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Chapter 4: Real World Path Loss Estimation

Another topic to consider is that the “Free Space Path Loss” calculations presented in this section presume that you are transmitting across a perfectly open space, with no terrain, obstacles, signal reflections/multi-path or other problems. In the real-world, this is very rarely the case!

Another topic to consider is that the “Free Space Path Loss” calculations presented in this section presume that you are transmitting across a perfectly open space, with no terrain, obstacles, signal reflections/multi-path or other problems. In the real-world, this is very rarely the case!

There are a number of algorithmic models available to better represent radio path-loss. These include “HAAT”, “Cost-HAAT” and “Longley-Rice Irregular Terrain Model”. The latter is freely available, and provides useful data in a timely fashion. Longley-Rice is generally referred to as the ITM model.

In order to produce an ITM plot, one needs to provide the altitudes of the transmitter and receiver, the altitudes (and number) of many points in between the two points, the frequency of the transmission, and a few variables defining climate, air conductivity and others. These are then run through an iterative algorithm that produces an estimate of actual path-loss across the defined terrain/atmosphere characteristics, as well as an “error-mode” that explains which of several types of calculation were applied (and provides a handy reference as to whether or not you have line-of-sight, or if the signal clipped a “horizon” in passing).

A complete discussion of the implementation of Longley-Rice could easily fill a small book (that book is available here: http://www.its.bldrdoc.gov/resources/radio-propagation-software/itm/itm.aspx ), and is far beyond the scope of what a WISP operator needs to understand. It is used by tools such as Radio Mobile and Splat! to determine coverage, so at the very least it is a good idea to be aware of its existence.

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