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Dummy Load Parts

Dummy Load Construction

Introduction

Seeking an interlude from the Kingsdon project. I decide to build the Dummy Load needed for transmitter testing. I had thought this would be a quick job, as indeed it should have been..

Construction

The resistors I had were 100 Ohm, I decided on a layout to give 50 Ohms and a max of 16 Watts which would be ample for my QRP projects. I trimmed the resistors then found this arrangement would not fit in the box that I had already drilled for the PL259. Oh.. a rethink was required. I experimented a little and soldered the resistors in place. Unfortunately as the ends of the resistors had been trimmed right back (Long story) it was very hard to solder them together in the required configuration. Despite all this the dummy load seems to perform it's simple task..

Learning Points

Dummy Load Parts
Dummy Load MK 1

Conclusions

There will be a Mk 2

Dummy Load Mk 2

Dummy Load Parts
Dummy Load MK 2

Mk I worked fine until it developed a rattle as a blob of solder came loose. Clearly this was less than ideal. Time for a rebuild with carbon resistors and a different layout. Now it is mechanically reliable, has a lower SWR from HF to 70cm's, plus the PL259 was replaced with a four nut fitting version, so it no longer works loose. The PL259 socket with a single retaining nut is indeed, the work of the devil.

 

Dummy Load Mk 3

Dummy Load MK 3
Dummy Load MK 3

Mk 2 worked fine, but it would be very handy to have a way of measuring power. I considered adding a capacitor and diode inside the dummy load to form an integrator so that peak voltage could be read using a DVM and power calculated. But this would have been rather inflexible, so instead I opted just to add terminals so a scope can be directly connected or a capacitor and diode added, depending on requirements.

Power Calculation

1. Peak voltage measured across the capacitor (excluding diode drop)

2. RMS voltage equals the peak voltage divided by the square root of 2

3. Power equals the square of the RMS voltage divided by the load impedence