Well you can… but it takes one of these zVPro modulators or something similar to do it, and they’re awfully expensive for home use. This used to be something people did all the time, so the question is, what happened?
When TV went to digital, it all got more complicated. Instead of simple analog modulation, now all TV had to be converted to digital and that meant more expensive, licensed chips. It also meant dealing with content providers who didn’t want their signals going out in pure digital form all over the world, and keeping them happy means additional licensing fees. (Sound like a shakedown to you? Sounds like one to us as well.)
That’s a big part of the reason you can’t have a $30 HD modulator, but the biggest reason is that… well… people don’t want them. Sure some people do but the big reason people wanted RF modulators in the first place was because their old TVs didn’t have composite inputs. In the 1980s and 1990s there were millions of old TVs that didn’t have the right inputs for the VCRs and DVD players that people wanted. RF modulators were in high demand, and that brought costs down. Today, pretty much every TV has a component or HDMI input, and those that don’t have a composite input. RF modulators aren’t really the hot item they once were, and while there are still probably several thousand people who would use an HD version of the old RF modulator, that’s not enough to drive costs down. You’d need millions of people to do that.
Of course, the old RF modulators are still available, but they won’t provide HD. If that works for you, they connect easily to satellite receivers and other devices with SD outputs and can run a signal anywhere in the house on a single wire.