RG6 vs RG58: Why 75 Ohm and 50 Ohm Cables Aren't Interchangeable

Published on 10/13/2026

A spare TV coax reel looks like an economical way to extend a radio antenna. The connectors can be adapted, and the thicker cable may even have lower attenuation at the band of interest. Yet the radio expects a different transmission-line impedance. That mismatch needs analysis, not a blanket approval or rejection.

RG6 is normally 75 ohm cable for video and related distribution systems. RG58 is a nominal 50 ohm RF cable family, although some RG58/U products specify 52 ohm. They are not automatic drop-in replacements. A deliberate design can use 75 ohm line with 50 ohm equipment, but it must account for mismatch, electrical length, loss and power limits.

This article distinguishes mechanical fit from electrical compatibility. It does not claim that an impedance mismatch always causes failure, or that an adapter can remove it.

What actually differs between RG6 and RG58

An RG number describes a cable family, not one uniform product. Check the exact construction. A professional RG6 video cable may use solid bare copper, while other RG6 products use copper-clad steel. RG58 products can use solid or stranded conductors and different jackets.

Why impedance mismatch matters

Characteristic impedance is not the cable's DC resistance. It relates voltage and current in a travelling wave. Connecting different impedances creates a reflected wave. A multimeter continuity test cannot determine whether an RF line has the required characteristic impedance.

For a single ideal 50 ohm to 75 ohm boundary, the reflection-coefficient magnitude is 0.2. The corresponding VSWR is 1.5:1 and return loss is about 14 dB. Reflected power is 4%. This does not mean every finite 75 ohm cable between 50 ohm ports has only 0.18 dB total loss or a constant 1.5:1 input VSWR.

The second boundary, line phase and load impedance interact. The input match can change with frequency and length. Loss in the cable can also hide reflections by attenuating them, but it still reduces the useful signal. An acceptable receive path may be unacceptable for a transmitter or precision measurement.

An adapter changes the connector, not the line

A simple F-to-SMA or BNC-to-SMA adapter is a mechanical transition. It does not inherently provide a 75-to-50 ohm match. A matching pad, transformer or designed transmission-line network is a different component and can add loss or constrain bandwidth.

BNC exists in both 50 ohm and 75 ohm versions. Some versions can mate mechanically, but that does not make their impedance equal. Other connector families need special care: never assume all 50 ohm and 75 ohm N-Type contact geometries are intermateable.

Confirm DC requirements as well. Active antennas and satellite systems may use a bias feed through coax. Check conductor resistance, current, voltage drop and whether any adapter, surge protector or matching network passes the required DC. Copper-clad steel cable can have different DC resistance from solid-copper cable.

When a deliberate 75 ohm design can work

A 75 ohm line can be used in a 50 ohm system when the expected input match, transmitted power and frequency behavior are acceptable or intentionally matched. It is not automatically unsafe or useless. The design must include the actual antenna impedance and operating band.

An ideal lossless half-wavelength line repeats its load impedance at its input. An odd-quarter-wavelength line transforms a resistive load according to Z_in = Z_0² / Z_load. These are frequency-dependent properties, not a broad-band replacement rule. Cable velocity factor determines the physical length for a given electrical length.

For a production assembly, specify the intended system and test planes. Read SigmaRF's Insertion Loss vs Return Loss vs VSWR guide for measurement terms and the RG58 cable guide for family selection. Use the RG58 vs LMR-240 comparison when the actual goal is lower loss without changing to a 75 ohm line.

Worked engineering example

Take an ideal lossless 75 ohm cable that is one quarter wavelength long, terminated in a 50 ohm resistive load. This isolates electrical-length behavior; it is not a model of every RG6 installation.

The example explains why one isolated-boundary calculation is not enough. Real cable loss, connectors and a reactive antenna change the result. Broadband operation needs a frequency sweep rather than a length chosen for one center frequency.

Mistakes to avoid

Frequently Asked Questions

Bottom Line

RG6 and RG58 serve different impedance systems. Keep the title's warning practical: do not substitute them without checking the electrical design. It is a mistake to call them interchangeable, and also a mistake to say every mismatch makes operation impossible.

For a new assembly, use the intended system impedance unless the design explicitly requires something else. When adapting an existing path, measure the complete result over the real operating band.

SigmaRF VNA-sweeps every assembly for VSWR and insertion loss before shipment. Its testing capability spans 100 MHz to 40 GHz; each assembly is specified for its own operating band. Builds are supported by the Vadodara volume facility and Cleveland-area precision facility. Lead times can be as short as 2 days, depending on the build and component availability.