Selecting transmission lines
Oct 1, 2008 12:00 PM, By Mick Bennett
Learn how to pick the most appropriate feeder solution for your specific application.
Rising from inside the transmission equipment room and reaching all the way to the antenna at the top of the tower is a vital link in the broadcast chain — the RF feeder or transmission line. There are various schools of thought as to what feeder solution is preferred out of three basic variants: semiflexible coaxial cable, rigid coaxial transmission line and rigid rectangular waveguide.
Each feeder option differs significantly in construction, electrical performance and maneuverability. And, no single solution is ideal in all cases. The challenge, therefore, is how to select the most appropriate feeder solution for a specific application.
Broadcast transmission sites display a wide range of electrical and physical requirements. Transmitter power levels can range from tens to hundreds of kilowatts. Depending on the length of the run and the frequency of operation, feeder insertion loss constraints can differ greatly, and other electrical parameters, such as VSWR, can assume different degrees of significance. Also, physical considerations, such as whether the feeder is located indoors or outside (or both), wind loadings and space restrictions, all come into play.
Finally, the installation periods associated with the three feeder options vary significantly and need to be factored into the selection process. Occasionally, one issue will be of such importance that it overrides other considerations.
The final selection, however, typically will be based on a combination of the various technical priorities and the associated cost impact. Let's look at different types of transmission lines and some key considerations.
Coaxial cable
In general, it is hard to beat semiflexible coaxial cable for the long run from the equipment room to the antenna at the top of the tower. Although the historical convention in North America has often been to deploy rigid coaxial transmission line, coaxial cable has several advantages, particularly for out-door applications.
The performance benefits attributed to using continuous coaxial cable include low loss and low VSWR. This is mainly due to the lack of points of connection, which would otherwise generate reflections. Sweeping bends in the feeder line are permissible and perhaps even preferred to straight lines. There are no 90-degree bends that can introduce reflections that may occur in rigid feeder systems.
Coaxial cables deployed for broadcasting applications often require relatively high-power ratings. To accommodate the heat that is produced, air dielectric coaxial cable construction may be preferred. At mid-UHF frequencies of 665MHz, 6-1/8in air dielectric cables are typically rated at 60kW. Higher-power versions, constructed with Teflon-based spacers between the inner and outer conductors, can accommodate powers of around 90kW. For lower-power applications, such as repeater sites or indoor routing of the signal between some equipment, foam-dielectric cables are a good solution.
Semiflexible coaxial cable is easy to install.
For outdoor installations, an air dielectric construction requires the use of pressurization equipment to ensure the transmission line is effectively protected against water ingress. This can be handled with bottles of dry nitrogen or, if the line is quite long or leaky, a dehydrator.
The ease of on-site pressurization, and the general lack of significant ongoing maintenance, can add to the advantages of using semiflexible cable for the tower-top feeder run. With no mechanical joints along the length of the cable, the opportunity for air leaks is reduced. This decreases the need for regular maintenance of the pressurization system.
A rigid view
The main alternative to semiflexible coaxial cable is rigid coaxial transmission line, or rigid line. Like conventional semiflexible air dielectric coaxial cables, rigid line has copper inner and outer conductors, separated by polyethylene spacers, with air as the main dielectric medium.
Unlike semiflexible cable, rigid line is installed in short straight lengths — typically up to 20ft — that are connected together at flanged joints with internal couplings to form the required transmission path. A 1000ft line would therefore have more than 50 mechanical joints. A large number of connections can make a rigid line installation prone to reflections and higher VSWR.
Despite this, many broadcasters still deploy rigid feed line to the antenna. Rigid coaxial transmission line has similar attenuation and power handling to feeder cable, but it is uniquely suited to deployment in tight spaces, where the minimum bend radius of cable is too great. Installation, however, is considerably more labor-intensive and time-consuming when compared with a flex cable. Each section of rigid line has to be hoisted for installation and then bolted to the tower using spring-loaded hangers, which accommodate expansion and contraction.
In contrast, semiflexible cable is installed in a single length using a process that is far simpler. A winch is attached to hoisting grips fitted to the end of the cable, which is then hauled up the tower, directly from the cable drum. The coax is then connected to the antenna and mounted on the tower with simple hangers or straps.
When comparing the installation times of the three feed line options, the rule-of-thumb is to allow two days for hauling and terminating coaxial cable. This compares favorably to a one-week allowance for rigid coaxial transmission line installations and two weeks for rigid rectangular waveguide.
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