Tower Top Amplifier Design with Improved Failsoft Performance (03-Feb-2003)
A tower top amplifier (TTA) can be used to improve base station receiver sensitivity by compensating for heliax cable losses. Should a failure occur, such devices are difficult to access and repair due to their mounting location at the tops of tall structures. Some TTA designs include an automatic bypass switch in case of amplifier failure. Although system performance is severely impaired by bypassing the amplifier, this prevents total loss of receiver operation. This paper describes a design enhancement that compensates for a TTA failure by switching in a redundant preamplifier located in the equipment cabinet at the base of the tower. With this improvement only a minor degra-dation occurs to receiver performance.
Tower Top Amplifier Design with Improved Failsoft Performance
By William H. Cantrell, Ph.D.
ABSTRACT
A tower top amplifier (TTA) can be used to improve base station receiver sensitivity by compensating for heliax cable losses. Should a failure occur, such devices are difficult to access and repair due to their mounting location at the tops of tall structures.
Some TTA designs include an automatic bypass switch in case of amplifier failure. Although system performance is severely impaired by bypassing the amplifier, this prevents total loss of receiver operation.
This paper describes a design enhancement that compensates for a TTA failure by switching in a redundant preamplifier located in the equipment cabinet at the base of the tower. With this improvement only a minor degra-dation occurs to receiver performance.
PROBLEM
Tower top amplifiers are sometimes used to maximize receiver sensitivity by placing a low-noise amplifier (LNA) within a few feet of the antenna. This improves receiver sensitivity (at great expense) by preventing heliax losses from playing a significant role in establishing the system noise figure.
In practice, it is difficult and expensive to access TTAs at the tops of towers. It can be hours or days before a tower crew can be assembled to make repairs.
Some TTA designs may include automatic switching to activate a back-up amplifier or (if that fails) bypass all failed amplifiers. System performance is severely impaired when a failed amplifier is bypassed because there is inadequate gain in the overall system lineup. Bypassing is still preferable to doing nothing, as a failed amplifier will cause a severe degradation to the system noise figure by introducing several dBs of additional loss. Bypassing a failed amplifier prevents total loss of receiver operation. It is very desirable to compensate for the lack of gain when a failed amplifier i...