Building a Hendricks Dummy Load and Power Meter

Having just completed building a Hendricks PFR-3 rig, I noticed that whenever I transmitted, the power output as read on my MFJ-941E power meter showed around 10W. This was strange as the rig is rated at around 5W.

I posed the question as to why on the PFR-3 forum and the general consensus was that these meters are not really accurate at QRP levels. What I needed, I was advised, was a piece of gear with which I could read RF voltages such as the Hendricks Dummy Load and Power Meter.

I placed my order and two weeks later the kit arrived. Time to melt solder once more.

The contents of the kit.

I started by soldering all the resisters, and there are a lot of them.

It´s handy using a vise to hold the PCB steady for soldering the leads of the resisters.

The dummy load is very neat and tidy and attaches to the BNC connector of the rig.

A piece of spare resister wire is used to connect the resisters to the BNC connector.

One the dummy load was complete (it took about an hour to build), I connected it up to the PFR-3 rig and applied 12V DC. Then it was a simple matter of taking a DC voltage reading with a digital multi meter across the two little terminals on the top of the PCB, and using the formula P=(E+0.3×0.707)squared/50, I would have the power put out by the rig. The formula takes into account forward voltage drop across the 1N5711 diode in the dummy load. That is the 0.3v added to the voltage E as read on the digital multi meter.

The dummy load in position ready for measurements.

So now I know that my rig puts out 5.9W on 20m, 5.15W on 30m and 5.29W on 40m.

Brilliant.

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