Wireless Data Acquisition and Control How Antennas Work Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 3 Element Yagi Yagi Horizontal

Wireless Data Acquisition and Control

How Antennas Work

Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved

3 Element Yagi

Yagi Horizontal Beam Pattern

Omni-Directional

ANTENNA GAIN

Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved2

ABOUT ANTENNAS

The antennas in a wireless system are extremely important to the reliable performance of the system. The transmitter antenna radiates a magnetic field that must couple to the receiver antenna with sufficient strength to create a power level above the minimum threshold of the receiver. The gain of the antennas, thepolarization of the elements, the height above ground or other objects, and the lack of obstructions between the two antennas play an important role in achieving satisfactoryperformance.

REFERENCE ANTENNASGAIN

RECIPROCITY


Antennas have a gain that is usually referenced to an isotropic source or a dipole*. The gain is the ratio of the power level out of the antenna of interest to the power level out an isotropic source or a dipole.

Gain referenced to an isotropic source is expressed in dBi while gainreferenced to a dipole is commonly expressed in dB or dBd. All antennas in this document have their gain expressed in dBi.

Antennas have perfect reciprocity. The have the same characteristicswhether used as a transmitting or receiving antenna. The gain isidentical for both applications.

*A dipole is 2 elements end to end with the RF signal fed to the ends

of the elements at the center of the 2 elements.

4

Gain is expressed as the ratio of the antenna power out (transmitting)

or in (receiving) to the reference antenna power out or in. Assume a

reference antenna is used to receive a signal from a transmitter. Then

the antenna of interest is used. The gain of the antenna of interest is:

Gain = 10Log(P2/P1) dBi

P2 = Power level of signal out of antenna of interest

P1 = Power level of signal out of reference antennaExamples:

A power ratio of 2 yields Gain = 10Log(2) Log 2 = 0.30 Gain = 3dBi

A power ratio of 10 yields Gain = 10Log(10) Log 10 = 1 Gain = 10dBi

GAIN


HOW ANTENNAS GET THEIR GAIN

(Reference Isotropic Source)

An isotropic source antenna radiates a

signal in a perfect sphere from a point

source. At a given distance from the

antenna, the signal level is the same at any

point on the sphere. There is no isotropic

antenna. It is a mathematical model only.

Gain in an antenna is achieved by shaping

the magnetic field to reduce the field in

some directions by forcing an increase

in other directions.


Isotropic SourcePerfect Solid Sphere

Radiation Pattern

6

VERTICAL ANTENNAS

This image is the vertical beam pattern of a 2.1dBi gain vertical antenna. The radiation angle has been changed by the design so it radiates more in a horizontal direction with a reduction of radiation vertically. It has a doughnut shaped pattern horizontally which gives it the name "omni-directional antenna". *Wavelength is the distance between 2 zeropoints on the RF signal (sine wave).Wavelength = Velocity of wave / Frequency¼ wave at 900MHz = 3.1 inchesAll antenna design is frequency dependent.


The simplest antenna is a vertical wire 1/4 wavelength* long with a

metallic plane under the bottom. It has a gain of 2.1dBi.

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VERTICAL ANTENNAS

This image is the vertical beam patternof a 6dBi gain vertical antenna. Theradiation angle is much narrower thanthe 2.1dBi gain antenna. The narrowradiation angle puts more power out ina horizontal direction and less vertically. A longer mechanical length creates thegreater gain by modifying the radiationpattern. It has a doughnut shaped pattern horizontally.


YAGI ANTENNAS

Yagi antennas have a greater gain capability than

a simple vertical antenna. This is possible due to

the ability to reduce radiation in a vertical as well

as horizontal direction and direct more energy in

a single direction. The horizontal beam pattern

shows the reduction in energy to the rear and

sides of the antenna. The vertical beam pattern

shows the reduction in energy to the top and

bottom as well as the rear of the antenna. This 3

element yagi has a gain of 8dBi.


YAGI ANTENNAS

This 10 element yagi has a gain of 14dBi. Note the

narrower beam width in the horizontal and vertical

planes. The higher gain is achieved by narrowing

the energy into this narrower beam in the horizontal

and vertical planes.

The insensitivity of the yagi to signals from the rear

makes it a good choice for reducing interference

from the back direction.


The difference in gain for the 2 directions is labeled the front to back ratio. This antenna has a front to back ratio of 20dB to 30dB, depending on the polarization used.

POLARIZATION

A vertical omni-directional antenna has vertical polarization. Any

antenna sending or receiving signals to/from it must have the same

polarization.

For the magnetic field around an antenna to be received by another

antenna, the magnetic field must be at a right angle to the

receiving antenna. Since the magnetic field is at a right angle to the

transmitting antenna, the 2 antennas must be parallel to each other.

A yagi must have its elements parallel to another yagi or an

omni-directional antenna. When antenna elements are cross

polarized, the signal at the receiving antenna can approach zero.Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved

10

POLARIZATION

Yagi antennas must be aligned pointing at each other and with the

same polarization. Yagi antennas must be aligned pointing at

omni-directional antennas and must have the same polarization.

Omni-directional antennas at different heights must be tilted to make

the elements parallel and the elements should be in the same plane.

Treat them as a single element yagi and point them at each other.

Antennas can provide a wide choice of gain to insure reliable operation.

The cables that connect the antennas to the receiver and transmitter

have a loss in RF energy. The resultant system gain is the antenna

gain minus the coaxial cable loss.


SHORT RANGE ANTENNAS

Small vertical antennas are used for short range communications

7” vertical dipole antenna for indoor mounting. Mounts by screwing on RPSMA bulkhead connector. Articulated 90 degree joint allows horizontal or vertical connector to be used. Line of sight range about 1500 to 2000 feet. 2.1dBi gain

2 3/4” vertical antenna for outdoor mounting. Mounts on a special bulkhead connector . A bracket is available for mounting on a flat vertical surface or a mast. This type antenna must mount on a “ground plane” such as a metal enclosure or a metal disk. Line of sight range about 1500 to 2000 feet. 2.1 dBi gain

Vertical omni-directional antenna for outdoor mounting. Type N female connector to accept coaxial cable. Has bracket for mounting on 1 ½” mast. Line of sight range several miles.

6dBi and 8.5dBi gain models. 27” and 60” lengths.


915MHz Band Antennas

LONG RANGE ANTENNAS

Yagi antennas with high gain are used for long range communications.

8 dBi gain 18” length




2 Stacked Yagi 17 dBi gain 48” lengthCopyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 13

MAST

RECEIVER SENSITIVITY

Receiver sensitivity is usually expressed as the threshhold power level at

which the receiver detects a useable signal level. The level is expressed in

dBm (decibels referenced to 1 milliwatt across 50 ohms). Antennas and

receiver input impedance is standardized at 50 ohms.

An outstanding receiver may have a sensitivity of -110dBm, while many will

have a sensitivity of -90dBm. The 20 dB difference is a factor of 100

difference in power sensitivity. A wireless system should be designed for a

20dBm signal safety margin.

1 milliwatt across 50 ohms = 0.223V.

-90dBm = 22.3µV. A 20 dBm Safety margin = -70dBm = 223µV

-110dBm = 2.23µV. A 20 dBm Safety margin = -90dBm = 22.3µV

Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 14

TRANSMITTER POWER

The FCC regulations for the 900MHz ISM band limits radiated power to 4 watts. If a

100mW transmitter is used, the antenna gain minus cable losses cannot exceed 16dB.

In a one way point to point system, receiver antenna gain can be anything possible. If a

transmitter is involved, the 16dB rule applies.

Transmitters radiate a sine wave AC signal. The sine wave is not perfect, therefore

harmonics of the fundamental frequency are radiated. The FCC requires the antenna to

appear to be, within limits, a 50 ohm resistor to the transmitter to keep down excessive

radiation of harmonics. To prevent improper antennas from being easily connected to ISM

transmitters, they are fitted with reverse polarity coax connectors. The reverse polarity

connectors are not easily found. A common connector is the SMA series. On a transmitter,

one will find an RPSMA connector.


MORE INFORMATION

Wilkerson Instrument Company is creating a group of Power Point presentations for wireless

applications, and other instrumentation problem solving applications.

We are also creating application notes in a pdf format for the same subjects.

The slide presentations and application notes can be viewed on our web site www.wici.com.

These aids can be downloaded.

The slide presentations and application notes will also be on our CD Wireless Data

Acquisition Products and Instrumentation Products catalogs. These catalog CD’s can be downloaded

or they can be requested from [email protected]. Tel .800 234 1343.

The application notes will be added to our email broadcast list. If you would like to be

added to our list, please send a note to [email protected].

Richard Huffman

President

Wilkerson Instrument Company, Inc

[email protected]


Need More Information!

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Toll Free: 800-234-1343 Phone: 863-647-2000 Fax: 863-644-5318 Email: [email protected]

Complete documentation is available for all Wilkerson Instrument Co. products at www.wici.com.


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Wireless Data Acquisition and Control How Antennas Work Copyright © 2008 Wilkerson Instrument Co., Inc All rights reserved 3 Element Yagi Yagi Horizontal