EIA TSB 179-A:2018

In the land mobile radio service there has historically been no clear relationship between the VSWR specifications of the individual components in the RF path and the expected resultant 'system' VSWR at the bottom end of the transmission line. This makes it difficult, at best, for system operators to set a realistic set of measurement expectations for installers and technicians to use for checking completed installations Various calculators and nomographs have been developed over the years to try to estimate what the expected system VSWR ought to be for the total RF path, but, at best they treat the combination of the individual component VSWR vectors as if they combine in a random fashion. This might be true most of the time, but there might be those instances where random vector combinations do not happen. This Bulletin presents some of the nomographs and calculators that have been employed over the years and introduces an improved calculator that shows both typical and worst case vector summation. It also presents some theoretical reference material detailing how the VSWRs of specific components might combine. [1]. The use of manufacturer's names is not an endorsement, rather an acknowledgement to the source of the material in this Bulletin.