A dip meter is used for adjustment of radio equipment and antennas. The DM-81 is a self-excited oscillator designed for external coupling to the equipment being tested. It features both inductive and capacitive coupling for measuring enclosed coils and toroidal coils (patent pending). This is not possible with conventional testing instruments. The DM-81 has the following two functions: Inductive coupling As shown (Figure 1A), place the coil unit of the dip meter in close proximity to the circuit being measured. Adjust the dial. When the dip meter oscillation frequency coincides with the resonant frequency (tuned circuit), oscillating energy is absorbed by the circuit, thus decreasing the oscillation strength. This strength is indicated on the meter. The pointer swings back momentarily at the resonant point. Since the meter pointer dips at a tuned point, this instrument is called a dip meter. Capacitive coupling This is a special feature of the DM-81 which is not found in any other dip meter. The resonant frequency can be checked simply by touching the capacitance probe to the hot side of the resonant circuit under test, instead of coupling the DM-81's coil and measuring inductively. The frequency is read directly on the dial. See Figure 1B. Radio equipment is miniaturized and most coils are enclosed in metal shields. Also, toroidal coils are used in many types of radio equipment, and these coils do not couple to conventional dip meters. The DM-81 has solved this problem.