Unless subjected to damage or unusual stress, runs of foam or air dielectric semi-flexible transmission line will typically require minimal maintenance over their operational lifetimes. Although not always performed, replacement of hardware, O-rings and bullets every few years is a wise investment. These changes allow for easy visual checks at connection points, which tend to be the weakest points in the system.
Similarly, rigid transmission lines require maintenance, although to a somewhat greater degree depending on the design. Because steel and copper expand and contract at different rates, a mechanism is employed in rigid line sections to address this issue. Although there are some bellows designs where the center conductor itself expands and contracts, most designs use a watchband spring. In these types, the watchband spring, so colloquially named because of its appearance, is the contact point between the center conductor and bullet, and over time with continued expansion and contraction, suffers a reduction in its surface area. The decrease in the surface area of these springs results in gaps between the spring and the center conductor leading to the potential for arcing. Arcing, of course, is the leading cause of burnouts in transmission line.
Depending on with whom you talk, the replacement cycle on watchband springs and associated hardware varies. I have heard time frames from as long as 15 years to as short as seven years or so. Replacement every 10 to 12 years coupled with biennial or triennial sweeping of the system should catch better than 95 percent of non-malicious events, thus limiting unplanned off-air time and collateral damage.
In the case of open wire line, maintenance is really quite simple, although it can be costly as are the installation costs, hence its minimal popularity. Typically, as long as there are no damaged insulators, support structures, breaks in the wires, or large amounts of corrosion or dirt present, this type of line will be in good shape. Due to the style of construction, visual inspections are quite simple to perform.
Finally, in the case of air dielectric lines, pressurization is a must. If pressurization is not used, the normal “breathing” of the transmission line at the joints and connections coupled with ambient temperature and humidity changes will allow for the ingress of water into the line. A minimal amount of water may have a substantial effect on the return loss of transmission line and ultimately can lead to failure via a variety of mechanisms. It cannot be stressed enough that air dielectric line with a substantial leak is a ticking time bomb that must be fixed as soon as possible.
The bottom line is that without maintenance, transmission line will ultimately fail. The good news is that with regular and periodic inspections, the likelihood of unplanned failures is greatly reduced. These inspections, however, are really a team effort between the transmitter engineer, tower crew and consultant. The seemingly large investment at the time amortized over several years really becomes a pittance when the number of hours in a year is considered, and is certainly far less than the incurred expense with catastrophic failure.
-- More examples of transmission line damage are on page 3.
Ruck is a senior engineer with D.L. Markley and Associates, Peoria, IL.