Definition
A test instrument used to measure the resonant frequency of a tuned circuit without making any electrical connection to it. The meter contains a small variable-frequency oscillator with a plug-in coil that is held near the circuit being tested. When the oscillator's frequency matches the resonant frequency of the circuit, the circuit absorbs energy from the oscillator, causing a noticeable dip on the meter's display. The frequency at which this dip occurs is read directly from the instrument's calibrated dial.
Plain English
A handheld tester that finds the natural tuning frequency of a radio circuit by being held next to it. When you sweep through frequencies and the meter's needle suddenly drops, you've found the circuit's frequency.
Context Anchor
Seen in avionics, radio, and antenna maintenance discussions, especially when checking or adjusting older radio-frequency circuits.
Derivation
Named for what it does and how it does it. 'Grid' refers to the control grid of the vacuum tube originally used in the oscillator -- the current flowing in that grid is what the meter measures. 'Dip' describes the sudden drop in that current when the test circuit absorbs energy at resonance. So the name literally describes the visible event: the grid current dips.
Why Pilots Care
Pilots themselves rarely use one, but understanding the term helps when reading maintenance reports or talking with avionics technicians about antenna tuning, radio alignment, or troubleshooting issues with onboard radio equipment.
Analogy
It is a little like humming near a guitar string until the string responds. When the meter and the circuit are on the same frequency, the instrument shows that match with a dip.
Intuition Check
Do not read “grid” as a map grid or chart grid here. In this term, “grid” refers to an older radio-tube part used in the original design of the instrument.
Example Sentence 1
The avionics technician used a grid dip meter to confirm the VHF antenna was resonant at the correct frequency before signing off the installation.
Example Sentence 2
During the radio bench check, the grid dip meter confirmed that the tuned circuit was operating at the correct frequency.