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Archives
 April 1999 Issue
Reference
Training
Evaluating Multiple Outlet Requrements, Part 1
By the NCTI
This months installment begins a series on evaluating the requirements for multiple outlets. The material is adapted from a lesson in NCTIs Installer Course. © NCTI.
Customers now are requesting more cable outlets within a single premises. Systems are designed to provide an adequate signal level at each tap port for a certain number of outlets at each customer premises. Beyond that, to provide adequate signal level to each outlet/wall plate, an auxiliary amplifier (house amp) is needed. Before connecting multiple output cables to a splitter, consider whether there is adequate signal level for the number of outlets requested. (Federal Communications Commission regulations require the signal level of any video carrier at the input to the customer premises equipment to be no less than 0 dBmV.)
The number of outlets/output cables to be installed at a premises determines the number of output ports on the RF signal splitter(s). Similar to coaxial cables, RF splitters reduce the signal level passing through them. The following presents a method to determine if the input signal level is adequate for the number of planned outlets, then provides approximate loss values for various splitters and splitter combinations that can be used to easily make rough estimates in the field.
Calculating outlet signal levels
To determine if the input signal level is adequate for the number of planned outlets: 1) connectorize the drop input cable, 2) measure the input signal level with a signal level meter at the highest channels frequency, 3) estimate the required cable length to the furthest outlet, 4) calculate the cable loss at that frequency, 5) add the cable loss to the splitters or combined splitters loss for the total loss, and 6) subtract the total loss from the measured input level. (See Figure 1.) If the result is below the minimum for your system-specified signal level for an outlet, install an auxiliary house amp. Because minimum cable outlet/wall plate signal level and auxiliary house amp installation policies vary, follow your systems procedures.
Estimating effects of two-way splitters
The amount of signal level attenuation a splitter causes, known as insertion loss, varies slightly with the signal frequency, number of output ports and manufacturer. Remember that a two-way splitter lowers the input signal level by about 4 dB at each output port (Figure 2) at frequencies between 50-550 MHz (slightly more at higher frequencies). Figure 3 provides examples of typical loss specification for two-way splitters. Check manufacturer specifications for the exact insertion loss of the splitters in your system. TB
The next installment will provide approximate insertion losses in three-, four- and eight-way splitters.
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