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411 As system traffic increases,the preclusive effect of In areas of very high cellular traffic density,such what we will call a Phase I base station becomes as major highway interchanges and downtown too great. Several changes must occur as the base areas,even Phase III base stations cannot cope station evolves into a higher capacity Phase II with the demand. Special techniques become facility to cope with growing demand without also necessary if the network is to provide enough precluding channel reuse nearby.First,antenna channels for the extremely high local demand for heights must drop to 100 to 150 feet.Second,ERP service. In a Phase IV base station,antennas must decrease to about 20 to 50 watts per channel. become even more directional,and ERP can Nondirectional antennas give way to directional decrease to only a few watts.The base station's antennas aimed to serve just a slice of the overall service radius shrinks to a mile or less.Sometimes, coverage area(a swath 60°to 120°wide).The even these dramatic changes are inadequate. number of active channels within the entire service Special microcell base stations will serve the area increases,to perhaps 30 to 45,but the number inside of an airport terminal,or a particular inter- in any one sector lessens. In some cases,what we section.Highly directional antennas, 100 feet or will call a Phase III base station will have an ERP less above ground,will focus on narrow swaths of below 10 watts per channel,and as many as 57 area,and ERP will be well under one watt per channels.'5 Phase II base stations must be substan- channel.16 tially closer together,within four or five miles. ANTENNAS AND ANTENNA GAIN directions,the antenna emits little or no power. Any transmitting antenna,when driven by a trans- The more directional the antenna,the narrower its mitter, emits a portion of the power applied to it as main beam,and the higher its gain in that direction. • a radio wave.All practical antennas emit more if you disassemble a flashlight and loosely inter- signal power in some directions than others. The connect the bulb and batteries with wire on your amount of signal power that escapes in a given tabletop,the bulb's filament glows,but not blindingly direction depends on the exact performance char- Let us suppose the filament radiates 50 milliwatts acteristics of the antenna.A simple half-wave of visible light perpendicular to itself,and little off dipole,mounted vertically,is omnidirectional in its ends(like a half-wave dipole).if you pass a light the horizontal plane.At a fixed distance out from meter over the bulb,tracing out a circle at a right the antenna(except in the so-called near field,in angles to the filament;you get a constant reading. the immediate vicinity of the antenna),the radiated The reading decreases if you move the meter to signal toward the horizon is the same at all points measure the light output off the filament's ends. on the compass.By contrast;the vertical dipole is Now reassemble the flashlight The reflector directional in the vertical plane. The antenna emits focuses the bulb's total light output into a tight little power off its ends(straight up and down), beam. The intense,narrow beam might illuminate intermediate power levels at angles in between a spot as well as the 5 watts of light from a 100- straight up or down and out toward the horizon, watt light bulb without a reflector.12 The reflector and maximum signal straight out toward the horizon. provides an optical gain of 100 times,also For some antennas,in a favored direction,the expressed in decibels as 20 dB.13 In the flashlight's radiated signal power can appear to exceed the small cone of light;the light meter's reading is RF power into the antenna,at least relative to the very high.But in most directions,the needle will signal power that a reference antenna(such as a not even budge. The tiny bulb's peak ERP is five half-wave dipole)would radiate when driven by watts, 100 times its total light-power output;but it the same transmitter.How can that be? After all, is nil outside the cone of light formed by the no device can generate more power than the total reflector. The flashlight's peak ERP matches the power applied to it five watts of visible light from the much larger, The answer lies in antenna gain.A directional unfocussed bulb,but only in the flashlight's narrow antenna redirects power that would otherwise beam. The diffuse light from the larger bulb illumi- radiate on certain bearings and concentrates it in nates a much larger area. • others. This redirection of power intensifies the signal level in the favored direction.But in other 9