LMR system tower site selection is fraught with challenges
Posted on November 9, 2022 by Alicia Trotman
In theory, selecting where to place land mobile radio (LMR) system towers is a straightforward and easy endeavor — simply choose the location that will deliver the largest signal coverage footprint. But in reality, site selection is anything but easy.Coverage depends on several factors. One is signal strength, which mostly is a function of the LMR system’s infrastructure and design. Another is propagation, which is defined as what happens to a radio signal as it travels from one point to another through a vacuum. But radio signals travel through the air, not a vacuum --- and there’s a lot that can affect what happens to the signal as it travels from transmitter to receiver.
Atmospheric conditions are a factor. So too is the frequency band. For instance, VHF signals travel farther than UHF signals, but the latter, given their shorter wavelength, penetrate buildings better. Speaking of buildings, they and other obstacles, such as foliage, will get in the way of radio signals — literally, which affects not only how far they travel, but also their quality when received. Finally, antenna height affects how far a signal travels — the higher the better.
The optimal situation then is a very tall tower — ideally one situated on a mountaintop, or at least a hill — that has clear line of site. But even when the ideal location is identified, the public-safety agency isn’t out of the woods yet. Here are just a few things that can go awry:
- The location is in an area that has been zoned residential, which will preclude tower construction; if that’s the case, the tower only can be constructed in areas that have been zoned commercial or industrial.
- The county or municipality might have ordinances that limit the type of tower that can be constructed; for instance, towers that are latticed or require guy lines might be prohibited and tower height might be limited.
- The NIMBY factor — even when a tower is allowed in a residential area, residents might not want it close to their homes (i.e., “not in my backyard”), schools or parks/playgrounds. And residents can raise quite fuss.
- Soil composition often is an issue, i.e., the soil is unable to support the tower’s weight without engineering solutions, e.g., the sinking of pilons, that could make the site cost prohibitive.
- Environmental issues sometimes are a problem too. For example, digging into a brownfield site, i.e., one where toxic chemicals are present in the soil, typically is prohibited by county and municipal governments for fear of contaminating air and water in the area surrounding the site. Tower sites that are near fault lines, in flood plains, near wetlands, or near airports also are problematic.
- In addition, the selected site must have a footprint that is adequate to accommodate the tower and its supporting infrastructure, e.g., commercial power and an equipment shed to house the electronics. Towers sometimes collapse for all sorts of reasons, so the site also must accommodate an adequate “fall zone,” especially if the tower is near a school. The zone should be proportional to the tower’s height, e.g., a 300-foot tower needs a zone with a 300-foot radius.
- Finally, the site might be subject to restrictions pertaining to tribal and historical site preservation.
Any of these factors can derail an LMR system implementation, but often more than one exists, which is even more vexing. But all is not lost — several mitigation strategies exist as well, which a future blog will dig into.
Alicia Trotman is an MCP technology specialist.
Email her at AliciaTrotman@MissionCriticalPartners.com.