In a frequency multiplier circuit, an inductance (L1) and a variable capacitor (C2) are connected in series between VCC+ and ground. The collector of a transistor is connected to a tap on L1. A fixed capacitor (C3) is connected between the VCC+ side of L1 and ground. The purpose of C3 is to:

In this circuit, C3 acts as a standard RF bypass or decoupling capacitor. It is placed between the DC power supply line (VCC+) and ground to provide a very low impedance path for Radio Frequency (RF) signals. By shunting any stray RF energy to ground, C3 prevents the RF signal from traveling back into the power supply, which could cause instability, parasitic oscillations, or interference with other parts of the radio. From an AC signal perspective, this low-impedance path effectively creates an "RF ground" at the VCC+ connection point of L1. This allows the L1/C2 tank circuit to operate properly as a closed AC loop without affecting the DC supply.