Quality RF Services, Inc. CATV / MATV Design Fundamentals

Page 3

Calculating Cable and Passive Losses Between Amplifiers
The nominal cable losses per 100 feet of some popular cable sizes is given in the chart below.  * The 0.412" and 1.000" cable losses are given as the maximum cable loss per 100 feet.   To discover the loss of a given length of cable at specific CATV channels or frequencies, divide the length of cable in feet by 100, then multiply by the appropriate dB figure from the chart below.  Doing this at the highest and lowest frequencies to be distributed will provide the losses needed to select a cable equalizer when reaching successive RF amplifiers.  This topic is discussed in another article on this web site called Fixed Equalizer Selection.  The formula for computing cable equalizer selection is explained and tables of signal level difference solutions are provided.
Cable Loss vs. Frequency Times Fiber T10 Drop Cable Maximum Losses @ 68 deg.F CommScope Parameter III Nominal Cable Attenuation at 68 degrees Farenheit in dB per 100 feet by cable sizes RG-59 RG-6 RG-11 .412" .500" .625" .750" .875" 1.000" 5 MHz 0.77 0.57 0.36 0.20 0.16 0.12 0.10 0.09 0.08 30 MHz 1.45 1.15 0.75 0.50 0.38 0.31 0.25 0.23 0.21 50 MHz 1.78 1.48 0.93 0.63 0.50 0.40 0.33 0.28 0.27 220 MHz 3.60 2.87 1.83 1.38 1.08 0.87 0.72 0.62 0.62 300 MHz 4.27 3.43 2.17 1.63 1.26 1.02 0.85 0.73 0.72 400 MHz 4.88 4.00 2.53 1.90 1.47 1.18 0.99 0.86 0.84 450 MHz 5.30 4.28 2.69 2.05 1.56 1.26 1.06 0.91 0.90 550 MHz 5.90 4.51 3.01 2.25 1.75 1.41 1.19 1.03 1.01 600 MHz 6.18 4.98 3.16 2.36 1.83 1.48 1.23 1.08 1.06 750 MHz 6.96 5.62 3.58 2.55 2.04 1.66 1.38 1.21 1.21 865 MHz 7.54 6.09 3.90 2.84 2.20 1.77 1.49 1.30 1.34 1000 MHz 8.09 6.54 4.23 3.05 2.41 1.95 1.62 1.42 1.44
Similar cable loss data is published by CATV amplifier companies in handy pocket reference books for Times Fiber Cable and Trilogy Communications MC˛ (M-C-squared) coaxial cable types.  The above chart is given as an example only.  The user should refer to the correct cable loss charts for actual signal loss calculations.
The following table gives typical insertion losses for Scientific Atlanta SAT4G-(db) family of 4-outlet taps. Freq (MHz) -8 -11 -14 -17 -20 -23 -26 -29 -32 5 to 10 N/A 3.2 1.6 1.0 0.6 0.5 0.4 0.3 0.3 11 to 300 N/A 3.0 1.7 1.0 0.7 0.7 0.5 0.5 0.3 301 to 400 N/A 3.2 1.9 1.3 0.9 0.9 0.6 0.6 0.5 401 to 450 N/A 3.5 2.0 1.4 1.1 1.0 0.8 0.7 0.7 451 to 600 N/A 3.7 2.2 1.5 1.2 1.1 0.9 0.8 0.8 601 to 750 N/A 4.1 2.6 1.8 1.5 1.4 1.3 1.2 1.2 751 to 900 N/A 4.3 3.0 2.2 1.7 1.7 1.5 1.4 1.4 900 to 1000 N/A 4.7 3.5 2.5 2.0 1.0 1.8 1.6 1.6 Tap Loss = 8 dB 11 dB 14 dB 16 dB 20 dB 23 dB 26 dB 29 dB 32 dB
The above data table contains the insertion loss through the tap to the next cable or tap at the frequency ranges specified.  The tap loss is the signal level loss that occurs from the input of the tap to each of the output ports to feed subscriber drop cables.  This chart indicates the loss differences based on frequency and tap values and will vary from one tap manufacturer to another.  Indoor taps that do not contain AC power passing circuitry may have better specifications.  Similar charts exist for 2-port and 8-port taps as well as for inline directional couplers and 2-port and 3-port splitters.  In the chart above, the 8 dB tap is a 4-way splitter and therefore has no through-port insertion loss, since all of the RF signals are directed to the 4 tap ports.  ( Click here for page FOUR )

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