equivalent of a coaxial cable

[student52]

Hi all,

I am designing a electrical equivalent of a coaxial cable. I have data of conductor resistance and Insulation resistance for required distance d. How to calculate effective resistance? I think they should be considered to be in series? Am I right ? Please elaborate!!!!!!!!!

Thanks in advance,

Regards.

 

[FvM]

Electrical equivalent in which regard? Modelling the transmission line properties? Ideal or lossy transmission line? Dielectric leakage can be usually ignored. For a lossy transmission line, frequency dependent losses are more important than DC resistance. Characteristic transmission line impedance is a completely different thing, created by specific L and C per length unit.

 

[student52]

I want to measure attenuation of signal through that cable so I would like to know its equivalent circuit. I have details from vendor like insulation resistance, conduction resistance. I have a circuit with me but not sure if it is apt. Please check this circuit and let me know if it is right or not. If it is okay how to calculate resistance? I think it should be series of insulation and conduction resistance.

--------inductance-------resistance-----
|
|
|
c
a
p
|
|
--------------------------------------|

 

[FvM]

Dielectric resistance is parallel to cable capcitance and surely not in series with copper resistance. As said, you can ignore it for the time being (unless your circuit impedance is in a Gohm range).

For higher frequency, the cable needs to be modelled by mutiple series connected LRC segments. You can refer to the lossy line model of circuit simulation programs like LTSpice.

 

[cks3976]

Try to get the Student version of HyperLynx from mentor Graphics (The EDA Technology Leader - Mentor Graphics). Modeling of transmission line can be done in a very systematic way in that tool and you can also probe on intermediate points of the cable to examine the losses, impedance variation etc.,

 

[volker_muehlhaus]

--------inductance-------resistance-----
|
|
|
c
a
p
|
|
--------------------------------------|

Ok for the series path with inductance and conductor losses.
For the dielectric losses, you need a resistor in parallel to the capaciator.

This is appropriate for a line segment that is shorter than 1/20 wavelength or so. For higher frequencies/longer lines, you need more segments cascaded.