Relative RF Power Meter

Add this handy device to your shack's arsenal.

The Relative RF Power Meter shown in Photo A is a. piece of test equipment useful for tuning RF circuits to maximum output power. The circuit does not measure absolute power, however; it instead measures the relative output power as the RF circuit is tuned.

When the meter shows maximum deflection, the output power is peaked. The meter has been tested from 2 MHz to I GHz over an input power range of approximately -10 dBm (0.1 mW) to 20 dBm (100 mW).

To use this instrument, the source resistance of the circuit being measured must be 50 ohms.

Circuit description

Fig. 1 shows the schematic diagram for the Relative RF Power Meter. J1 is the RF input. J2 is the connector for the 50-ohm terminator. The RF signal enters at JI. is rectified by D3. and filtered by FB1 and FB2, and charges Cl. RU, RIO, and R3 form a voltage divider which determines the meter's sensitivity to input power. Overvollage protection of Ul's input is provided by

R9 and D2. The gain of U1 is set at about 200. Offset voltage is eliminated by adjusting R5. R1 and R2 form a voltage divider with thcxenter connection used as a ground. When the meter is turned on. D1 is illuminated.

Construction

First, drill holes in the-front of the case for Jl, J2, Rll, R10, and the LED clip. Use a nibbling tool to cut a hole

Photo A. Front panel view-

34 73 Amateur Radio Today • May 2002

Photo A. Front panel view-

34 73 Amateur Radio Today • May 2002

J1 input 03 FB2 FB1 NTE583

J1 input 03 FB2 FB1 NTE583

Fig. J. Relative RF power meter .schematic.
Fig. 2. Potentiometer designations.

To P3© C To Battery GND

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