# primary line constants

Discussion in 'General Electronics Chat' started by gorgondrak, Nov 23, 2014.

1. ### gorgondrak Thread Starter Member

Nov 17, 2014
61
1
Is resistivity (rho) measured in ohm-meters , used as a primary line constant to find characteristic impedance?

2. ### MrChips Moderator

Oct 2, 2009
12,636
3,455
Characteristic impedance is a function of resistance and reactance. It is independent of the length of the transmission line.

See Characteristic Impedance.

3. ### gorgondrak Thread Starter Member

Nov 17, 2014
61
1
Is specific resistance used as a primary line constant to calculate characteristic impedance? If so, is there also such a thing as specific inductance?

4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
I'm not familiar with the use of the qualifier 'specific' in this context. By analogy to other fields where the modifier is used e.g. "specific gravity" or "specific heat", I'm going to take a wild guess and assume that you mean the distributed parameters of resistance, inductance, and capacitance. The distributed parameters have units of ohms/meter, henries/meter, and farads/meter. You can also speak of distributed reciprocal parameters of conductance, susceptance, and admittance with their appropriate units. These parameters are used to construct the differential equation of a transmission line. In the process of analyzing that differential equation certain combinations of things show up as a constant. Characteristic impedance is one of those constants. As you can see from the following article on coaxial cable:

http://en.wikipedia.org/wiki/Coaxial_cable

It can be derived from the distributed properties of inductance an capacitance, or from the geometric properties of the coaxial cable itself. Naturally there is a correction for a cable with non-negligible resistance, and there are completely different geometric formulas for other cable constructions.

I suggest Gonzales, Microwave Transistor Amplifiers, [1997], for a rigorous and thorough treatment of transmission lines.

http://www.amazon.com/Microwave-Transistor-Amplifiers-Analysis-Design/dp/0132543354/ref=sr_1_sc_1?ie=UTF8&qid=1416796315&sr=8-1-spell&keywords=Microwave Transistor Amplifers