this has been posted before.
Read this page.
https://antiqueradio.org/HomeTVTransmitter.htm
the tricky bit will be to find and Agile modulator that they are NOT asking too much for, Phil got his for under $30 but that was years ago before they knew what they could do, now they are not so cheap, but you may find other variants than he got that will work more affordable, AM60-550A AM40-550A AM60-450A and so on as is explained in the page.
the one I have is a SINGLE CHANNEL unit, BLONDER TONGUE MICM-b, non programmable, much less power than the Agile modulator, and not of much use as a transmitter w/o a RF power amp, these are factory set at their channels are VHF/UHF ones are getting very hard to find now, but the CATV ones seem to be all over the place.