StacyDrake
11/22/11
Antenna Types
Research the different types of antennas that are included under the three main categories:
Omni-Directional, Semi-Directional, and Highly-Directional. Draw or print an example of each type
and summarize when it is used, what is its maximum transmission distance, and its advantages
and disadvantages.
According to a few online sources, Antennas are most often used to increase the range of
a wireless LAN system. Proper antenna selection can also enhance security of a wireless
LAN. Antennas are most sensitive to RF signals whose wavelength is an even multiple of
the antenna’s length (including fractional multiples – such as ½ or ¼). Some of the basic
antenna principals includes; the antennae convert electrical energy to RF waves in the
case of transmitting or converts the RF waves into electrical energy in the case of
receiving antennae. The physical structure of an antenna is directly related to the shape of
the area in which it concentrates most of its radiated RF energy. Generic categories of RF
antennas includes, Omni-directional, Semi-directional, or Highly-directional. Each category
has multiple types of antennas, each having different RF characteristics and appropriate
uses.
One of the most common wireless LAN antenna is a Omni directional (Dipole) antenna. A
Dipole antenna is a standard equipment on most access points. It also radiates energy
equally in all directions around its axis. It Radiates in a 360-degree horizontal beam. The
Dipole antenna radiates in all directions around axis, but does not radiate along the length
of the wire itself. See page 2 for diagram. One of the advantages the Dipole is it does
not require external power. Some of the Disadvantage are as the gain increases, its
coverage becomes more focused, Highest-gain antennas can’t be used for mobile users
because of their tight beam and Active gain involves an amplifier. The Omni directional
antenna is used when coverage is required in all directions around the horizontal axis.
Most effective when large coverage areas are needed around a central point.
Commonly used for point-to-multipoint designs.
Semi-directional antennas direct energy from the transmitter significantly more in one
particular direction. The antenna is often radiate in a hemispherical or cylindrical coverage
pattern.
Have back and side lobes that, if used effectively, may further reduce the need for
additional access points. Frequently used types: Patch & Panel– flat, designed for wall
mounting. Focus coverage in horizontal arc of 180° or less. Yagi – elongated, ribbed, and
usually housed in an enclosure for moisture protection. Semi-directional antennas are
common vertical & horizontal beam widths of 90° or less. 30° or less average. Semidirectional antennas are Ideally suited for short and medium-range bridging.In some cases,
semi-directional antennas provide such long-range coverage that they eliminate the need
for multiple access points in a building. Patch or panel typically used on short range
building-to-building & in-building directional links. Yagis most often used on short to
medium length building-to-building bridging up to 2 miles (3.3 km).
Highly-directional antennas are High-gain antennas that emit the most narrow signal beam
of any antenna type. Greatest gain of any of the types. Typically concave, disk-shaped
devices (similar to satellite TV antenna) Parabolic dishes. Some are grids (grid antennas) –
provides good resistance to wind loading. These antennas are ideal for long distance,
point-to-point wireless links. Highly-directional antenna can transmit at distances up to 58
km (about 35 miles).
StacyDrake
11/22/11
Modulation Techniques
Analog amplitude modulation, frequency modulation, and phase modulation, along with their digital
counterparts of AFK, FSK, and PSK, all have their advantages and disadvantages. Research
these types
of modulations along with their respective strengths and weaknesses. Include coverage regarding
how they are used today.
Modulation is defined as the process where a Radio Frequency or Light Wave's amplitude,
frequency, or phase is changed in order to transmit intelligence. The characteristics of the carrier
wave are instantaneously varied by another "modulating" waveform.
Amplitude Modulation (AM)
Amplitude Modulation occurs when a voice signal's varying voltage is applied to a carrier
frequency. The carrier frequency's amplitude changes in accordance with the modulated voice
signal, while the carrier's frequency does not change.
When combined the resultant AM signal consists of the carrier frequency, plus UPPER and
LOWER sidebands. This is known as Double Sideband - Amplitude Modulation (DSB-AM), or
more commonly referred to as plain AM.
The carrier frequency may be suppressed or transmitted at a relatively low level. This requires
that the carrier frequency be generated, or otherwise derived, at the receiving site for
demultiplexing. This type of transmission is known as Double Sideband - Suppressed Carrier
(DSB-SC).
It is also possible to transmit a SINGLE sideband for a slight sacrifice in low frequency response
(it is difficult to suppress the carrier and the unwanted sideband, without some low frequency
filtering as well). The advantage is a reduction in analog bandwidth needed to transmit the signal.
This type of modulation, known as Single Sideband - Suppressed Carrier (SSB-SC), is ideal for
Frequency Division Multiplexing (FDM).
Another type of analog modulation is known as Vestigial Sideband. Vestigial Sideband modulation
is a lot like Single Sideband, except that the carrier frequency is preserved and one of the
sidebands is eliminated through filtering. Analog bandwidth requirements are a little more than
Single Sideband however.
Vestigial Sideband transmission is usually found in television broadcasting. Such broadcast
channels require 6 MHz of ANALOG bandwidth, in which an Amplitude Modulated PICTURE
carrier is transmitted along with a Frequency Modulated SOUND carrier.
Frequency Modulation (FM)
Frequency Modulation occurs when a carrier's CENTER frequency is changed based upon the
input signal's amplitude. Unlike Amplitude Modulation, the carrier signal's amplitude is
UNCHANGED. This makes FM modulation more immune to noise than AM and improves the
overall signal-to-noise ratio of the communications system. Power output is also constant, differing
from the varying AM power output.
The amount of analog bandwidth necessary to transmit a FM signal is greater than the amount
necessary for AM, a limiting constraint for some systems.
Phase Modulation
Phase Modulation is similar to Frequency Modulation. Instead of the frequency of the carrier wave
changing, the PHASE of the carrier changes. As you might imagine, this type of modulation is
easily adaptable to data modulation applications.