It is fairly easy to find online path loss calculators (or versions built into downloadable spreadsheets). A person designing a radio system might think that they've found a great resource when they stumble across one of these calculators. Such tools can be useful, but can also be very misleading and possibly dangerous.
This article will show first an online calculation of a typical path, and then a calculation using a well known professional software package.
First, take a look at this typical online calculation page. It shows values for a seemingly simple real-world path. Typical numbers are used:
|Center Operating Frequency:||915 MHz (902-928)|
|Path Length:||1.4 miles (2.24 km)|
|Transmit Power:||30 dBm (1 watt)|
|Transmit Cable Loss:||3 dB (incl connectors)|
|Transmit Antenna Gain:||7.15 dBi (5 dBd)|
|Receive Antenna Gain:||12.15 dBi (10 dBd)|
|Receiver Sensitivity:||-108 dBm (K Series at 32 kbps)|
|Receive Cable Loss:||2 dB (shorter cable)|
The calculated receive sensitivity sounds great. At -54.5 dBm this would theoretically provide a fade margin of 53.5 dB, for presumably a “bullet-proof” path. (most paths only need a 20 dB fade margin to be reliable) But on this path, the reality is much different, as the following graphics will show.
Below is a profile of the same path, generated from a terrain database using a professional path loss calculation package. Included are approximations of trees (green vertical lines) and buildings (grey vertical lines) as seen on an online map website. Discussions with the customer provided approximate tree heights.
It can now be seen that certain factors were not considered when running that simple online calculator. The trees, buildings and terrain can all get in the way of the radio signal, unless very tall towers are used. In this case a 15 foot mast was used for the site on the left (Well #3) and a 35 foot structure for the master site on the right. (WWTP)
A path worksheet generated by the professional software package is shown below. It clearly includes many more details than the online software does, but all of the input parameters included above are the same. The key item among them is called Diffraction Loss. In this example, you can see that the Diffraction Loss is 30.76 dB. This is a lumped value which includes path degradation for each hill encountered, and for each block of trees or buildings.
This huge difference in path losses takes the fade margin down to just over 22 dB. While this is still a workable path, (anything over 20 dB is usually ok) it does clearly show the need for the antennas and structures specified. If a lower gain antenna or shorter tower were used, or if the path length were much greater, the fade margin might easily drop well below 20 dB.
In order to get completely above the trees, considerably taller structures would be required. The red line shows the “line of sight” path. It clearly pushes through obstacles much of the way. The blue line shows something called the 60% F1 zone. Technical knowledge beyond the scope of this document would be required to fully explain Fresnel zones, but suffice it to say the following:
- To avoid any path losses due to diffraction around obstacles or absorption in trees & buildings, the blue line (60% of the F1 zone) would have to remain entirely above any of the obstacles seen in the graphic. Only then would the simple online calculator be accurate, as there would no longer be any diffraction losses.
Typically, only microwave paths are designed to avoid all obstacles. The higher in frequency a radio signal is, the less it is able to diffract around obstacles, and the more it is absorbed by trees and buildings. Such system designs will be much more expensive, as taller towers are needed. In this case, instead of 15 and 35 foot structures, 70 foot structures would be required!
In order to design a reliable radio system when sources of diffraction or signal absorption exist, a professional path study should be performed. This study may not be free, and it may require several days or weeks to complete. It will however result in a system which will be brought online with minimal changes required, and which will remain highly reliable in poor weather or with significant degradation due to equipment maintenance issues.
Simple free space path loss calculators do have their place. They can give an idea of whether a path is theoretically possible at the specified frequency and distance, and they will work well for paths with no obstacles. But before moving on to build a system, such simple tests should be followed up by a path analysis that includes real world factors such as terrain, trees, buildings and other obstacles.