Quick Answer: Match impedance before choosing an adapter. Consumer coax for TV, satellite and S/PDIF is 75 ohm, and every RCA phono jack on consumer equipment is 75 ohm unbalanced, so an F-to-RCA adapter is electrically transparent only when the cable behind it is also 75 ohm. A 50 ohm cable adapted into an RCA input reflects part of the signal, costs about 0.5 dB at 100 MHz and far more above 300 MHz, and makes digital audio drop out on peaks. Keep analogue line-level runs under 25 ft (7.6 m) and digital coax under 50 ft (15 m).
The plug fits, so the signal is assumed to follow. An RCA jack has a 3.2 mm centre pin and a return shell, and the cable feeding it has to present 75 ohm across the band in use. When that match is missing the symptom is rarely silence. It is a hiss, a low hum, or a digital input that unlocks on loud passages.
Impedance decides whether the adapter works
Impedance comes from the ratio of inner conductor diameter to dielectric diameter, not from the connector shell. RG-6 is 75 ohm with an 18 AWG copper-clad centre and a foam dielectric; RG-58 is 50 ohm with a stranded centre. Neither cable is superior; they are matched to different systems, and the mismatch, not the quality, is what you hear.
75 ohm is the consumer default
Baseband video, satellite intermediate frequency at 950 to 2150 MHz, cable television up to 1 GHz and S/PDIF coax all sit in 75 ohm systems. RG-6 and RG-11 belong here. A 75 ohm RCA jack is the correct termination for any of them.
50 ohm belongs to radio and test gear
Two-way radio, scanner feeds and lab instruments use 50 ohm with BNC or N connectors. Adapting that cable to an RCA input produces a standing wave ratio of about 1.5 to 1, which reflects roughly 4 percent of the power and distorts the level response rather than merely lowering it.
What an F-to-RCA adapter changes
A barrel adapter is a mechanical transition, nothing else. It changes an F female thread to an RCA male plug and adds 0.1 to 0.3 dB of insertion loss. It does not transform impedance, add gain or convert digital to analogue. Short moulded adapters with an unshielded stub are the usual source of 60 Hz hum, because the screen is broken for 10 mm or more inside the moulding. Where the source is a laptop with no analogue output, the conversion belongs in an active device rather than a passive adapter, such as a usb c to rca cable with a DAC in the plug.
S/PDIF over coax is the clean case
Coax S/PDIF carries 192 kHz sample rates on a 0.5 V peak-to-peak signal into 75 ohm, and it is the one place where an RCA jack and a coax cable are genuinely the same system. Use a 75 ohm RCA plug, not a 50 ohm BNC, and never run the signal through a passive splitter to feed two devices at once. A resistive splitter halves the level and doubles the reflections; a distribution amplifier or a second feed from the source is the correct answer.
Run length, loss and shield coverage
Loss rises with frequency, so a cable that is fine at 1 MHz can be unusable at 1 GHz. Figures below are typical for copper braid plus foil constructions and should be treated as rules of thumb, since jacket and braid quality move them by 20 percent either way. For a long feed to a receiver, a 50 ft rca cable built on RG-6 keeps loss near 1 dB at 100 MHz, which is why the construction matters more than the length.
| Cable | Impedance (ohm) | Loss at 100 MHz (dB per 100 ft) | Sensible use |
|---|
| RG-6 | 75 | 2.0 | TV, satellite, S/PDIF to 50 ft |
| RG-59 | 75 | 3.2 | Short patch leads under 25 ft |
| RG-11 | 75 | 1.4 | Drops over 100 ft |
| RG-58 | 50 | 4.5 | Radio only, never to an RCA jack |
| RG-8X | 50 | 2.7 | Bench radio feeds |
Shielding matters as much as loss. A braid gives 90 percent coverage at best, foil plus braid reaches 100 percent at low frequencies but cracks when flexed. On a run near mains wiring, use quad-shield RG-6 and bond the shell at both ends.
Practical scenarios
- Situation: A set-top box has one F output and a stereo receiver has RCA inputs. Use a 75 ohm F-to-RCA adapter on RG-6, set the box to stereo downmix, and keep the whole chain under 50 ft (15 m).
- Situation: A 25 ft (7.6 m) run passes along a wall with mains conduit. Choose quad-shield RG-6 in a CL2 jacket for in-wall work and a 25 ft rca cable for the exposed section, routed 12 in (300 mm) away from the conduit.
- Situation: A turntable needs to reach an amplifier 6 ft (1.8 m) away. Do not adapt coax here at all; the cartridge needs 47 kohm loading and a phono stage, so a 50 ohm cable and an RCA adapter will only add capacitance and hum.
- Situation: Four cameras feed one monitor through a passive four-way splitter. Replace it with an active distribution amplifier, because each passive output drops 7.5 dB and the monitor then sees a level 14 dB below nominal.
FAQ
Can a 50 ohm cable be used with an RCA jack if the run is short?
It will usually work at line level over 3 ft (1 m), because the mismatch loss is small at audio frequencies. It is not correct practice. Above 300 MHz the reflection grows quickly, so the same cable that passes audio cleanly will break a satellite or S/PDIF feed.
Why does the adapter introduce hum?
Hum appears when the screen is interrupted, which happens inside moulded adapters with an unshielded stub and at loose F threads. The fix is a one-piece machined adapter and a bonded shell, not a ground lift. Never cut a screen to cure hum on a coax feed.
Does the length of the coax change the level?
Yes, and predictably. At 100 MHz, RG-6 loses about 2 dB per 100 ft (30 m), so a 50 ft run costs roughly 1 dB. Loss rises with the square root of frequency, so the figure at 1 GHz is about 6.5 dB per 100 ft.
