Two devices need to be connected with 20 feet of cable, but the cable available does not work at that length. Why?

Two devices need to be connected with 20 feet of cable, but the cable available does not work at that length. Why?

Two devices need to be connected with 20 feet of cable, but the cable available does not work at that length. Why? To understand this, first you must understand the basic cables and what each cable type's capabilities are.

There are four basic cable types. These types are twisted pairs cable, multi-conductor cable, coaxial cable, and fiber optics cable. The type of cable chosen depends on the application the cable will be used for.

Twisted pairs cable consists of pairs of conductors that are twisted together. This cable is specifically intended for signal carrying. This type of cable was invented in the 1880's for the specific intent of wiring up early telephone systems. Twisting the pairs of conductors gives the cable some immunity to interference.

Multi-conductor cable is cable that is made up of many insulated conductors. This type of cable is common in control applications but is almost never used in signal applications.

Cable types listed above are considered balanced line configurations. This means that two conductors are kept very close together, normally by twisting them. Conductors must be the same length, size, and have a constant distance between them.

Coaxial cable is the other popular configuration for cable. The signal on the two conductors in coaxial cable is not the same because the shield carries the ground and the signal. Since the signal is not the same on both conductors this configuration is an unbalanced line. Coaxial cable does not have the immunity to interference that twisted pairs cable does, but the performance may be far stabler with coaxial cable then with twisted pairs cable because of the fact that various parts of the cable are “knitted' together.

There are six basic configurations of shields involved in cable. These are Unshielded, Braid Shields, Serve/Spiral shields, French Braid Shields, Foil Shields, and Combination Shields. Shields are added to prevent noise interference and radiation.

Unshielded cable is used where there is no crosstalk or noise present. There is no shield in this cable type. This configuration can also be used if noise is not important or will not substantially affect the cable signal. These cables are used prevalently for the data world where conductor pairs have extremely tight twists or use conductors that have been bonded together. These high performance pairs of conductors have great immunity to interference and radiation.

Serve/Spiral Shields are manufactured to be extremely flexible. Serve Shield effectiveness are compromised by the fact that every time the Serve Shield is flexed it opens up. A Spiral Shield has an affect on the inductance of the shield and so these shield configurations are normally not used for video, only audio. People who choose this type of cable usually are trading performance in exchange for flexibility.

Braid Shields are manufactured by spinning conductors or groups of wires around a center core. The braiding process is very labor intensive and slow to produce, which makes it the most expensive step of the cable making process. The braiding can be single or double braid, with double braiding being more effective. Since there are always holes where the wires cross, braiding is never 100% effective. The higher the frequency the less effective a Braid Shield is.

French Braid Shields are a combination of a Serve Shield and a Braid Shield. This shield consists of two Serve Shields braided along a single axis. This shield gives excellent flexibility and RF performance. The French Braid Shield takes the same effort and material as a Braid Shield so the manufacturing process costs roughly the same for both.

Foil Shields are the cheapest and easiest cable shield to apply. They consist of two different layers. One layer is metal and the other layer is a polyester substrate. This shield can be applied just as quick as the cable can be run. These shields show performance for low-frequency to be poor but performance for high-frequency to be excellent. Since Foil Shields are made with foil that has no holes, they can be 100% effective.

Combination Shields are cable shields that use a combination of braiding and foil. Sometimes there are multiple layers of each. Combination Shields are the most expensive of all shields. These shields give the best broadband coverage because the foil works for high frequencies and the braiding works for low frequencies.

Fiber optic cable is divided into three kinds: Plastic Fiber, Multi mode Fiber, and single mode Fiber. Plastic fiber is the largest fiber in fiber optic cable, is made of plastic, and is usually used in high end audio signals. Multi mode fiber is made of glass, ranges in diameter, and is used in the data world. Single mode fiber is considered the king, or the best, because the fiber is too small to see without a microscope. This fiber gives the best performance but is very difficult to connectorize because of fiber size and hardness.

The usual specification length of maximum run for HDMI cable is normally about 50 feet, or approximately 15 yards. The maximum cable run that will be efficient will be different for each individual cable, depending on the quality specifications of the cable manufacturer. A higher quality cable will cost more, but will also run further before any additional devices and technology are needed. Generally, the thicker the copper wire is in the pairs, the more costly the cable is, but the longer that cable can be run before experiencing signal problems.

If the cable that is being used does not work properly, then it is the wrong cable for the application it is being used for. Check the cable specifications and switch to an appropriate cable for this application use.