Quick answer: A coaxial input expects a 75 ohm source, a shielded cable and a connector that seats fully. Coaxial names the geometry: a centre conductor inside a dielectric inside a shield. Impedance comes from those dimensions, so a cable built to the wrong ones reflects part of the signal back. On a digital link that reflection causes dropouts or a total loss of lock, not a gradual loss of quality.
The socket says coaxial, the plug goes on, and there is no sound or no picture. Most of these cases come down to three things: the plug belongs to another family and only appears to fit, the cable is 50 ohm rather than 75 ohm, or the run is lossy enough that the receiver never locks. None of them are cured by turning the volume up.
What the socket is actually expecting
A consumer coaxial input is an unbalanced 75 ohm connection. The cable and the connector are part of that impedance, and the receiver front end is matched to it. Over a metre or two almost any shielded cable passes a signal, which is why a wrong cable can seem fine until the run gets longer.
Impedance is geometry, not a label
Impedance is set by the ratio of centre conductor diameter to the inside diameter of the shield, together with the dielectric material. A cable made for radio equipment at 50 ohm has different dimensions from a 75 ohm video cable, and no printed word on the jacket changes that.
What a mismatch does to a digital stream
Part of the signal reflects at the transition, travels back to the source and returns late, smearing the edges. Once the receiver cannot separate the levels reliably it mutes or drops lock. There is no middle ground where the sound is slightly worse, so the same cable works or fails depending on the day.
Why an analogue signal just gets weaker
An analogue signal tolerates the same mismatch by losing level and picking up standing waves. The result is a softer picture rather than a hard failure, so one bad cable can look acceptable on an analogue input and fail on a digital one.
The connector that will not seat
Three families appear on coaxial sockets: the push on RCA for digital audio and composite video, the threaded F type for antenna and satellite, and the bayonet BNC on test gear. Each has its own pin diameter and shield contact, so they are not interchangeable even when one slides onto the other.
Adapters that look right and are not
An F to RCA adapter works because both sides keep the 75 ohm geometry. A 50 ohm BNC to RCA adapter does not, because pin and dielectric differ, and a loose plug contacts the centre pin alone while the shield floats.
Cable choice for a short digital drop
For a metre or two, almost any 75 ohm cable with a moulded plug carries a digital audio stream. As the run grows, attenuation rises with frequency and thin foil construction loses the highs first. Quad shield helps where the cable shares a path with mains wiring.
Where the shield stops helping
Shielding blocks interference but does nothing for loss. A long run of thin cable with heavy shielding is still thin cable, so check the conductor before adding foil.
Blaming the input last
Before replacing an amplifier, try the same cable on another input, try a known good short cable, and confirm the source sends a format that input accepts. An input receiving a format it cannot decode goes silent rather than reporting an error, which sends people hunting a socket that works.
| Input type | Impedance expected | Connector | What a mismatch looks like | Fix |
|---|
| Digital audio coax | 75 ohm | RCA | Mutes, then re-locks | 75 ohm cable, short run |
| Antenna or cable TV | 75 ohm | F type | Weak channels, block errors | Seat the F nut fully |
| Satellite feed | 75 ohm | F type | No signal at the receiver | Replace the outdoor connector |
| Subwoofer LFE | 75 ohm, line level | RCA | Hum, low output | Shorter shielded cable |
Practical scenarios
- Soundbar silent on coax: the source sent a surround bitstream the input could not decode, and switching it to two channel PCM restored audio with no cable change.
- Wall plate with an F socket: a push on RCA plug held only by friction lost the shield contact, and a proper F to RCA adapter cured the intermittent drop.
- Fifteen metre antenna run: thin foil cable lost the higher channels, so the fix was heavier conductor rather than a new receiver.
- Hum on the subwoofer input: a long coiled cable picked up mains noise, and a shorter shielded run removed it.
FAQ
Can I use a 50 ohm cable on a 75 ohm input?
It passes a signal and reflects part of it. On analogue video that shows as ringing; on a digital stream it shows as dropouts.
Why does the plug feel loose?
Because it is the wrong family. An F plug on an RCA socket has no shield contact, and a BNC will not latch at all. Use an adapter that keeps the 75 ohm geometry.
Does cable length matter for digital coax?
Yes, through attenuation at the signal’s highest frequencies. A short run hides a poor cable, and the same cable fails once the run is long enough.
Next step
Identify the socket first with the coax connector types guide, then size the run using the attenuation versus frequency data, and read quad shield explained before choosing a cable for a noisy path.
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