Any calculator (like the Windowʼs calculator in scientific mode) can by used to perform conversions. For example, consider a 100 mW access point. If you enter 100 into your calculator and press the LOG button youʼll get the base-10 logarithm for 100 (which is 2). Now multiply by 10 and youʼll find that 100 mW = 20 dBm. Going the other way, if you start with 20 dBm you can convert back to mW by using the X^Y function on your calculator (first value, x, raised to the second value, y, power). Enter 10 into your calculator. Press the X^Y key and enter the dBm value divided by 10, which is easy enough since you only have to mentally move the decimal point one place to the left. If

the dBm value is negative use the +/- key to change the sign of the number. Try this out by converting -95 dBm into mW and your answer should be 0.000000000316 mW.

You can also remember that 20 dBm = 100 mW and that +3 dB doubles the mW and -3 dB halves it. Consequently you know that 17 dBm = 50 mW, 14 dBm = 25 mW, and so forth. More importantly, you know that if you measure -85 dBm in one location during a site survey and you measure -91 dBm in another location the second location has only 25% of the signal strength that the first one has. This is because when you go from -85 dBm to -88 dBm the power goes down by half. Going from - 88 dBm to -91 dBm cuts the power in half again, resulting in 25% of the original measurement.

Magnetic Fields

Andre Ampereʼs work in the study of electric currents and magnetism laid the groundwork for todayʼs science of electrodynamics. In 1820 he announced the discovery that the magnetic needle of a compass moved in response to a nearby electric current flow. He demonstrated that the magnetic field moves away from and around a conducting wire in direct relationship to the direction of current flow through the wire. Ampere’s theorem stated the relationship between the strength of a magnetic field and the strength of the electric current producing it.

Magnetic fields develop perpendicularly around an electric current carrying wire (Figure 2.1, below). The perpendicular aspect refers to the fact that the circular, rotating magnetic field is expanding (developing) in a direction that is 900 to the direction that the current is flowing in the wire. They expand as the electric current (the electrons moving in the radiating element) reach their maximum speed. The magnetic characteristic of the field causes electrons to move in a receiving element resulting in the receiver developing an electric current.

Figure 2.1 The Magnetic Field Surrounding a Current Carrying Conductor

Copyright 2003 - Joseph Bardwell