Any VCR with antenna output has a built-in RF modulator.
Generally they transmit on channels 2-4. Here are the US frequencies:
from wikipedia http://en.wikipedia.org/wiki/Television_channel_frequencies
The signal is weak, not intended for broadcast. Not legal to broadcast… but hypothetically, amplifiers and antennas could be connected.
This audio signal is from an iPod playing through a VCR received by rtl-sdr in Max on 65.75 MHz. (channel 3) using a random length wire connected to the antenna output.
Notes about RF modulators:
I have tried this with some small RF modulator boxes. One of the problems is that you need to send a signal to the video input, or the modulator won’t run. You can get a rough signal by patching one of the audio channels into the video input jack. Or a better signal by using the video composite output of a raspberry-pi. Although the VCR gives a much cleaner signal