This month I'll look at some of the challenges New York City broadcasters face restoring TV coverage to what existed Sept. 10, 2001. As promised last month, I also will outline some of the reasons DTV to analog (and vice versa) interference in some cases is likely to be greater than what was previously predicted.THE EMPIRE CHALLENGE
Broadcasters have long recognized that it is best if as many TV stations as possible in a given market are in the same location. It makes it easier for viewers and cable companies to receive the signals as one fixed antenna can be used for most of the stations. Furthermore, in areas where reception from the common site is poor, everyone is likely to receive local programming over cable or satellite rather than live without most of the local stations. In New York City, the primary TV site was the World Trade Center.
All of the New York City market VHF stations (WCBS, WNBC, WNYW, WABC, WWOR, WPIX and WNET) and two of the area UHF stations, WNJU and WPXN, were transmitting from the World Trade Center the morning of Sept. 11, 2001. In the days after the disaster, some stations began broadcasting from Armstrong tower in Alpine, N.J. WCBS had a backup facility at the Empire State Building and was able to go to it immediately. Now, all but one of the World Trade Center TV stations is transmitting from the Empire State Building, most with reduced power and coverage.
This has solved the location problem, but has created another one. Building a permanent facility at the Empire State Building will require work in or near the aperture where most New York City-area TV and FM stations are transmitting. If you have ever worked with other stations in an antenna farm or on your tower to coordinate off-air times for maintenance, you can imagine the difficulty New York City stations face even to improve their temporary facilities. The amount of time available to work on the new antenna systems will be limited.
Broadband antennas can allow several stations to share the same aperture, both at UHF and VHF. However the antenna system will have to be large enough to handle enough power to allow TV stations to maintain the effective radiated power they have now at Empire or had at the World Trade Center. In addition to finding a place for existing stations on Empire to broadcast from while this large antenna system is constructed, erecting large panel antenna systems raises structural issues that have to be studied before construction can begin.
The challenges do not end with the antennas and supporting mast. The use of multichannel antennas reduces the number of feed lines somewhat, but to service multiple high-power transmitters, larger transmission lines will be needed. Photo 1 shows some of the transmission line run inside the mast at the World Trade Center. I took this photo in October 1999 before the WNBC multichannel DTV antenna system was installed. The open space was quickly filled! The WPXN rectangular waveguide is visible on the left, smaller VHF rigid coax lines are in the center of the picture and the WNJU 8-inch and 6-inch rigid lines are on the right. Some of the lines are hidden behind other lines in this photo. There wasn't enough room to get a shot including all the lines at that elevation.
| Photo 1. WNBC's multichannel DTV antenna system was installed shortly after this photo of the World Trade Center's tower mast was taken in 1999.|
Transmission line routing at the Empire State Building, as at any transmission facility with such a large number of antennas in a small space, will be a challenge. To avoid pattern scalloping, panel antennas have to be mounted close together. This conflicts with the need for space for transmission lines to the panels and to antennas above the panels. The transmission line situation at Empire will be worse than at the World Trade Center because in addition to the stations that were broadcasting from the World Trade Center, the three existing UHF analog full-power TV stations at Empire have to be considered as well as the WPIX DTV antenna and the WCBS multichannel DTV antenna.
The list of challenges does not end here. The power risers at Empire were not designed to handle additional VHF and high-power UHF transmitters and, eventually, UHF DTV transmitters. A new riser had to be installed.
Broadcasters, ironworkers, riggers, electricians, other contractors and Empire State Building management are working hard to overcome these obstacles. As stations move from temporary antennas mounted outside windows to antennas higher on the mast with more omnidirectional antennas, New York City broadcasters that lost facilities in the World Trade Center will begin to approach the coverage they had before.
These broadcasters, however, will not be able to duplicate the height and, in some cases, the power that they had at the World Trade Center.
It isn't surprising that with the difficulty in locating this many TV stations on one building and the desire to regain the height of the World Trade Center, New York City TV broadcasters are looking at another location for TV transmission. (See Frank Beacham's interview with Ed Grebow and Altan Stalker of the Metropolitan TV Association on p. 8 for details on how the search is progressing.)
Everyone hopes there will never be another attack like the one that destroyed the World Trade Center a year ago, but like the rest of the country, should such a disaster strike again broadcasters will be better prepared. Most TV broadcasters in New York City are maintaining their emergency transmission facilities at Alpine New Jersey. Broadcasters in other cities with vulnerable transmission sites are considering backup facilities.DTV/ANALOG TV INTERFERENCE - PART 1
Interference between new DTV stations and existing analog stations has become a hot topic recently. As more DTV stations begin broadcasting and as those DTV stations already on the air increase power, interference to existing analog stations is likely to become more of a problem. Because the FCC is likely to view this interference as a temporary condition that will end when analog broadcasting ends, don't expect them to get involved in these interference disputes if the DTV station complies with the FCC rules and if engineering studies did not predict unacceptable interference.
Why is interference from DTV stations becoming a problem in some cases when FCC Bulletin OET-69 studies predict it won't be? Part of the problem lies in the method the FCC chose for determining interference. Most readers know the FCC uses the Longley-Rice terrain sensitive point-to-point propagation model as the basis for determining DTV interference.
As I explained five years ago in my August 1997 TV Technology column, FCC Bulletin OET-69 lists the specific parameters and methods to be used applying the Longley-Rice model to DTV interference studies. I'll try to post this article on www.transmitter.com soon. In particular, the FCC specified one value for surface refractivity. In reality, surface refractivity can vary substantially from location to location and from season to season. In another case, OET-69 specifies default transmit elevation antenna patterns for UHF and VHF DTV stations when in many cases the actual elevation pattern is quite different. Finally, as has been pointed out by Hammett and Edison engineers at numerous conferences when OET-69 is discussed, the FCC handling of cells with Longley-Rice Error Code 3 flags as having coverage and no interference can overstate coverage and understate the impact of interference.
Before blaming the FCC for this situation, it is important to remember the task they had in 1997 - finding a DTV channel for every U.S. full-power TV station while replicating service and minimizing interference to existing analog TV. This required some compromises, including reducing the number of variables that had to be calculated, Computers were not as powerful five years ago as they are now.