Katrina Covington, age 11, of New Brunswick, N.J., for her question:
WHAT IS VHF AND UHF ON TELEVISION?
Before a television signal can be broadcast, a transmitter must boost the rate of vibration or frequency. A high frequency is needed to carry the picture information through the air. The transmitter increases and amplifies both the video and audio signals by a process called modulation. The video signal is amplified to a power of 1,000 to 100,000 watts.
A transmission line is used to carry the television signal from the broadcast studio to the transmitting antenna, where the signal is released into the air. TV stations build tall antennas on high buildings or towers on tops of hills and mountains so that the signal can reach as far as possible. Maximum range of most TV signals is from 75 to 150 miles.
Each television station in a given area transmits on a different frequency or rate of vibration. With this system, one TV station's signal will not interfere with another. The group of frequencies over which one station broadcasts is known as a channel.
There are a total of 68 channels available for television broadcasting in the United States. The channels are divided into two groups: channels numbered 2 through 13 are called VHF channels (very high frequency) and channels number 14 through 69 are called UHF channels (ultra high frequency).
VHF television channels, FM radio stations and " ham " radio operators have been assigned portions of the VHF band by the Federal Communications Commission. VHF refers to the band of electromagnetic waves that range in frequency from 30 megahertz (30 million cycles per second) to 300 megahertz (300 million cycles per second).
UHF waves are very high frequency waves. They range from 300 million to 3 billion hertz, or cycles, per second.
VHF television signals are those with a frequency between either 54 megahertz (54 million vibrations per second) and 72 megahertz, 76 and 88 megahertz or 174 and 216 megahertz. UHF signals, on the other hand, have a frequency between 470 and 806 megahertz.
UHF waves, when sent from reflectors or directional antennas, are transmitted in a narrow path and travel in a straight line. UHF waves can also be reflected, like light waves.
UHF waves and microwaves, which are super high, have been developed to their greatest usefulness in television, radar, guided missiles and radio aircraft equipment. These frequencies are far above the ordinary broadcast band and can therefore be used without interference. They are similar to light in that they can be projected into a beam in any direction desired.