In a 50-ohm BNC-BNC coaxial cable, is 50 ohm the resistance between the core and the outer mesh?

Thread Starter

vishnu.mec

Joined Aug 6, 2010
23
This is for calculating the load for a power source. Suppose I have a 50 V DC source with an output resistance of 10 ohms (let's consider a purely resistive situation). Now, what will be the current through the circuit if I connect this source to a 100-ohm resistor via a 50-ohm BNC-BNC coaxial cable? Will it be \[ \frac{50 V}{10+50+100} \] or \[ \frac{50 V}{10+\frac{50*100}{50+100}} \] ?
 

DickCappels

Joined Aug 21, 2008
9,524
The resistance of the cable is a minor part of the impedance. The impedance is largely determined by the distributed capacitance and distributed inductance of the cable or transmission line. A termination equal to the cable’s characteristic impedance will prevent the power fro being reflected back toward the signal source.
 

WBahn

Joined Mar 31, 2012
27,946
The DC resistance of coax depends on the specific coax and the length used. But you will need quite a bit of it (perhaps 500 feet) before the total resistance of the core going out and the sheath coming back will get to even 1 Ω.

The "50 Ω" is the "characteristic impedance" of the transmission line at high frequencies and is dominated by the sqrt of the ratio of inductance to capacitance. If you look at that ratio, you will see that the units turn out to be those of impedance, namely volts/ampere.
 

MrChips

Joined Oct 2, 2009
27,742
This sounds like a trick question in a job interview.

The DC resistance of a 50Ω coaxial cable is not 50Ω.
You can make the assumption that the DC resistance along the central conductor of a short length of cable is 0Ω.
 

panic mode

Joined Oct 10, 2011
2,366
neither...

the 50 Ohm is the characteristic impedance of the cable, when it is used as a transmission line (for high frequencies). DC is the opposite end of the spectrum. inductive and capacitive effects practically do not exist when used with pure DC so you should see open circuit between coax core and the shield. resistance of conductors themselves (core or shield) will be very low and it usually can be considered to be close to zero Ohm.
 
Last edited:

crutschow

Joined Mar 14, 2008
31,156
Measure the end to end resistance of both connections, and the resistance from the center conductor to shield, to see what you actually have.

Below are the characteristics of a common 50Ω coaxial cable:
1666017470303.png
 

Lo_volt

Joined Apr 3, 2014
280
A cable with no termination at the far end will have very high DC impedance controlled by the leakage current (very, very small) through the insulation dielectric. You won't be able to measure it with a normal ohmmeter or multimeter. For the calculations you are asking about it can be considered an open circuit.

Others are mentioning characteristic impedance, etc. Don't be confused. They are talking about reactive impedance.
 

Thread Starter

vishnu.mec

Joined Aug 6, 2010
23
Thank you very much for all your replies. I understood that the coaxial cable's impedance is mostly caused by the capacitive and inductive reactances, which can be neglected for a DC signal.
 
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