RG-6 Cable Loss by Frequencyby Frank Gates
RG-6 cable is the coaxial cable most commonly used to distribute cable television or DBS (Direct Broadcast Satellite) signals throughout the home. RG-6 is very popular because it loses less signal than its smaller-diameter brother RG 59 and is more flexible than its larger-diameter brother RG 11. As signal travels through RG-6 cable, it loses strength. The farther it must travel, the more strength it will lose. How fast the strength dissipates through the cable depends not only on the characteristics of the cable but on the frequency of the signal. Lower frequencies will travel farther through this coaxial cable than higher frequencies.
In 1923, American Telephone and Telegraph Company introduced the decibel (dB). Named after Alexander Graham Bell, the decibel is 1/10 part of a "Bel." It is important to understand and know what a decibel is because signal strength in coaxial cable is measured in decibels. The decibel is a measure of power (strength) and it measures ratios, not absolute units like volts or ohms. For example, you might say, "The loss through this piece of RG-6 cable is 3 dB or one-half of the signal strength." Or you might hear, "There is 10 dB of the signal available at the beginning of this piece of RG-6 cable, and after 100 feet there is only 3.5 dB of signal left." You can see that these examples are talking about ratios of signal strength. Measuring signal strength in decibels is very popular because you can simply add or subtract instead of multiply or divide different measurements. And while these examples explain what happened to the signal as it traveled through the coaxial, they are still not entirely accurate without identifying the frequency of the signal.
Using U.S.A. TV channels as an example, channel 2 is located at 54 MHz and channel 69 is located at 806 MHz. Any cable that can carry all of the signals from channel 2 through channel 69 will have a bandwidth of 752 MHz (806 MHz minus 54 MHz). Coaxial cable loses more signal at the higher frequencies than it does at the lower frequencies; 806 MHz would be the frequency to be measured in dB. Here is an example of RG-6 loss across three specific frequencies: 55 MHz/100 feet equals 1.6 d six B of loss, 1,000 MHz/100 feet equals 6.5 dB of loss and finally 2,250 MHz/100 feet equals 10 dB of loss. You can see that the highest frequency (2250 MHz) loses six times the signal strength than the lowest frequency (55 MHz).
Measuring and Calculating Loss
You can measure signal loss in dB by using an expensive signal strength meter. These meters allow you to specify the frequency that you would like to measure, or they may give you the entire bandwidth measurement in a numeric and/or graphic display. A less expensive option is to use an online calculator. There is a link in the Resources section to the Times microwave site, which offers a free calculator. With these calculators it's a matter of simply entering the manufacturer's published loss characteristics for the cable and the frequency and cable length that you are using. Another option is to calculate this loss by hand, using logarithmic tables. This is an extremely complex calculation and it is rarely performed in the field.
RG-6 Loss Table
There are numerous loss tables available online through the cable manufacture. If you check the DBS Install link in Resources, you will find a loss table for RG-6 cable. Here is an extract from the table for RG-6 Quad CCS cable: 55 MHz/100 feet equals 1.6 d six B of loss, 1,000 MHz/100 feet equals 6.5 dB of loss and finally 2,250 MHz/100 feet equals 10 dB of loss. Keep in mind that not all RG-6 cables are identical. Different shielding, different applications that require different cable construction techniques can all have an impact on the loss characteristics of the coaxial cable.
- photo_camera Ryan McVay/Photodisc/Getty Images