Assuming that RF stands for the usual (Radio Frequency), an RF Voltmeter is a piece of electrical test equipment designed to measure the voltage of electrical signals whose frequencies are in the radio frequency range (usually considered to be anything above about 1 MHz. The frequency coverage of any one such Voltmeter can vary considerably depending on the application. If you are interested mainly in AM broadcast radios, for example, then a meter covering 0.5 to 2 MHz is probably sufficient. For FM broadcasts, the range would have to be increased to at least 110 MHz. For WIFI and other digital radios, the range would again have to be increased to several GHz. Most RF Voltmeters have an input impedance of 50 ohms since that matches the impedance of RF transmission lines and is a widely used impedance level in high frequency circuits. The input level range again will vary with the application but can range from microvolts to volts, with the former for measurements on receivers and the latter more for measurements on signal generators and low power transmitters. RF Voltmeters are manufactured by many companies including Agilent. Just google RF Voltmeter for more information or,if you provide a more specific question, I will expand this response.