Many passive field strength meters have appeared in the past, typically using a 50mA analog meter movement if reasonable sensitivity was to be obtained. This circuit is similar but has the advantage that it works with the high-impedance load of a digital multimeter, typically switched to the 200mV range. The sensitivity is adequate for low power equipment like CB radios, cordless phones and model R/C sets (cars, model airplanes, etc).
For best results, use OA81 or similar germanium diodes. Modern Schottky signal diodes could also be used but the results are not as good. The circuit can be wired directly into a small plastic box with protruding banana posts to match the terminals on your DMM. A banana jack can also be used for the antenna which could be just a 500mm length of wire as a starting point.
Author: Gerard La Rooy Copyright: Silicon Chip Electronics