N5ESE's RF Power Meter

Sorry! Under Construction! but here's what we have so far...

I'm not sure I'll ever finish this one, but I'll post it here because it illustrates a good application for an Altoids tin ;-)

This project was patterned after an article in SPRAT (Issue 48, Autumn 1986) by Ian Braithwaite, G4COL. I was fascinated with this unique rf-power measuring circuit based on detecting power in an incandescent lamp. Some have claimed claimed 30 dB or more of accurate power measurement range down into the milliwatt range, and I built it to see if that was possible. I was unable to duplicate those results.

The idea of the circuit was to apply RF power to the lamp, such that it lit somewhat (but not too much). The lamp formed the first part of an optical coupler, illuminating a sensitive photocell. In the picture below, you can see the coupler marked "Pwr Coupler" and you can see the base of the bulb on the right, and the leads of the photocell on the left. Black heatshrink covers the light path, so that low light levels (corresponding to low RF power) could be detected.

[Click on the above image to see a larger version]
A servo (amplifier with feedback) circuit maintained constant current into the lamp, and provided a measure of the current it was providing to an external Digital Voltmeter. If the rf power into the lamp increased, the servo circuit decreased it's current, so that the entire lamp current stayed the same. By measuring the difference between the servo circuit's output both with and without the RF applied, one could accurately calculate the input power.

There were three reasons I abandoned this project:

  1. The measurement dynamic range was only about 10-15 dB;
  2. Having to use a calculator was NOT a feature I liked;
  3. Some RF signal "leaked" around my 20 dB attenuators.

All the same, this was a fun investigation, and I learned that "simple" carries with it some highly probable limitations.

Click -here- for an additional text narrative explaining the therory behind the circuit, from Alex, AI2Q.

Monty N5ESE

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Overseer: Monty Northrup ... n5ese@n5ese.com ... leave e-mail ...