When selecting a cellular amplifier there are many factors to consider. Two key specifications are amplifier gain and output power.
Cellular Amplifier Gain
Cellular amplifier gain is the number of times the power into the amplifier is increased at the output of the amplifier. Amplifier Gain is expressed in dB’s. (ex: 10 dB is a gain of 10 times, 20 dB is a gain of 100 times). For example, 1 watt into an amplifier which has 10 dB gain puts out 10 watts.
What if the amplifier is only capable of 3 watts output?
The amplifier is then over-driven. Too much input power results in 3 watts of distortion, just like yelling into a microphone and distorting a hi-fi system. The same happens to the cell phone amplifier, which causes it to interfere with adjacent channels on the cell site.
The maximum power that can be applied to a 10 dB 3-watt amplifier is .3 watts, which gives 3 watts of undistorted output. A cellular device (phone or datacard) connected to a direct connection amplifier with normal cell phone maximum power (.2 watts) going into the amplifier and a gain of about 12 dB gives 3 watts of undistorted output power to the cell site. Any more input power would not increase the output, but only distort it, which causes harmful interference. A gain of 15 times is 11.8 dB.
Why do we need higher gain amplifiers?
An example is a 40dB, 3-watt cellular amplifier. We need more gain because when the cell phone has to transmit to and receive from the amplifier’s inside antenna (which is about 1-2 feet from the cell phone), the signal is much weaker into the amplifier than the .2 watts that was going directly into the direct-connection amplifier. The signal is now approximately 660 times weaker when it enters the amplifier’s input. It just so happens that a 40 dB gain amplifier will put out 3 watts with that input.
Different Gain for Different Applications
You can see that different gains are needed for different applications. Also, in an automobile situation with the outside antenna on the roof of the car and the inside antenna near the headrest, 40 dB is generally the maximum gain before the amplifier starts oscillating. A good cellular amplifier will shut down once oscillation occurs as to not cause harmful interference with other users on a cell tower.
With in-building applications, the cell phone is much farther from the inside antenna; therefore, a higher gain amplifier is needed. A 50 to 65 dB gain will generally give good coverage in most building applications dependent on outside signal strength. Amplifiers with higher gain require much greater antenna separation. For example, a 72 dB gain amplifier requires 4 times more antenna separation distance than a 60 dB amplifier.
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