he US the FCC specifies a maximum transmitter power of 1 Watt for FHSS and DSSS transmitters in the 2.4 and 5.8 GHz ISM bands. In the UK the Radio Communications Agency (RA) specifies that these devices must have a maximum effective isotropic radiated power (EIRP) in the 2.4 GHz band of 100 mW or 20 dBm. As described below, EIRP is a combination of the transmitter's power and the antenna gain. Antenna Gain An antenna converts the power from the transmitter into electro-magnetic waves that are radiated to the receiver, and the type of antenna affects the pattern and power density of this radiation, and therefore the strength of signal seen by a receiver. For example, a simple dipole antenna emits radiation relatively evenly in all directions apart from along its axis, while a directional antenna emits radio waves in a narrow beam. Typical radiation patterns of a dipole and a directional antenna are shown in Figure 4-28. The ratio of the maximum power density at the centre of the radiation pattern of any antenna to the power density of the radiation from a reference isotropic antenna is known as the antenna gain (GTX or GRX), and is measured in dBi units. The effective isotropic radiated power, or EIRP, of a radiating antenna is then, the sum of the dBm power arriving at the antenna from the transmitter plus the dBi antenna gain. The types of antenna that can be used in wireless Chapter Four -3 dB -3 dB -6 dB -6 dB -9 dB Figure 4-28: Radiation Pattern from Dipole and Yagi Antennas networks were described in the Section "Wireless LAN Antennas, p. 55" which illustrated antennas with gain ranging from 0 dBi for an omnidirectional dipole antenna to +20 dBi or more for a narrow beam directional antenna. The cables and connectors that link the transmitter or receiver to the antenna also introduce a loss into the system that can range from a few dB to tens of dB depending primarily on the length and quality of cabling. An equally important aspect of transmitter to antenna or receiver to antenna connections, is the matchin