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.
High-power alternative
For higher-power applications of up to 200kW, rigid waveguide provides another RF feeder option. Comprising a rectangular aluminum waveguide cross-section, it is typically used as a single run to the tower top in place of a pair of parallel cables. Notwithstanding the lack of redundancy, the main advantage to waveguide, in addition to the power rating, is its remarkably low insertion loss. This is typically around 0.0007dB/ft, compared with values of 0.15dB/ft for 6-1/8in semiflexible cable, at the mid-UHF frequency of 665MHz.
The low insertion loss of rectangular waveguide, however, comes at a cost, both electrically and physically. Bandwidth and VSWR rigid-line performance are typically inferior to both coaxial cable and rigid line. Importantly for digital TV transmission systems, which are sensitive to nonlinear phase distortions, the group delay response of waveguide is also inferior. Additionally, installation and maintenance practices are far more complex, time-consuming and costly.
Even so, the most practical application of rectangular waveguide is for extremely long runs and/or very high powers, where minimizing insertion loss is of prime importance, and the reduced performance in other areas is regarded as secondary.
Rigid line indoors
Indoor applications, where pressurization is unnecessary, often rely on unflanged rigid line. This line is mainly used for RF connections between transmitters, filtering equipment, combiner chains and switch frames, and for the link to an external wall where the antenna feeder is connected. The transmission path often comprises short sections and multiple 90-degree bends. These applications are far more easily resolved with rigid line than semiflexible coaxial cable because of semiflexible cable's inability to handle small radii bending. This is especially the case where high powers (and accompanying large diameter cables) are concerned.
The advantages are multiplied when space is at a premium. Using 90-degree and 135-degree elbows to achieve bends, unflanged rigid line is the most flexible option for tight indoor applications. Moreover, the unflanged rigid line is easily handled on-site. Once the various transmission path lengths have been determined, the rigid line is easily cut and joined, even in confined spaces.
Joining the sections of rigid lines indoors is a simple process. Instead of a braised- or soldered-on flange, the join is achieved by means of a bullet-style inner connector and an external sleeve, held in position by hose clamps. If the system needs to be relocated as the transmission site grows and evolves, the connections are easily removed, adding an element of future-proofing to the installation.
Cable selection
Each of the above cable options has advantages in particular situations. The choice of which feeder solution to select is largely dependent on the transmitted power, space availability at the site, whether the deployment is indoors or out, and other project-specific requirements.
In general, however, a good rule-of-thumb is to consider using semiflexible coaxial cables for the run up the tower and rigid line indoors. Such a solution provides a good combination of features and benefits. It has the advantage of lower cable-to-tower mounting cost while still providing ease of installation in small transmitter room spaces.
Mick Bennett is global product manager, broadcast and defense systems, Radio Frequency Systems.
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