Tutorial on Wireless LAN Antennas
(continued)

Radio Math

In the simple block diagram (Figure 1), we have a transmitter (xmit) with Transmit Final Output power (TFO) line losses and antenna gain. The math is relatively simple. Gains are added to the TFO, and losses are subtracted to give us an Estimated Isotropic Radiated Power (EIRP).

Figure 1  System Block Diagram

In radio theory, we use milliwatts, watts, or kilowatts; however, the numbers can get rather big and unmanageable, and so many engineers convert watts or milliwatts into logarithmic references. It does not matter which base you choose -- whether it is milliwatts, watts or kilowatts -- the formula is the same: dBy = 10 LOG (power in y).

If you work in milliwatts, the expression is dBm. If you work in watts, the expression is dBW. With Kilowatts, the expression is dBK.

A "generic" look at the math in Table 1 shows the power in watts (y) as converted to dBy, where y can be m, W, or K. What is important to note in this chart is that when power doubles, the gain is 3 dB.

Table 1: Generic Power-to-dB Chart

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