Note signal generator on left has shielded cable output. Both signal generator and radio connected to good ground connection.
In this circuit, the hot lead from signal generator connects to components in center (dummy antenna) and right to radio receiver inductance such as the antenna coil.
Normally, signal generator hot lead connects in series with capacitor to protect signal generator from high DC points in radio receiver. Check signal generator manufacturer instructions.
Rule of thumb guidelines. Use 100 nF capacitor at 600 volt rating for IF and AF signal into tube radio set. For RF signals use 250 nF at 600 volt rating capacitor in series with hot lead. For shortwave use a 400 ohm resistor.
