In a coaxial signal path, 50 Ohm and 75 Ohm describe two common characteristic impedance standards. The difference affects how a cable interfaces with the source, load, connectors, and other transmission line components. A direct 50 Ohm to 75 Ohm impedance step creates a reflection, while the response of a complete mixed impedance cable section also depends on its electrical length and operating frequency.
What Is Impedance in a Coaxial Cable
Characteristic impedance, measured in ohms (Ω), describes the voltage to current relationship of a traveling signal on a coaxial transmission line. It is different from the DC resistance measured through the conductor. Cable geometry, the dimensions of the inner and outer conductors, and the dielectric material between them determine the nominal characteristic impedance. For this reason, a cable marked 50 Ohm or 75 Ohm is describing its transmission line behavior, not the amount of conductor resistance measured with a multimeter.
The same nominal impedance can be produced by different coaxial constructions. Two 50 Ohm cables may use different conductor sizes, dielectric materials, shielding structures, and outside diameters while still presenting the same nominal characteristic impedance. Their attenuation, power capability, flexibility, and usable frequency range can therefore be very different.
Why Do 50 Ohm and 75 Ohm Coaxial Cables Exist
The 50 Ohm standard reflects a compromise between power capability and attenuation in coaxial transmission line design. A lower impedance does not automatically mean better power performance, and 50 Ohm is not the theoretical impedance for maximum possible power handling. Its value provides a useful balance for RF transmission systems where both power transfer and manageable attenuation matter.
The 75 Ohm standard developed around signal transmission systems where lower attenuation was a stronger design priority. With comparable geometry and materials, a coaxial design near 75 Ohm can provide lower attenuation than a comparable 50 Ohm design. This relationship does not mean that every 75 Ohm cable has lower loss than every 50 Ohm cable.
These two impedance values became established in different equipment and signal ecosystems. Once the equipment port, termination, connector, and cable are designed around one nominal impedance, maintaining that impedance through the transmission path reduces discontinuities and makes RF behavior more predictable.
What Happens at a 50 Ohm to 75 Ohm Impedance Step
A 50 Ohm to 75 Ohm impedance step reflects part of the incident wave back toward the source. The mismatch appears as a nonzero reflection coefficient, higher VSWR, and finite return loss. The amount of reflection depends on the impedance values at that transition.
For one ideal impedance step from a 50 Ohm transmission line to a purely resistive 75 Ohm load, the reflection coefficient magnitude is 0.20. This corresponds to 4% of the incident power being reflected at that interface. The same ideal mismatch gives a VSWR of 1.5:1 and a return loss of approximately 14 dB.
The approximately 0.18 dB mismatch loss in this calculation describes one ideal interface. It does not represent the total insertion loss of a cable assembly containing a 75 Ohm cable section between 50 Ohm equipment. Cable attenuation, connector loss, additional impedance transitions, and the electrical length of the mismatched section still affect the complete result.
Why a Mixed Impedance Cable Section Is Different from One Mismatch
A 75 Ohm cable section inserted between two 50 Ohm interfaces creates two impedance discontinuities, one where the signal enters the 75 Ohm section and another where it returns to 50 Ohm. The reflection produced at the second transition travels back through the cable and combines with reflections from the first transition.
The phase relationship between these reflections changes with electrical length. Electrical length depends on physical cable length, signal wavelength inside the cable, and velocity of propagation. A cable section that produces one response at one frequency can therefore produce a different input reflection and transmission response at another frequency.
Does 75 Ohm Coax Always Have Lower Loss Than 50 Ohm Coax
No. A 75 Ohm cable does not automatically have lower attenuation than a 50 Ohm cable. Impedance is only one electrical parameter of a coaxial cable. A coaxial cable type is defined by a combination of conductor size, dielectric material, shielding configuration, outer jacket, and manufacturing standard. As a result, multiple cable types can share the same nominal impedance while exhibiting very different mechanical and electrical performance.
A 75 Ohm coaxial design can provide lower attenuation than a geometrically comparable 50 Ohm design, but impedance alone does not determine cable loss. Conductor size, cable diameter, dielectric properties, conductor construction, and operating frequency all influence attenuation. A larger low loss 50 Ohm cable can therefore have lower attenuation than a smaller 75 Ohm cable at the same frequency.
FAQ
Conclusion
50 Ohm and 75 Ohm describe two established characteristic impedance systems with different transmission line design priorities. A direct 50 Ohm to 75 Ohm transition creates a measurable reflection, while the response of a finite mixed impedance cable section also depends on frequency and electrical length. Impedance therefore needs to be considered together with the equipment ports, connectors, cable construction, and required RF performance. If a cable path needs to cross between 50 Ohm and 75 Ohm interfaces, prepare the port impedance, operating frequency, cable length, connector interfaces, and return loss or insertion loss requirements before confirming the cable assembly.