Explanation: Copper oxide rectifiers, being an older technology, have significant internal or parasitic shunt capacitance. At radio frequencies (RF), this relatively large shunt capacitance presents a low impedance path for the RF signal. Recall that capacitive reactance (Xc) decreases as frequency increases (Xc = 1 / (2πfC)). A large capacitance at high frequencies effectively acts like a short circuit, shunting the RF signal *around* the rectifying junction of the copper oxide device. This prevents the rectifier from properly converting the AC RF signal into a measurable DC voltage. Consequently, the meter will either read very low or nothing at all, making the rectifier unsuitable for accurate RF voltage measurements. Modern silicon diodes, especially small signal types, are designed with much lower shunt capacitance, making them more suitable for RF applications.