Shadow Studies and Longley-Rice

Q: Why does my coverage using a Shadow Study look so different from the same area using the Longley-Rice Propagation model?

A: The shadow study and Longley-Rice are a little bit of apples and oranges. While Longley-Rice computes actual field strength values, the shadow study is only looking at line of sight. And depending on how you define "coverage" the results from the two approaches can look very different.

Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.

The shadow study and Longley-Rice are a little bit of apples and oranges. While Longley-Rice computes actual field strength values, the shadow study is only looking at line of sight. And depending on how you define "coverage" the results from the two approaches can look very different.

For Longley-Rice (or any of the propagation models such as Bullington), you can define the "required field" value - how much signal is enough?

That will depend on a number of factors such as the mobile hardware, but also what environmental effects you want to consider (building losses, body losses, etc.)

You can define various levels to display, so that different parts of the display indicate various levels of coverage (such as good coverage if the losses are minimal, or good coverage even under situations with a lot of loss)

Shadow studies, on the other hand, don't compute an actual field strength (that can be translated to a power input level). Instead, shadowing computes a ratio of how clear or blocked the path is for the line of sight and Fresnel zone:

The shadow study gives a relative measure of the path from the base station to different locations in the coverage area - the signal to clear locations is going to be better than the signal to blocked locations.

But if you are looking at the "coverage" for a shadow area defined by clear or grazing paths, the results will be much more limited (usually) than any propagation study, especially if you are working in VHF or lower UHF bands. The reason is that until you get to higher UHF or microwave, most of the propagation models will show some coverage even in areas where the Fresnel or even the line if sight are obstructed, since rf energy can be diffracted over minimal obstacles. So a location that shows "blocked" line of sight in a shadow study may show a usable signal level based on Longley-Rice.

Shadowing studies are useful for site selection (set up a hypothetical fixed facility at a desired coverage location, then run shadowing to see where there are possible sites in the area that will provide the best line of sight). Shadow studies are also sometimes used for ground-to-air studies, where the mobile antenna elevation (e.g., 30000ft MSL) is beyond the nominal parameters of most of the propagation models (for example, Longley-Rice specifies an antenna height limit of 1000 meters). In earlier versions of TAP, shadow studies were also faster than most of the propagation models, so running several shadow studies was a handy (and quicker) way to evaluate several potential repeater sites (for example) than running the same studies using Longley-Rice.

In general, you will get more definitive results for the actual radio performance using the Longley-Rice model.

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