A golden opportunity at Round Mountain
A cooperative effort between public safety and private industrial radio users leads to the development of a reliable radio communications site. A new philosophy of site selection for year-round radio coverage and creative antenna system design play key roles.
Two common problems faced by site planners and system managers are the high cost and difficult access often associated with the most desirable locations for a wide-coverage, multi-agency communications site.
This was the case in Central Nevada, where the Nevada Highway Patrol (NHP), Nye County and several other users depended on Shoshone Peak (9,200 feet) for radio coverage in the Big Smoky Valley north of Tonapah. Police, fire, roads and public works systems were all located on the peak.
Flanked to the east and west by the 12,000-foot Toquima and Toiyabe mountain ranges, the Big Smoky Valley curves gently for about 100 miles. State Route 376 is the only highway through the middle of the valley, and it is the main link between Tonapah and Austin, on Highway 50, to the north.
Shoshone Peak had the advantage of an existing dirt access road, circumventing Bureau of Land Management regulations that prevent road building on most publicly owned peaks. Budget limitations prevented construction of facilities for commercial power, and a combination of solar panels and battery banks provided power to the various repeater systems and remote receivers.
Accessible in winter only by a slow, dangerous climb in a snow cat, the site is vulnerable to frequent damage from ice storms that regularly tear antennas off their mounts, leaving troopers and county personnel without radio coverage.
Making matters worse, the mountain micro-climate often produces thick fog, cutting off light to the solar panels. Although extra batteries and remote start generators were considered, it was not possible to sufficiently harden the site against winter storms, and three years ago the search for a replacement began.
Efforts by both agencies to relocate the site were slowed by the same budgetary limitations that brought about its creation. The costs of land acquisition, restrictions on road building and extension of commercial power made development of another high-elevation site nearly impossible. Early in 1997, Nye County and NHP came together in their search. Both agencies made extensive use of VHF highband channels, ensuring technical compatibility. They made a decision to emphasize year-round access and availability of commercial power rather than elevation.
This would require a site below the annual snow line, but high enough to cover the 2 ,000-square-mile valley, with its two tall mountain ranges cutting off radio coverage from outside. During the previous year, Nye County Communications Manager Terry Payne had approached Round Mountain Gold Corporation to discuss the possibility of locating a radio site on their property. Several small hills above the eastern side of the valley, some as high as 1500 feet, were particularly attractive.
Named for the small nearby town of Round Mountain, which sprang up with the discovery of gold in the late 1800s, the mine has grown rapidly during the last 20 years to become one of the largest in the world. Mine officials, already considering communication improvements, quickly realized that participation in the new site would allow them to upgrade their outmoded simplex VHF radio system, and they provided access to a prime location. Because the site was low enough to allow some activity during the winter, construction of a tower, a shelter and utility service began in late 1997 and progressed rapidly.
NHP communications manager Mel Pennington contacted Telewave to provide antenna and combining equipment, as well as overall system design, based on several previous successful installations for his department. Pennington outlined his desire for a system that would cover the valley and its two-lane highway but would not waste power in unneeded directions. This ruled out the use of collinear or yagi antennas because of sharp nulls in their patterns and the problem of coverage overshoot close to the site. These antenna types would also cause severe multipath distortion in this environment because their highly compressed patterns would scatter signal power off the mountains. Another concern was the gain and bandwidth requirement. Existing (and future) users operated over a frequency range of more than 15MHz, and continuous radio coverage was needed over a minimum 75-mile span.
The combination of existing users and the need to allow for uncomplicated future expansion created a wide range of frequencies that could not be accommodated within one combiner or antenna system. A computer analysis also indicated significant intermodulation problems among several transmitters and receivers. As work began on the difficult task of minimizing interaction between transmitters while keeping signal loss as low as possible, our system engineers determined that the different transmit frequencies would have to be handled by at least two separate antennas.
This required the design of a transmit antenna array with four identical, horizontal bays mounted vertically, allowing selection of two non-interfering transmit frequency groups, handled by separate combiners. Although the use of hybrid combining would have simplified the design, the much higher loss of this type of combiner ruled out their use in the system.
The multibay antenna system is based on the ANT150D6-9 folded dipole array. The combination of medium gain, wide bandwidth and mounting flexibility made this antenna the only feasible choice. This specific dipole array is capable of creating custom radiation patterns for special requirements, and this was the critical factor for the Round Mountain site. The corporate feed system allows individual configuration of each element to create shaped coverage, reducing nulls and pattern distortion. Our antenna manager, John Hilmer, had to contend with both a relatively short 80-foot tower that created reduced isolation between transmit and receive antennas, and the unusual coverage requirements.
“Not only does the valley curve,” Hilmer said, “but the highway bends right toward the antenna site, and the multipath off the mountains from a conventional antenna would have made this site unusable.”
Using computer modeling and data gathered through hundreds of installations, we created a four-bay (two-by-two) transmit array that uses the tower to shape the radiation pattern and fits into a compressed vertical space to maximize separation from the receive system. The “coverage shaping” directs signal power up and down the highway and keeps multipath to a minimum.
“The coverage of the antenna system is superb,” said NHP’s Pennington. “It’s literally molded to the shape of the valley. Nothing is wasted, and we don’t have any nulls anywhere on the highway.”
Final system antenna gain is just over 8dBd, and received signal strength exceeds 50mV over most of the desired coverage area.The combining system is based on M108-150 eight-channel VHF cavity-ferrite combiners. The main design goal of combining is always to control intermodulation.
Computerized frequency analysis allows the selection of two groups of channels that are fed to separate combiners and, in turn, to each of two transmit antenna arrays. The smaller coverage requirement of the gold mine made it possible to further reduce transmitter interaction by placing RMGC’s three repeaters on a third combiner and single bay antenna system, with room for expansion. Although only seven channels were installed for immediate use, other users occupying five additional channels were accommodated for future expansion, bringing the total to 12 channels. The lowest frequency will be used by local amateurs at 147.330MHz, and a weather service transmitter will operate at 162.550MHz. An FCC waiver allows the use of three marine ship-to-shore channel pairs for the mine repeater operations at 157/162MHz.
The receiver system also required special engineering for tight frequency spacing. An expansion receiver channel for the Nevada Division of Wildlife on 151.160MHz was affected by Nye County Public Works at 151.055 as well as a future NHP repeater, also in the 151 MHz band.
“UHF and 800MHz systems are relatively easy to design, because transmit/receive spacing is consistent, and receivers are high or low, depending on the band,” said system engineer Bill Kubicki. “With a complex VHF system, the rules go out the window because frequencies are not assigned in standard pairs, and receivers often end up very close to transmitters.”
TWPC-1505-2 dual high-Q pass cavities were used for all receiver paths, with the addition of crystal filters for spacings of less than 100kHz. Low-noise amplifiers feeding power splitters allowed custom tuning of signal level to each receiver, fed by a 2-bay dipole array at the top of the tower. For the gold mine, the separate antenna system composed of two single dipoles provides coverage of the entire 400-square-mile property, including the 1600-foot-deep main pit, an area of prime concern.
“The coverage for radios in the pit has always been a problem, and it got worse as we dug deeper,” said RMGC Chief Engineer Dan Moore. “The new system allows even a 5W hand-held to talk out of the pit to any other part of our property.” Potential commercial development of surrounding land owned by the mine also will be aided by improved radio communication.
The cooperation of several organizations with similar goals has resulted in a successful and reliable radio communications site at Round Mountain. A new philosophy of site selection for year-round radio coverage has given the NHP, Nye County and RMGC an improved ability to provide public and private safety services to residents and mine employees. Cost savings to Nevada taxpayers, compared to a conventional mountaintop site, are considerable, both for initial development and ongoing operation.
The coverage of the valley is now better than before, with more consistent signal levels and greater penetration of previously shadowed areas. The lower site elevation has also eliminated interference to sites outside the valley, not to mention stormy snowcat rides, formerly a fact of life for users of the mountaintop.
