RG58 Coax Cable: The 50 Ohm Exception
Quick answer / TL;DR: Short answer: RG58 is a 50 ohm coaxial cable with a stranded centre conductor, commonly 20 AWG, used with BNC connectors for test equipment, radio, laboratory RF and legacy 10BASE2 thin Ethernet. It is not interchangeable with RG6 or RG59, which are 75 ohm. Mismatching impedance at a connector causes reflections and standing waves, so the cable must match the port, not merely fit it. RG58 is also a poor choice for long runs because its loss at high frequency is high.
Three coaxial families: BNC for CCTV, F-type for TV, N for RF – each with its impedance.Almost every coax cable in a building is 75 ohm: video, CATV, satellite. RG58 is the common exception at 50 ohm, and that single number changes what it can be connected to.
It survives because a large installed base of lab equipment, radio gear, test instruments and legacy thin Ethernet still expects 50 ohm with a BNC connector. Buying it by appearance rather than by impedance is the mistake that produces a working-looking link with poor signal quality.
The Impedance Number Is the Specification
Impedance is set by the ratio of centre conductor diameter to dielectric diameter, and it is not a quality grade. A 50 ohm cable and a 75 ohm cable can look similar in a photograph and behave completely differently in a circuit, because the reflection coefficient at a mismatch is determined by the two impedances.
| Cable | Impedance | Centre conductor | Typical connector | Primary use |
|---|
| RG58 | 50 ohm | 20 AWG stranded | BNC | lab RF, radio, 10BASE2, test leads |
| RG59 | 75 ohm | 22 to 23 AWG | BNC or RCA | short video runs, legacy CCTV |
| RG6 | 75 ohm | 18 AWG | F type | satellite, CATV, CCTV |
| RG11 | 75 ohm | 14 AWG | F type or compression | long 75 ohm runs |
A 50 ohm source into a 75 ohm cable produces a reflection of about 20 percent of the incident voltage at the transition. In a video system that shows as ghosting or a soft picture; in a digital system it shows as a link that works at short length and becomes unreliable as the reflection interacts with the data.
Where RG58 Still Earns Its Place
Test and measurement
Bench instruments, signal generators and oscilloscopes commonly present 50 ohm. A short RG58 jumper with BNC connectors is the correct lead: it matches the port, it is flexible enough to route on a bench, and the loss over one or two metres is irrelevant.
Radio and antenna feeds
Amateur radio, public address links and short antenna feeds often use 50 ohm with BNC or PL259 connectors. RG58 is acceptable for short runs at moderate frequency, but its loss rises quickly with frequency, so a longer feed at VHF or UHF usually calls for a foam dielectric cable with a larger conductor.
Legacy thin Ethernet
10BASE2 ran on 50 ohm coax with BNC T pieces and terminators, limited to segments of about 185 metres. It is obsolete as a network technology but still present in industrial equipment and legacy control systems, which is where most RG58 demand now comes from.
Laboratory and instrument interconnection
Where a signal is a low-level measurement rather than a video feed, the shield quality and the connector repeatability matter more than the loss. A double-shielded RG58 variant is worth the small extra cost in that role.
Why the Wrong Cable Looks Like It Works
A mismatched cable will often pass a basic continuity test and even deliver a picture. The failure appears as degradation rather than absence: reduced level, visible ghosting on analogue video, higher bit error rate on a digital link, or a device that works until the cable is moved.
- Check the port marking first: 50 ohm ports are usually BNC, 75 ohm video ports are BNC or F type, and BNC alone does not tell you the impedance
- Never assume a BNC connector means 50 ohm: 75 ohm BNC exists and is common on video equipment
- Keep 50 and 75 ohm stock physically separated, because the connectors look identical at a glance
- Label both ends of every jumper with the impedance, especially in a shared lab
- When adapting between impedances, use a proper matching pad or transformer rather than a passive plug
Loss and Length
RG58 has a relatively small centre conductor and a solid polyethylene dielectric, so its attenuation at high frequency is significant compared with larger cables. The practical consequence is short runs: a few metres on a bench is fine, tens of metres at UHF is not, and any run where the receiver sensitivity is close to the signal level needs a bigger cable.
The stranded centre conductor is a deliberate trade. It makes the cable flexible and resistant to fatigue, which suits a test lead that is handled constantly, at the cost of slightly higher loss than a solid conductor of the same gauge.
Real-World Applications
Bench jumper between instrument and device under test
Keep it as short as the bench allows, use BNC connectors matched to the port impedance, and replace jumpers that have been flexed repeatedly. A jumper that has developed a partial shield break shows as noise that appears only when the cable is moved.
Legacy industrial control link
Where 10BASE2 equipment is still in service, the segment needs 50 ohm terminators at both ends and T pieces at each node. Adding a node without a T piece, or removing a terminator, breaks the segment rather than degrading it. Keep a spare terminator on site, because they are the part that goes missing.
Short antenna feed on a radio installation
For a run of a few metres at moderate frequency, RG58 with the correct connector for the radio is adequate. For a longer feed, specify the loss at the operating frequency rather than at 100 MHz, because the difference between those two figures is where installations fail.
Standards and Verification
RG58 is defined by published construction and interface standards, which is what makes impedance and connector compatibility checkable.
- MIL-C-17: the Radio Guide designation system that defines RG58 construction, impedance and conductor form
- IEC 61196 series: coaxial communication cable construction and test methods, including characteristic impedance and attenuation
- IEC 61169 series: coaxial connector interfaces, including the 50 ohm and 75 ohm BNC variants
- IEEE 802.3 clause for 10BASE2: segment length, termination and cable requirements for thin Ethernet
- UL 444 and the NEC: communications cable safety where the cable is installed in a building
Verification is a measurement, not a visual check. Measure characteristic impedance with a time-domain reflectometer, or measure attenuation over a known length at the operating frequency. Then check the connector interface: a 50 ohm BNC plug into a 75 ohm jack will mate, which is precisely why the mistake is common and why the marking on the port should be read before the cable is chosen.
Related reading: Coaxial cable guide: impedance, families and construction — blogs/news/coax-cable
Related reading: 75 ohm BNC cable: the video-side counterpart — blogs/news/bnc-cable-75-ohm
Related reading: RG59 cable: the other small 75 ohm type — blogs/news/rg59-cable
Frequently Asked Questions
Is RG58 50 ohm or 75 ohm?
RG58 is 50 ohm. RG6, RG11 and RG59 are 75 ohm. The two families are not interchangeable, and a BNC connector can be either impedance, so the port marking has to be checked rather than assumed.
Can I use RG58 for CCTV?
Not properly. CCTV is a 75 ohm system, so a 50 ohm cable creates a mismatch at both ends that shows as reduced level and reflections. Use RG59 or RG6 for CCTV.
What is RG58 used for today?
Test and measurement leads, radio and antenna feeds, laboratory RF interconnection, and legacy 10BASE2 industrial networks. Its main modern role is short 50 ohm jumpers.
Why does RG58 have a stranded centre conductor?
Flexibility and fatigue resistance. A test lead is handled constantly, and a stranded conductor survives repeated bending better than a solid one, at a small cost in attenuation.
How long can an RG58 run be?
It depends on frequency and on the receiver's sensitivity. Short bench jumpers of a few metres are fine at any reasonable frequency; tens of metres at UHF needs a larger cable, and the loss figure at the actual operating frequency is the number to check.
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