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A 50 ohm coax cable is a transmission line with a characteristic impedance of 50 ohms, and it is the default choice in most RF systems because it balances low signal loss with reasonable power-handling capability. The “50 ohm” rating does not refer to a DC resistance you can measure with a multimeter; it describes how voltage and current travel along the cable at radio frequencies. This article explains what 50 ohm impedance really means, why the industry adopted it, how it compares with other impedances, and what to check before you buy. If you need a full step-by-step selection method, see our RF coaxial connector guide.
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What Does 50 Ohm Mean in a Coax Cable?
The characteristic impedance of a cable is the ratio of voltage to current in a wave traveling along it, and it is fixed by the cable’s geometry and dielectric material. For a coaxial cable, the impedance is approximately Z0 = (138 / sqrt(εr)) × log10(D/d), where D is the inner diameter of the outer shield, d is the outer diameter of the inner conductor, and εr is the relative permittivity of the dielectric.
This impedance is independent of cable length. A 10 m and a 100 m length of the same 50 ohm cable will both show roughly the same characteristic impedance when measured with a network analyzer. A DC ohmmeter will only read a low series resistance, often near 0 ohm for a short piece or several ohms for a long run, so it cannot be used to verify a 50 ohm rating.
- Characteristic impedance is set by construction, not by length.
- The impedance value is meaningful only at frequencies where the signal wavelength is comparable to or smaller than the cable length.
- A mismatch creates reflections, which increase insertion loss and signal distortion.
Why 50 Ohm Became the RF Standard
The 50 ohm standard emerged because it was the best practical compromise between low attenuation and high power handling for air-dielectric cables. For a coaxial line with air dielectric, the maximum power-handling peak is near 30 ohm and the minimum attenuation is near 77 ohm. The geometric mean of those two values is approximately 48 ohm, which rounded naturally to the 50 ohm we use today.
When solid polyethylene and other low-loss dielectrics became standard, the optimum values shifted slightly, but 50 ohm remained close enough and was already widely used by telecommunications and test-equipment engineers. Radar, radio transceivers, laboratory instruments, and later cellular and wireless equipment all standardized on 50 ohm.
50 Ohm vs 75 Ohm vs Other Impedances
For general-purpose RF work, choose a 50 ohm cable; choose a 75 ohm cable only for low-loss receiving applications such as broadcast video or television, where power handling is less critical. The table compares the main differences.
| Impedance | Typical use | Signal loss | Power handling | Connector types |
|---|---|---|---|---|
| 50 ohm | Wireless, test equipment, radio, antenna feeders, IoT | Low | Moderate to high | BNC, N, SMA, TNC, UHF |
| 75 ohm | Broadcast video, CCTV, cable TV, telephony | Lower than 50 ohm for the same outer size | Lower | BNC, F, RCA |
| 93 ohm | Legacy data transmission, early LANs | Very low for certain low-speed data | Very low | BNC (rare) |
Using a 75 ohm cable in a 50 ohm system creates a mismatch that reflects part of the signal. At low frequencies and short lengths the effect may be negligible, but above a few hundred megahertz or for long cable runs the added ripple and loss usually make it unacceptable.
How to Choose a 50 Ohm Coax Cable
Start by setting the maximum acceptable loss at your operating frequency, then verify power handling, mechanical flexibility, shielding, and impedance tolerance. The most common mistake is buying the cheapest RG-58 lookalike without checking attenuation at the actual frequency of interest.
- Determine the highest frequency you will transmit. Loss rises with frequency, so a cable that works at 100 MHz may be unusable at 6 GHz.
- Calculate total loss from the cable’s attenuation per metre, often specified in dB per 100 m at 1 GHz; multiply by your run length and add connector loss.
- Check the outer conductor construction. Braid is flexible but leaks; foil plus braid gives better shielding; semi-rigid and corrugated copper give the lowest loss and best shielding for fixed installations.
- Check impedance tolerance. A good 50 ohm flexible cable typically holds characteristic impedance within ±2 ohm; poor-quality cables can drift more and cause higher VSWR.
- Verify the connector family and mating interface. A 50 ohm SMA connector is not the same as a 50 ohm N connector mechanically or electrically, and neither should be forced onto a 75 ohm BNC port.
Common generic 50 ohm cable families include RG-58/U for short flexible patch leads, RG-213/U for higher-power base-station runs, and low-loss foam-dielectric cables for long feeders. The actual performance depends on the construction quality, so always compare datasheet attenuation curves before ordering.
Matching Connectors to a 50 Ohm Cable
Every 50 ohm cable needs a 50 ohm connector that matches the cable diameter and the target port; mismatching connector impedances creates reflections and can damage the interface. For example, a 75 ohm BNC connector can mate with a 50 ohm BNC connector, but the impedance step at the junction increases VSWR and the centre pins are not always dimensionally compatible. N-type and SMA connectors are far more consistent in 50 ohm systems, while TNC provides a threaded, weather-resistant version of BNC.
SMA Connectors
SMA is the first choice for frequencies up to 18 GHz and for compact modules such as radios, antennas, and test fixtures. Choose an SMA connector that matches your cable type, whether flexible, semi-rigid, or microstrip.
SMA RF Coaxial Connectors for Broadband ApplicationsExplore SMA connectors with 50 ohm impedance, supporting DC to 18 GHz on semi-rigid cable. They suit compact modules and high-frequency circuits mentioned in the preceding text.View Product →
N Connectors
N connectors are rugged, 50 ohm connectors with outstanding repeatability from DC to 11 GHz or higher. They are common on base-station filters, RF amplifiers, and outdoor antenna feeders.
N RF Coaxial Connectors with Screw CouplingThese rugged 50 ohm connectors operate up to 11 GHz on semi-rigid cable. Ideal for base-station filters and outdoor feeders, as noted in the surrounding discussion.View Product →
BNC Connectors
BNC connectors are convenient for quick connections, but verify that the specific BNC is rated for 50 ohm and for your frequency range. The same shell is also used for 75 ohm video circuits, so check the datasheet before mating.
BNC RF Coaxial Connectors with Bayonet CouplingCheck these 50 or 75 ohm BNC connectors for quick connect-disconnect needs. The context warns to verify frequency and impedance ratings before mating, making this page useful.View Product →
When you assemble a cable, confirm that the connector is specified for the exact cable family you are using, such as flexible braided cable or semi-rigid cable. If the two port interfaces are different, use a low-loss 50 ohm RF adapter rather than adapting with a long cable.
Typical Applications and Installation Checks
You will find 50 ohm coax cable in wireless infrastructure, test and measurement, avionics, IoT devices, and any system where a transceiver drives an antenna. In these applications, the cable impedance is part of a matched chain from the connector on the PCB to the antenna feed point.
- Bench test leads and spectrum-analyzer connections use short 50 ohm patch cables.
- Base-station antenna feeders use low-loss 50 ohm cable with N-type connectors.
- Software-defined radios often route RF into a 50 ohm SMA female port.
- GPS receivers, telematics, and radio modules share the same 50 ohm impedance convention.
For outdoor installations, check whether the cable has a weather-resistant jacket, and add a surge protector or lightning protection component near the entry point. A protector designed for 50 ohm operation will maintain the system’s characteristic impedance through the surge path.
Frequently Asked Questions
These are the questions engineers most often ask before selecting a 50 ohm cable.
Can I use a 75 ohm cable in a 50 ohm system?
Yes, but only for a short temporary test and not at high power or high frequency. The impedance mismatch creates a reflection coefficient of about (75-50)/(75+50), or 0.2, which means roughly 4% of the signal power is reflected. Return loss is only about 14 dB, which may be acceptable for bench work but is rarely acceptable for a transmitter.
Does a 50 ohm cable measure 50 ohm on a multimeter?
No. A multimeter applies DC and measures the series resistance of the centre conductor and shield, which is usually below 10 ohm for a short cable. Characteristic impedance is an AC wave property and can only be measured correctly with impedance analysis at RF, for example with a network analyzer.
What is the difference between 50 ohm and 75 ohm BNC connectors?
The basic shell is the same, but 75 ohm BNC connectors have a different centre-conductor profile and are not interchangeable with 50 ohm types without degrading the interface. A 50 ohm plug will physically mate with a 75 ohm jack, but the impedance change and poor contact force can increase VSWR and damage the jack over repeated connections.
What cable should I buy for a 50 ohm antenna feeder?
Choose the thinnest cable that passes your loss budget. For short indoor runs, RG-58 type is convenient. For long outdoor runs or frequencies above 1 GHz, choose a low-loss 50 ohm cable with a closed-cell jacket and N-type connectors, and verify its attenuation at your actual frequency.
