Broadcast Writer

Site Management & Technology (SMT) magazine,

By Vicki W. Kipp

April 1, 2002  

TRANSITION TO DTV

To leap into the FCC-mandated ATSC broadcast standard, your TV station will need to install a new digital transmitter, additional transmission line, and an antenna for your digital channel. The FCC digital channel allocation plan is based on using your existing tower.

Ideally, your existing tower is able to support the addition of your digital broadcast equipment. However, you may need to modify your existing tower, or build a new tower for DTV to accommodate the increased wind loading from your digital channel transmission line and antenna.

STRUCTURAL ANALYSIS

The first step is to hire a tower consultant to perform a structural analysis of your current tower. Tower manufacturers, structural firms, or tower engineering firms should be able to complete the structural analysis.

When choosing a firm, ensure that they are familiar with ANSI (American National Standards Institute), TIA/EIA (Telecommunication Industries Association)/ (Electronic Industries Association), and ASCE (American Society of Civil Engineers) standards.

The analyst will need to know your tower’s history. The history includes tower drawings, age, inspection reports, previous analysis, modification reports and drawings, and guy tension measurement reports. The age of a tower may not be significant if the tower has been well maintained.

Tower analysis should determine loading based on standard ANSI/TIA/EIA RS 222-F. The report should include an inspection of the physical condition of the tower. It should note the condition of the antennas, grounding system, lighting system, paint, climbing and safety devices, transmission lines, hangers, foundations, guy wire anchors and hardware, loose or missing bolts, bent or missing members, and corrosion.

Analysis of a self-supporting tower should account for new DTV antennas and transmission lines that add wind load, leg/bracing members that might be overstressed, narrowed tops that may be weak, and foundations that may be overloaded.

Analysis of a guyed tower should account for new lines and antennas that add wind load, leg/bracing members that can be overstressed, and for guy wire and anchor improvements. The analysis should be done using software that is designed for guyed towers. Either weather conditions or human error can cause tower failure.

Weather failures are due to wind storm, ice storm, or large ice loads. Tower collapse from human error can occur when tower members or top-mounted antennas are being replaced. It is recommended that safety frames or temporary guys be used during these processes, and that an experienced tower engineer be on site.

Structural analysis of your tower will tell you what structural standards the tower meets. Tower standards, in order of least current to most current, are RETMA TR-116 from 1949, EIA RS222 from 1959, ANSI A58.1 from 1980, RS-222-C, RS-222-D, RS-222-E/F, and RS-222-F from 1996. RS-222-F is also known as ANSI/TIA/EIA RS-222-F.

It is possible that a tower built prior to RS-222-F may still meet that standard. But if your tower doesn’t meet RS-222- F, and you modify it, you may then be required to bring it up to the RS-222-F standard.

RS-222-F explains how appurtenances are to be positioned on a tower for minimum wind load. An ‘appurtenance’ is something added to another, more important thing. In this case, ‘appurtenance’ refers to an antenna that is added to the tower.

Tower loads include dead load, wind load, ice load, and seismic load. Dead load can be antennas, transmission lines, waveguides, lighting systems, conduit and junction boxes, ladders and safety climbs, work platforms, guy wires, and elevators.

Adding a digital antenna and new transmission line will increase the dead load.

Wind load increases with height. Ice load is considered a serious risk since it is the leading cause of tower failure.

Seismic load, subject to the vibration or movement of the ground, includes forces such as shear force, bending moment, and axial load.

Once the outcome of the structural analysis is complete, you can decide on a tower plan and draft a new budget. Structural analysis can result in no retrofit being required, a minor retrofit being required, a major retrofit being required, or a complete replacement of the tower.

TOWER MEETS NEEDS

Although it would be nice to have a tower where no retrofit is required, this is uncommon. In this case, you can proceed to install your new transmission line and antenna.

IMPROVEMENT NEEDED

This is a more common scenario. You may need to replace bracing members and upgrade legs to avoid overstressing before you can begin installing transmission line.

The majority of towers, which would include most towers 20 years old or older, require a major retrofit. This may include removing or rebuilding large portions of the tower, replacing or adding bracing members, upgrading tower legs, and modifying or replacing guy wires.

UPGRADE

The bracing members can be upgraded by increasing the bracing size, adding cover plates on existing bracing, and adding internal bracing for overstressed girds. If the leg members of a tower need to be upgraded, there are several options. By adding leg bracing to a leg, you reduce the length of unbraced leg and can double the axial load capability of the leg. This is the most cost-effective option.

You could fill the legs with high strength concrete or grout to increase the stiffness and capacity of the legs.

You could weld or bolt plates, angles, or channels to legs. The drawback of this option is that it creates new potential rust spots.

For guyed towers, several options exist for upgrading guy wires and anchors. Existing guy wires can be replaced with larger sized guys. A new guy level can be added in between existing guys. Adding new anchors or improving existing anchors will also help.

Overloaded tower foundations will need to be upgraded by adding weight or bearing area to the foundation system. In some cases, reinforcement of towers may be impractical because the tower base plate is unable to carry the increased load. The base plate’s and surrounding soil’s ability to support the extra weight should be considered.

Shielding coaxial cables and transmission lines to reduce wind load can reduce tower loading. Sharing a transmission line will also help.

OTHER CONSIDERATIONS There are FAA issues as well. The FAA must be notified if you change the height of your tower. This could occur if you stack or modify your antenna. Tower owners must pay attention to public safety. Security measures should be taken to limit access to the tower grounds. Use the appropriate signage to post RF hazards. Have a responsible tower maintenance plan in place.

Insurance is another concern. You must request a “Certificate of Insurance” from the tower contractor when your tower is being modified. Once the upgrade is complete, you may have an issue with your tower’s insurance company. They may be unwilling to insure a tower that has been modified. Insurers are hesitant to cover towers that have had their supporting members reinforced because reinforcement usually means additional steel welded to the existing tower legs. The welding process is different than gluing two pieces of metal together. When metal is melted, it is crystallized. The original dynamic capabilities of the metal are changed.

REPLACE TOWER

If the cost to upgrade your current tower exceeds the useful life of the tower or the cost/value of a new tower, then building a new tower may be the best choice.

Building a new tower is no small undertaking. Tower designers  are in short supply and tower crews are in even shorter supply. It is worthwhile to seek out professional help for permit and zoning presentations and for public relations.

Consider partnering with others in this venture. The benefit of multiple user towers include a lower cost per station, limited liability, management handled by someone else, greater acceptance from the community, and simplified antenna aiming for viewers.

TRANSMISSION LINE AND ANTENNA

Most stations will have to purchase another transmission line and antenna for their DTV channel. Under the right circumstances, however, it is possible to share a single transmission line or a single transmission line and a single antenna for your NTSC and DTV channels. The ability of your station to share a single transmission line and antenna depends on the channel band (See figure 1) relationship (in-band versus crossband) and channel separation between your NTSC channel and DTV channel.

Both channels can share the same antenna if the antenna bandwidth is adequate to provide a good match at both channels. Sharing a single transmission line for both channels has several benefits. For one thing, there is less wind loading. Another advantage is the cost savings of only buying and maintaining one transmission line instead of two.

When you share a transmission line, you make some compromises. If there is a problem with the transmission line, both channels are affected. The power rating of the NTSC feedline may not be adequate to handle the additional DTV power.

Three-inch coaxial line, which has been commonly used for NTSC, may not have an adequate power rating to carry both signals. It is probably only practical to share a three-inch transmission line if the DTV power level is low and the tower absolutely can’t support one more transmission line. Six-inch transmission line is recommended for dual-channel transmission line (figure 2). 

When stations combine two channels on one transmission line, the line must be broadbanded to work favorably on both channels. Transmission line elbows (figure 2) can be a problem when there is a complex installation requiring multiple elbows. In these cases, the elbows need to be factory tuned for the expected channels. Plan the transmission line run to minimize the number of elbows required. Semi-flexible line could be substituted in places that would require a large number of elbows.

It is possible to share a single transmission line while having separate antennas for your NTSC and DTV channels.

However, sharing a single antenna for both channels has benefits. Advantages would be: a common radiation center and HAAT; comparable azimuth and elevation patterns for both channels, reduced tower clutter, and lower initial and operating expense.

Sharing of transmission line requires a combiner. The NTSC and DTV transmitter outputs are both fed to a combiner. Both In-Band and Dual Band combiners are available.

The combiner feeds combined NTSC/DTV to the common transmission line. The signal travels up the tower in the transmission line. At the top of the tower, the combined signal is fed to a dual channel broadband antenna or separated by a second combiner (splitter) and fed to the separate NTSC and DTV antennas.

CONCLUSION

When transitioning to DTV, you must consider tower structure, transmission line, and antenna issues. Next month, we’ll continue our discussion of the tower industry by exploring tower siting issues.  Information for this article came from the following sources: Joe Fedele, “More Crews Needed for DTV Towers” TV Technology May 22, 1997; Harris/PBS DTV Express “Transmission System/Towers” DTV [Transmission Course] 1998; Don Markley, “Transmission lines for DTV” Broadcast Engineering, June 2000; NAB Engineering Handbook, Sixth Edition. 

Channel, Channel Band

2-6 Low Band VHF (Very High Frequency)

7-13 High Band VHF

14-69 UHF (Ultra High Frequency)

Figure 1. Television channel bands. 

Figure 2. WISC-DT’s Six-inch transmission line with elbow.