Typical Coax Cable Technical Features
The largest single passive device in an RF distribution system is the coaxial transmission cable itself. The purpose of the tansmission cable is to carry the RF signal with a minimum amount of loss. At the RF frequencies involved in CATV and MATV distribution systems, however, characteristics of the cable and losses in the cable must be taken into careful consideration. The Frequency Selective Voltmeter provides a good way of checking coaxial cable to determine if it is performing as it should with minimum loss.
One of the losses associated with coaxial cable is signal leakage. Signal leakage occurs when the coaxial cable can not contain the whole RF signal, and allows some of it to leak out into free space. Leakage loss should be identified and corrected. This is discussed in an earlier portion of the Application section.
Two other types of cable loss, dielectric loss and resistance loss. All coaxial cables have a specific amount of dielectric and resistance loss. These losses are taken into account when the distribution system is designed and built. Any changes in these parameters after the system is operating, however, may severely affect the performance of the distribution system. For this reason, it is important to briefly review cable loss and its affect on a distribution system in order to better understand how the Frequency Selective Voltmeter may be used to check for cable loss.
Resistance loss is by far the largest contributor of llosses in coaxial cable. Losses caused by the resistance of the inner conductor vary with the cross sectional area of the conductor. Most of the loss, however, is frequency related, a condition called "skin effect." Skin effect describes the condition where, as the frequency of the signal increases, the signal is carried through the conductor further and further away from the center. Thus, the resistance loss in any given cable type varies in direct proportion to the frequency of the RF signal-the higher the frequency the greater the loss.
Table above lists some commonly used RF distribution cables and the typical losses for each. Use this chart as a guide to determine the normal amount of attenuation to expect in a piece of cable. Any attenuation which differs substantially from this amount indicates a problem with the cable, such as a poor connector, physical damage such as a sharp crimp or bend, or moisture in the cable.
Using the chart for an example, we see that 30 m of RG 59/U cable normally attenuates the signal at channel 2 by 2.6 dB. Measuring a 30 m drop of RG 59/U cable with the Frequency Selective Voltmeter, however, indicates 6.2 dBmV at one end of the cable and 1.1 dBmV at the subscribers drop. This 5.1 dB loss indicates a problem somewhere within the length of drop cable.
We are often asked: "How long can the cable be from our Active Antennas to the Headend ??"
Coax Cable "Tilt" or "SLOPE"