Hartley Oscillator Calculator

Ralph Hartley, AT&T researcher and later Bell Labs staffer, patented the tapped-inductor LC oscillator in 1915. His circuit was designed for long-distance telephone repeater applications but became foundational for early radio transmitters and receivers. Hartley also made major contributions to information theory — Hartley\'s law (bits per second = 2B·log₂(s)) is his eponymous result.

Hartley oscillators were the standard transmitter topology for 1920s-30s ham radio because matched variable capacitors for Colpitts were expensive, but tapped-coil Hartley needed only one variable cap to tune across a whole band. Every 1930s ARRL handbook devoted chapters to Hartley circuit design.

Hartley is rarely used in modern integrated RF designs because matched on-chip capacitors are cheap while multi-tapped on-chip inductors are expensive and lossy. It survives in discrete ham-radio and educational circuits, and in specialized applications like plasma-RF generators and induction heating — where the physical coil geometry makes a tapped inductor natural.

Enter L1 (collector/drain section, usually larger), L2 (emitter/source feedback section, usually smaller), tuning capacitor C, and optional coupling coefficient k (0 for separate coils; 0.5-0.9 for tightly-wound coils on a common former). The tool computes mutual inductance M = k·√(L1·L2), L_total = L1 + L2 + 2·M, and oscillation frequency f = 1/(2π·√(L_total·C)).

For reliable startup: L2/L1 ≈ 0.1-0.2 (β ≈ 0.1-0.15), and amp gain > 10× β reciprocal. Use silver-mica or NP0 ceramic for C (low temperature drift) and air-core or powder-iron coils for the inductor (low loss). Everything runs client-side.