Sixty years ago, the January 1965 issue of 73 Magazine showed how to make this simple Grid Dip Oscillator (GDO or “Grid Dipper”.)

The GDO is handy for quickly measuring the resonant frequency of a tuned circuit or of an antenna. The GDO’s coil is exposed, and it’s poked into the coil of the circuit in question. For an antenna, a small loop of wire attached to the antenna goes around the coil. The frequency is adjusted, and at the resonant frequency, the meter showing the grid current drops precipitously. If you need the exact frequency, you can find the signal of the GDO on a receiver, or use a frequency counter.

This example uses a 3S4 tube, and has the convenience of being able to run from batteries. Of course, these days, the same thing can be done with transistors. Even though transistors have no grid, many old timers still refer to the instrument as a Grid Dipper. Hams these days are more likely to use an antenna analyzer, but the Grid Dipper gets the job done in an elegant analog fashion.