Schottky Field Strength Meter

Posted on Jun 14, 2012

When an antenna is attached, (or even if not, in the presence of a strong RF field) the field strength meter has a moving coil meter to indicate relative field strength. The box holds a Schottky diode detector, a DC amplifier, and a meter display. If the parts used in construction are the same values shown in the schematic, the meter shows full scale (`+3db` on the surplus store VU meter movement) when the output of the Schottky diode detector is between 1.6 millivolts and 160 millivolts, depending up the setting of the gain control. Since opamps are used as amplifiers, the range of the gain control can be modified easily. The actual sensitivity in terms of field strength is primarily a function of the antenna system, and that is well beyond the scope of this web page.

Schottky Field Strength Meter
Click here to download the full size of the above Circuit.

An RCA connector with which you can attach an antenna (as in the photograph near the top of the page), of if desired signal preconditioning circuits such as a filter and/or preamp. I expect that for most of my low power uses, a short whip antenna or no antenna at all will be sufficient. A stereo headphone jack that provides a way to connect the amplified detector output to a digital voltmeter, chart recorder, oscilloscope, or other instrument, and also allows the application of an offset voltage from an external device. I had imagined adding an external logger with an automatic offset adjust function. An offset knob allows correction for drift in the Schottky detector as well as offsets created by interfering signals. A coarse offset control on the circuit board allows for compensation for the mismatch between components. A power switch and pilot light complete the set of user controls. The Schottky diode detector is built on a single sided fiberglass circuit board that is separate from the amplifier board. This was mainly because the detector board needed to be close to the RCA input connector, which would be on the front panel. With the presence of the other connector and the controls on the front panel, there was no room for a larger circuit board to accommodate the DC amplifier. Whip antennae require a ground structure in order to work. The large copper areas on the detector board, as show in the...

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