Now the real work begins
The previous articles in this series have focused on how to effectively plan a land-mobile radio system. It now is time to discuss the implementation and integration of such systems.
The distinct purpose of an LMR system is to provide radio coverage for mobile and portable radio units and fixed base or control stations, over a certain geographic region. If the purpose of the system is to cover a very large geographic area, typically countywide or statewide, and only mobile units or control stations are to be deployed, then sites that have very high antennas throughout the area will be used.
However, if the system will be used by portable radios inside of buildings in an urban environment, then multiple sites utilizing lower antennas and located in the heart of the areas to be covered will be required. The sites that are chosen will have the most influence on the range of the system.
The sites can be guyed or self-supported towers, or monopoles. Antennas also can be mounted on top of tall urban buildings or on water tanks. Regardless of configuration, it is important that a site has primary power available for the system, as well as backup power if the system cannot be off the air during power interruptions or failures. The backup power can be in the form of batteries and chargers, inverters, generators and transfer panels, or fuel cells. Many sites use a combination of these solutions. Some sites, such as mountaintop sites, will use batteries and photocells as the only power source for the site.
In addition, the site will need the following:
- Grounding to help protect it from lightning damage.
- Sealed containment to keep insects, mice, snakes, and other predators out.
- Fences and restricted access to keep out thieves and vandals.
- Fire protection.
- Environmental heating and cooling.
- Enough space to accommodate all of the equipment needed to operate the system.
When trying to find a suitable location for the site, an engineer will perform a site survey to ensure that all of the pieces will fit. Pictures, drawings, equipment-layout diagrams, and myriad other items need to be recorded for each potential site to ensure that everything is in order. A frequency and antenna inventory also needs to be completed, in order to conduct an interference analysis.
Site surveys can take anywhere from two hours to a full day to complete. The information they generate is used to prepare detailed engineering drawings that are used in the structural analysis of the radio tower pursuant to its location. These drawings often are attached to the legal documents related to the leasing or ownership of the tower and associated floor space. A site-survey kit should include; digital camera, GPS device, distance meter (either mechanical or laser), binoculars or field glasses, digital voice recorder, and a notebook.
Once a location is chosen, then radio engineering can be performed for the site. This will include determining the required effective radiated power and transmitter power, the antenna type and orientation, the transmission-line parameters, and the receiver-sensitivity requirements for the site. This information is needed for the coverage studies that ultimately will determine whether the site is suitable for the LMR system, or what needs to be done to make the site suitable. For example, many sites will need tower-top amplifiers for the receiving side of the system.
Some sites will require special filters on the transmitters and receivers to prevent interference. All of these are factors to be considered when preparing a site survey.
In addition to the system’s main channel parameters, consideration must be given to the backhaul part of the system. This can range from a simple leased telephone line back to the central transmitter or control point, or a microwave or satellite system that requires more equipment and engineering than the primary system. Backhaul has important implications on site construction due to the potential power and space requirements.
Placing a new system into an existing radio site has certain advantages, as many of the necessary components already are in place. So, the addition of another radio system is less costly to implement and integrate. The disadvantage of putting a new system into an existing site is the radio frequency interference (RFI) that is present at all sites. The RFI can be caused by transmitter noise, high noise floor, intermodulation, harmonic energy, spurious transmitter energy, or any combination of these. The LMR industry rule of thumb is that the last user to apply to integrate radio equipment at a site is responsible for interference.
Numerous tasks must be performed when attempting to develop a new site. First, the site must be located and surveyed, and the land owner or building manager will need to be contacted regarding a suitable lease or purchase deal. Before you lock into such a deal, make sure that antennas and towers are allowed on the site; be especially vigilant in determining whether local zoning or land-use covenants prevent or limit the deployment of such equipment. Next, you will need to ensure that there is full access to the site so that the equipment can be brought in, and that your maintenance people also can have access to the site, typically on a 24/7/365 basis.
Many sites require environmental impact studies that address the special handling of any trees and other plants that will need to be relocated if such items happen to be in the way of the new site. There also are endangered animal and historic zones to consider. Some sites will require that you have enough land area so that, should the tower fall, it would land on the property that you own or lease, and not on any neighbor’s adjoining property.
During the zoning process, do not be surprised by the neighbors objecting to the eyesore impact of the tower, or by their fears that the radiation emanating from your site will cause various cancers in them. The NIMBY (not in my back yard) factor always comes up in new sites, and neighbors may need to be educated. Due to the plethora of factors involved in a new site development, it is prudent to perform the radio engineering before you go through the trouble of site acquisition. It also is prudent to have identified alternate locations, in the event that the neighbors succeed in their efforts to stop you from completing the site.
Once a site is secured and the radio engineering confirms that the coverage will be sufficient, you will need to apply for FCC (or other regulatory entity) authorization. In the United States, an LMR system application first must be submitted to a frequency coordinator for approval. The coordinator confirms the following:
- That the chosen channels are correct for your location and service purpose.
- That they match the assignments of the frequency steps.
- That they meet the power and height limitations of the channels.
- That no interference issues exist regarding co-channel users or adjacent channel users that are in close proximity to the site.
After the coordinator approves the location, Form 601 will need to be filed with the FCC. It also is feasible that the FAA will need to be notified.
If the tower is less than 200 feet in height and more than 20,000 feet from the closest airport runway, then you only have to deal with local zoning and building-code issues. However, if your tower is more than 200 feet tall or within 20,000 feet of an airport runway, then you have to apply to the FAA before you can proceed with any part of the tower construction. In addition, you have to ensure that the tower conforms to painting and/or lighting standards per the FAA rules and regulations.
For these reasons, many system operators purposely keep their towers under the 200-foot threshold in order to avoid having to comply with the FAA’s rules and regulations. Some voluntarily light or paint their towers even though they are not required to do so; it is important to note that those who do still are obligated to follow the relevant FAA rules.
Clearly, there are many factors and issues to consider above ground when deploying a new LMR site. However, the issues below ground are nearly as formidable. During the excavation phase, you will need to be vigilant that no utilities are cut. You also will need to take core samples of the soil and have them analyzed so that a licensed engineering firm can tell you how much concrete you will need for the tower foundation. Once you pour the concrete into the foundation, you will need to perform concrete strength tests to determine whether the mixture is correct. Finally, you will need to ensure that the tower’s leg supports are level and set to the correct dimensions. You cannot have any error here, or the tower eventually will fall.
Once you compare building a new site and tower with adding new equipment to an existing site, you will see why most system engineers opt for the latter.
Part 1: Class is in session: Basic LMR and FCC definitions
Part 2: Start at the beginning: Understanding LMR user needs
Part 3: The devil's in the details: Conducting a user-needs survey
Part 4: Decisions, decisions: Understanding the LRM procurement process
Part 5: Let's get started: System engineering begins with RF planning
Part 6: The lynchpin: Receiver planning and noise interference
Part 7: Connecting the dots: How to connect LMR sites
Part 8: The next piece of the puzzle: Understanding dispatch communications
Part 10: The bane of your existence: How to deal with RF interference
Part 11: Winning the battle: What causes radio frequency interference
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved in the radio communications business since 1966. He is a senior member of the IEEE and has been a licensed amateur radio operator since 1963. He can be reached at [email protected].
Robert C. Shapiro, P.E, is the senior manager-systems engineering for PlantCML, an EADS Company. He serves on the TIA TR8 committee as the TSB-88.4-C task-group chair and is a senior member of the IEEE. He can be reached at [email protected].