You cannot connect a coaxial cable directly to an HDMI port, since they carry completely different types of signals. What you need depends on what’s connected to the other end of the coaxial cable: an antenna, a cable or satellite receiver, an old VCR, or a game console.

Your source (coax)	What you need	Typical price
Over-the-air antenna	ATSC digital converter box with HDMI output (a tuner)	about $20–$50
Cable TV or satellite box	Usually nothing: use the box’s own HDMI output	$0 (HDMI cable only)
Old VCR, DVD recorder or retro console with RF/coax output only	RF-to-HDMI converter (with analog tuner)	about $25–$60
Same old device, but it also has yellow/red/white RCA jacks	AV (composite)-to-HDMI converter. Better picture than RF	about $10–$25
HDMI source, TV with only a coax input	HDMI-to-coax RF modulator	about $30–$80

First, check whether you actually need an adapter—surprisingly, many people buy adapters they don’t need.

• Antenna and a modern TV: most TVs sold in the US in the last decade have a built-in digital tuner. Plug the antenna cable into the TV’s ANT/CABLE IN port and run a channel scan. No converter needed.
• TV has no antenna port at all: this is the case where you need a converter box.
• Cable or satellite box: the provider’s box almost always has an HDMI output. The coax cable goes into the box, and an HDMI cable goes from the box to the TV.

Connecting an Antenna to a TV or Monitor via HDMI

An antenna receives radio-frequency signals that must be tuned and decoded before they can be displayed on a screen. This is handled by a digital TV tuner compatible with the broadcasting standard used in your region, such as ATSC or DVB.

Most TVs have a built-in tuner, so you can connect a compatible antenna directly to the TV’s antenna input. However, if your TV or monitor does not support the required broadcast standard or lacks an antenna input, you will need an external digital TV converter with an HDMI output. The converter receives the antenna signal through a coaxial cable, decodes it, and sends the video and audio to the display through HDMI.

What you’ll need: an antenna, a coaxial cable with F-type connectors, a digital TV converter with an HDMI output, an HDMI cable, and the converter’s remote control.

How to connect the antenna

1. Connect the antenna’s coaxial cable to the ANT IN port on the digital converter.
2. Connect one end of the HDMI cable to the converter’s HDMI OUT port and the other end to an available HDMI port on your TV or monitor.
3. Plug the converter’s power adapter into an electrical outlet and turn on both the converter and the display.
4. Select the HDMI input to which you connected the converter.
5. Using the converter’s remote control, open its menu and start an automatic channel scan. Wait for the scan to finish so the converter can detect the available channels.
6. If some channels are missing or reception is poor, adjust the antenna’s position and run the channel scan again.

Connecting an Old VCR, DVD Recorder, or Retro Game Console to HDMI

Many older devices output video via an RF (coaxial) connector, usually on channel 3 or 4. To display this video on a TV with an HDMI port, you’ll need an RF-to-HDMI converter equipped with an analog tuner.
Before buying: check the back panel of the device. If it also has yellow, red, and white RCA jacks (composite signal), use them with an AV-to-HDMI converter. The picture will be clearer, and these converters are cheaper and easier to use.
Setting up the RF-to-HDMI converter:

1. Connect a coaxial cable from the RF OUT port on your old device to the coaxial IN port on the converter.
2. Connect an HDMI cable from the HDMI OUT port on the converter to your TV.
3. Turn on the converter, your old device, and the TV.
4. Set the RF output of the old device to channel 3 or 4 (many have a small switch on the back panel).
5. Set the converter to the same channel (3 or 4).
6. Switch the TV to the corresponding HDMI input.

HDMI-to-Coaxial Connection for a TV with Only a Coaxial Input

If you have an HDMI signal source, such as a streaming device, set-top box, or game console, and an older TV with only a coaxial input, you will need an HDMI-to-RF converter. This device converts the HDMI signal into an RF signal that can be transmitted to the TV through a coaxial cable.

1. Connect the signal source to the converter’s HDMI IN port.
2. Connect a coaxial cable from the converter’s RF OUT port to the TV’s antenna or coaxial input.
3. Turn on the converter and set its output channel, typically channel 3, channel 4, or another available channel.
4. Scan for channels on the TV or manually tune it to the same channel selected on the converter.

The picture quality will be lower than with a direct HDMI connection. Most RF modulators are limited to standard-definition resolution, although some models output a digital signal that complies with a digital TV broadcasting standard.

Check the connectors before purchasing a converter. Some devices use a BNC connector for converting HDMI signals to coaxial connections. BNC connectors are commonly found in security camera systems and professional video equipment. Although they may look similar to the connectors used for antenna cables, they are not compatible with the standard F-type connectors found on most TVs and antenna cables. Always check the product description to ensure that the device has a Type F connector or explicitly supports coaxial RF output.

You can buy a converter like this on Amazon