Tower-Top Amplifiers

The use of tower-top amplifiers has been deployed in numerous communication
sites and is anticipated to be used in the introduction of 2.5G and 3G
also. The use of a tower-top amplifier has occasionally been misapplied in
that the gain exhibited from the tower-top amplifier is added directly to the
link budget. However, the purpose of the tower-top amplifier is to improve
the noise figure for the receive system.
The noise figure is improved by having the first amplification stage placed
as close as possible to the antenna itself, thereby eliminating the loss experienced
due to the feedline that connects the antenna to the rest of the receive
system. The location of the tower top amplifier is shown in Figure 9-6
The tower-top amplifier has to have a minimal gain of 10 dB and,
because the feedline usually has 2 to 4 dB of loss, the additional gain needs
to be attenuated by the insertion of a resistive pad, shown in Figure 9-6.
The typical improvement in the receive path due to the introduction of
the tower-top amplifier is equal to the line loss that would have been attributed
to the feedline, nothing else. The negative issues with tower-top amplifiers include the requirement for power, in DC, to be supplied to the unit
and increased maintenance issues in the event of a failure. Another problem
is the increased system noise due to the amplifier having a less-thanoptimal
receive filter due to the size and weight restrictions imposed with
installing the unit on a tower.