AP or a wireless device in order to
increase the probability of receiving the signal in a multipath
Note: In addition to the Cisco Aironet Antennas that work for larger
are High-Gain Omnidirectional antennas supported by Cisco for Small Office
With the directional antennas, you can divert the RF energy in a
particular direction to farther distances. Therefore, you can cover long
ranges, but the effective beamwidth decreases. This type of antenna is helpful
in near LOS coverage, such as covering hallways, long corridors, isle
structures with spaces in between, etc. However, as the angular coverage is
less, you cannot cover large areas. This is a disadvantage for general indoor
coverage because you would like to cover a wider angular area around the AP.
Antenna arrays should face in the direction where the coverage is
desired, which can sometimes make mounting a challenge.
As 802.11 devices operate in the unlicensed bands, this makes it
available for anyone to use. WLAN interference comes from other similar devices
and other sources such as microwave ovens, cordless phones, radar signals from
a nearby airport, etc. Interference is also found from other technologies that
use the same band as Bluetooth or security devices. In the 2.4 GHz unlicensed
there are limited channels to utilize to avoid interference, with only three
non-overlapping channels available.
Interference and multipath cause the receive signal to fluctuate at a
particular frequency. This variation of signal is called fading. Fading is also
frequency selective, as attenuation varies with frequency. A channel can be
classified as either a fast fading channel or slow fading channel. This depends
on how rapidly the transmitted base band signal changes. A mobile receiver that
travels through an indoor environment can receive rapid signal fluctuations
caused by additions and cancellations of the direct signals at half wavelength
Interference increases the requirement of signal to noise ratio (SNR)
for a particular data rate. The packet retry count goes up in an area where
interference or multipath is very high. A change in the type of antenna and
location of the antenna can eliminate multipath interference. Antenna gain adds
to the system gain and improves signal and interference to noise ration (SINR)
requirements as shown here:
Figure 9: Noise Floor and Signal and Interference to Noise
Although directional antennas help to focus the energy in a particular
direction which can help to overcome fading and multipath, multipath itself
reduces the focusing power of a directional antenna. The amount of multipath
seen by a user at a long distance from the AP can be much more.
Directional antennas used for the indoors typically have a lower gain,
and as a result, have a lower front-to-back and front-to-side lobe ratios. This
results in less ability to reject or reduce the interference signals received
from directions outside the primary lobe area.
While directional antennas can be of great value for certain indoor
applications, the vast majority of indoor installations utilize omnidirectional
antennas for the reasons cited in this document. The selections of an antenna,
directional or omnidirectional, should be strictly determined by a correct and
proper site survey.