Archives

The Physics of Loss And Temperature
By David Devereaux-Weber

• Joseph A. Ferreri, Times Fiber
• Randy Altergott, Cox in Phoenix
• The point

	David Devereaux-Weber

As often happens on the List, he got numerous responsesalmost 30. Here are two edited answers.

Joseph A. Ferreri, Times Fiber

The answer resides in the discussion of resistivity of metals in the conductors. The resistivity (Rho) of a material is defined as the electric field (E) required to produce a unit current density (J). The resistivity can be written mathematically as Rho = E/J.

A practical definition of resistivity is the resistance of a length of wire 1 foot by 1 mil in diameter, measured at 20° C. The resistance of a conductor increases as temperature rises because of the increased molecular movement within the conductor, which hinders the flow of charge (current).

The resistance of a material will increase almost linearly (straight line) as temperature rises. Conversely, the resistance of a material will decrease linearly as temperature falls until the inferred temperature of zero resistance for that material (-234.5° C for copper and -228.1° C for aluminum).

The "loss" or attenuation of a coaxial cable is defined by parameters such as impedance, dissipation factor, frequency, conductor dimensions and conductor resistivities.

Because the resistivities of the inner and outer conductor reside in the numerator of the attenuation equation for coaxial cable, the attenuation is proportional to the square root of conductor resistivity. As defined by the physics/mathematics, the attenuation of coaxial cable increases with increasing temperature (and vice versa).

Randy Altergott, Cox in Phoenix

Ive been researching attenuation in detail for the last three weeks, and here are some answers Ive found.

Attenuation stems from several basic factors:

1) The number of valence electrons in each atoms most distant orbit. The fewer valence electrons, the better the conductivity1 electron for copper.

2) The length of the conductor. Longer length equals more resistance.

3) The diameter of the conductors. Larger diameters equal less resistance.

4) Inductive reactance. Even a single, straight conductor exhibits some inductance, which creates a magnetic field. This magnetic field (also known as the countermagnetomotive force, or CMMF) opposes the normal flow of electrons in a circuit and is measured in ohms.

5) Capacitive reactance. The capacitive properties of a conductor, such as coaxial cable, also increase the opposition to current in a conductor. This opposition to current also is measured in ohms.

6) The temperature of the conductor. Yes, you were correct. The temperature of the conductor determines the density of the atoms that make up the conductor. At colder temperatures, the density of the conductor increases, making it easier for electrons to move from one atom to the next (since the atoms are closer together). Higher temperatures actually decrease the atomic density, which causes an increase in resistance.

7) Frequency. As you already know, the higher the frequency, the higher the attenuation.

The point

Its pretty safe to say that Mays got an answer to his question, possibly far more answer than he expected. This is what the List is all about, thoughproviding a forum in which we can help each other solve problems and learn more about our business.

Dave Devereaux-Weber, P.E., is a network engineer at the University of Wisconsin-Madison. He is a senior member of the SCTE, and he can be reached via e-mail at .

Back to August Issue

Access Intelligence's CABLE GROUP
Communications Technology | CableFAX Daily | CableFAX's CableWORLD | CT's Pipeline
CableFAX Magazine | CableFAX databriefs | Broadband Leaders Retreat | CableFAX Leaders Retreat