RG58 Impedance - Page 1
Forum section: electronics / beginners

author: Shocker
author_rank: Contributor
timestamp: January 19, 2016, 04:52:48 pm
content: Hi,What is the DC resistance of the cable? And what is the impedance? Any datasheets or anything official that states these?I think the DC resistance is about 0.05 ohms per foot and 100pF/m, 250nH/m, 50 ohms characteristic impedance but i'm not sure how correct these values are because i obtained them from a forum.Thanks

author: TimFox
author_rank: Super Contributor
timestamp: January 19, 2016, 05:04:17 pm
content: Are you looking for mil-spec RG-58/U cable or one of many variations?There are important differences between resistance and attenuation/length..This is Belden's spec sheet for their 9201, which is RG-58/U:http://www.belden.com/techdatas/metric/9201.pdf.Here is another manufacturer's comparison of variations on RG-58:http://www.prioritywire.com/specs/c-3.pdf

author: Shocker
author_rank: Contributor
timestamp: January 19, 2016, 05:32:18 pm
content: Quote from: TimFox on January 19, 2016, 05:04:17 pmAh cheers.Resistance differs from Atten/length because Atten/length is frequency dependent, yes?The conductance of the cable, is that as simple as 1/DC resistance? If not, how does one calculate it?G = 1/(Zo^2/(R+wL)) -C ?

author: TimFox
author_rank: Super Contributor
timestamp: January 19, 2016, 11:47:47 pm
content: Yes, attenuation is frequency dependent. The frequency-dependent variables that enter into it include skin effect in the conductors and dielectric loss in the insulation. Foamed polyethylene tends to have lower dielectric loss and therefore lower attenuation (all other things being equal).The characteristic impedance of the cable depends only on the capacitance and inductance per unit length. For cables that do not include permeable (ferrous) material, the velocity depends only on the dielectric constant of the insulation. See the normal textbooks or Wikipedia for the formulae.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 20, 2016, 12:54:10 am
content: Quote from: Shocker on January 19, 2016, 04:52:48 pmThe DC resistance depends on the gauge of the wire and the length. It is generally quite low (a few ohms or less than than 1 ohm)."RG58" is a generic description for a type of coaxial cable. Different physical sizes and different manufacturers may have somewhat different DC resistance. Of course we are talking about the DC resistance between the center conductor at one end, and the center conductor at the other end. Or the outer shield at one end over to the outer shield at the other end. The DC resistance between the center conductor and the outer shield is close to infinity (open circuit).QuoteThe "nominal" impedance of RG58 is 50 ohms. At some point in ancient history it may have referred to 52 ohm cable as well.Quotehttps://en.wikipedia.org/wiki/RG-58And each different brand of cable will likely have a data sheet for their various kinds of RG58.Google should return the data sheet if you tell it the brand and "RG58"QuoteYou can't state the DC resistance as any sort of given. It depends on the wire gauge and the length.Also, the capacitance depends on the length and the construction of the particular cable.There are many sources of official data available on the internet. You don't need to rely on the opinions of others.

author: LukeW
author_rank: Frequent Contributor
timestamp: January 20, 2016, 02:50:51 am
content: If you care about having very well defined information for your cable, rather than just "generic RG58" properties, then get the datasheet from the manufacturer you're buying the cable from. And/or take a sample of the cable and measure its properties with appropriate instruments.250 nH/m and 100pF/m is sort of roughly in the right ballpark, but it depends on exactly which RG58 variant it is. Some of them are 52 ohms rather than 50.sqrt(L/C) will give you the characteristic impedance of the cable, which is obviously 50 ohms for the above numbers.

author: TimFox
author_rank: Super Contributor
timestamp: January 21, 2016, 02:56:10 pm
content: Again, "RG-58/U" is an official military designator for a well-defined cable."RG58" is a generic number that can refer to a range of similar cables.