Remember television? For most of its history, TV wasn't cable, satellite or YouTube. It was radio with low-res moving pictures. On the transmitting side it was an extension of radio, with transmitters on towers, mountains and high buildings, serving viewers with signals within a range limited by frequency and terrain. Like FM radio, TV was on VHF bands. In the U.S., channels 2-6 were spread from 54 to 88MHz (ending just below the FM band) and channels 7-13 ran from 174-216Mhz.
Not all TV signals are the same. For a signal with a given power and antenna height, range varies with frequency. Higher frequencies have less bending properties than lower ones. Energy is also more quickly absorbed by atmosphere and reduced or halted by buildings and terrain. This is why (again in the U.S.) visual signals for channels 2-6 had maximum powers of 100,000 watts, while those from 7-13 had "equivalent" maximum powers of 316,000 watts.
Range for a given power became more critical when TV expanded to the UHF band in the 1950s. UHF was problematic from the beginning. Receiving it required a new or expanded tuner, plus a new kind of antenna specially designed for the very short UHF waves. This is why TVs from the 50s to the 90s came with little "loop" or "bowtie" antennas, in addition to rabbit ears. The latter were for UHF while the former were for VHF.
For TV stations and networks, VHF channels were much more desirable than UHF ones — not just because VHF channel numbers were lower and more easily remembered, but because UHF signals had much less range than VHF ones. To compete with VHF signals, UHF stations could operate with a maximum power of 5,000,000 watts. And even those signals were never equivalent. If you're looking to push signals out past the horizon, VHF is much better than UHF.
In recent years the UHF band has also become more cramped. UHF TV originally ran from channels 14-80 (470-894Mhz). Since then everything upwards of channel 51 (698-869MHz) has been lopped off for a variety of other uses. Pagers use the 806-824MHz band (formerly TV channels 70-72). Analog mobile phones got 824–849 MHz (formerly TV channels 73-77). Public safety got the 849-869MHz band (formerly TV channels 77-80). And the "700MHz" band (formerly TV channels 52-69) was auctioned off in March of this year.
Yet UHF is winning, by federal mandate. On February 17, 2009, all U.S. television stations will be required to switch off their analog transmitters and use digital transmission exclusively. Nearly all of that will happen on what's left of the UHF band. Most stations will maintain their old channel branding, and still be known as "Channel 2" or "Channel 12", but their signals will in nearly all cases be coming in on a new UHF channel. For example, WNBC-TV in New York will move from Channel 4 to Channel 28. WABC-TV will move from Channel 7 to Channel 45. Both are already broadcasting on those channels in digital form. Come next February, all of them will, even if all they're doing is putting out low-def pictures over a high-def signal. In the Boston area, where I live most of the year, only one of the four major commercial networks broadcast their evening news in HD. The rest put SD (standard def) pictures on an HD signal.
Most of us are satisfied with the analog signals of cable. US channels will be digitized in Feb 2009. Our channels will be digitized in 2020.