The 50-ohm impedance is standard in RF design, while 75 ohms is also used, so it’s important to understand the attributes of each and not confuse them.
One of the issues that beginning electronic engineers must grasp is the concept of impedance and impedance matching, especially concerning transmission lines, cables, and connectors.
This FAQ will look at the two most commonly used “standard” impedances of 50 Ω and 75 Ω, why they are what they are, and the role of each.
Q: What is the relationship between a coaxial cable and a transmission line?
A: Transmission lines such as coaxial cables appear to be simple passive components consisting simply of an inner conductor and outer conductor (or shield) (Figure 1 (top)).
In the simplistic view, it is just a conductor (wire) fully surrounded by a 360° shield for confining any electromagnetic energy radiating from the center-conductor wire while preventing outside energy from impinging on the wire. But there’s much more to the coaxial cable/transmission line situation than that.
Q: What’s the bigger story?
A: Even though there are physically only two conductors – center wire and shield – a source or transmitter sees a cable seemingly made out of distributed inductance and capacitance, as shown in the equivalent circuit (Figure 1 (bottom)) — the coaxial cable functions as a waveguide for supporting the propagation of electromagnetic energy and confining it.
The transmission line has a characteristic impedance, usually designated as Zo. Cable characteristic impedances can have many possible values depending on the conductor dimensions, internal spacing, and dielectric properties of the spacing insulator between the cable’s inner conductor and outer shield, with 50 Ω or 75 Ω the most common values.
Q: What’s the relationship between those two impedance options?
A: When RF engineers think about the impedance of their project’s transmission lines, they may automatically assume that these lines all have a nominal impedance of 50 Ω. That makes sense, as so much of today’s RF design work is based around that value. It’s not an arbitrary number, either, as there are good technical reasons for using 50 Ω.
However, there are also many RF applications where the transmission-line impedance has a 75-Ω value. These are mostly related to video signals and cable TV, which includes the many related functions in this large market, such as building-wide distribution amplifiers. To designers and end users in these areas, 75 Ω is the “normal” impedance while 50 Ω is the oddball value (you can even buy 75- Ω cables at Home Depot and similar stores).
Q: Using two very different impedances raises some interesting questions. Why are there two standard impedances? Which is “better” where, and in what way? Why do they have those values? Does using one versus the other matter, and if so, in what specific ways?
A: There’s much more behind the story of those simple-looking coaxial cables and their connectors than just a solid inner conductor with some surrounding shielding; coaxial cable is a precisely dimensioned waveguide for electromagnetic energy (Figure 2).
The answers to the impedance questions have both historical and technical roots. They begin with the work done by Lloyd Espenscheid and Herman Affel, who developed and analyzed the first coaxial cable in 1929 while working for the legendary Bell Labs [1]. Their goal was to find a transmission medium for propagating a 4 MHz signal (a very wide bandwidth in those early days of long-distance telephony), which was needed to carry about 1000 bandwidth-limited analog voice calls for hundreds of miles. Doing so required a transmission line that could handle high voltage and power.
Q: What did they determine?
A: The two researchers analyzed the tradeoffs among key transmission-line parameters of attenuation, voltage rating, and power rating (Figure 3).
Q: What did they determine?
A: Their analysis looked at the performance of three characteristics as a function of impedance:
- Attenuation (loss) is largely a function of the dielectric in the cable. For the air-filled coaxial cable they analyzed, the lowest loss is at about 77 Ω, around 50 Ω for some dielectrics.
- The voltage maximum is a function of the intensity of the electric field between the coaxial outer conductor and the inner conductor. For coaxial cable supporting RF signals in the TE10 electromagnetic-field (EM) waveguide mode, the e-field is maximum at around 60 Ω.
- The power-handling capability is determined by the breakdown field and impedance (V2/Z); for air-filled coaxial cables operating below the TE11 mode cutoff frequency, the power transfer is at its maximum at around 30 Ω.
Q: So what is the best value to use?
A: As with most engineering decisions, there is no “ideal” impedance value; instead, the “best” choice involves balancing tradeoffs. The 50-Ω value is a good compromise for power and voltage, such as from the output of a source such as a transmitter or line driver. In contrast, for situations where low attenuation is the primary goal, such as with low-level signals from an antenna or an analog video link, 75 Ω is a better choice.
Q: Are there other reasons why 75 Ω is a desirable impedance?
A: The “natural” impedance of a standard half-wave dipole antenna at its resonant frequency is 73 Ω, while the widely used folded dipole antenna impedance is 300 Ω. This means that 75 Ω is a near-perfect match for the larger dipole, while it also is easy to provide a close match of the folded dipole to a 75- Ω transmission line using a basic 4:1 balun.
Q: How does this affect a practical design?
A: In practice, using different impedances to realize different objectives in a single design complicates the design. For example, the difference in loss over a short run of a few centimeters may be negligible. Further, the voltage standing-wave ratio (VSWR) when connecting a 75-Ω cable to a 50-Ω one is 1.5:1, which may be an acceptable non-unity value as in many low- or medium-power situations, a VSWR below 2:1 is considered acceptable [2].
Q: Can you relate the cable and impedance theory to practical, real-world coaxial cables?
A: Translating electromagnetic field theory and analysis into real transmission lines is the function of coaxial cables, most of which have an “RG” designation (RG, or Radio Guide, comes from the original WWII military specification for coaxial cables).
Although there are well over a hundred RG-x varieties, nearly all have either a 50- or 75-Ω nominal impedance. The difference among the many versions relates to their physical size, insulation specifics, shielding type and performance, dielectric material and construction, power rating, environmental ruggedness, fire-related ratings, and other factors. The RG-X number often has a letter suffix after the “X” number to identify a specific non-impedance attribute such as a ruggedness rating.
Q: Why are so many coaxial cable models (not counting spooled length)?
A: There are many tradeoffs in cable electrical and mechanical performance factors beyond impedance. In general, lower frequencies need cables with a larger diameter to accommodate the RF wavelength with low attention, while thinner cables work for higher frequencies. For low-power RF into the tens of gigahertz, the coaxial cable may have a diameter of only one or two millimeters. At the same time, thinner cables cannot handle large amounts of RF power, so they must migrate to thicker cables with appropriate higher-frequency performance.
Q: What kinds of coaxial cables are available?
A: Vendors provide many RG-cable types, usually on large spools. For example, among their many 50-Ω cables, Belden Inc. offers the 8240 010500 RG-58A coaxial cable with a 20 AWG solid inner conductor, polyethylene insulation, 95% tinned copper braid shield, and a PVC Jacket (Figure 4). It is rated to 530 watts at 50 MHz and has attenuation specified to 1000 MHz (1 GHz).
Q: What about cable terminations?
A: There’s more to using a coaxial cable than just the cable itself, as it needs to be terminated with a connector designed for the correct impedance and coaxial-cable size and shielding. Among the commonly used connectors for 50-Ω coaxial cable is the venerable PL-259, available in many versions from many vendors (Figure 5), and the wideband Type N (Figure 6).
Many 75-Ω systems, especially those in low-cost consumer applications, use the Type F connector (Figure 7). These are available in many versions, including push-on, thread-on, and “bayonet” retention.
Q: Why should designers look closely at the nominal impedance of their design?
A: First, they shouldn’t assume the system is 50 Ω when it may be 75 Ω – or the reverse.
Also, when making out the all-important bill of materials (BOM), it is critical to check the impedance value of the coaxial cable and associated connectors or any pre-cut and terminated cable being specified. It’s easy to inadvertently pick a 75-Ω coaxial cable instead of the intended 50-Ω one or vice versa. Further, connectors such as the classic BNC style (among the oldest still used) come in both 50- and 75-Ω versions, which look the same, having only slight differences in dimensions – they can even be mated with a little “pressure” (Figure 8).
Related EE World content
What is characteristic impedance?
What is the common impedance of a twisted pair?
VSWR and impedance, Part 6: Microstrip and stripline
VSWR and impedance, Part 5: Making a match
Coaxial cable myths and misunderstandings
Why is Micro-Coaxial Preferred for High-Speed Applications?
Microwave/Millimeter Wave interconnects, Part 1: Coaxial cables
References
[1] U.S. Patent 1,835,031, “Concentric Conducting System”[2] Circuit Design, Inc., “RF Design Guide”
[3] Belden, “50 Ohms: The Forgotten Impedance”
[4] Tech Play On, “Why is the characteristic impedance of RF transmission lines kept 50 ohms?”
[5] Altium, “The Mysterious 50 Ohm Impedance: Where It Came From and Why We Use It”
[6] Microwaves 101, “Why 50 Ohms?”
[7] Power Signal, “Should I buy a 50‑ohm or a 75‑ohm cell signal booster?”
[8] Solid Signal, “Which is better: 50 Ohm cable or 75 Ohm cable?”
[9] Ham Stack Exchange, “Using 75 Ω instead of 50 Ω coax feed”
[10] Wikipedia, “Coaxial cable”
