Quick Answer: You cannot send HDMI down coax as HDMI. You convert it to a television channel with an HDMI to coax modulator, feed that channel into the existing coax splitter, and every television tunes to it. All screens show the same source, the ceiling is usually 1080p, audio is stereo or analog, and HDCP is stripped by the modulator, which is why this method works on old TVs and fails on copy-protected streaming apps.
The appeal is that the coax is already in the walls. A house built before Ethernet has one coax outlet per room and nothing else, and a modulator turns that dead infrastructure into a one-channel distribution system for a few tens of dollars. The cost is resolution, control and content rights, and those limits decide whether the project is a success or a complaint.
What the Modulator Actually Does
It takes an HDMI input, decodes it, and re-encodes it as an analog RF channel in the VHF or UHF band, the same way a cable box once did. You pick a channel number that is empty in your area, set the modulator to it, combine the output with your antenna or cable feed through a two-way combiner, and rescan on each television. From that point every TV behaves as if a broadcast station exists inside the house.
| Method | Best resolution | Independent feeds | HDCP content | Cost level |
|---|
| HDMI to coax modulator | 1080p, often 1080i | No | Blocked | Low |
| HDMI splitter plus long cables | 4K60 at short runs | No | Passes | Low to medium |
| HDBaseT over Cat6 | 4K60, 100 m | No | Passes | Medium |
| HDMI over IP matrix | 4K60 with 10GbE | Yes | Passes | High |
The HDCP Problem, Stated Plainly
Every streaming service and most discs assert copy protection. A modulator cannot pass that handshake to a tuner that has no HDMI port, so the source either refuses to output or the modulator refuses to encode. Many cheap modulators ignore the flag and produce a picture; that works until the source firmware tightens enforcement, and it does not make the practice lawful for redistributed content. In practice this method is reliable for a locally generated feed: a security camera page on a kiosk browser, a digital signage player, a laptop driving a training loop, or a set-top box that outputs unprotected HDMI.
Wiring Order That Avoids Ghosting
Modulator output to the combiner input, antenna or cable feed to the other combiner input, combiner output to the existing splitter, splitter outputs to the rooms. Keep the modulator as close to the splitter as possible, use a channel at least two numbers away from any local broadcast, and terminate unused splitter ports with 75 ohm caps. Then rescan every television, because most tuners will not show an analog channel that was not found during a scan.
Practical Scenarios
- Sports bar, six televisions, one satellite receiver: one channel everywhere is exactly the requirement, and the coax is already installed. Set the modulator once and label the remote.
- Whole-house signage on a legacy coax network: a signage player at 1080p into a modulator beats replacing every television with a networked model.
- One room that needs 4K: do not downgrade the whole house. Feed that room over Cat6 with HDBaseT and leave coax for the 1080p sets.
- Mixed old and new displays: where a set has no tuner, add a coax to HDMI converter at that one location instead of replacing the display.
Choosing the Modulator
Look for a selectable UHF channel rather than a fixed one, an adjustable output level so the channel does not overload the splitter, and stereo audio input. A unit with a loop-through HDMI output lets the local television stay digital 4K while the remote sets receive the analog channel. Our notes on an HDMI to coax modulator cover the settings that most often cause a weak or noisy channel, and the companion guide to an HDMI coax modulator compares fixed and agile models.
Channel Planning and Signal Level
Analog channels are sensitive to whatever else occupies the band. Leave two channels of guard space below and above strong local broadcasts, avoid the FM band entirely, and set the modulator output so the channel arrives at roughly the same level as the broadcast stations on the same splitter. Too high and the tuner overloads, producing a herringbone pattern; too low and snow appears. A two-way splitter costs about 3.5 dB per output, a four-way about 7 dB, so a modulator feeding eight rooms through three splitter stages will need an amplifier or a higher output setting. Label the chosen channel inside the media cabinet, because the next person to rescan the televisions will not guess it.
FAQ
Can I send 4K over coax this way?
Not with an analog modulator. Digital coax distribution of 4K uses QAM or IP gateways at broadcast-grade cost. For residential work, coax means 1080p.
Can different televisions watch different sources?
Only by using one modulator per channel, one channel per source, and having each television tune to a different channel. Beyond two or three sources, the cost and channel planning make a network solution cheaper.
Why is the picture grainy?
Usually an output level that is too high for the splitter, a channel adjacent to a strong broadcast signal, or a corroded splitter. Lower the modulator level first, then replace the splitter.
