Bill Hayes /
07.24.2002 06:40 PM
DTV Antenna Dilemmas
A significant part of planning any project is to look for methods to maximize the purchasing power of the capital investment. When reviewing our transmission facilities at Iowa Public TV, we saw that there would be considerable benefit in purchasing DTV transmitters from the same manufacturer. Aside from the cost savings we saw in quantity purchasing, there was the benefit of improved serviceability based on common models of hardware. However, when considering antennas for multiple transmission sites, the unique nature of each broadcast facility led us to conclude that IPTV's long-term interest would be better served by fitting the best-performing antenna for each station. This analysis involved looking at antenna gains, vertical and horizontal patterns, directivity, mechanical specifications and additional external factors that vary greatly from site to site.
Currently, IPTV has four antenna projects in various stages of completion. Our Des Moines facility is complete and operational, using a Dielectric axial stack on which our Channel 50 is the support structure for the NBC affiliate's DTV antenna. At the Cedar Rapids site, tower modifications are just beginning, but the Dielectric wideband panel antenna has been in storage at the Cedar Rapids Gazette's printing facility since June 2001. Our Red Oak facility also has an antenna in storage at the site, which is a Dielectric slot antenna as well. Finally, we are evaluating the antennas for our site in Sioux City and have received proposals from Dielectric and Andrew. I suspect that by the time this article goes to print we will have made our selection.
DIFFERENT CITIES, DIFFERENT REASONS
Now I know that by looking at the three antennas we have purchased and the fact that they are all Dielectric, it would appear that we should have just signed a group purchase agreement with Dielectric and been done with it. However, things are not always as they seem and there are very different reasons why we have Dielectric antennas at these three locations.
Cedar Rapids is actually pretty easy to explain. We did our research and analysis and, based on that information and the need to work with the tower owner, we determined that the Harris wideband panel antenna was the best choice. We actually had the quote and were in the process of purchasing the antenna from Harris when Dielectric acquired the antenna division from Harris. In that particular location, because of a negotiated settlement with the tower owner and IPTV's desire to improve our coverage, a wideband antenna was a given. We evaluated all the wideband designs that were available along with the tower owner and concluded that the Harris design offered the best solution, so we had actually rejected the original Dielectric offering.
At our Red Oak facility, coverage was only one of the factors we considered. Since that facility is an N-1 and the existing analog antenna is a topmount on a tower that we own, our plan was to replace the existing antenna with one that could carry both signals. We also had a structural analysis done because we wanted to minimize any tower modification. The conclusion was that since we were proposing to replace the existing RCA antenna, Dielectric had a version of the original design that would handle both our analog and DTV signals and would require no tower modifications. We saw that as the deciding factor since we determined that any savings from a different provider would be spent a couple times over on tower modifications.
In the case of our Sioux City facility, where we acquired the tower on which our existing analog station is sidemounted, our DTV plan called for us to replace the old owner's topmounted Channel 14 antenna, which had a gain of 45. Our basic technical plan for installing a topmount Channel 28 antenna with a gain of no more than 25 allowed us to avoid significant structural modifications. Since this facility is an N+1 assignment we looked at combining the signals on a common topmount antenna, but the coverage tradeoff for analog was too great for the slight increase in vertical elevation - not to mention that it is always better to deal with a single-channel antenna that can be optimized for a particular station's requirements.
DES MOINES DRAWBACKS
I purposely saved Des Moines for last. Regular readers of this column recall my recounting of the horror tales that came out of this tower project. Sadly, in my view, the broadcasters made one of the larger errors involved as the site was being planned.
There are five UHF stations - analog and digital - on this tower. All these stations have omnidirectional patterns. The tower is a very complex and expensive candelabra with each station having its own individual antenna mounted in stacks on the arms. To me there were any number of reasons to look at using a wideband panel antenna to accomplish the same project. But for some reason, this was not done.
This is certainly not an indictment of the facilities involved since panel antennas are still fairly new to the U.S. television market and cooperative projects between competing commercial stations still give general managers queasy stomachs. However, in the case of the Des Moines tower, there could have been tremendous savings for a number of the stations. In the case of IPTV and WHO, which share an analog high-band VHF antenna, the new DTV site is a temporary facility that we will ultimately leave when we return to our VHF channels for digital broadcasting. The ABC affiliate in our market is currently trying to work out a low-power location to get on the air. The station is in the unenviable position of having Channel 59 as its DTV assignment and Channel 5 as its analog channel. The station would certainly like to return to Channel 5 if DTV works at low-band VHF but there is so little field data available, it cannot be sure that it will return to Channel 5. Meanwhile the station knows that Channel 59 will not be its ultimate home.
I can't say that I blame the station for not wanting to invest in a full-power Channel 59 DTV facility. The folks there have the additional problem of being licensed to Ames, Iowa, which is over 20 miles north of the tower sites in the opposite direction from Des Moines - which is where the majority of the population lives. The Fox affiliate in the market has just recently relocated to the new tower from its old location. Its motivation was to improve its coverage since the station was on a significantly shorter tower in the same location as the rest of the Des Moines stations. It is also an N-1 so it was able to put up a single antenna to accommodate both signals. The WB station, which was the impetus for the construction of the new tower to begin with, does not yet have a DTV assignment since they were authorized after the FCC cutoff date. As a result, the station has an offset stack that doesn't yet have a bottom antenna - just a pole doubling as a space holder and support for its analog antenna.
Conceivably, all these stations could have shared a single high-power wideband antenna with each station paying one-fifth the cost for the hardware.
As I said, this is not meant to be a finger-pointing statement, especially since I am one of the people living with the results of this project. However, the majority of U.S. commercial stations have applied for extension of the DTV conversion deadline. Many of those stations are no doubt considering low-power alternatives for their initial entry. Low-power over the city of license does have some drawbacks, disenfranchising potential viewers outside of the limited coverage area. Those folks are going to convert to digital whether they can receive the local DTV stations or not.
Don't make this decision in a vacuum, but meet with the other stations in your market to at least explore the idea of a joint facility. You may find that there is little real savings in the low-power alternative compared to a shared facility that provides DTV to the entire market and ensures that as the audience converts to digital, your station will have an opportunity to be a part of their decision.