Point in the right direction
Following the theme of the ongoing LMR 200 series on resource management, antenna systems play an important role in maximizing the use of radio frequency spectrum, which generally is in short supply. After selecting the proper radio frequency band and balancing the link, the antenna system must provide the match between the available radio sites and the coverage requirements.
Antenna systems in land mobile radio networks consist of coaxial jumpers; coaxial transmission lines; lightning surge protection; earth-grounding connections; waveguide, coaxial or electronic filtering; and of course, the antennas themselves.
Antennas come in all shapes and sizes. They can be tall or short, wide or narrow. The exterior can be made out of metal or synthetic materials, such as fiberglass. They can be vertically or horizontally polarized. They can be directional or omnidirectional.
The type of antennas being used in a radio system is a critical factor in the design that the engineer will use to ensure that the system meets the demands of the end user. In addition, the antenna type that is chosen may meet other design parameters, such as using a lower-gain antenna to accomplish fill from a very high site, or using a highly directional antenna to keep co-channel interference from being an issue on a new or existing radio channel. By matching the system design to the antenna type, the engineer has great latitude in designing a good system for the end client.
Available antenna types include the following:
- Unity-gain folded ground plane
- Ground plane
- Corner reflector
- Passive reflector
- Downtilt gain
Besides the antenna type and size, the weight and wind-loading are additional factors that the engineer must keep in mind when putting together a radio system. All towers and some building rooftops have a maximum weight load and wind-load rating. These factors also must be considered when selecting the antenna type.
Finally, the band of operation also plays a part in selecting the antenna type. As the frequency is increased, the size of the quarter wavelength decreases in direct proportion. In addition, smaller antennas allow for higher gains at the higher frequencies.
Broadcasting 1000 watts from a mountaintop site using high-gain antennas may look great on paper, but if the site’s desired coverage area is in the valley below, then sending signals 100 miles away to the next state may not be the best idea. Instead, a low-gain or a down-tilted antenna will place the radiated signals to where the coverage is needed. This saves the frequency for another business or agency to use 70 miles away, or prevents self-generated interference.
Meanwhile, broadcasting 10 watts from a 100-foot tower in the valley using a unity-gain or an omnidirectional antenna might seem like the only solution when the desired coverage area needs to extend to the next valley. But if the valley site can be linked with a mountaintop site, then a Yagi antenna may be a better choice.
These examples are just a small sample of the ways to use antenna systems to maximize limited resources such as radio frequency channels, electrical and radio frequency power, and available base station sites, as well as capital and operational budgets. Other examples include rooftop installations where the antennas must cover the streets below the site and not the next city; sites on the edge of the desired coverage area that do not need to reach past a certain distance; and very tall television or FM towers on the edge of the desired coverage area.
Citywide and dense urban designs are the biggest challenge for antenna systems in terms of spectrum efficiency. Some urban areas, such as Manhattan and Chicago, are laid out in a street-grid format—as soon as the subscriber turns left or right from the normal “ducting” of the radio signals along a street or avenue, the signal is lost. Antennas placed on tall rooftops with low-gain or down-tilted antennas can fill in the areas where right and left turns may affect the ability of subscribers to receive signals from antennas placed lower towards street level.
On the other hand, placing antennas on tall rooftops, even when using low-gain or down-tilted antennas, limits the use of the valuable spectrum. Antennas placed on the corners of strategic buildings at a lower level can balance the need to cover avenues and streets, and keeps the radiators at a low-enough level to allow frequency reuse or prevent interference to co-channel users.
There are many other antenna applications for dense urban coverage, such as layering high and low sites to pick up the 0-20 stories, 21-50 stories, and 50 stories and above. In this case, panel antennas are used to beam the signals to the proper locations. Another application involves using antennas to collect signals that are then rebroadcast inside the buildings as part of a distributed antenna system. These antennas, either yagi or corner reflector, are strategically placed on rooftops or on the side of buildings, and feed fiber optics or coax-based DAS.
Base station sites, which are located on the edge of the service area, can use offset or cardiod antennas to beam the signals into the desired coverage area and to prevent signals from interfering with co-channel or adjacent channel users in neighboring areas. The antenna can be top-mounted on the structures with the proper electrical or mechanical radiation properties, or can use the structure as a reflector—or as a blocking barrier—to prevent the signal from going in a certain direction.
When an antenna is side-mounted on a tower, the antenna’s signal pattern is distorted, a condition known as “shadowing.” Many towers today are used for cellular, PCS, and now LTE 700 MHz service, and the multiple feed lines going to the panel antennas look like solid barriers to the radio signals emanating from the side-mounted antennas on the same towers. The result can be a 20 dB to 30 dB signal loss within the shadow area on towers that had just a few decibels of distortion in the pattern caused by the structure itself. This significant loss of signal equates to major holes in the coverage area behind the obstructions that the engineer had not expected or accounted for in the overall system design.
There are many systems in use today that are not working properly or as designed because the tower is no longer a transparent factor in the coverage of the jurisdiction, due to the signal being blocked by the transmission lines. This can be overcome by going to separate antennas on the same tower to eliminate the shadow effect on the signal. If your system used to work properly but no longer does, a field trip to the tower will be useful in light of the shadow effect being so great on the new systems being installed.
Test equipment can be used to measure the signal from the antenna on the ground, and this signal can be measured 360 degrees around the tower to see the effects of the distortion. Software also exists that simulates the effect of tower obstructions on side-mounted, non-panel antennas. This software uses ray-tracing theory to determine the gains and losses from the phasing of the direct and indirect rays, but the accuracy must be determined on the ground with test equipment, because modeling the obstructions is very difficult and can lead to inaccuracies if not properly vetted.
Subscriber antennas also play a major role in the link budget on a system. A mobile antenna shows directivity unless it is mounted in the center of the roof of the vehicle. Because not every vehicle can have the antenna mounted there, the offset pattern must be factored into the link budget to ensure full coverage across the jurisdiction. Gain antennas and longer-length antennas can be used to accomplish higher signal factors in these mobile applications. The newer canister antennas mounted on the roof of a vehicle have shown to have the same range as the larger antennas mounted there—or anywhere else on a vehicle.
Portable radios also have antenna issues, and that must be considered when determining the link budget. Radio signals will be shadowed by the body of the person wearing the device on the hip. This can be overcome by using remote speaker microphones that also elevate the antenna to the shoulder of the person wearing the radio. Another way is to use satellite receivers in areas where portable radios are used. By giving the signal the ability to be received in multiple directions from a given site, the shadowing of the portable radio is less severe.
Yet another issue with portable radios is the detuning of the antennas as they get closer to the body of the person wearing the radio. This is more severe at the lower frequencies (VHF and UHF), but it also occurs in the 700/800/900 MHz bands. Finally, the bandwidth of the antenna itself may cause some antennas to exhibit no resonance on the extreme edges of a given band. The lower power of the portable radios also causes an imbalance between the higher power of the base stations and the subscriber units in the field. These factors also must be considered in the system design.
The effects on the link budgets are remarkable and engineers can miss the mark by 5 or 10 dB simply by not considering the effects of antenna distortion stemming from towers, body losses (portable radios) and mobile antennas that do not have a proper ground plane at the user end. If the system range is 20 miles and the negative antenna distortions are not considered, a change of just 3 dB can reduce power by 50%. The result would be a 15% reduction in range, or 3 miles.
Similarly, using a unity-gain antenna value in the link budget for the portable antenna—where the device is mounted on the hip and the body loss is 10 dB—can throw off system range by a factor of 10, since a 10 dB reduction equates to 10 times lower power. This can reduce system range from 20 miles to 10 miles from the site given the previous example. The size of the person and the direction the person is facing, either towards the tower or away, also can affect the calculations. Fortunately, many articles and research are available on this topic, and the conclusion is to use the worst-case value in the link budget to ensure that the maximum losses are accounted for properly.
The link budget effects of mounting mobile antennas on the vehicle roof are less dramatic than that of side-mounting a portable device on the hip, but are important nonetheless. Most mobile antennas require an external ground plane to match its impedance properly. A 3 dB gain on paper is more like a 3 dB reduction when the ground plane is not adequate. Again, it is best to use the worst-case design values of unity gain and attempt to center the mobile antenna on the roof or trunk.
Clever selection of antennas allows the design engineer to tailor the radio frequency design to meet the system’s coverage objectives. But caution is recommended due to the distortions caused by the tower structure (in the case of side-mounted base station antennas), the side mounting of portable antennas on the hip, and the improper mounting of the mobile antennas on vehicle roofs. All can create inaccuracies in the design. n
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 a consulting engineer who has been in land-mobile radio since 1984. He serves on the TIA TR8 committee (TSB-88) as vice chair and is a senior member of the IEEE. He can be reached at [email protected].
LMR 200 Series
Part 1: School's back in session — LMR in real-world applications
Part 2: Where it all begins — Pros, cons of primary Part 90 spectrum bands
Part 3: Spectrum redux — Part 90 spectrum bands in real-world applications
Part 4: The power of reduced power — Smart technologies optimize radio performance
Part 5: A balancing act — Balancing uplink and downlink paths to improve system performance
LMR 100 Series
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 9: Now the real work begins: How to select a suitable LMR site
Part 10: The bane of your existence: How to deal with RF interference
Part 11: Winning the battle: More causes of RF interference
Part 12: Now the fun begins: Installing the LMR system
Part 13: Dotting Is and crossing Ts: Choosing the LMR project, program managers
Part 14: Now you're done: Maintaining the LMR system