Start with traceable identification, not cable thickness or connector appearance. A readable cable or connector part number can usually be checked against its datasheet. If the markings are missing, a multimeter can check continuity and shorts but cannot identify 50 Ohm or 75 Ohm characteristic impedance. An unmarked cable can then be verified with a TDR or equivalent controlled impedance measurement.
50 Ohm vs 75 Ohm Identification Path
Use the strongest information available first. A complete part number that leads to a datasheet is a much stronger identification point than cable color, diameter, connector finish, or a resistance reading.
| What You Have | What to Check Next | Identification Confidence |
|---|---|---|
| Full cable part number | Find the exact datasheet and read the characteristic impedance specification | High |
| Partial cable marking or RG designation | Use it to identify the exact cable product, then confirm the datasheet | Medium until the exact product is confirmed |
| Full connector part number | Check nominal impedance and compatible cable in the connector datasheet | High |
| Connector appearance only | Look for a part number, drawing, packaging record, or cable specification | Low |
| Multimeter only | Check conductor continuity and center to shield shorts | Does not identify 50 or 75 Ohm characteristic impedance |
| No markings or documentation | Measure the cable section with TDR or equivalent impedance measurement | High when the cable section can be measured clearly |
Identify the Cable from Its Marking
The most useful jacket information is a manufacturer or product number that can be traced to an exact datasheet. After locating the product, look in the electrical specifications for Characteristic Impedance or Nominal Characteristic Impedance. That field provides a direct confirmation of whether the cable is specified as 50 Ohm, 75 Ohm, or another impedance.
An RG designation is useful as a lookup key, but it is not the final confirmation. RG58 products are commonly found in 50 Ohm cable families, while RG59 and RG6 are commonly associated with 75 Ohm cable families. Use the RG designation to narrow the search, then confirm the exact cable from its product specification.
If only part of the jacket print remains readable, record it before removing the cable. A partial model number, RG designation, approval marking, or manufacturing code may still lead to the original specification. If the remaining text cannot be tied to one exact product, keep it as a clue and continue with the next identification step.
What Counts as Confirmation
Strong identification
- Exact cable part number with a matching datasheet
- Datasheet showing nominal characteristic impedance
- Original cable drawing or equipment documentation with a traceable cable part number
- Jacket marking that directly identifies an exact cable specification
Useful clues, but not final confirmation
- RG designation
- Cable diameter
- Equipment type
- Connector family
Cable color, jacket color, connector plating, or approximate thickness is not enough to assign a 50 Ohm or 75 Ohm rating.
Check the Connector Without Guessing from Appearance
A connector family can contain more than one impedance version. BNC is a common example because both 50 Ohm and 75 Ohm versions are available. Visual appearance alone is not a dependable way to determine which version is installed.
When a connector part number is readable, check its nominal impedance and compatible cable in the corresponding specification. The connector part number is more useful than plating color, shell finish, or a visual comparison of the center contact.
Physical mating also does not confirm impedance. A 50 Ohm and a 75 Ohm connector may share a mechanically compatible interface in some configurations, while the electrical characteristics remain different. A connector that fits the port therefore still needs its impedance specification checked.
When the Cable Has No Reliable Identification
A multimeter can help check whether an unknown coaxial cable has a basic wiring fault, but it cannot identify 50 Ohm or 75 Ohm characteristic impedance. Use it to check center conductor continuity, shield continuity, and whether there is a direct short between the center conductor and shield. These checks can confirm basic cable condition, but they do not establish transmission line impedance.
When no cable marking, part number, or documentation can identify the cable, TDR provides the more useful verification method. The practical goal is to read the stable cable section rather than the first local change created by the test connector, adaptor, or launch. In other words, the useful impedance result comes from the main cable body, not from the first spike or dip at the test connection.
A practical reading sequence is simple: first identify the transition created by the test connection, then locate the more stable section that represents the cable body, and finally read whether that section is closer to 50 Ohm or 75 Ohm. If the cable is very short and the stable cable section cannot be separated clearly from the connector transition, the cable should remain classified as unidentified rather than being assigned a 50 Ohm or 75 Ohm rating from one local feature.
FAQ
Conclusion
Identify a 50 Ohm or 75 Ohm coaxial cable from the strongest available evidence. Start with the cable or connector part number and confirm the nominal impedance in the corresponding specification. Treat RG designations and appearance as clues, not final proof. A multimeter can check continuity and shorts but cannot identify characteristic impedance. If no traceable identification remains, use TDR or another suitable impedance measurement to verify the cable section before assigning a 50 Ohm or 75 Ohm rating.