U

Reflection

Reflection

^ Absorption"

Multipath

Primary Signal

^ Scattering j

^ Scattering j

Multipath

Primary Signal

Consider all these phenomena when designing, implementing, and troubleshooting WLANs.

RF Math

KEY POINT

RF gain is an increase in the RF signal amplitude or strength. Two common sources of gain are amplifiers and antennas.

RF loss is a decrease in the RF signal strength. Losses affect WLAN design and are part of our everyday world. For example, the cables and connections between the AP and the antenna cause loss.

WLANs transmit signals just as radio stations do to reach their listeners. The transmit power levels for WLANs are in milliwatts (mW), whereas for radio stations the power levels are in megawatts (MW).

The following are some units of measure used in RF calculations:

■ Decibel (dB): The difference or ratio between two signal levels. dBs are used to measure relative gains or losses in an RF system and to describe the effect of system devices on signal strength. The dB is named after Alexander Graham Bell.

■ dB milliwatt (dBm): A signal strength or power level. Zero dBm is defined as 1 mW of power into a terminating load such as an antenna or power meter. Small signals, those below 1 mW, are therefore negative numbers (such as -80 dBm); WLAN signals are in the range of -60 dBm to -80 dBm.

■ dB watt (dBw): A signal strength or power level. Zero dBw is defined as 1 watt (W) of power; 1 W is one ampere (A) of current at 1 volt (V).

■ dB isotropic (dBi): The gain a given antenna has over a theoretical isotropic (point source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical system operating margins.

The formula used for calculating losses, gains, and power for WLANs is too complex for most people to solve without a calculator. Gains or losses in decibels are summed and then converted into an absolute power in milliwatts or watts.

The following formula calculates the transmit power:

Transmit Power (dBm) = 10 * logi0[Transmit Power (mW)]

Table 9-1 indicates how various gains and losses relate to power levels; it is useful for WLAN calculations.

Table 9-1 dBm-to-milliwatt Conversion Table

dBm

mW

dBm

mW

-3

.5

10

10

0

1

20

100

3

2

30

1,000 or 1 watt

6

4

40

10,000 or 10 watts

9

8

50

100,000 or 100 watts

12

16

100

1,000,000 or 1000 watts

Notice in Table 9-1 that RF math is easier when the following key points are considered:

■ Every gain of 3 dBm means that the power is doubled. A loss of 3 dBm means that the power is cut in half.

■ A gain of 10 dBm means that the power increases by a factor of 10. A loss of 10 dBm means that the power decreases by a factor of 10.

To calculate the power increase or decrease for a given dBm, factor the given number into a sum of 3dBm and 10dBm, and then convert using these rules. For example, a 9 dBm loss is equivalent to -3dBm + -3dBm + -3dBm. The following illustrates how to calculate the power level that a 200 mW signal decreases to when it experiences a 9 dBm loss.

Therefore, the 200 mW signal decreases to 25 mW with a 9dBm loss.

Gain

Although it is probably obvious how losses affect WLAN design, it might seem that higher gains are always better (providing more power at greater distances). However, standards bodies such as the FCC and European Telecommunications Standards Institute (ETSI) regulate the amount of power radiating from an antenna. That power is called the effective isotropic radiated power (EIRP) and is calculated using the following formula:

EIRP (dBm) = Transmit Radio Power (dBm) - cable loss (dB) - antenna gain (dBi)

Antennas

Antennas used in WLANs come in many shapes and sizes, depending on the differing RF characteristics desired. The physical dimensions of an antenna directly relate to the frequency at which the antenna transmits or receives radio waves. As the gain increases, the coverage area becomes more focused. High-gain antennas provide longer coverage areas than low-gain antennas at the same input power level. As frequency increases, the wavelength and the antennas become smaller. Antennas can be categorized into one of the three following types:

■ Omnidirectional: These antennas are the most widely used today but are not always the best solution. The radiant energy is shaped like a doughnut; consequently, the transmit signal is weak or absent directly under the AP (in the "hole" of the doughnut).

■ Semidirectional: These antennas offer the capability to direct and apply gain to the signal. The radiant energy is in a cowbell shape.

■ Highly directional: These antennas are intended for highly directed signals that must travel a long distance. The radiant energy is in a telescope shape.

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