Like this box? This one only does UHF, which won't be suitable for our VHF only vintage TV sets. For placing multiple VHF NTSC channels from multiple HD converter boxes. To drive a house cable system.
http://www.surveillance-video.com/chpl54fochvi.html?productid=chpl54fochvi&channelid=SHOPC

http://cgi.ebay.com/RCA-TAG001-Cable-Service-Signal-Generator_W0QQitemZ230254605724QQihZ013QQcategoryZ 73173QQcmdZViewItem

This will generate any VHF or Cable channel. Use an RCA TV remote to select the channel. Needs 12 volts at about an amp. I've used one in my shop for about 12 or 13 years now. pretty cool.

I've been looking at these by Channelplus. Many variations available. Frequency agile for the channels we need when analog goes dark. It can be injected into analog cable if the vacant channel is not carrying a digital that we cannot see.

And if someone can figure out a way to shoot it to an antenna, that would be golden.

http://www.channelplus.com/products_rf.html

Dave A