Large Power Coupling Coils

In the tank circuits of large power amplifiers, the coupling coils are arranged to couple the antenna to the power amplifier, and the equivalent circuit is like that of Fig. 172. Optimum coupling between tank and antenna circuits is given by equation 103. The construction of the coupling coil itself is usually similar to that of Fig. 174(b), with the coupling coil on the outside and spaced from the main tank coil to reduce capacitive coupling. Taps are provided on the coupling coil for frequency and antenna resistance adjustments.

When the secondary circuit is untuned, and the secondary load is reactive, all the secondary volt-amperes (which may exceed the secondary watts many times) flow through the transformer windings. It is then necessary that tight coupling be used between primary and secondary in order to prevent loss of power, due to current circulating in the primary without corresponding current flowing in the load. If the load power factor is less than 20 per cent, currents and volt-amperes in the circuit may be considered independent of the winding and load resistances. In Fig. 171, let the load Z₂ be inductive, comprising L₃ and R_L. Also let

R₁ = 0
L₁ = primary self-inductance
L₂ = secondary self-inductance
L_m = mutual inductance
k = coefficient of coupling = L_m/√(L₁L₂)

[108]

This equation is plotted, together with values of ratio L₃/L₂ for maximum power transfer, in Fig. 181.

Fig. 181. Effect of coupling on maximum volt-amperes in untuned load.

If perfect coupling could be attained, all the primary volt-amperes could be transferred to the load. iron-core transformers operate at the extreme right of Fig. 181. With air-core transformers it is often difficult to approach this condition, especially if voltages are high.