Equivalent isotropically radiated power (EIRP) is the highest RF signal strength that is transmitted from a particular antenna. To understand this better, think of our fl ashlight example for a moment. Let’s assume that the bulb without the lens generates 1 watt of
power. When you put the lens on the fl ashlight, it focuses that 1 watt of light. If you were to look at the light now, it would appear much brighter. If you were to measure the bright- est point of the light that was being generated by the fl ashlight, because of the effects of the lens it may be equal to the brightness of an 8-watt bulb. So by focusing the light, you are able to make the equivalent isotropically radiated power of the focused bulb equal to 8 watts.
It is important for you to know that you can find other references to EIRP as equivalent isotropic radiated power and effective isotropic radiated power.
The use of EIRP in this book is consistent with the FCC definition, “equiva- lent isotropically radiated power, the product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna.” Even though the terms that the initials stand for at times differ, the definition of EIRP is consistent.
As you learned earlier in this chapter, antennas are capable of focusing, or directing, RF energy. This focusing capability can make the effective output of the antenna much greater than the signal entering the antenna. Because of this ability to amplify the output of the RF signal, regulatory bodies such as the FCC limit the amount of EIRP from an antenna.
In the next section of this chapter, you will learn how to calculate how much power is being provided to the antenna (IR) and how much power is coming out of the antenna (EIRP).
Why Are IR and EIRP Measurements Important?
As you learned in Chapter 1, the regulatory domain authority in an individual country or region is responsible for maximum transmit power regulations. The FCC and other domain authorities usually defi ne maximum power output for the intentional radiator (IR) and a maximum equivalent isotropically radiated power (EIRP) that radiates from the antenna. In laymen’s terms, the FCC regulates the maximum amount of power that goes into an antenna and the maximum amount of power that comes out of an antenna.
You will need to know the defi nitions of IR and EIRP measurements. However, the CWNA exam (CWNA-106) will not test you on any power regulations because they vary from country to country. It is advisable to educate yourself about the maximum trans- mit power regulations of the country where you plan on deploying a WLAN so that no violations occur. The transmit power of most indoor WLAN radios varies in a range between 1 mW and 100 mW. Therefore, you usually do not need to concern yourself
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Units of Power and Comparison
When an 802.11 wireless network is designed, two key components are coverage and per- formance. A good understanding of RF power, comparison, and RF mathematics can be very helpful during the network design phase.
In the following sections, we will introduce you to an assortment of units of power and units of comparison. It is important to know and understand the various types of units of measurement and how they relate to each other. Some of the numbers that you will be working with will represent actual units of power, and others will represent relative units of comparison. Actual units are ones that represent a known or set value.
To say that a man is 6 feet tall is an example of an actual measurement. Since the man’s height is a known value, in this case 6 feet, you know exactly how tall he is. Relative units are comparative values comparing one item to a similar type of item. For example, if you wanted to tell someone how tall the man’s wife is by using comparative units of measure- ment, you could say that she is fi ve-sixths his height. You now have a comparative mea- surement: If you know the actual height of either one, you can then determine how tall the other is.
Comparative units of measurement are useful when working with units of power. As you will see later in this chapter, we can use these comparative units of power to compare the area that one access point can cover vs. another access point. Using simple mathematics, we can determine things such as how many watts are needed to double the distance of a signal from an access point.
Units of power are used to measure transmission amplitude and received amplitude. In other words, units of transmit or received power measurements are absolute power mea- surements. Units of comparison are often used to measure how much gain or loss occurs because of the introduction of cabling or an antenna. Units of comparison are also used to represent a difference in power from point A to point B. In other words, units of compari- son are measurements of change in power.
Here is a list of the units of power, followed by another list of the units of comparison, all of which are covered in the following sections.
Units of power (absolute)
■ watt (W)
■ milliwatt (mW)
■ decibels relative to 1 milliwatt (dBm) Units of comparison (relative)
■ decibel (dB)
■ decibels relative to an isotropic radiator (dBi)
■ decibels relative to a half-wave dipole antenna (dBd)