Quick Answer: No passive adapter turns HDMI into coax. HDMI is four shielded pairs carrying TMDS at 18 Gbit/s for 2.0 and 48 Gbit/s for 2.1, while coax carries one modulated channel over 75 ohm. A working product is always a pair: a transmitter that converts HDMI into a modulated RF channel or a baseband stream, and a receiver at the far end. Budget two boxes, power at both ends, and RG-6 rather than RG-59 beyond 100 ft (30 m).
A 6 MHz television channel carries about 19 Mbit/s of usable payload after error correction, while uncompressed 1080p60 at 8 bit needs 3.7 Gbit/s. That ratio explains every product in this category: conversion, compression, or both, never a cable with two different plugs on the ends.
Why a passive adapter cannot exist
HDMI sends four differential pairs at 100 ohm plus a low-speed control channel,. Coax is a single 75 ohm conductor with a screen, so the impedance, the pair count and the signalling all differ. A balun can match impedance for composite video because that signal is one analogue channel under 6 MHz; it cannot carry 3.7 Gbit/s of TMDS.
Two architectures that do work
RF modulator into a spare channel
A modulator takes the HDMI input, decodes it and re-encodes it as a television channel that any tuner can receive. In the US that means an analogue channel on 3 or 4 for legacy distribution, or an ATSC encoder for digital sets. One 6 MHz channel carries one programme, so four sources need four modulators and four channels.
Baseband extender pair
An extender pair keeps the HDMI signal digital and sends it as a baseband stream over one 75 ohm cable, with an equaliser at the receiver. This is the option for a single link between a source and a display: 1080p to 300 ft (90 m) and 4K30 to 150 ft (45 m) are typical published figures. Latency stays under 1 ms.
Options compared
Resolution limits below assume one RG-6 run with compression F connectors and no intermediate splitters.
| Option | Max resolution | Latency | Coax run | HDCP 2.2 |
|---|
| Analogue RF modulator | 1080i | 20 to 60 ms | Whole building | No |
| ATSC encoder and modulator | 1080i | 40 to 120 ms | Whole building | No |
| Baseband extender, 1080p | 1080p60 | under 1 ms | 300 ft (90 m) | Yes |
| Baseband extender, 4K30 | 3840 x 2160 at 30 Hz | under 1 ms | 150 ft (45 m) | Yes |
| Encoder and decoder over IP | 1080p60 | 80 to 200 ms | Via network | Yes, if rated |
Coax specification for the run
Use 75 ohm RG-6 with an 18 AWG copper centre and a foil plus braid screen of at least 60 per cent coverage. RG-6 loses roughly 6 dB per 100 ft (30 m) at 1 GHz, and RG-59 loses about half as much again, so a 150 ft (45 m) RG-59 run can cost 18 dB before the receiver equaliser even starts. Fit compression F connectors rather than twist-on types,.
Practical scenarios
Where one source has to drive two displays before the coax stage, the splitter is the part that decides which resolution both ends receive; the EDID behaviour compared in best hdmi splitter is the reason a 4K panel and a 1080p panel settle on the lower timing.
- Situation: Lobby display 120 ft (36 m) from a media player. Use a baseband extender pair on RG-6, keep 1080p60.
- Situation: Four sources to twelve legacy TVs on existing coax. Use four analogue modulators on channels 3, 4, 7 and 9,.
- Situation: Bar with satellite boxes and a 4K panel. A modulator will not carry 4K, so run the panel from a baseband extender or an HDMI matrix and leave the RF network for the 1080i sets.
- Situation: Retrofit where only RG-59 is in the wall. Limit the baseband link to 100 ft (30 m) and 1080p, or pull RG-6; a 4K30 link on 150 ft (45 m) of RG-59 will not lock.
Failure symptoms
A picture that sparkles white or drops for a frame at a time is a link margin problem, not a source problem, so shorten the run or fit a better cable. No picture with a valid signal usually means the receiver is not completing the HDCP handshake, and the fix is to put both ends on the same power strip and connect the display first. A system that works at 720p but fails at 1080p is running out of bandwidth, which is where a certified hdmi cable on the short legs and a higher-grade coax on the long leg both matter. Choose the HDMI leg by the bandwidth table in hdmi 1.4 vs 2.0 rather than by price.
FAQ
Can I use a coax output on a set-top box to feed HDMI?
Not directly. The coax output is a modulated RF channel intended for a tuner, so it needs a demodulator or a TV tuner at the far end before anything can drive HDMI.
Does an HDMI to coax converter carry audio?
Yes, because the audio is embedded in the HDMI stream and travels with it. An RF modulator places that audio on the channel’s subcarrier, and a baseband extender keeps the original format.
How many displays can one transmitter feed?
A baseband pair is one to one, with no splitting on the coax. An RF modulator reaches every tuner on the same coax network. For two HDMI displays from one source, a splitter is the correct part, and one 6 MHz channel still carries one programme, so an RF network needs a modulator per source.
