Coax Splitter & Tap Loss Chart
The dB arithmetic behind every coax distribution problem: what each splitter costs, why the losses stack the way they do, when taps replace splitters, and the signal-level windows the design must land in. Splitter values are datasheet-typical; the DOCSIS limits are spec.
Splitter insertion loss
| Splitter | Loss | Why |
|---|---|---|
| 2-way splitter | 3.5 dB per output | 3 dB halving + ~0.5 dB insertion |
| 3-way balanced | 5.5–6 dB all outputs | — |
| 3-way unbalanced | 3.5 / 7 / 7 dB | Two cascaded 2-ways inside — low-loss leg feeds the modem |
| 4-way splitter | 7 dB per output | Two 2-way levels |
| 8-way splitter | ~11 dB per output | Three 2-way levels |
Signal-level targets (cable modem downstream)
| Level | Meaning | Basis |
|---|---|---|
| 0 dBmV | Ideal cable-modem downstream | rule-of-thumb |
| −7 to +7 dBmV | Recommended downstream window | guideline |
| −15 to +15 dBmV | DOCSIS 3.0/3.1 hard limits | spec |
Designing the tree
Start from the demarc level, subtract cable loss (per the coax comparison chart) and each split, and check every endpoint lands in its window. Two rules keep the tree honest: amplify before splitting, where the signal is still clean — an amp boosts noise too, and unused splitter ports get 75 Ω terminators, not air. On MoCA-equipped plants, a moca point-of-entry filter keeps the in-home moca band out of the neighborhood plant (stop-band edges vary by filter).
Common questions
How much signal does a splitter lose?
Every 2-way split halves the power (3 dB) and adds insertion loss — 3.5 dB per output is the working number. Splitters are internally trees of 2-ways, so the losses stack by level: 4-way = 7 dB, 8-way = ~11 dB per output. And the label figure is measured at the low band; at 1 GHz real loss runs 0.5–1.5 dB worse.
What signal level should a cable modem have?
Ideal is 0 dBmV downstream; −7 to +7 is the recommended window, and DOCSIS itself tolerates −15 to +15. The arithmetic is unforgiving: a +7 dBmV feed through an 8-way splitter lands at −4 — fine; through an 8-way plus 150 ft of RG6 at high band, it doesn't. Put the modem on a 2-way or the low-loss leg of an unbalanced 3-way.
What is a directional tap and when do you use one instead of a splitter?
On risers and MDU distribution. A tap drops a fixed, chosen level (values 4–35 dB) to each unit while passing the trunk through with minimal loss — higher tap value = lower through loss; the lowest-value tap ends the run. Cascaded splitters would starve the far end of the run; a ladder of taps delivers roughly equal levels to every floor.
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