Friday, May 27, 2011


Well, the transition from analog television transmission to digital ended nearly two years ago and a lot has been learned since then. In the early days of analog operation, most television stations operated on Channels 2 through 6 (low-band VHF channels) and Channels 7 through 13 (high-band VHF channels). These stations had significant coverage, indoor and outdoor, for miles from the transmitter site. As that spectrum became congested, Channels 14 through 69 (UHF channels) were allocated to new analog television stations. Due to the fact that UHF stations required 10 to 20 times more power than VHF stations to cover market populations, they were the poor step-children of the television spectrum. In addition, since the network programmed stations (ABC, CBS, NBC and eventually FOX) had the VHF channels in most major markets, the UHF channels were home to more of the niche programmers. As a result of these issues, many VHF stations decided early on in the digital allocation game to stick with their analog channels, convert them to digital and give back to the FCC their digital UHF allotments at the end of the transition. Channel branding and lower operating costs were usually quoted as the main reasons for keeping their original analog channels.

One of the biggest concerns that has emerged, since the end of the transition, is the inability of these digital VHF stations to cover all of the viewers of the original analog station. One reason is that digital signals do not exhibit analog’s “graceful degradation” as one approaches the edge of the coverage area. Either there is enough signal to translate into a picture or there isn’t. In addition, it is now clear that the FCC’s planning factors did not adequately provide for enough power for VHF stations to even come close to covering their old analog audience (especially east of the Mississippi River). The reason for this is probably that the FCC needed to restrict power levels of such stations in spectrum-congested areas of the country in order to allot every station a second channel for digital operation prior to the end of transition. Not only are the power levels restricted, but the required level of signal for outdoor reception of a digital VHF station are severely over-stated by the FCC. Adequate reception of a 36 dBu signal from a Channel 12 station would probably require a high-gain outdoor antenna mounted 30 feet above the ground, pointed at the transmitter site, have no intervening impediment (terrain, building or vegetation) and at least one signal booster in the receive system. These are unreasonable go/no-go requirements for most viewers living at the edge of a station’s service area.

Since most digital VHF stations have maximized their effective radiated power values, there are not many options to improve station coverage. The exception is the addition of a vertical component to their existing facilities, if they are presently operating with an antenna that is horizontally polarized. The addition of a vertically-polarized antenna to the existing transmitting facility, if properly designed, can increase the coverage of the station in a couple of ways. For indoor reception, the addition of a vertically polarized signal to the present horizontally polarized signal can double the signal density at the receive location and reduce the need to optimize the receive antenna in a particular direction. In addition, service at the fringe areas of the station’s coverage can be improved due to rotation of the vertical signal into the horizontal plane as it travels along the earth. Since most outdoor antennas are horizontally polarized, more signal can be picked up by the viewers in the fringe area.

The three down sides to adding a vertically-polarized antenna to an existing VHF facility are: the supporting structure (tower) may not be able to handle the additional antenna, either from a space or a loading standpoint (or both); the vertical antenna must be properly phased with the horizontal antenna in order to avoid causing interference to the latter; and, additional transmitter power, a significant cost factor, is required for the vertically-polarized antenna.

Besides the coverage benefits of adding a vertically-polarized antenna to the existing system, the addition of the antenna does not require prior approval from the FCC as long as the power in the vertical component does not exceed that authorized to the station. Once the new antenna is added and becomes operational, a 302-DTV (license application) is filed with the new parameters within ten days.