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Repeaters: how they work and the environment for which they are designed
Repeaters are normally used in fixed situations (buildings) where the signal outside of the building is good but the signal inside of the structure is attenuated or diminished due to the structure of the building (a metal or heavy concrete building or an underground garage for example).
Signal originating at the tower is received by the external antenna from outside of the building, this signal travels through the antenna cable to the repeater where it is amplified, sent out of the repeater through the antenna cable leading to the internal antenna which broadcasts the amplified signal into the building.This signal is then received by the cell phone antenna.
When a phone call is engaged, the signal that is transmitted by the cell phone antenna is received by the internal antenna, this signal travels through the internal antenna cable to the repeater, where it is amplified and sent out through the external antenna cable to the external antenna where it is transmitted to the air by the external antenna and is then received by the tower.
A repeater amplifies the originating external signal and internal signal, in the order of 10,000 to 100,000 or greater than the signal received by the either antenna, however, the resultant amplified signal at either antenna is not very strong because the originating signal is extremely low to begin with, the tower is usually nearby and the coverage inside of the building is relatively small, therefore repeaters usually have high gain which amplifies very low signal many times but the resultant output signal at either antenna is not extremely powerful. In a sense, each of the antennas acts like a microphone and a speaker at the same time.
To draw a comparison to things we can hear one can use this analogy: the phone makes a sound, this sound travels through the air and loses energy (free air loss) on its way to the internal antenna which acts like a microphone and picks up the sound at a very low level like a whisper. The repeater acts like an amplifier and makes the whisper louder and sends it to the external antenna which acts like a speaker that broadcasts the phone's sound to the outside and this sound travels through the air to the tower. The same is the case in the reverse where the external antenna acts like a microphone which picks up the sound from the tower etc.
There is one criteria that always has to be met for a repeater to work; it is imperative that the signal that is transmitted by the internal antenna does not reach the external antenna and visa versa, hence, the antennas require attenuation (separation) from each other. If the signal from one antenna reaches the other, the repeater will oscillate (feedback), this is the same effect as the squeal that can be heard when a microphone is brought too close to a speaker. If this happens with a repeater one cannot hear the oscillation nor can it be detected without instruments and the repeater will simply overdrive itself until it burns out and ceases to function. When the repeater oscillates it will also generate RF noise which will interfere with both the signal coming from the tower to the phone and the signal going from the cell phone to the tower. The RF noise will render the site where the repeater is being used a "non-service" area as long as the oscillation occurs.
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