You need a card or adapter that takes the external out from vmix and converts it to analog signal. Personally i have the blackmagic intensity 4k pro card that has analog (rca/s-video/composite) cables that you can connect in an analog source. Or other solution (if you allready have an external card that gives hdmi out or sdi) take a (cheap but also crap) hdmi/sdi to analog converter to do the connection. Now if need RF modulator because the analog tv transmitter doesnt have rca/scart connection input connection and only RF you need a proper adapter