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Chu Harrington Limit
Often referred to as the Chu limit, this is a graphical limit line on a chart relating a radio antenna's volume to the Q that the antenna delivers.

Some antennas are physically enormous. Some, like the antenna structure in a cell phone are only a few centimeters across. All antennas need to be thought of, not by their size in meters or feet, but in wavelengths. The active element of a 900kHz AM broadcast antenna might be 83 meters tall, and the active element of a 70cm amateur radio antenna might be .18 meters tall but both of these are one quarter wavelength in length. Both of these are the same size electrically.

The x axis of the Chu limit chart is the volume of the antenna in wavelengths.