Antennas

RF
RF systems complement wired networks by
extending them.
Different components may be used
depending on the frequency and the distance
that signals are required to reach
g
Two RF components are of particular interest
to wireless users:
Antennas
Amplifiers

Amplifiers
A lifi
Amplifiers make signals bigger.
Signal boost, or gain, is measured in
g , g,
decibels (dB).
Amplifiers can be broadly classified into
three categories:
low-noise
high-power
other

Low N i A lifi
L Noise Amplifiers (LNA )
(LNAs)
Low-noise amplifiers (LNAs) are usually
connected to an antenna to boost the
received signal to a level that is recognizable
by the electronics the RF system is
connected to.
LNAs are also rated for noise factor, which is
the measure of how much unwanted
information the amplifier introduces. Smaller
noise factors allow the receiver to hear
smaller signals and thus allow for a greater
range.
range

High Power A lifi
Hi h P Amplifiers (HPA )
(HPAs)
High-power amplifiers (HPAs) are used
to boost a signal to the maximum
g
power possible before transmission.
Output power is measured in dBm
Amplifiers generate heat in addition to
amplifying the signal.
signal

Antennas
At
They convert electrical signals on wires into
radio waves and vice versa.
An antenna must be made of conducting
material. Radio waves hitting an antenna
cause electrons to flow in the conductor and
create a current.
Likewise, applying a current to an antenna
creates an electric field around the antenna.
As the current to the antenna changes so
changes,
does the electric field. A changing electric
field causes a magnetic field, and the wave is
off.
off

The size of the antenna you need depends on the
frequency: the higher the frequency, the smaller the
frequency
antenna.
The shortest simple antenna you can make at any
frequency i ½ wavelength long (though antenna
f is l th l (th ht
engineers can play tricks to reduce antenna size
further).
This l f th b
Thi rule of thumb accounts for the huge size of
t f th h i f
radio broadcast antennas and the small size of
mobile phones.
An AM station broadcasting at 830 kHz has a
wavelength of about 360 meters and a
correspondingly large antenna
But an 802.11b network interface operating in the
2.4-GHz band has a wavelength of just 12.5
centimeters. With some engineering tricks, an
antenna can b incorporated into a PC Card, and a
t be i t di t Cd d
more effective external antenna can easily be carried
in a backpack.

Antenna Theory
At Th
A theoretical
isotropic antenna
has
h a perfect 360º
ft
vertical
and horizontal
beamwidth
This is a reference
for ALL antennas

Antenna Variables
At V i bl
Bandwidth
Beamwidth
Gain
Polarization
P l i ti
Diversity
Power

Bandwidth
B d idth
The bandwidth of an antenna is the band
of frequencies, over which it is considered
frequencies
to perform acceptably. The wider the
range of frequencies a band
encompasses, the wider the bandwidth of
the antenna.

Beamwidth
B idth
Beamwidth is a measurement used
to describe directional antennas.
Beamwidth is sometimes called
half-power beamwidth. It is the total
width in degrees of the main radiation
lobe, at the angle where the radiated
power has fallen below that on the
centerline of the lobe, by 3 dB (half-power).

Gain
Gi

Polarization
P l i ti

Polarization
P l i ti cont.
t

Radiation Patterns
R di ti P tt

Dipole Antennas
Di l A t

Diversity
Di it
Space diversity

Diversity cont.
Di it t
Frequency diversity

Omnidirectional A t
O idi ti l Antennas
An omnidirectional antenna is designed to
provide a 360 degree radiation pattern
This type of antenna is used when
yp
coverage in all directions from the antenna
is required.
q

Omnidirectional A t
O idi ti l Antennas cont.
t
In a perfect world, all the omnidirectional
antennas would radiate perfectly i all di ti
t ld di t f tl in ll directions.
Unfortunately, this is not the case.
H-Plane E-Plane

Omnidirectional A t
O idi ti l Antennas cont.
t

Antenna Theory- Di l
At Th Dipole
Energy lobes are
gy
‘pushed in’ from
Side View
the top and bottom (
(Vertical Pattern)
)
Higher gain Vertical Beamwidth
Smaller vertical New Pattern (with Gain)
beamwidth
Top View
Larger horizontal (Horizontal Pattern)
lobe
Typical dipole
pattern

Directional A t
Di ti l Antennas
Lobes are pushed
in a certain
direction, causing Side View
(Vertical Pattern)
the energy to be
co de sed
condensed in a
particular area
Top View
Very little energy is (Horizontal Pattern)
in the back side of
a directional
antenna

Directional A t
Di ti l Antennas
This type of antenna does not offer any added power to the
signal, and instead simply redirects the energy it received
from the transmitter. By redirecting this energy, it has the
effect of providing more energy in one direction, and less
energy in all other directions
gy

Directional t
Di antennas l antennas cont.
ti t
Patch
The patch antenna is great for indoor and outdoor applications.
Mechanically they
M h i ll th are small rectangles about a 1/2 i h thi k Th
ll t l bt inch thick. The
patch works well and fits aesthetically into most work environments.
6 dBi Patch Antenna – 65 degree

Directional antennas cont.
Di ti lt t
Yagi antennas
Yagi antennas are directional and designed for long
distance communication
Horizontal Vertical

Directional antennas cont.
Di ti lt t
Solid dish antennas
H-Plane
H Pl E-Plane

Back lobe d id lobes
B k l b and side l b

Beamwidth vs. G i
B idth Gain

High Gain Omni-Directionals
Hi h G i O i Di ti l
More coverage
area in a circular
pattern
Energy level
directly above or
below the antenna
will become lower

Link Engineering and
RF Path Planning

Path Considerations
P th C id ti
Radio line of sight
Earth bulge
Fresnel zone

Line f Sight
Li of Si ht
The following obstructions might obscure a
visual li k
i l link:
Topographic features, such as mountains
Curvature of the Earth
C t f th E th
Buildings and other man-made objects
Trees
Line of sight!

Longer Di t
L Distances
Line of Sight disappears at 6 miles
(
(9.7 Km) due to the earth curve
)

Fresnel Z
F l Zone

Fresnel Z
F l Zone cont.
t

Improving F
I i Fresnel Effect
l Eff t
Raise the antenna
New structure
Existing structure
Different mounting point
Remove trees

Site to Sit F
Sit t Site Fresnel Zone
lZ
Antenna Height
Fresnel zone consideration
Line-of-Sight over 25 miles (40 Km) hard to
implement
Antenna
Height
(Value
(V l “H”) Total Distance
Fresnel @ 60% (Value “F”)
Earth Curvature (Value “C”)