Since RF modulators are used to convert a coaxial signal to an RCA signal, an RF demodulator works in reverse to convert an RCA signal to a coaxial signal. If you need to convert the RCA signal from your audio/visual device to a coaxial signal, you can do so by creating a makeshift RF demodulator with a VCR and ordinary audio/visual cables.
Connect one end of your RCA cables to your audio/visual device. Connect the other end of your RCA cables to the RCA input ports on your VCR.
Connect one end of your coaxial cable to the coaxial output port on your VCR. Connect the other end of your coaxial cable to the coaxial input port on the back of your television.
Turn your television, VCR and audio/visual device on. Press the "Input" button on your VCR to switch to your audio/visual device's signal. You will see the signal from the device on your television, sent through the RF demodulator.