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-   -   Longley-Rice vs Pathloss diffraction algorithm (http://pathloss.com/forums/showthread.php?t=461)

JoelW 07-22-2008 07:19 PM

Longley-Rice vs Pathloss diffraction algorithm
 
I just compared a challenging UHF path in Pathloss with the same path in Radio Mobile. Pathloss showed it as being a good path, with about a -81 dBm Rx level. Radio Mobile instead showed a -100 dBm level. Radio Mobile uses the Longley-Rice algorithm, no choices. Pathloss by default uses the "Pathloss" algorithm.

The difference between these two is dramatic, enough so that if I were to recommend this path (trusting Pathloss) I'll guess that my customer would be very unhappy with me. There's a 200 foot tall hill about 80% of the way along the path that sticks up like a sore thumb and is the cause of the trouble.

Two questions:

- Does anybody have a good reason for why the two calculations would provide such dramatically different results? (19 dB!!)

- Should I be running all my paths using two or three algorithms, and then using the worst-case result? How do I decide which to go with?

Yes, I have read the brief discussion on this topic in the Pathloss manual. It's so short that it's clearly meant for people who are already path study experts. What I need is some good clear discussion on the topic. (that hopefully won't require me to lock my office door for 6 months and study the topic at a PhD level)

Joel

ImranSabir 07-23-2008 06:14 AM

On obstructed paths, the Longley-Rice irregular terrain algorithm estimates the loss using the weighted average of the smooth earth loss and the loss of a double knife edge. The weighting factor is based on empirical data and considers the following parameters:


terrain roughness effective antenna heights
frequency horizon angles

For smooth paths, the loss approaches the smooth earth value. For very rough paths the loss approaches the double knife edge value. The Pathloss automatic diffraction algorithm defaults to irregular terrain when the path cannot be characterized as a single or multiple isolated obstacle(s).

Lars H-L 07-24-2008 11:44 AM

Which diffraction method/algorithm to use depends on the path in question. From the sparse information you give, it seems to me that you should use the isolated obstacle algorithm. This method accounts for the rounding (profile radius) of the obstructing hill, which can be set independently.

If you need a reliable (final) value for the diffraction loss and its variability a field radio test may be the only way.

JoelW 08-18-2008 06:32 PM

All well and good, BUT...
 
Unfortunately I spend much of my time doing other work. I'm not a PhD candidate, nor do I have a day or a week to spend on each and every path. Typically I actually spend an hour or less on each path.

This makes it very difficult to be spending much time performing complex analyses of the terrain. That's why I bought the software, and I was really hoping it would be capable of performing a reasonably accurate analysis after it studied the terrain. What else is it for, after all? Is Path Loss just an expensive HP calculator?

If anyone knows of a website or document which discusses the various algorithm choices in layman's terms I would greatly appreciate hearing about it.


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