VK3AQZ RF power meter (RFPM1)

This article documents my build of Lou Destefano's (VK3AQZ) RF Power Meter kit.

The power meter is based on probes using an AD8307 logarithmic detector.

RMPM00

Above, the RF Power meter with two probes. (The ferrite sleeves were not part of the kit.)

Build

RFPMM01Above, a view of the top side of the meter PCB.

RFPMM02Above, the underside of the meter PCB. A single sided board that is an easy build.

RFPM-probe-tAbove is a view of the top of the probe boards. The top board is notched for an N type connector, but BNC connectors have been used on this build.

RFPM-probe-bAbove is a view of the underside of the probe boards. Parts alignment is less than perfect, but otherwise the build quality is good.

RFPM-probe

Above, the detail of my changes to the probe assembly. I used a Jaycar LF1260 ferrite sleeve to reduce the risk of common mode noise, and attached a male BNC cap to protect the connector when not in use. The screws used at the BNC end are M3x8mm SS button head screws (widely used in the RC model industry), the extra length provides good purchase for clamping the connector flange against the folded over copper foil. Once satisfied that the probe is stable, I may shrink some clear tube over the probe body.

Testing

RFPMtest

Above is the RF Power Meter under test. In this case, an automated test to explore the accuracy of the instrument.

Clip 050

A plot of red probe calibration at 10MHz. Each data point is an average of 10 successive readings.

Clip 051

A plot of blue probe calibration at 10MHz. Each data point is an average of 10 successive readings.

Both of these could be adjusted for a little better slope, but the calibration is unlikely to hold with temperature variation… it would be pretending that they are more accurate than they are.

Clip 052Above, the frequency response of the red probe from 0.5 to 500MHz. Each data point is an average of three successive readings.

Clip 053

Above, the frequency response of the blue probe from 0.5 to 500MHz. Each data point is an average of three successive readings.

References / links