An apparatus is provided for heating a hydrocarbon resource in a subterranean formation having a wellbore extending therein. The apparatus includes a radio frequency (RF) source, an RE antenna configured to be positioned within the wellbore, and an RF transmission line configured to be positioned within the wellbore and couple the RE source to the RE antenna. The RE transmission line defines a liquid coolant circuit therethrough. The apparatus further includes a liquid coolant source configured to be coupled to the transmission line and to provide a liquid coolant through the liquid coolant circuit having an electrical parameter that is adjustable.