Tech Talk: Coax Isolation on Shared AM Towers
Most people think of an isocoupler as a capacitor or loop pair inserted in a coax line of sufficiently low impedance at the coax frequency to pass that signal without undo loss and VSWR, while looking like a high impedance at the AM frequency so as to not disturb the base impedance too much.
There are a number of serious problems with traditional isocouplers: “lightning gap” failure and burnup, pass frequency case radiation safety and EMI hazards, pass loss and vswr, single frequency pass, significant and unpredictable impedance loading across the base insulator, and no DC pass capability. This made them unsuitable for high reliability, complex, wireline carrier operations.
We developed a standardized system for wireline carriers based upon an inductor formed of coaxial sections. When tuned, the impedance is stable and very high across the AM base insulator. The coax path is uninterrupted and totally frequency insensitive, so can accommodate any frequency, or mix of frequencies, including DC powering (needed for tower top amplifiers and antenna positioners).
CoLoCoil® systems are packaged for ease in adding new cable sets on the tower without major system retunes. The packaging is also such that all AM “hot” functions can be isolated on one side of the fence from carrier functions on the other. All carrier tests can be made from the cold side, so access to the AM “hot” area is not routinely required. CoLoCoil® capacity is typically 3 or 6 coax paths.
The first CoLoCoil®.was deployed on a directional station about 10 years ago. LBA has CoLoCoil® systems supporting colocation on many AM stations, for most all of the wireless carriers. We have had no known failures or station instability problems in that time.
Using the same principal, we have fabricated larger CoLoCoil®s to isolate high power FM and TV coax (3 1/8”+)from high power (100 KW+) AM towers. We also offer quite a popular small unit dubbed the “CAMI™” for single coax low power STL, RPU, or translator use. It is a direct substitute for the traditional isocouplers, but without its negatives.
You can explore more about LBA colocation systems, and our other broadcast products through https://www.lbagroup.com/associates/am-wireless-colocation.php.
LBA would be glad to support any isolation requirement you have. Contact sales@lbagroup.com
