(CABLE TELEVISION) 7/8 Air dielectric Zo = 75 S Andrew HJ series Frequency (MHz) Figure 17.5 Attenuation-frequency response for 7/8-in. coaxial cable, Zo = 75 S2, Andrew HJ series helix. (Courtesy of Andrew Corp.) 1-in. 1 coaxial cable. Such a length of cable would have 5.45-dB loss at 54 MHz and 13-dB loss at 300 MHz. The equalizer would probably present a loss of 0.5 dB at 300 MHz and 8.1 dB at 54 MHz. 17.3.8 Taps A tap is similar to a directional coupler. It is a device inserted into a coaxial cable, which diverts a predetermined amount of its RF energy to one or more tap outputs for the purpose of feeding a TV signal into subscriber drop cables. The remaining balance of the signal energy is passed on down the distribution system to the next tap or distribution amplifier. The concept of the tap and its related distribution system is shown in Figure 17.6. Trunk amplifiers Bridger amplifier Headend +10.5 dBmV To Subscriber TV Sets Figure 17.6 A simplified layout of a CATV system showing its basic elements. The objective is to provide a + -10.5-dBmV signal level at the drops (tap outputs). LEA, line extender amplifier. 17.4 HYBRID FIBER-COAX (HFC) SYSTEMS Taps are available to feed 2, 4, or 8 service drops from any one unit. Many different types of taps are available to serve different signal levels that appear along a CATV cable system. Commonly, taps are available in 3-dB increments. For two-port taps, the following tap losses may be encountered: 4, 8, 11, 14, 17, 20, and 23 dB. The insertion loss for the lower value tap loss may be on the order of 2.8 dB and, once the tap loss exceeds 26 dB, the insertion is 0.4 dB and remains SO as tap values increase. Another important tap parameter is isolation. Generally, the higher the tap loss, the better the isolation. With an 8-dB tap loss, the isolation may only be 23 dB, but with 29-dB tap loss (two-port taps), the isolation can be as high as 44 dB. Isolation in this context is the isolation between the two tap ports to minimize undesired interference from a TV set o