You’re still being vague, so I’m going to assume you want to watch TV on a computer monitor. Why am I assuming this? Because trying to bridge pure analog signals (like S-Video, component, composite, or SCART) with a hybrid VGA signal is a massive headache. You'd need an active converter, which isn't cheap, and most of those aren't even designed for monitor use—they're built to send a PC signal to a TV, not the other way around. By the way, when I asked about connectors, it doesn't matter what ports your monitor has; it's about what video outputs your graphics card actually provides. But whatever...
If you're just pulling a signal from a digital antenna receiver and sending it elsewhere, you're only getting one stream: whatever channel is currently selected. That would mean your monitor is basically just acting as a second screen mirroring your TV. Change the channel on the box, and it changes on the monitor too. They'll always show the exact same thing. I don't think that's what you're after; I'm guessing you want to watch two different shows on two different screens at once.
First question: are you picking up an over-the-air signal via an antenna, or are you getting your service through a provider via phone lines, Ethernet, or cable? If you're using a major US provider like Comcast or Spectrum, you're probably out of luck. Their set-top boxes are locked down specifically to prevent you from watching their content on multiple devices.
Now, if you really are just pulling an over-the-air signal from an antenna, then yeah, it's totally doable. You just need a tuner designed for a PC. These come as internal PCIe cards you slot into your desktop or as a USB flash drive that plugs right into a port. You feed the signal into them via a coaxial cable.
It looks something like this:
https://www.amazon.com/tv-tuners-0519One end plugs into your PC (if it's a USB flash drive style, obviously into a USB port), and the other end connects to the antenna cable providing the signal. Once you install the software, you can sit back and watch TV on your monitor. Better yet, you can record shows on a timer, play back recordings, or grab still screenshots. You control everything with your keyboard or mouse, and some even come with a remote so you can kick back on the couch.
Aside from letting you watch TV independently of your main receiver, these solutions are way cheaper than buying converters. For example, you won't need anything more expensive than this:
https://www.amazon.com/tv-tuner-usb...nski-051903087http://www.bestbuy.com/product.asp?produc...&code=19358ADMThe only trick is making sure you "tap into" the antenna signal before it even hits your receiver. You need the raw over-the-air signal. This is easy; you can grab a coax splitter with one input and two outputs at any hardware store or Best Buy for a few bucks. If you try to split the signal *after* it leaves the receiver, you'll only ever see the one channel that's currently playing on the box, so your USB tuner will be stuck on that single channel.
If I guessed right, great. If not, let me know... 😬