Originally featured on BroadcastEngineering.com
DTV tower modifications expect planning
The tower in the foreground was heavily loaded before the addition of any DTV appurtenance. Now it is also home to two DTV antennas.
Installing a DTV antenna requires a minimum of three months lead time, even if no modifications are needed for the tower. But most towers need strengthening, which requires analysis and engineering, before the modifications can take place. Thus most DTV installations require six months lead time for the tower part of the project.
Companies such as American Tower and Spectrasite build and lease towers that are being used for some DTV installations. Some broadcasters have found that leasing tower space costs less than modifying their present tower or constructing a new one. Tower construction and modification aside, side-mounting a DTV 1000 feet up a tower can cost $200,000 or more.
It is not uncommon for full power analog and DTV antenna combinations to weigh in close to 50,000 lbs. That can be a significant percentage of the actual tower weight. One thousand-foot towers without antennas usually weigh between 150,000 to 250,000 lbs.
The tower should be analyzed to insure that it meets current TIA/EIA RS-222 standards. Strengthening a tower usually involves the replacement of diagonal braces of the tower, which make the tower more rigid and able to handle increased shear forces. This is typically done one diagonal brace at a time by first installing a temporary frame at the diagonal being replaced.
Complicating matters is that many stations share towers, and often it makes sense to consider sharing actual antennas, such as wideband panel antennas. The dilemma many broadcasters face is whether they will stay on their new DTV channel when the transition is complete, or move back to the current analog one. This choice can temper the amount of capital invested and the amount of cooperation competitors will want to give to community tower and antenna projects.
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