Bài giảng Antennas

Bài giảng Antennas

Antennas

(1 September, 2006)

● Explain the types of wireless LAN

antennas and how/when to use them

● Describe the various wireless LAN

accessories and where they are used

Objectives

Upon completion of this chapter you will be able to:

External Antenna Classifications

● Omni-directional Antenna

✔Mast mount omni

✔Pillar mount omni

✔Ground plane omni

✔Ceiling mount omni

● Highly- directional Antenna

Omni-directional Antenna

✔ Mast mount omni

✔ Blade omni

✔ PC Card integrated omni

✔ Pillar mount omni

✔ Ground plane omni

✔ Ceiling mount (blister) omni

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✔ It radiates energy equally around the antenna axis- 360 degrees.

☛ However it dos not radiate along the length of the antenna hence the radiation pattern is in the shape of a doughnut

☛ Anything radiating in all directions equally (the sun) is called and isotropic radiator

☛ The gain of an antenna is referenced to an isotropic Radiator

☛ The higher the gain the more horizontally squeezed the

Omni-directional Antenna Radiation Pattern

Dipole/Omni Antenna

Omni Ceiling Mount Antenna

Omni Pillar Mount Antenna

Omni Ceiling (Blister) Mount Antenna

Omni-directional Antennas

2.4 Gz 10 dBi Omni-Directional Antenna

2.4 Gz 5 dB Magnetic Mount Omni Antenna

Cantenna Waveguide Antenna

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5.2 dBi Omni-directional Antenna

Vertical Pattern

5.2 dBi Omni-directional Antenna Contd

Frequency Range 2.4-2.83 GHz

VSWR Less than 2:1, 1.5:1 Nominal

Polarization Vertical

Azimuth 3dB BW Omnidirectional 360 degrees

Elevations Plan (3dB BW) 50 degrees

Antenna Connector RP-TNC

Dimensions (H x W) 11.5 x 1.125 in

Mounting Mast mount—indoor/outdoor

✔Golf Courses, etc

● They are most often used for short distances (2-3 miles) to bridge two buildings.

Patch Antenna

Patch Antenna

Patch Antenna Contd

Semi-directional Antenna Radiation Pattern

Directional Patch Antenna

Main lobe

Back Lobe

2.4 Gz 13.5 dBi Radome Yagi

Vertical Pattern

Horizontal Pattern

2.4 Gz 13.5 dBi Radome Yagi

Semi-directional Antenna Radiation Pattern

Directional Yagi Antenna

Main lobeSide Lobe

Azimuth and Elevation Charts

Azimuth

Elevation

Point-to-Point Wireless Bridge Link

Highly- Directional Antenna

✔ Parabolic dish

✔ Grid antenna

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Highly-directional antenna

● Highly-directional antenna radiate in a cylindrical coverage pattern that is highly flattened Not used for communicating with clients.

● Used for

✔ Pt-to-Pt communication links

✔ Blast through obstructions

✔ Must be accurately aligned.

● They are most often used for long distances (35 miles) for pt-to-pt communication.

2.4 Ghz 21 dBi Parabolic Dish Antenna

2.4 Ghz 21 dBi Parabolic Dish Antenna

Frequency Range 2.4-2.83 GHz

VSWR Less than 1.8:1, 15:1 Nominal

Power 5 watts

Front to Back Ratio Greater than 25 dB

Maximum Side Lobe -17 dB

2.4 Gz 19 dBi Reflector Grid Antenna

Grid Dish Parabolic Antenna (2400 to 2485 MHz Operation)

✔15dBi, 19dBi and 24dBi models

✔ Rugged and Weatherproof

✔ Ultra Low Wind Loading and Low Visual Impact

✔ Vertical or Horizontal Polarization

✔ 2.4GHz Wireless LAN Applications • Point to Point Backhaul

Line of sight

Fresnel Zone

✔ The Fresnel Zone occupies a series of concentric circles around the RF LOS.

✔ Objects in the Fresnel Zone can diffract or reflect the RF wave away from

receiver thereby changing the RF LOS.

✔The formula to calculate the "60 percent unobstructed radius" is:

√

d/4f

r = 43.3 x

✔ Typically 20%-40% Fresnel Zone blockage introduces little or no interference.

✔ Attempt to Design the link with 0% blockage If this is not possible then allow no more than 20% blockage of the Fresnel Zone

Line of Sight

Line of sight

There are several options to establish or improve the line

of sight:

✔ Raise the antenna mounting point on the existing structure

✔ Build a new structure, i.e radio tower, which is tall enough to mount the antenna

✔ Increase the height of an existing tower

✔ Locate a different mounting point, i.e building or tower, for the antenna

✔ Cut down problem trees

Earth Bulge

Line of Sight

✔ The earth's horizon can obstruct the Fresnel Zone if the distance

between antennas is greater than 7 miles.

✔ The formula for calculating the additional antenna height needed

to correct for the communication links greater than 7 miles is:

H=D 2 /8

✔ The formula for calculating the minimum antenna height for

communication links over 7 miles is:

D 2 /8

√

D/4F) +

H = (43.3 x

Diverse Antennas

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Diverse Antennas

● Diverse antennas are used to overcome multipath distortion

(multipath fading)

✔This uses two antennas separated by at least one wavelength

● Multipath distortion occurs at the receiver when the wavelength

travels multipaths from the transmitter to the receiver

✔The reflected wave travels farther than the desired wave arriving later in time

✔The reflected wave travels farther and loses more RF energy than the direct wave

✔The signal will lose energy when reflected

Multipath Fading

Multipath

Reception

Distortion

Diverse Antenna Switch

● Diverse antennas include two antennas that are connected to an RF switch which in turn is connected to the receiver

✔ The receiver switches between antennas sampling the

preamble

✔ It then selects the best antenna for receiving the signal

✔ The transmitter then uses the same signal for transmission

● Antenna's have four fundamental concepts:

✔ Polarization: The orientation of the Electric

component of the electro-magnetic field

✔ Gain: A measure of the increase in power.

✔ Direction: The shape of the radiated

transmission pattern

✔ Free Space Path Loss: RF signal loss due to

transmission distance.

Antenna Concepts

● The Radio wave is made up of an oscillating electro-magnetic field

composed of two planes:

✔ H-Plane: The Magnetic plane is created perpendicular to the

antenna

✔ E-Plane: The Electric plane is created parallel to the antenna

and defines the orientation of the radio waves radiated from the antenna

☛ Vertical polarization has the E-Plane perpendicular to the

earth Most WLAN antennas are vertically polarized

☛ Horizontal Polarization has the E-Plan horizontal to the

earth

H-Plane

Different Polarized Antennas

Vertically Polarized Antenna Horizontally Polarized Antenna

Gain and Direction

● Gain is the increase in energy that an antenna appears

to add to the RF Signal.

● An antenna has passive gain – they do not increase the

power input to them.

✔ Real antennas reshape the radiation pattern by

simply redirecting the energy through reflection – think of

a flashlight that has the ability to change its beamwidth.

✔ It provides more energy in one direction and less in another.

✔ As the Antenna gain increases the angle of radiation decreases:

☛ This provides greater coverage distance but

Less energy in one direction More energy in one direction

Beamwidth Contd

✔ Narrowing or focusing antenna beams increase or decrease the

antenna's gain – dBi

✔ The beamwidth is composed of a vertical and a horizontal element

Directional Antenna

Horizontal Beamwidth

Gain and Direction Contd

● Basic Antenna gain is rated in comparison to an Isotropic radiator.

✔ An isotropic radiator is a theoretical construct that

radiates equally in all direction simultaneously.

● The Antenna's gain is measured in dBi –

decibels relative to an Isotropic radiator.

✔ The dBi rating is used to compare the power level of a

given antenna to the theoretical isotropic radiator

✔ For example, an isotropic radiator has a gain of 0 dBi whereas a dipole is rated at 2.14 dBi

Free Space loss

● Free Space loss is the loss incurred by an RF signal as it travels over

distance

✔ The signal disperses (broadens) over distance

✔ The power decrease is inversely proportional to the distance

traveled and proportional to the signal wavelength

✔ The 6dB rule states

☛ A 6dB increase in EIRP doubles the range.

☛ A 6dB decrease in EIRP cuts the range in half

End of Lecture