Providing for Coax Replacement, Additions
Rarely is a collocation tower dressed with lines and antennas, then forgotten. Rapid technology changes in carrier systems result in almost routine changes or additions of antennas and lines. The isolation method and infrastructure deployment must give maximum flexibility to the carrier, and keep tower crew workloads to a minimum. The old quarter wave stub isolation method, used for years for its simplicity in isolating a single coax, fails seriously when confronted by modern cable installation requirements, and is rarely, if ever, employed today.
Isolation Device Considerations
Modern carrier systems often employ tower top amplifiers and antenna positioning devises powered over the signal coaxial cable. Devices such as isocouplers do not pass DC or AC, and thus have limited utility. The isolation system should be designed for end-to-end DC and AC connectivity, for which devices like folded unipoles and ColoCoils are well suited.
Because of the wide range of frequencies employed by carrier systems (700 – 3600 MHz), and stringent VSWR specifications on the system, it is preferable to have no cable discontinuities. Devices, such as isocouplers, that are tuned to a pass frequency seriously limit broadband carrier installations. The most desirable installation is on a folded unipole tower where no interruptions in the coaxial cable are required at all.
Folded Unipoles can Improve Bandwidth
Folded unipoles are time tested in their ability to both match an AM tower, and make it “cold” at the same time. Towers with folded unipoles installed may be ready for tower collocation with little, if any work. In many cases, the folded unipole will need to be replaced with one that is built specifically for tower collocation. These systems employ wires that are spaced and insulated in such a way as to allow safe tower climber access. They are also built to accommodate clearance to multiple coaxial lines and ice bridges.
Non-directional series-fed towers can often be converted to folded unipoles with only replacement of the antenna tuning unit. The complexity and economics are a bit different with towers in a directional array, however. Since the folded unipole significantly alters the impedance and phase matching conditions at the base, you will likely need a new ATU to accommodate these parameters. This will usually require redesign by your consulting engineer, retuning of the array, and a partial proof of performance.
The upside of a unipole is not only isolation for unlimited coaxial lines, but better bandwidth and lightning protection. It also eliminates the need for tower lighting and sample loop isolation.