Juggling Antennas for 2009 Requires Planning

The last FCC "use-it-or-lose-it" deadline has passed and most stations with a tentative DTV channel designation on their existing DTV channel, have constructed their fully maximized facilities.
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The last FCC "use-it-or-lose-it" deadline has passed and most stations with a tentative DTV channel designation on their existing DTV channel, have constructed their fully maximized facilities. For many stations, more work lies ahead when analog broadcasting shuts down in February 2009.

After the FCC releases its post-transition DTV Table of Allotments, the current freeze on increasing DTV coverage beyond existing contours is likely to be lifted. Stations then will have an opportunity to file for facilities offering improved DTV coverage.

With little if any revenue from DTV broadcasting, many stations have positioned DTV antennas below analog antennas, often side-mounting on existing towers. Once analog broadcasting ends, these stations may want to move their existing DTV antennas to the top position or install new antennas in place of analog antennas if they aren't returning to their analog channels.

TOWER LOADING

It isn't too early to start planning for 2009! In addition to the coverage and interference studies, stations planning to move or replace DTV antennas will have to consider how they will juggle antennas on the tower. If multiple broadcasters are on the same tower, this becomes more complicated, especially for stations moving to a new channel after analog ends. The FCC has not indicated how much time stations will have to build out their post-analog facilities, but it is probably best to plan for months rather than years.

Juggling antennas will certainly affect tower loading. Sunday afternoon's NAB Broadcast Engineering Conference session included a paper by Thomas Hoenninger, vice president of operations and chief engineer at Stainless LLC, on how removing analog equipment affects a tower. You may be surprised to learn that less equipment on a tower doesn't always mean less stress on a tower-especially if it is guyed tower!

Hoenninger points out that if a topmounted analog antenna is removed from a guyed tower and the side-mounted DTV antenna is left on the tower, the tower mast in the top span will probably be overstressed.

Fixing this condition will require adjusting guy tensions. Mast members are likely to require reinforcement if the tower was not originally designed with both antennas on the tower.

The paper includes an analysis of a 1,507.5-foot tower (1,442 feet excluding the topmount analog antenna) based on the ANSI/TIA/EIA-222F tower standard. For the analysis, the maximum stress level at six locations on the tower were all set to 100 percent-the top guy level, the top midspan, guy level No. 7, the first guy level, the tower bottom midspan and the tower base.

The percentage of stress contributed by beam action stress and column action stress was calculated for the initial condition-a topmounted analog antenna (85-square-feet EPA (effective projected area) weighing 10,475 pounds fed with 6 1/8-inch rigid line) and a side-mounted DTV antenna (1,350-feet center of radiation in the top span of the tower, 100-square-feet EPA, weighing 4,000 pounds, and fed with 6-1/8-inch transmission line) as well as two post-analog conditions-a topmounted DTV antenna alone and the existing sidemounted DTV antenna without the topmount antenna.

STRESS LEVELS

The single topmounted DTV antenna used for this study weighed 10,000 pounds (less than the analog antenna) but had a greater EPA-100 square feet. In this situation, the stress at all levels except the top guy level dropped. However, at the top guy level, while the column action stress dropped slightly (to 74 percent), the beam action stress increased to 34 percent, increasing the maximum stress level to 108 percent compared with the original condition. This is due to the beam action changing when the sidemount antenna was removed. If other antennas were sidemounted on the tower, especially on different legs, the impact would likely be greater.

The calculated increase in stress for the third condition, where the topmounted antenna was removed and only the sidemounted antenna remained, was greater. In this case, the maximum stress level at all locations dropped except at the top midspan level, where it increased to 120 percent of the stress under the original condition. As in the second condition, the main increase was in the beam action stress level, which increased from 50 to 73 percent. The small reduction in column action stress from 50 to 47 percent did not offset that increase. In this case, the stress increase is consistent with having a load removed from the cantilever portion of a cantilevered beam.

Tom Hoenninger's paper "Tower Effects from Removing Your Analog Equipment" is available from the Stainless LLC Web site resources page at http://www.stainlessinc.com/Resources.htm.

Interaction between antennas has to be considered when they share aperture space, as is the case for towers with a T-bar or star-mount on top. In many markets, DTV allotments are on adjacent channels. If two antennas operating on adjacent channels are located side by side, RF energy from one antenna will couple into the other. The mask filter located on the output of each DTV transmitter will reject most of this energy, but if the sample port used for adjusting DTV exciter precorrection is located between the mask filter and the antenna, as is usually the case, the energy from the adjacent channel may prevent dynamic linear precorrection while the adjacent channel station is operating.

Depending on the type of mask filter used, adjacent channel energy may be reflected from the filter back up to the antenna, creating a ghost on the adjacent channel. Obviously, stations with adjacent final DTV channels that plan to share the same tower will need to take this into consideration when planning final antenna locations and transmitter configuration.

Although the FCC indicated in a Public Notice listing tentative designated DTV channels that it may modify channels before the final post-transition DTV allotment table is released, it seems unlikely there will be significant changes. Here are some items to start considering.

If your station is remaining on its existing DTV channel, will you use your current DTV antenna? If so, what happens to your analog antenna? If the plan is to move your existing DTV antenna to the location of the analog antenna, will you need a temporary antenna to stay on the air during the move?

Some stations will be moving to channels other than their current analog or DTV channel. If sufficient space isn't available for a new antenna in the preferred position, this means one of the antennas will have to come down. This will have to be done post-transition and within the time limit the FCC sets for moving to the new DTV channels.

While this may seem simple, consider that another station may be moving to the analog or DTV channel you are giving up. Any channel change will have to be coordinated with that station, which could be in a different market.

Stations sharing a tower with other stations have some real juggling to do depending on what channels they and the other stations plan to use. American Tower has started planning for these changes on the towers they lease to broadcasters. When workers are on the tower, stations on the tower will have to shut down or reduce power, depending on their location on the tower. Coordinating antenna changes among users will help limit off-air or reduced power operation.

Do you want to improve coverage? It is possible the de minimis interference criteria adopted during the DTV transition will not be as generous once analog ends. If coverage area can't be increased because of interference to other stations, it may still be possible to increase height and increase signal strength inside your coverage contour by increasing electrical beam-tilt with the possible addition or modification of mechanical beam-tilt. If a stacked antenna configuration is planned, modifying mechanical beam-tilt will affect the antennas above the one being modified!

As always, comments and questions are welcome. You can e-mail me at dlung@transmitter.com.