Quick answer: A cable modem needs a 75 ohm RG6 drop with as few splits as possible, compression F connectors, and a splitter rated to at least 1 GHz. Every two-way split costs about 3.5 dB, so a modem four splits deep from the entry point has already lost 14 dB before the cable is counted. Downstream bonding tolerates loss better than the upstream path, which is why instability usually appears as rising transmit power rather than as a slower download.
The cable between the wall and the modem looks trivial and is the most common cause of intermittent internet. Providers publish upstream and downstream power windows, and when the modem sits at the edge of the upstream window it will drop channels and re-register, which the user experiences as a connection that dies for a minute several times a day.
What travels on the cable
Downstream and upstream bands
Downstream channels occupy roughly 100 to 1000 MHz in most plants, and DOCSIS 3.1 can extend higher. Upstream sits low, typically 5 to 42 MHz or up to 85 MHz on upgraded networks. The upstream band is where ingress hurts most, because household noise sources such as cheap chargers and bad connectors radiate into it easily.
Why the return path is fragile
The modem transmits upstream and the node receives it after passing through every splitter and connector in the path. Each of those adds loss the modem must compensate for by transmitting harder. Once it reaches its maximum output, it cannot compensate further and the channel drops. That is why a single loose F connector can take a whole connection down while the downstream signal still looks fine.
Splitters and loss budget
| Element | Loss | Notes |
|---|
| Two-way splitter | 3.5 to 4 dB | Per leg, plus 0.5 dB insertion |
| Four-way splitter | 7 to 7.5 dB | Per leg |
| RG6 at 1 GHz, 10 m | About 2 dB | RG59 is roughly 3.5 dB |
| Poor F connector | 1 to 6 dB, variable | Often frequency dependent |
| Flat window cable, 3 m | 2 to 3 dB | Avoid on a modem feed |
Getting the physical layer right
One split to the modem
The best arrangement is a two-way splitter at the entry: one leg to the modem, the other to the television distribution amplifier. If the modem must sit further downstream, use a balanced splitter rather than a tap, and avoid stacking splitters to make up a four-way: two two-way splitters in series lose 7 dB plus two insertion losses, which is worse than one four-way.
Connectors and cable
Use compression F connectors on RG6 with a foil plus braid shield. Twist-on connectors work loose and let ingress in through the shield gap. Keep the bend radius at 10 times the cable diameter, and do not staple through the cable: a crushed dielectric changes the impedance at that point and reflects energy back toward the modem.
Reading the modem’s numbers
The modem’s status page shows downstream power, signal-to-noise ratio and upstream transmit power per channel. Downstream around 0 dBmV plus or minus a few dB is healthy, SNR above 35 dB is good, and upstream transmit power well below the maximum means the return path has margin. If upstream sits near the ceiling, remove a split before replacing the modem.
For a permanent installation, run a dedicated RG6 from the entry point to the modem and terminate it on a wall plate rather than through a hole in the floor. The RG6 cables overview covers shield construction, the RG6 vs RG59 comparison explains why the thinner cable is a bad idea on a modem, and the coaxial connectors and adapters page covers fittings that keep the impedance constant.
Grounding and bonding
The coax shield is bonded to earth at the point of entry, which protects the building and gives interference a path away from the signal. Inside the house the shield should stay continuous: a splitter with a poor ground, or a wall plate that does not bond the shield, turns the cable into an antenna. Do not add a separate earth to a splitter unless the provider specifies it, because two earth paths at different potentials create a current in the shield that shows up as hum on analogue channels and as upstream noise on the modem.
Practical scenarios
- Modem and three televisions: one two-way split at the entry, modem on one leg, a four-way amplifier on the other, and the amplifier powered from the same outlet as the modem.
- Internet drops every evening: check upstream power first; if it climbs in the evening, ingress from a neighbour’s appliance is likely and a new drop may be needed.
- Moving the modem to another room: extend with RG6 and a compression splice or a wall plate, never with a flat cable under a door.
- New build: pull two RG6 to the modem position, one active and one spare, because a damaged drop is expensive to replace later.
FAQ
Does a better coax cable increase internet speed?
It can restore speed that loss was taking away. Speed tiers are provisioned by the provider, so a healthy drop does not exceed the plan, but a marginal drop causes dropped channels and slower peak-time performance. Fixing connectors and removing splits often recovers stability without changing the plan.
Can I use RG59 for a cable modem?
It will often work at short lengths, but RG59 loses about 50 percent more than RG6 at 1 GHz and has a weaker shield, which matters on the upstream path. For a temporary link it is acceptable; for a permanent drop, RG6 is the minimum.
Do I need a filter on the modem line?
Modern installations do not use the old low-pass filters that separated television from data, because the services share the spectrum. If a filter is present from an older installation, ask the provider before removing it; some plant designs still rely on it.
