An off-air antenna system is one of the simplest coaxial installations there is, and one of the least forgiving. The signal arriving at the antenna is weak, digital reception does not degrade gradually, and every avoidable loss in the chain comes straight off the margin.
Almost everything that goes wrong in these systems is a loss or an impedance problem.
Quick answer
| Question | Answer |
|---|---|
| What cable for an antenna run? | RG6, 75 ohm |
| Does the run length matter? | Yes. Loss accumulates with distance, in proportion |
| Where does a preamp belong? | At the mast, before the cable loss |
| Can I use any splitter? | Only one rated for the band, with unused ports terminated |
| Does the antenna’s output impedance matter? | Yes. Everything in the chain should be 75 ohm |
| What happens when the signal is marginal? | Digital reception fails abruptly rather than getting gradually worse |
Everything in the chain is 75 ohm
The domestic television system is a 75 ohm world: the antenna output, the cable, the splitters, the wall plate and the television’s input. Mixing in a 50 ohm component — a cable from the radio drawer, a connector from another system — introduces a mismatch, and a mismatch reflects part of the signal instead of passing it. The impedance distinction is covered here.
Older antennas with a balanced 300 ohm output are the one legitimate exception, and they need a balun to convert to 75 ohm unbalanced. Modern antennas with an F socket are already 75 ohm, and adding a balun to one of those does nothing useful.
Loss is the whole design
An antenna run has a source level set by the antenna and the signal strength at the location, and a receiver that needs a certain minimum. Everything between them is loss.
| Source of loss | How it scales |
|---|---|
| Cable | With length, and more at higher frequencies |
| Splitters | With the number of outputs |
| Connectors and joints | With the number, and much more when badly made |
| Wall plates and patch leads | Small individually, but they add up |
The attenuation table gives the cable figure at the frequencies in use. RG59 has noticeably more loss than RG6 at the same length, which is why RG6 is the default for anything beyond a short patch.
Why digital reception fails suddenly
Analog television degraded gracefully. A weak analog signal produced a snowy picture, and viewers learned to tolerate a certain amount of it.
Digital reception has a threshold. Above it, the picture is perfect. Below it, the picture breaks up or vanishes. There is very little in between.
That changes the design objective. The aim is not a signal that is good enough on a sunny day, but a signal with margin — enough headroom that rain attenuation, a slightly less than ideal connector, or a new splitter added later does not take the system below the threshold.
Where a preamp helps, and where it does not
This is the most commonly misunderstood part of antenna work.
A preamp at the mast, ahead of the cable, takes the antenna’s signal and lifts it before the loss happens. The cable then removes its share of a larger number, and the signal reaching the receiver is stronger and the noise figure of the system is better. It genuinely helps a long run or a weak signal.
An amplifier at the receiver end takes a signal that has already lost its margin and multiplies it, noise included. The level on the meter goes up; the quality does not. Sometimes it gets worse, because a strong signal overloading the receiver’s input produces distortion.
The rule that follows: amplify at the source or not at all, and remember that an amplifier cannot create signal-to-noise ratio that the antenna never delivered.
Grounding, which is a safety matter
The mast is an outdoor metal structure, and the coaxial cable connects it to the building’s wiring. Both need attention.
The mast should be bonded to the building’s grounding electrode system, and the coaxial cable should pass through a grounding block at or near its entry point, which bonds the shield. Your local code governs the details, and it is worth getting right because a surge on the antenna system has a direct path into the building’s equipment. The grounding arrangement is covered separately, and it applies to any outdoor coaxial run.
Scenarios
A single television on a roof antenna. One RG6 run, compression F connectors, and a wall plate. No splitter, so no splitter loss.
Four televisions on one antenna. A single four-way splitter near the head end rather than cascaded two-ways, because each stage adds loss. Check the budget; a preamp at the mast may be needed to make it close. The splitter arithmetic is here.
Reception that works in good weather only. Before adding anything, check every connector, then look for corrosion at the outdoor joints. A failing connector looks exactly like a marginal signal.
A distant transmitter and a long cable run. Preamp at the mast, RG6, and as few joints as the layout allows.
A house where one outlet is worse than the others. That outlet’s lead, connectors and wall plate are the first suspects. A problem that is uniform across all outlets points at the antenna or the splitter.
FAQ
What coax for a TV antenna? RG6, 75 ohm. RG59 works over short distances but has higher loss.
How long can an antenna cable be? There is no fixed limit. Work the loss budget: cable loss at the frequencies in use, plus splitters and joints, against the margin the receiver has.
Do I need a preamp? Only for a weak signal or a long run. Put it at the mast, not at the television.
Can I use two splitters in series? You can, and each one costs about 3.5 to 7 dB. Fewer, larger splitters usually work better than several small ones.
Why does my picture break up but not disappear? A digital signal near its threshold produces exactly that. The fix is margin, not a different television.
Does the antenna need grounding? Yes, and so does the coaxial cable where it enters the building. That is a code matter.
Related reading on this site
- Coax attenuation: why loss rises with frequency
- Coax splitter loss
- RG6 vs RG59
- Coax grounding and surge protection
- Satellite dish coax runs