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Chapter 3. Coaxial Cable

Chapter 3. Coaxial Cable

Coaxial cables have been used to construct the cable TV distribution network. At the beginning, a cable TV distribution network was very simple and consisted of coaxial cables connecting an antenna on a hillside to many homes that did not get good reception using their rooftop antennas. To compensate for signal losses caused by branch splitting and cable attenuation, TV signals picked up by the hillside antenna are usually enhanced by a wide-band amplifier, whose bandwidth covers TV channels of interests, raising the signal level by 10 to 25 dB. The off-the-air TV channels of VHF (Very High Frequency) band cover frequencies from 54 to 72 MHz (channels 2, 3, and 4), 76 to 88 MHz (channels 5 and 6), and 174 to 216 MHz (channels 7 to 13). A wide-band amplifier might only cover a group of off-the-air TV channels and a number of wide-band amplifiers; signal splitters and combiners might be required to cover all TV channels of interests. There are also off-the-air TV channels allocated in the UHF (Ultra High Frequency) band of between 300 and 3000 MHz. UHF TV channel signals over a cable TV distribution network also need to be amplified and sometimes down-converted to an unused VHF TV channel.

Within a particular TV broadcast region, there are always some spare TV channels available. In fact, off-the-air TV programs in the same region are not allocated next to each other to avoid interference between adjacent channels. Exceptions are channels 4 and 5 and channels 6 and 7 because there are already frequency gaps between them. A TV channel signal usually has a little energy, at below –42 dB, leaked into its adjacent channels. According to the regulation, the carrier-to-noise or to adjacent channel interference ratio should be larger than 36 dB to maintain good-quality TV reception. Because of different transmission conditions, TV signals from different antenna sites of the same region can easily have a signal strength difference of more than 12 dB. That situation could result in a Signal-to-Noise Ratio (SNR) of less than –42 + 12 = 30 dB. Therefore, the leakage signal of an off-the-air TV channel with a relatively high signal level could disturb that of an adjacent TV channel if they occupy adjacent channels without any frequency gap in between. On the other hand, TV channels can be placed next to each other in a cable TV system as long as signal strengths of adjacent channels are adjusted to similar levels.


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