Published on 10/13/2026
Open any RF lab and almost every cable, connector and instrument is 50 ohm. Cable TV is 75 ohm, twisted pair Ethernet is 100 ohm, and nobody asks why. So why did RF engineering settle on 50 ohm? The popular story says it was an accident of history. The real answer is a compromise between two physical limits, plus a few decades of standardization.
Short answer: 50 ohm sits between the impedance for lowest loss (about 77 ohm) and the impedance for highest power handling (about 30 ohm) in an air-filled coaxial line. It is close to the geometric mean of the two, 48 ohm. Standards and test gear then locked it in.
Here is the physics, the history as it is usually told, and what it means when you choose between 50 and 75 ohm today.
On This Page
A coaxial line has an inner conductor of diameter d and an outer conductor of inner diameter D. The characteristic impedance is Z0 = (60 / sqrt(er)) x ln(D / d), where er is the dielectric constant.
Changing the D/d ratio changes the impedance, and different goals pick different ratios. For an air-filled line:
| Design goal | D/d ratio | Impedance |
|---|---|---|
| Maximum power handling | About 1.65 | About 30 ohm |
| Maximum voltage breakdown | About 2.72 | About 60 ohm |
| Minimum conductor loss | About 3.59 | About 77 ohm |
Key idea: Low loss and high power ask for different geometries. 50 ohm is the middle ground: you give up a little of each rather than optimizing for one.
A 0.141 in semi-rigid cable (RG402 type) has a PTFE dielectric of about 3.0 mm OD around a 0.91 mm center conductor. That ratio, about 3.3, gives close to 50 ohm. The geometry is a direct result of this compromise.
The commonly cited account says early work at Bell Labs on coaxial lines settled around 77 ohm for lowest loss in long-distance telephone trunks, which is why 75 ohm cable became the standard for cable TV and telecom. Radar and microwave work in the 1930s and 1940s needed high power handling too, and 50 ohm (and for a while 51.5 ohm) emerged as the practical compromise in U.S. military standards.
The exact decisions are hard to trace to a single document, and many versions of the story disagree on small details. What is not in dispute is the physics above and that military specification (the JAN and later MIL standards) made 50 ohm the default for RF cable and connectors.
Need 50 ohm assemblies built to length?
SigmaRF builds 50 ohm assemblies from 100 MHz to 40 GHz, every one swept on a VNA for VSWR and insertion loss before shipment.
Talk to an EngineerReasons it stuck
| Impedance | Typical use | Strength |
|---|---|---|
| 50 ohm | RF, microwave, wireless, test | Balanced loss and power |
| 75 ohm | Cable TV, video, broadcast, telecom | Lowest loss per length |
| 100 ohm | Twisted pair, differential | Different topology |
PCB traces usually target 50 ohm single-ended too, which is why a 50 ohm cable connects without a transformer. When you mix 50 and 75 ohm parts, see our guide to using 75 ohm cable in a 50 ohm system on this blog. For connector fit and selection, read the cable selection and design guides listed on our blog.
Why is coaxial cable 50 ohms?
It is a compromise between the lowest-loss impedance (about 77 ohm) and the highest-power impedance (about 30 ohm) for an air-filled coax. It is close to the geometric mean of the two, 48 ohm.
Why is cable TV 75 ohm?
75 ohm is close to the lowest-loss geometry, which matters over long runs where power handling is low.
What is the best impedance for coax?
It depends on the goal: about 77 ohm for lowest loss, about 30 ohm for highest power, and 50 ohm as a balance.
Is 50 ohm better than 75 ohm?
Neither is better overall. 50 ohm suits RF transmit and test, 75 ohm suits receive and video distribution.
Is 52 ohm cable the same as 50 ohm?
Close. Some cables are 52 ohm nominal and work in 50 ohm systems with a VSWR under 1.05:1.
What impedance is a 0.141 semi-rigid cable?
50 ohm, with a PTFE dielectric and a D/d ratio of about 3.3.
50 ohm is not magic. It is a geometric compromise between loss and power that the military and the test equipment industry then standardized. Use 50 ohm for RF and test work and 75 ohm for video and distribution, and match each system end to end.
SigmaRF builds custom 50 ohm cable assemblies in India (Vadodara) and the USA (Cleveland area) with lead times as short as 2 days.
Build a 50 ohm assembly
Standard or custom, swept before shipment.
Standard AssembliesCustom AssembliesTalk to an Engineer