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Demarcation line, (generally) the outside limit of an object or area or surface; a place farthest awayfrom the center of something; "the edge of the leaf is wavy"; "she sat on the edge of the bed"; "thewater's edge"

ISOTROPIC ANTENNAAn isotropic antenna is an ideal antenna that radiates its power uniformlyin all directions. There is no actual physical isotropic antenna. However, an isotropic antenna isoften used as a reference antenna for the antenna gain.

Antenna gain ISOTROPIC The antenna gain is often specified in dBi, or decibels over isotropic.This is the power in the strongest direction divided by the power that would be transmitted by anisotropic antenna emitting the same total power.

Antenna gain An antenna focuses the radio waves in a certain direction. Usually, this is called themain direction. Because of that, in other directions less energy wil be emitted. The gain of anantenna, in a given direction, is usually referenced to an (hypothetical) isotropic antenna, whichemits the radiation evenly strong in all directions. The antenna gain is the power in the strongestdirection divided by the power that would be transmitted by an isotropic antenna emitting the sametotal power. In this case the antenna gain (Gi) is often specified in dBi, or decibels over isotropic.

Other reference antennas are also used, especially:gain relative to a half-wave dipole (Gd), when the reference antenna is a half-wave dipole antenna;gain relative to a short vertical antenna (Gv), when the reference antenna is a linear conductor,much shorter than one quarter of the wavelength.

Gain is not a quantity which can be defined in terms of a physical quantity such as

the Watt or the Ohm, but it is a dimensionless ratio. Gain is given in reference to astandard antenna. The two most common reference antennas are the isotropic

antenna and the resonant half-wave dipole antenna. The isotropic antenna radiatesequally well in all directions. Real isotropic antennas do not exist, but they provide

useful and simple theoretical antenna patterns with which to compare real antennas.

Any real antenna will radiate more energy in some directions than in others. Sinceantennas cannot create energy, the total power radiated is the same as an isotropicantenna. Any additional energy radiated in the directions it favors is offset by equally

less energy radiated in all other directions.

The gain of an antenna in a given direction is the amount of energy radiated in that

direction compared to the energy an isotropic antenna would radiate in the samedirection when driven with the same input power. Usually we are only interested in

the maximum gain, which is the gain in the direction in which the antenna isradiating most of the power. An antenna gain of 3dB compared to an isotropic

antenna would be written as 3dBi. The resonant half-wave dipole can be a usefulstandard for comparing to other antennas at one frequency or over a very narrow

band of frequencies. To compare the dipole to an antenna over a range of

frequencies requires a number of dipoles of different lengths. An antenna gain of 3dBcompared to a dipole antenna would be written as 3dBd.

The method of measuring gain by comparing the antenna under test against a

known standard antenna, which has a calibrated gain, is technically known as a gaintransfer technique. Another method for measuring gain is the 3 antennas method,

where the transmitted and received power at the antenna terminals is measuredbetween three arbitrary antennas at a known fixed distance.

Decibel The bel (symbol B) expresses the ratio of two powers by the decimal logarithm of thisratio. This unit is not often used, having been replaced by the decibel (symbol dB) which is one-

tenth of a bel.

For example, if P1 and P2 are two powers, their ratio expressed in decibels is:

10 10log (P1 / P2)

Originally the dB is used to express the ratio of two powers, such as:

the signal-to-noise ratio,the ratio of received power to the transmitted powerthe ratio of output and input power, e.g.:the gain of an amplifier,

antenna gain.

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The decibel is also used to express the signal power itself. The signal power is then given as a ratioto 1 Watt. Sometimes the symbol dBW is used to denote this ratio. The signal power can also begiven as a ratio to 1 mW (1/1000 Watt). In that case the symbol dBm is used. A power in dBm isequal to a power in dBW + 30.

The decibel is also used to express the ratio of two field quantities, such as voltage, current, soundpressure, electric field, charge velocity or density, the square of which in linear systems isproportional to power. To obtain the same numerical value as a power ratio, the logarithm of thefield quantity ratio is multiplied by the factor 20, assuming that the impedances are equal.

For example, if V1 and V2 are two voltages, their ratio expressed in decibels is:

20 10log (V1 / V2)

The relationship between a current or voltage ratio and that of the corresponding power ratio isimpedance dependent. Use of the decibel when the impedances are not equal is not appropriateunless adequate information is given concerning the impedances involved.

Calculating in dB's

In the following table a number of ratios and their equivalent in decibels are given.

Power ratio dB

1 0

1.5 1.8

2 3.0

3 4.8

4 6

5 7

Power ratio dB

6 7.8

7 8.5

8 9

9 9.5

10 10

100 20

Other values can be calculated with the use of the table and the following rules:

log (a b) = log(a) + log(b) and log (ax) = x log (a)

For example, an power ratio of 42 (6 7) is equal to 7.8 + 8.5 = 16.3 dB and a power ratio of 1000(103) is equal to 3 10 = 30 dB.

Directivityis the ability of an antenna to focus energy in a particular direction whentransmitting, or to receive energy from a particular direction when receiving. If awireless link uses fixed locations for both ends, it is possible to use antenna

directivity to concentrate the radiation beam in the wanted direction. In a mobileapplication where the transceiver is not fixed, it may be impossible to predict where

the transceiver will be, and so the antenna should ideally radiate as well as possiblein all directions. An omnidirectional antenna is used in these applications.

Free-space path loss formula

Free-space path loss isproportionalto thesquareof the distance between the transmitterand receiver, and also proportional to the square of the frequency of the radio signal.

The equation for FSPL is

where:

is the signal wavelength (in metres),

is the signal frequency (in hertz),

is the distance from the transmitter (in metres),

http://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Square_(algebra)http://en.wikipedia.org/wiki/Square_(algebra)http://en.wikipedia.org/wiki/Square_(algebra)http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Hertzhttp://en.wikipedia.org/wiki/Proportionality_(mathematics)http://en.wikipedia.org/wiki/Square_(algebra)http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Hertz

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is the speed of light in a vacuum, 2.99792458 108metres per second.

This equation is only accurate in thefar field; it does not hold close to the transmitter. As[1]

put it, "If the separation d is continually decreased, eventually the received power appearsgreater than the transmitted power which is [obviously] impossible in reality, since free

space is not an amplifier."

[edit] Free-space path loss in decibels

A convenient way to express FSPL is in terms ofdB:

where the units are as before.

For typical radio applications, it

is common to find measuredin units ofMHz and in km, in

which case the FSPL equation

becomes

For in statute miles, the constant becomes .

http://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Metres_per_secondhttp://en.wikipedia.org/wiki/Metres_per_secondhttp://en.wikipedia.org/wiki/Far_fieldhttp://en.wikipedia.org/wiki/Far_fieldhttp://en.wikipedia.org/wiki/Free-space_path_loss#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Free-space_path_loss#cite_note-0%23cite_note-0http://en.wikipedia.org/w/index.php?title=Free-space_path_loss&action=edit&section=2http://en.wikipedia.org/wiki/Decibelhttp://en.wikipedia.org/wiki/MegaHertzhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Metres_per_secondhttp://en.wikipedia.org/wiki/Far_fieldhttp://en.wikipedia.org/wiki/Free-space_path_loss#cite_note-0%23cite_note-0http://en.wikipedia.org/w/index.php?title=Free-space_path_loss&action=edit&section=2http://en.wikipedia.org/wiki/Decibelhttp://en.wikipedia.org/wiki/MegaHertz